Issuu

Geological Society of Australia NUMBER 14

ABSTRACTS

THIRD CIRCUM-PAGIFIC TERRANE CONFERENCE EXTENDED ABSTRACTS

SYDNEY, 1985

Geological Society of Australia Abstracts NUMBER 14

THIRD CIRGUM-PAGIFIC TERRANE CONFERENCE

EXTENDED ABSTRACTS

EDITED BY EVAN LEITCH UNIVERSITY OF SYDNEY 1985 (i)

THIRD CIRCUM-PACIFIC TERRANE CONFERENCE

Organising

Committee E. C. Leitch

Convenors

E. Scheibner

M. J. Lennox

Secretary B, J, J. Embleton

M- W. McElhinny

D. G. Howell

G. M. Philip

N. L. Markham

S. Uyeda

Program Committee

B. J. J. Embleton

E. C. Leitch

R. A. Glen

E. Scheibner J. Roberts

Published by the Geological Society of Australia Incorporated Challis House, 10 Martin Place, Sydney ISSN

0729-011X

ISBN 0 909869 41 3

(ii)

ACKNOWLEDGEMENTS Many p e r s o n s , companies and other organizations have r e n d e r e d a s s i s t a n c e m u c h a p p r e c i a t e d by the O r g a n i s i n g C o m m i t t e e of the T h i r d C i r c u m - P a c i f i c T e r r a n e C o n f e r e n c e . We p a r t i c u l a r l y a c k n o w l e d g e the a s s i s t a n c e r e c e i v e d from the following c o m p a n i e s and organizations:

A m e r i c a n A s s o c i a t i o n of P e t r o l e u m A u s t r a l i a n A c a d e m y of

Circum-Pacific Council for Energy Mineral Resources CRA E x p l o r a t i o n Pty CSR

Geologists

Science and

Ltd

Earth Resources Esso Australia

Foundation Ltd

G e o l o g i c a l Society of

Australia

G e o l o g i c a l S u r v e y of N e w S o u t h W a l e s , D e p a r t m e n t of M i n e r a l R e s o u r c e s Goldfields E x p l o r a t i o n Pty New Zealand Geological Placer Pacific Pty SANTOS

Ltd

Survey

Ltd

Stanford

University

U n i v e r s i t y of

Sydney

Western Mining Corporation

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Ltd

NOTES

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EDITORAL

STATEMENT

The initial p o l i c y of the o r g a n i s i n g c o m m i t t e e was to r e p r o d u c e a b s t r a c t s as r e c e i v e d , except for those from c o n t r i b u t o r s who do not h a v e E n g l i s h as a first l a n g u a g e . A n u m b e r of a b s t r a c t s in the latter c a t e r g o r y h a v e u n d e r g o n e some editing and h a v e been r e t y p e d . B e c a u s e of time c o n s t r a i n t s it has not been p o s s i b l e to d i s c u s s m o d i f i c a t i o n s with the a u t h o r s p r i o r to p r i n t i n g , and I a p o l o g i s e both to the a u t h o r s and to r e a d e r s for any c h a n g e s of m e a n i n g that h a v e a r i s e n . Not all a u t h o r s were able to comply with our layout and o r g a n i s a t i o n i n s t r u c t i o n s and in o r d e r to m a i n t a i n some c o n s i s t e n c y amongst the c o n t r i b u t o r s several o t h e r a b s t r a c t s were also p a r t l y or c o m p l e t e l y r e t y p e d . No a t t e m p t was m a d e to edit these c o n t r i b u t i o n s aside from c o r r e c t i n g o b v i o u s t y p o g r a p h i c a l errors, and the p o s s i b i l i t y that some such errors h a v e been i n t r o d u c e d during retyping is admitted. E. C.

(V)

Leitch

Abstracts are arranged in alphabetical order. Those that pertain to poster displays only are indicated by the letter 'P' in the top right hand corner.

(vi)

EOCENE TECTONICS AND SEDIME^TIATION IN SOUTHWEST OREGON AND THEIR RELATIONSHIP WITH THE ORIGIN OF OREGON CASCADE VOLCANIC ARC Raisuddin Ahmad School of Earth Sciences, Macquarie University, North Ryde, N.S.W. 2113, Australia Eocene sedimentary rocks of the Coast Range in southwest Oregon are tectonically juxtaposed with the pre-Tertiary rocks of the Klamath Mountains; their eastern borders are delineated by the western edge of the Cascade Volcanic Arc (Fig.l). Various models have been proposed (snavely and Wagner, 1963; Dickinson, 1976; Simpson and Cox, 1977; Hanmond, 1979) to explain the origin and evolutionary pattern of the Coast Range and/or the Cascade Volcanic Arc. These models, in part, contradict each other, and none adequately explains scsn.e important geological features of the region. This paper reviews previous models and presents a new model based on modifications of these earlier models as well as recent sedimentologic and stratigraphic studies. Eocene stratigraphy of southwestern Oregon Coast Range was described in detail by Baldwin (1975). In this study, particular attention is given to the Roseburg and Lookingglass formations that make up bulk of the Eocene sedimentary record. The Roseburg Formation is the oldest formation in the Coast Range with total thickness of about 3,000 m. The basal 600 m or more (?) is predoninantly basalt flows, pillow lavas, or breccias interfingered with tuffaceous sandstone followed by about 2,400 m sequence of turbidite sandstones (Baldwin, 1975; Ahamad,. 1981) . Thin sections of sandstones frcm outcrops at Agness, Remote, Red Hill and sane other localities show presence of andesitic volcanic. ^v V V V V V V « \V V V V V COtTAt^ V V VV1 |v V V V V

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sedimentary and metaniorphic rock fragnents, mono- and polycrystalline quartz, chert, feldspars and heavy minerals. Average grain size decreases stratigraphically upward fron coarse to fine sand size. K-Ar dates of the basalts range fron 53 to 62 m.y. (Duncan, 1982). Microfossils date the lower sandstone sequence as late Paleocene and the upper sequence as early Eocene (Thans, 1965; Miles, 1977). The Lookingglass Foiination, approximately 1,500 m thick, unconformably overlies the Roseburg Fonnation; it is ccmposed of about 300 m of massive bedded basal conglanerate followed by coarse- to medium- to fine grained txarbidite sandstones and siltstones (Baldwin, 1975; Ahmad, 1981). Thin sections of the sandstones fron outcrops at Agness, Renote, Lookingglass, Glide and other localities reveal ccinpositions similar to those of the underlying Roseburg Fonnation. The Lookingglass sandstones are dated as early to lower Middle Eocene (ThonSp 1965; Miles, 1977). Point-counts on the Roseburg and Lookingglass sandstones show a gradual northward increase in the contents of andesitic volcanic rock fragments, in the ratio of mono- to polycrystalline quartz grains as well as in the ratio of K-feldspar to plagioclase. Conparison of the detrital framework modes of these lithic sandstones with the selected sandstone suites derived frcm various known provenances suggests that the Roseburg and Lookingglass sediments were derived mainly from two source areas. The principal source area was the melange terrane of Klamath Mountains to the south, and a secondary source supplying andesitic volcanic rock fragments was the lower part of the Western Cascade Group (WCG) of Hammond (1979) located to the east (Fig. 2) . Petrographical and paleontological evidences suggest a lower continental shelf to continental slope environments of deposition for sandstones of both of the formations, and a fluvio-neritic environment for conglonerates of the basal Lookingglass Formation. The Klamath Mountains represent essentially a melange terrane consisting of tectonically stacked sedimentary and metasedjjnentary rocks intruded by dioritic and ultramafic igneous bodies (Dott, 1965). The volcanic rocks of the Cascade Volcanic Arc in Oregon are separated stratigraphically into a Western Cascade Group (WCG) with age ranging fron 50 m.y. to 30 m.y. (Eocene to early Pliocene) and a High Cascade Group (HCG) of Plio - Quaternary age (Hammond, 1979) Hammond subdivided the VCG stratigraphically into a lower, middle, and upper part. The lower part of the WCG is 1 to 5 km thick consistdLng of, in order of abundance, andesitic, basaltic, and silicic volcanic rocks. Snavely and Wagner (1963) presents a paleogeographic model for Western Oregon and Washington. According to this mode, a N-S eugeosyncline occupied the area of present Coast Range during early Eocene. They suggest that early in the history of this geosyncline, a thick sequence of basaltic pillow lavas and breccia forming an oceanic seamount province was in existence on its western side; sediments including andesitic volcanic rock fragments were being derived fron south and eastern sources during early Eocene tme to form graywacke beds (the turbidite sandstones of the Roseburg and Lookingglass formations of Baldwin, 1975, and Ahmad, 1981) . This model, however, appears to be based on limited sedimentological and stratigraphic data, and does not fully explain the origin of the eugeosyncline and the Cascade Volcanic Arc. Dickinson (1976) postulated that basaltic seamounts collided with the North American continent during middle Eocene to form the Coast Range and a magmatic arc stepped rapidly westward frcm northeastern Washington and central Idaho to form the Cascade Range during middle and late Eocene. However, the presence of 50 m.y. old volcanics of the lower WCG (Hairtnbrid, 1979) suggests an early Eocene origin of the Cascades; this supports an easterly source of the andesitic rock fragments of the uppermost Paleocene to early Eocene Roseburg turbidite sandstones (Ahmad, 1981; Baldwin, 1975; Snavely and Wagner, 1963) contrary to Dickinson's middle Eocene origin of the Cascades.

Fig.2. Schanatic diagram shewing depositional environments pertinent to Roseburg and Lookingglass strata. SEsouthwest, NW - northwest, CV - Cascade Volcanics, PKHL Proto Klamath Highlands, LFC Lookingglass Formation conglomerates, UTS - upper trench slope, LTS - lower trench slope, FP - Farallon plate, RF - Roseburg Formation, NAP North American plate.

Based on paleonagnetic directions, Simpson and Cox (1977) determined that part of the Oregon Coast Range rotated about 70® clockwise to its present position during middle Eocene to middle Miocene. They proposed tivo paleogeographic models to explain the position of the Coast Range. In model 1, the Coast Range block, as a slab of oceanic plate undergoing subduction, rotated during early Eocene fron a southern point near the Klamath Mountains. The block rafted against the continent by middle Eocene but continued to rotate until middle Miocene. In model 2, the Coast Range, together with the Klamath Mountains, after being rafted against the continent in middle Eocene as an oceanic aseismic ridge, thereafter rotated fron a pivot point at its northern end. Although Hammond (1979) supports model 2 of Simpson and Cox, he does not give a full account of the origin and tectono-stratigraphic development of the early phases of the Coast Range and the Cascade Volcanic Arc. Based on detailed petrological study of the Roseburg and the Lookingglass formation sandstones, geophysical evidences (Duncan, 1982; Simpson and Cox, 1977) stratigraphical analysis of the southern Oregon Coast Range (Baldwin, 1975; Ahmad, 1981) and Cascade Volcanic Arc (Hammond, 1979), paleotectonics of Dickinson (1976) and paleogeography of Snavely and Wagner (1963), the following alternative model for the origin and early phases of evolution of the southern Oregon Coast Range and the Cascade Volcanic Arc is proposed: Basalts of the Roseburg Formatipn erupted at Kula-Farallon ridge during 60 - 53 m.y. B.P. as per Duncan's 1982 model. Initiation of the shift of subduction zone frcm northeastern Washington and Idaho to its present position possibly started at about 55 m.y. B.P. Initially subduction along this new siobduction zone was probably sluggish, thereby resulting in a collisional conponent which led the oceanic slab of the Farallon plate caught up by subduction-shifting process to collide with the North American plate fron about 55 m.y. B.P. to 50 m.y. B.P. During this collisional phase, the colliding oceanic slab/plate rotated clockwise fron a southern point in Klamath Mountains while sediments of the lower part of the Roseburg Formation were being deposited as an onlapping facies over oceanic basalts and basaltic mounts being rafted by the colliding plate. At 50 m.y. B.P. collision ceased; the subduction became fully active, and the Cascade Volcanic Arc was originated by outpouring of andesitic and silicic lavas of the lower WCG along the then coastal areas. At about that time, a forearc basin was formed floored with basaltic sea moun£s interfingered with basaltic and silicic sediments and occupying the continental shelf and slope. Sediments started pouring into that forearc basin frcm the pre-

Tertiary melange terrane of the Proto Klamath Mountains to the south and fron the andesitic volcanic source of the lower WZG to the east (Fig. 2). After deposition of the Roseburg strata during 55 - 48 m.y. B.P. there was a tectonic upliftment of the basin when the Coast Range formed by folding and faulting of the Roseburg strata. Since the origin of the Coast Range and the Cascade Volcanic Arc at about 48 to 50 m.y. B.P. these tectonic elanents together with the Klamath Mountains started rotating clockwise about a pivot in the Olympic Mountain to the north; this rotation continued until middle Miocene. With continuing siabduction of the Farallon plate and subsequently the Juan de Fuca plate underneath the North American plate, repeatedly tectonic uplift and subsidence of the basin and the coeval arc activity, the Coast Range and the Cascade Volcanic Arc gradually achieved their present configurations. References Ahmad, R., 1981, Stratigraphy, structure, and petrology of the Lookingglass and Roseburg formations, Agness-Illahe area, southwestern Oregon: Unxv. Oregon M.S. thesis, 150 p., unpub. Baldwin, E.M., 1975 Revision of the Eocene stratigraphy of southwestern Oregon, Weaver, D.W., Hbmaday, G.R., and Tipton,A., (eds.), Paleogene Symposium & Selected Technical Papers, Annu. Meeting - Pacific sections - AAPG - SEPM SBG, Long Beach, California, April, 1975, 49-63. Dickinson, W.R., 1976, Sedimentary basins developed during evolution of Mesozoic - Cenozoic arc-trench system in western North America, Can. Jour. Earth Sci., 13, 9, 1268-1287. Dott, R.H., Jr., 1965, Mesozoic - Cenozoic tectonic history of the southwestern Oregon Coast in relation to Cordilleran Orogenesis, Jour. Geophys. Res. 70, 4687-4707. Duncan, R.A., 1982, A captured island chain in the Coast Range of Oregon and Washington, Jour. Geophys. Res., 87, 10827-10837. Hanmond, P.E., 1979, A tectonic model for evolution of the Cascade Range, in Aimentrout, J.M. Cole, M.R., and TerBest, H., Jr. (eds.) Cenozoic paleogeography of the western United States, Soc- Econ. Paleontol. Mineral., Pac. Sec. Pacific Coast Paleogeography Symposium 3, 219-237. Miles, G.A., 1977, Planktonic foraminifera of the lower Tertiary Roseburg, Lookingglass, and Floumoy formations, southwest Oregon: Univ. Oregon PH.D. dissert., 360 p., unpub. Slnpson, R.W., and Cox, A., 1977, Paleanagnetic evidence for tectonic rotation of the Oregon Coast Range, Geology, 5, 585-589. Snavely,P.D., Jr., and Wagner, H.C.,1963, Tertiary geologic history of western Oregon and Washington: State Wash. Div. Mines and Geol. Rept. Invest, no.22, 25p. Thons, R.E., 1965, Biostratigraphy of the Ontipqua Formation, southwest Oregon: Univ. Calif., Berkeley Doctoral Dissert., 219 p., unpub.

PERMIAN AND TRIASSIC MARINE FAUNAS OF THE SOUTH WEST PACIFIC N.W. Archbold CSIRO Division of Geomechanics, Melbourne, Australia

The Southwest Pacific, incorporating for this review the areas of New Guinea, Australia, New Zealand and New Caledonia, displays a variety of Permian and Triassic marine faunas. Some exhibit endemism while others are transitional to faunas of adjacent regions. Hence an analysis of the faunas is critical in order to determine such features as palaeogeographical relationships and palaeoclimates during the PermoTriassic. Allochthonous terranes become evident by faunal and deduced palaeoclimatological anomalies. Permian Fa\inas The Permian marine faunas of the South West Pacific fall within three of the faunal provinces of Gondwana as suggested in previous work (Archbold, 1983a); specifically the Cimmerian, Westralian and Austrazean Provinces (Fig. 1) . The faunas of Irian Jaya, still marginal to Australia as they were during the Permian, demonstrate the important threefold transition from cold to cool temperature to warm temperature features from Tastubian to Kungurian times. Significant links with Westralian faunas of comparable ages exist (Archbold, 1983b) and similar faunal relationships are evident in the early Permian faunas of Southern Thailand (e.g. Waterhouse 1982), the Lhasa Block (e.g. Sun, 1984), Afghanistan (Termier et al., 1974) and the Pamirs (Grunt and Dmitriev, 1973). These are all areas which were rifted off the Northern margin of Gondwana during the Mesozoic. As yet Timor lacks distinctive Asselian - Sakmarian benthonic faunas but Kungurian faunas show significant links with Irian Jayan counterparts. The distinctive ammonoid Pseudoschistoceras is as yet only known from Timor, Irian Jaya and Western Australia (Glenister et. al., 1984) and the marginiferid brachiopod Retimarginifera also displays a Cimmerian Westralian distribution (Archbold, 1984). The mixed Glossopteris - Cathaysian floras of Thailand and Irian Jaya independently support the marginal Gondwanan nature of those regions. A mixed Gondwana - Cathaysian flora has also recently been described from the Gangdise (= Lhasa) block (Li et. al., 1985). The broad conclusion is that all such regions were peripheral to Gondwana during the Permian. The short duration larval stages of marine benthonic organisms (especially Brachiopoda) were able to migrate throughout the Cimmerian Province and those with a somewhat greater temperature range tolerance were able to penetrate the Westralian Province. The marine faunas of the western Australian basins have, since early this century, been known for their Tethyan aspect. Significant links with the classic Salt Range, Himalayan and Timor faunas have for long indicated that western Australia was at the southeastern extremity of the warm Tethyan seas characteristic of the Cimmerian Province. However, despite the occurrence of such distinctive Tethyan genera as Demonedys, Comuquia, Dyschrestia and Costiferina as minor elements within the Westralian faunas, the Westralian Province can be recognised by the somewhat lower diversity of its faunas. The development of many species of Neochonetes (Sommeriella), Streptorhynchus, Taeniothaerus and its allies, Neospirifer, Fusispirifer and small Tomiopsis is characteristic of the Province as are endemic genera such as Lyonia Permorthotetes and several spiriferid genera now in press. Notable absentees are Martinia and its allies, Richthofenia and other common Cimmerian genera.

Figure 1. Permian marine invertebrate provinces of the Gondwanan Realm (from Archbold, 1983a).

Striking differences between the Permian faunas of western and eastern Australia appear to reflect the absence of direct marine communication between the two regionso Diversity data and generic distributions indicate cold temperatures for Tasmanian faunas with more temperate conditions further north progressively from the Sydney Basin to the Bowen Basin and Gympie area. Brachiopoda are particularly distinctive between the Westralian and eastern Australian faunas. Some 65 genera occur in the Westralian faunas and some 50 genera in eastern Australian faunas. The 16 shared genera invariably demonstrate a considerable difference in species diversity between the two regions -and only two or three species may be shared. Faunal links are strongest during the early Permian (Tastubian) and between the Westralian Province and Queensland. Other groups such as Bivalvia, Gastropoda and Bryozoa show stronger links at the generic level but few species in common. Approximately half of the known Foraminifera species are shared between the Westralian Province and eastern Australian faunas. New Zealand faunas demonstrate a strong endemic eastern Australian aspect and hence both areas are grouped into the Austrazean Province<> Even the Brachiopoda reveal a close similarity. About 50 genera occur in the New Zealand faunas of which about 35 are shared with eastern Australia. 18 are shared with Westralian faunas of which only four are not yet known in eastern Australia. Some 10 genera are not yet known from the Australian mainland but it should be noted that many eastern Australian brachiopods are poorly known and faunas require m o d e m analysis and description. New Zealand faunas are in general terms, closest to those of Queensland (Bowen Basin and Gympie area). It can be noted here that suggestions that the Gympie terrane was tectonically transported to its present position (Harrington, 1983) may be consistent with structural data but faunal data (Runnegar and Ferguson, 1969) indicate a Permian position considerably closer to Queensland than to New Zealand for the Gympie Terrane. Meagre faunas from New Caledonia are of New Zealand character and relative positions adjacent to north east and east Queensland have been suggested for the faunas (Waterhouse, 1969). The anomalous New Zealand fusulinid and coral faunas have been considered to be allochthonous by various workers (e.g. Ozawa and Kanmera, 1984) but recent work (Ramsay and Moore, in press) on detailed field relationships does not support that conclusion. Subtropical offshore currents and a "Queensland" position for much of New Zealand during the later Permian would probably account for the occurrences.

Triassic Faunas Marine Triassic faunas of the South West Pacific fall into two distinctive provinces - specifically faunas of Tethyan (and wider) aspect and those of the Maorian Province which may represent a cool water remnant of the extensive Gondwanan Realm of the Permian (see Fleming 1979). The distinctive Maorian faunas occur in New Zealand and New Caledonia and investigations continue to demonstrate the similarities of the two regions (Campbell, 1984). Benthonic Brachiopoda and Bivalvia are generally endemic at the generic level in the province. Ammonites as free-swimming organisms and pterioid bivalves with longer duration pelagic lairvae, migrated freely into the Maorian Province and are of Tethyan and cosmopolitan affinities. The bulk of the Maorian Triassic faunas are of Middle and Late Triassic age but several Early Triassic ammonites are known. Early Triassic faunas are known from two onshore areas of the Australian mainland (the Perth Basin, Western Australia and the Gympie area, Queensland) and a Middle Triassic fauna is known from the north-western Australia Sahul Shoals No. 1 borehole on the continental shelf (Skwarko and Kummel, 1974). Ammonites and benthonic elements of these faunas are essentially Tethyan in character although the Queensland faunas chiefly consist of endemic species of Tethyan and cosmopolitan genera (e.g. Runnegar, 1969). No Late Triassic faunas are known from Australia. Middle and Late Triassic ammonite and benthonic faunas of Papua New Guinea are essentially Tethyan and cosmopolitan in character although, as with the Queensland Early Triassic, benthonic elements are generally distinct at the species level. The occurrence of the distinctive brachiopod Clavigera in the Kuta Formation of Papua New Guinea is a noteworthy link with the Late Triassic faunas of the Maorian Province (Skwarko et. al., 1976). If the Maorian areas of New Caledonia and New Zealand occupied a position northeast of Queensland in the Triassic as suggested for the Permian (Waterhouse, 1969) then the province boundary may have been delineated by deeper as well as cooler water, hence providing a double limit on migration potential. References below are not exhaustive but provide important supportive data and additional references on the topics discussed. References Archbold, N.W., 1983a, Permian marine invertebrate provinces of the Gondwanan Realm, Alcheringa, 7, 59-73. Archbold, N.W., 1983b, The Permian brachiopod faunas of Irian Jaya, Indonesia: Gondwanan or South East Asian?, Geological Society of Australia Abstracts, 9, 226-227. Archbold, N.W., 1984, Western Australian occurrences of the Permian brachiopod genus Retimarginifera, Alcheringa, 8, 113-121. Campbell, H.J., 1984, New records and taxa of Permian and Triassic fossils from New Caledonia and New Zealand, Alcheringa, 8, 151-167. Fleming, C.A., 1979, The Geological History of New Zealand and its life, Auckland University Press, 141 pp. Glenister, B.F., Glenister, L.M., & Skwarko, S.K., 1983, Lower Permian cephalopods from Western Irian Jaya, Indonesia, Geological Research and Development Centre, Paleontology Series, 4, 74-85. Grunt, T.A., & Dmitriev, V. Yu., 1973, Permskie brakhiopody Pamira, Akademiya Nauk SSSR Trudy Paleontologicheskogo Institute, 136, 1-212.

Harrington, H . J . , 1983, Correlation of the Permian and Triassic Gympie Terrane of Queensland with the Brook Street and Maitai Terranes of New Zealand, Geological Society of Australia, Queensland Division, Permian Geology of Queensland, p p . 431-436. L i , X.-X, W u , Y . - M . , F u , Z.-B., 1985, Preliminary study on a mixed Permian flora from Xiagangjiang of Gerze District, Xizang and its palaeobiogeographic significance, Acta Palaeontologica Sinica, 2 4 , 150-170o Ozawa, Tc , & Kanmera, K . , 1984, Tectonic terranes of Late Paleozoic rocks and their accretionary history in the Circum-Pacific Region viewed from fusulinacean paleobiogeography, Proceedings of the Circum-Pacific Terrane Conference, Stanford University Publications, Geological Sciences, 18, 158-160. Ramsay, W . R . H . , & Moore, P . R . , in p r e s s . Mineralogy and chemistry of a pillow lava, Northland, N e w Zealand, and its Tectonic significance. New Zealand Journal of Geology and Geophysics. Runnegar, B . N . , 1969, A Lower Triassic ammonoid fauna from southwest Queensland, J o u r n a l of Paleontology, 4 3 , 818-828. Rxmnegar, B.N<. , & Ferguson, J.A. , 1969, Stratigraphy of the Permian and Lower Triassic marine sediments of the Gympie District, Queensland, University of Queensland Department of Geology Papers, 6 , (9), 247-281. Skwarko, S . K . , & Kummel, B . , 1974, Marine Triassic molluscs of Australia and Papua N e w Guinea, Bureau of M i n e r a l Resources, Geology and Geophysics, Bulletin, 150, 111-139. Skwarko, S.K., Nicoll, R.S. & Campbell, K.S.W., 1976, The Late Triassic molluscs, conodonts, and brachiopods of the Kuta Formation, Papua New Guinea, BMR J o u r n a l of Australian Geology and Geophysics, 1 , 219-230. Sun T e . , 1984, Late Carboniferous - Early Permian strata and fauna in North A l i District, Xizang (Tibet), China, Exploration of N a t u r e , 3 , 71-83. Termier, G . , Termier, H . , deLapparent, A . F . , & M a r i n , P . , 1974, Monographie du Permo-Carbonifere de Wardak (Afghanistan Central), Documents des Laboratoires de Geologie de la Faculte des Sciences de Lyon, Hors Serie, 2 , 1-167. Waterhouse, J . B . , 1969, A new Permian fauna from New Caledonia, and its relationships to Gondwana and the Tethys. I n , Gondwana Stratigraphy (I.U.G.S. Symposium), p . 249-272. Waterhouse, J . B . , 1982, An early Permian cool-water fauna from pebbly mudstones in south Thailand, Geological Magazine, 119> 337-354.

THE SIGNIFICANCE OF SUSPECT TERRANES IN THE STUDY OF PROTEROZOIC

AND

EARLY PALEOZOIC

METALLOGENY IN NORTHEAST QUEENSLAND

J.H.C. Bain^ § I.W. Withnall^ Bureau

Mineral Resources, Canberra, Australia^, and 2 Geological Survey of Queensland, Brisbane, Australia

Several extensive fault-bounded geological units in the eastern part of the Proterozoic Georgetown Inlier are characterised by different associations of mineral deposits, whose ages and origins are mainly unknown. Attempts to explain their metallogenesis in terms of common or related processes & events have followed naturally from an assumption that these adjacent units have a common geological history-—being either facies variants or fault-telescoped parts of a single stratigraphic sequence. However recent work by the Geological Survey of Queensland (Withnall, 1982;1985) has demonstrated the possibility that this area contains at least three different tectonostratigraphic terranes of unknown age juxtaposed against the Georgetown craton by large scale transport from diverse origins in the east. Even the eastern part of the craton has been sliced into several major fault blocks with displacements of up to 50km. Consequently it is important to consider the possibility of unique and unrelated metallogenic events for each of the suspect terranes. The eastern part of the Georgetown cratonic terrane(GCT) is characterised by early Proterozoic amphibolite to granulite grade gneiss, schist, quartzite, & amphibolite containing clusters of metamorphosed stratiform polymetallic massive sulphide deposits at one general stratigraphic level (Bain & Withnall, 1980).The deposits vary from copper dominant (Einasleigh:FeCuAgAu) to lead-zinc dominant (Mt.Misery:FeZnPbCuAg), and most are associated with garnet-magnetite-epidote quartzite; some with barite. They are found generally in the vicinity of rare lenses of leucogneiss near the vertical transition from a mostly calcareous psammitic sequence to a mostly psammopelitic one.The depositional environment is thought to have been a shallow marine shelf, mostly devoid of volcanics(only the leucogneiss lenses can be interpreted as metavolcanics). Stanton(1982) and Patrick(1978) have shown that the deposits are accompanied by intense metasomatic alteration of the type usually associated with submarine hydrothermal systems and "volcanogenic" or ^exhalative" massive sulphide deposits. The general absence of metavolcanic rocks 4 discordant pre-metamorphic hydrothermal alteration indicative of fluid feeder zones beneath the deposits suggests that the deposits are distal with respect to both volcanic & exhalative sources. The factors that controlled the distribution of deposit clusters are not known. The westernmost of the possibly allocthonous terranes —Balcooma terrane(BT)— occurs in two separate fault-bounded slices each immediately east of a major mylonite zone, and one elongate enclave in a Silurian granite batholith.The terrane comprises lower to middle amphibolite grade rhyolitic to dacitic pyroclastics, mica

schist and quartzite; the latter mainly in the lower & mid parts of the unit. This sequence is thought to have formed during Cambro-Ordovician time, in a continental margin environment with most or all of the volcanics deposited subaqueously. Metamorphosed polymetallic sulphide deposits (eg Balcooma:FeCuZnPbAg & Surveyor:FeZnPbCuAgAu) also characterise this terrane. These lie within predominantly rhyolitic volcanics just above a pelitic sequence, 4 have recognisable stringer zone mineralisation. They are thought to have formed as a result of submarine hydrothermal activity, possibly related to the emplacement of an extensive subvolcanic porphyritic microgranite. Small auriferous quartz veins, possibly related to the basemetal deposits but geographically separate from them, are the only other known deposits.Similar rocks & mineral deposits (eg.Thalanga:FeZnCuPbAg & LiontownsFeZnPbCuAgAu) in the Cambro-Ordovician Seventy Mile Range Group, 250km to the SE, may be correlatives« East of the BT & separated from it by a Silurian batholith is the possibly composite Lucky Creek terrane(LCT) . The eastern half is mostly phyllitic metasediment. Most of the western half of this terrane is metavolcanic, & contains all the known mineralisation« Unlike BT much of it consists of metabasic (andesitic) submarine volcanics, 4 can be classified as an island arc terrane. It is thought to be of Cambro-Ordovician age (Withnall, 1985). A wedge of similar but less metamorphosed volcanics 4 Ordovician sediments lying 15 km to the east may be part of this terrane. Perhaps the most significant mineralisation is that of the Lucky Creek goldfield comprising lenticular auriferous quartz reefs conformable with the enclosing metavolcanics 4 metasediments. Little is known about these deposits, but they may be promising indicators of volcanogenic gold mineralisation. To the north are several small basemetal deposits; the largest,Wyandotte (containing <500000t of 2.7!tCu ore in foliated altered amphibolite) is regarded by some as a volcanogenic deposit. The easternmost of the suspect terranes, Greenvalia(GT), occurs in several fault blocks,the largest separated from the LCT by a mylonite zone and bounded on the east by faults and the superjacent rocks of the Ordovician to Carboniferous Broken River terrane(BRT), as are the smaller southern blocks. Most of the northern half of GT is concealed by Cainozoic basalt. The terrane comprises coarse mica schist, minor quartzite and pegmatite, and three ultramafic complexes. The latter are mostly amphibolite and serpentinite with relict pods of less altered ultramafic rocks and gabbro. One includes some tonalite and trondhjemite bodies and swarms of metaandesite and metabasalt dykes. GT has been affected by lower amphibolite facies metamorphism and two associated regional deformations that produced penetrative schistosities. Several subsequent events have refolded these rocks. Rubenach(1982) has suggested that the ultramafic complexes were originally parts of Proterozoic oceanic lithosphere (ophiolite) thrust from the east into marine sedimentary sequences, then metamorphosed 4 deformed at relatively shallow levels in the crust during the Proterozoic. GT can be regarded as a composite terrane accreted to the GCT, or a zone of interaction between the eastern edge of the GCT 4 obducted ophiolite. Two related groups of mineral deposits —podiform chromite 4 nickel-cobalt bearing laterites— characterise the ultramafic rocks whilst small copper deposits (eg.Hallos Reward) of unknown origin fill shears in the metasediments 4 metabasic intrusives. Some alluvial gold near the southern (Gray Creek) chromite occurrences, but east of the terrane, may be derived from quartz veins in the terrane.The Greenvale deposit (originally 45mt § 1 .SitNi ,0.1 iCo) is the largest deposit 4 the only currently active

mine in the terrane. Similar but smaller deposits are Minnamoolka (26mt § O.T^Ni) & Qunnawarra (21mt § 1.0$Ni). The deposits formed through successive lateritisation of serpentinised ultramafics during prolonged weathering & erosion in the Cainozoic, Additional deposits may remain buried beneath younger Cainozoic basalts of the McBride Volcanic Field. Other types of deposits that may occur in ophiolite & hence in this terrane are: massive, net textured & disseminated sulphide NiCuCo & PGE deposits in the relatively unserpentinised cumulate peridotite; vein & fracture fill NiCo arsenide deposits in highly serpentinised cumulate peridotite; & vanadiferous magnetite in gabbro (Page & others, 1982). The volcanic layer is apparently missing from the ophiolite sequence, & no Cyprus type deposits of massive sulphide in pillow basalt are known. References Bain,J.H.C., & W i t h n a l l . W . , 1980, Mineral deposits of the Georgetown region, northeast Qld., in, Henderson,R.A. & Stephenson, P.J. (eds). The Geology & Geophysics Of Northeastern Australia, Geological Society of Australia Queensland Division, 129-1^8. Page,N.J.,Foose,M.P.Lippin, B.R., 1982, Characteristics of metallic deposits associated with ultramafic & mafic rocks, in Erickson,R.L.,(ed),Characteristics of Mineral Deposit Occurrences, USGS Open File Report 82-795,1-12. Patrick,J.P., 1978, The geology & origin of the sulphide deposit & the high grade Precambrian metamorphic rocks at Einasleigh,NE Australia, MSc. thesis, James Cook University of North Qld,(unpubl). Rubenach,M.J., 1982, Metamorphosed mafic-ultramafic complexes of the Greenvale area, in, Withnall,I.W.,(ed), 1982 Field Conference, Charters Towers-Greenvale area. Geological Society of Australia Queensland Divsion, 47-66. Stanton,R.L., 1982, Metamorphism of a stratiform sulphide orebody at Mount Misery, Einasleigh,Australia: 1-Observations, Transactions of Institution of Mining & Metallurgy (Section B: Applied earth science), 91, B47-B71. White,D.A., 1965, The geology of the Georgetown/Clarke River area,Queensland, Bureau of Mineral Resources, Australia, Bulletin71. Withnall,!.W., 1982, The geology of the Greenvale-Balcooma area, in, Withnall,I.W.,(ed), 1982 Field Conference, Charters Towers-Greenvale area. Geological Society of Australia Queensland Division, 31-46 Withnall,I.W., 1985, Suspect terranes along the Precambrian/Palaeozoic margin, Greenvale area, north Queensland, ext. abstr., this conference.

ALLOCHTHONOUS TERRANES AND STRIKE-SLIP FAULTING IN THE ADSTRALIA-BANDA ARC COLLISION ZONE A.J. Barber Royal Holloway & Bedford New College, University of London, United Kingdom Between Sumba and the Kai Islands in Eastern Indonesia the northern margin of Australia is in collision with the Banda Island Arc System. The northern continental margin of Australia is a t y p i c a l p a s s i v e c o n t i n e n t a l m a r g i n , c o n s i s t i n g of a continental basement, broken into horsts and graben, overlain by 6-7 km of Permian to Recent continental shelf sediments. Seismic reflection profiles show that Australian continental shelf sediments dip northwards beneath the floor of the Timor Trough where they are overlain by a sedimentary infill which thickens northwards. On the north wall of the trough, trough sediments are uplifted in fold and thrust slices to form an accretionary complex, analogous to those seen associated with oceanic subduction zones. In the Kolbano region, on the south coast of Timor, Jurassic clastic sediments, early Cxetaceous radiolarites and late Cretaceous to Pliocene calcilutites, interpreted as part of the Australian continental shelf sequence, are folded, thrust and imbricated, evidently representing an uplifted earlier segment of the accretionary complex. Further north, in the Kekneno area, Permo-Triassic clastic sediments of Australian affinity show abundant evidence of overthrusting, contrary to earlier conlusions concerning the tectonics of Timor (e.g. Grady, 1975). The geological map of West Timor (Rosidi et al., 1981) shows that towards the north successively older units are brought up by thrusting, indicating that the decollement level extends progressively deeper into the Australian margin sequence in this direction. The structure of Timor may be interpreted as a developing foreland fold and thrust belt, formed as the northern margin of Australia is forced beneath the Banda Arc system by continuous plate movements. A group of allochthonous units, unrelated to the northern margin of Australia, form the highest mountains in Timor. These units are considered to rest on a roof thrust above a duplex formed from the Australian continental margin sediments. These allochthonous units include: 1. Palelo Group; imbricated Jurassic cherts. Cretaceous limestones and Cretaceous to Palaeocene clastic sediments, interpreted as a fragment of an accretionary complex formed in front of the Banda Arcs from ocean floor sediments which lay to

the north of Australia before the Australian continental margin arrived at the subduction zone (Barber, 1979). Low grade metavolcanicsr including p i l l o w lavas and lustrous slates, previously attributed to the Lolotoi Onit (Barber & A u d l e y Charles, 1976) may also form part of this assemblage. 2e M u t i s (Lolotoi) C o m p l e x ; serpentinite and high grade m e t a m o r p h i c rocks, some of g r a n u l i t e facies, showing varying degrees of retrogression to a m p h i b o l i t e and g r e e n s c h i s t facies assemblages (Earle, 1981). Element partitioning in minerals of pelitic rocks s h o w s that they u n d e r w e n t m a s s i v e uplift from depths of 35 km to 7 km without concomitant fall in temperature. Rb/Sr whole rock dating suggests that this event occurred in midCretaceous time (Brown & Earle, 1983). This unit is interpreted as a f r a g m e n t of uplifted continental b a s e m e n t w i t h a t t a c h e d mantle peridotite (Barber, 1979). 4. M a u b i s s e - A i l e u unit; vesicular pillow basalts with interstitial carbonate, associated w i t h highly f o s s i l i f e r o u s P e r m o - T r i a s s i c crinoidal l i m e s t o n e interpreted as a b a s a l t i c plateau with overlying c a r b o n a t e bank (Audley-Charles et al«, 1972). In East T i m o r this unit passes n o r t h w a r d s into the increasingly arenaceous clastic Aileu Group of P e r m i a n to M e s o z o i c age. The carbonates often show p r e s s u r e solution cleavage and the clastic sediments become increasingly deformed and more highly metamorphosed until on the north coast they are schists of high a m p h i b o l i t e facies (Barber & A u d l e y - C h a r l e s , 1976; Berry & G r a d y , 1982). These a l l o c h t h o n o u s units w e r e probably derived from the margin of Asia at an earlier stage by the development of marginal seas, strike-slip faulting or a combination of these mechanisms (Carter et al., 1976; H a m i l t o n , 1979; Barber, 1979). By analogy with the present situation of the microcontinental block of the island of S u m b a to the w e s t , they occupied the forearc zone of the Banda island arc system before the northern m a r g i n of A u s t r a l i a arrived at the subduction zone. W i t h the a r r i v a l of Australia the allochthonous units were thrust across the marginal s e d i m e n t s , generating the foreland fold and thrust belt, w h i c h following uplift now constitutes the island of Timor. Stages in this uplift are marked by the sediments deposited upon and around the collision c o m p l e x , including the P l i o Pleistocene Viqueque Group, composed of bathyal to shallow water sediments and Pleistocene coral reefs, now elevated to heights of up to 1200 m. Following the collision Timor has been divided up into a series of fault-bounded segments by major wrench faults. V e r t i c a l wrench faults cut through the thrust units, releasing overpressured shales in a phase of shale diapirism on a massive scale, with the formation of mud volcanoes and the escape of oil and gas. W r e n c h faulting appears to be related to oblique convergence between Australia and the Banda Arc System.

References Audley-Charles, M.G., Carter^ D.J. & M i l s o m , J.S., 1972, Tectonic d e v e l o p m e n t of e a s t e r n I n d o n e s i a in r e l a t i o n to G o n d w a n a l a n d dispersal, Nature, Physical Science, 239, 35-39. B a r b e r , A.J., 1 9 7 9 , S t r u c t u r a l i n t e r p r e t a t i o n s of the i s l a n d of T i m o r , Proceedings of the South East Asia Petroleum Association, 4 , 9-21. B a r b e r , A.J. & A u d l e y - C h a r l e s , M.G., 1 9 7 6 , The s i g n i f i c a n c e of the m e t a m o r p h i c rocks of T i m o r in the d e v e l o p m e n t of the B a n d a A r c s , Tectonophysics, 30, 119-128. B e r r y , R.F. & G r a d y , A.E., 1 9 8 1 , D e f o r m a t i o n and m e t a m o r p h i s m of the A i l e u F o r m a t i o n , n o r t h c o a s t . E a s t T i m o r and its t e c t o n i c significance. Journal of Structural Geology, 3, 143-167. B r o w n , M . & E a r l e , M.M., 1 9 8 3 , C o r d i e r i t e - b e a r i n g s c h i s t s a n d g n e i s s e s from T i m o r , e a s t e r n I n d o n e s i a : P - T c o n d i t i o n s of metamorphism and tectonic implications. Journal of Metamorphic Petrology, 1 , 183-203. C a r t e r , D.J., A u d l e y - C h a r l e s , M . G . & B a r b e r , A.J., 1 9 7 6 , Stratigraphic analysis of island arc-continental margin collision in E a s t e r n I n d o n e s i a , J o u r n a l of the G e o l o g i c a l S o c i e t y of London, 132, 179-198. Earle, M.M., 1981, The metamorphic rocks of Booi, T i m o r , Eastern I n d o n e s i a , In: The G e o l o g y and T e c t o n i c s of E a s t e r n I n d o n e s i a , A.J. B a r b e r & S. W i r y o s u j o n o (Eds.), G e o l o g i c a l R e s e a r c h a n d Development Centre, Special Publication N o . 2 , 239-251. Grady, A.E., 1975, A re-investigation of thrusting in Portuguese T i m o r , J o u r n a l of the G e o l o g i c a l S o c i e t y of A u s t r a l i a , 2 2 , 223227. H a m i l t o n , W., 1 9 7 9 , T e c t o n i c s of the I n d o n e s i a n r e g i o n , Geological Survey Professional Paper, 1078.

U.S.

R o s i d i , H.M.D., S u w i t o d i r d j o , K . & T j o k r o s a p o e t r o , S., 1:250,000 geologic map of the Kupang-Atambua Quadrangles, Timor, Geological Research and Development Centre, Bandung.

TECTONOSTRATIGRAPHY OF THE TUMUT TROUGH AND ADJACENT TERRANES H. Basden^, B.J. Fr?nklin^, B. Marshall^ and A.E. Waltho^ ^New South Wales Department of Mineral Resources, Sydney, Australia. ^Department of Applied Geology, The New South Wales Institute of Technology, Sydney, Australiac

The Early Silurian to Early Devonian Tumut Trough occupies the southern section of a structural unit known as the Bogan Gate Synclinorial Zone, which occurs between the Girilambone - Wagga Anticlinorial Zone to the west and the Forbes Anticlinorial Zone to the east, in the southeastern Lachlan Fold Belt. These three structual units occur within the Girilambone, Wagga - Oneo and Molong - Monaro Terranes respectively. The Tumut Trough occurs between two elements of the Molong Volcanic arc which is represented in the present day Lachlan Fold Belt by disconnected outcrops of Ordovician intermediate and mafic metavolcanic rocks. These outcrops form a 450 kilometre long, roughly north trending belt which extends from south of Cabramurra, near the New South Wales -> Victorian border, to a point to the northwest of Molong, in central New South Wales, where the Palaeozoic Lachlan Fold Belt disappears beneath the Mesozoic cover of the Great Artesian Basin. The two elements of the Molong Volcanic arc in southern New South Wales are the Nacka Nacka Metabasic Igneous Complex and the Gooandra Volcanics which outcrop to the west and east of the Tirniut Trough respectively. The Molong Volcanic arc is thought to have been a Marianas-type, as distinct from a Chilean-type arc, implying that the arc developed on oceanic crust to the east of the Gondwana craton. Flysch-like metasediments of the Wagga Metamorphic Belt^ which occurs within the Wagga - Cmeo Terrane, were deposited in a marginal basin between the arc and the craton. Although it occ\irs within the Molong Volcanic arc, the Tumut Trough is not thought to be a product of relatively local tensional forces generated within the arc by the downgoing plate, subduction and arc related volcanism having ceased prior to the initiation of rifting responsible for the trough's formation in the earliest Silurian (--445 Ma) . Instead, the Tumut Trough is thought to have developed in response to fold beld wide transtensional tectonics which persisted until the Early Devonian. The oldest rocks in the Txamut area belong to the Jindalee Group and comprise quartz-rich flysch, possible oceanic crust, and upper mantle (Figure la, b). These rocks formed basement blocks underlying parts of the Txrniut Trough sequence and are thought to be of Cambro-Ordovician age. Deposition of quartz-rich flysch in the Wagga Marginal Basin and tholeiitic, submarine volcanism (Nacka Nacka Metabasic Igneous Complex) on the western edge of the Molong Volcanic arc occurred during the Middle to Late Ordovician. At the end of the Ordoviciain, or in the earliest Silurian, a mainly massive, regional aureole, S-type granite, the Green Hills Granodiorite, intruded the Wagga Metamorphic Belt sediments, probably as a result of partial melting of basonent to the Wagga Marginal Basin during the Benambran Deformational Event (-509 Ma) . Development of the Tumut Trough due to rifting of the western edge of the deformed Molong arc in the Early Silurian was probably influenced by pre-existing crustal faults. Major faiiLts, now the Mooney Mooney Fault System and the Gilmore Fault Zone, bounded the trough while subsidiary faults, the Killimicat and Lacmalac Faults divided the trough into three basins: the Gundagai, Brungle, and Bongong Basins (Figxires la, lb, 2) .

Figure 1a Generalised Geological Approximate

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Map of the Central Tumut Trough 000

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Initial A-type acid volcanism, related to the rifting, and terrestrial sedimentation (Frampton Volcanics) were closely followed and partly overlapped by basic volanism. Basic igneous activity, both in the Tumut Trough and in the bounding Wagga Arch and Yass-Canberra High, occurred as a result of the crustal extension. Peripheral initial rifting in the Wagga Arch yielded the Avenall Basic Intrusive Complex while major peripheral rifting in the Yass-Canberra High yielded the Micalong Swamp Basic Intrusive Complex. Extensive rifting in the Brungle and Bogong Basins yielded the Honeysuckle Metabasic Igneous Complex which has strong analogies with modern day ocean floor complexes formed in inter-arc basins. The Coolac Serpentinite represents the depleted upper mantle, and the two units together form the Coolac ophiolite suite. The Coolac ophiolite suite is best described as a dismembered ophiolite, emplaced onto the continental margin of the Yass-Canberra Rise during the mid Late Silurian ( 417 Ma). Although parts of it are tectonically reduced and disrupted, the northern portions contain a completely developed ophiolitic section of over five kilometre thickness (North Mooney Complex). Here the ophiolite can be subdivided into a basal plutonic unit of rythmically and cryptically banded, cumulate ultramafic rocks, underlying a middle unit of high level massive gabbros, diorites and plagiogranites which in their turn are overlain by a small sheeted dyke complex (0.5 - 1km thick) and extrusive pillow basalts. The petrological character of the cumulate rocks is "wehrlitic" aind serpentinization is less than 40 percent. Small bodies of deformed, K-rich granodiorite intrude the ophiolite near its margins. These represent crustal melts associated with its anplaconent onto the continental margin.

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The Long Tunnel, Snowball and Valley View Metabasic Igneous Complexes, all in the Gundagai Basin, and the Tumut Pond Serpentinite Belt may also be ophiolite complexes, the intensity of rifting possibly being less than that which formed the Honeysuckle Metabasic Igneous Complex. The Brungle Creek (Br\ingle Basin) and Wermatong (Bogong Basin) Metabasalts probably formed in an oceanic island setting. Each of the three basins in the Tumut Trough had a different sedimentary sequence. In the Gundagai Basin the sequence was of the order of 8000 metres thick and comprised andesitic to dacitic volcaniclastics, minor volcanics and limestones (Jackalass Slate, possibly related to dioritic intrusions, eg: Blacks Flat Diorite), quartz-rich flysch (Bumbolee Creek Formation: derived from Wagga Metamorphic Belt metasediments), and dacitic volcaniclastics (Blowering Formation: derived from the Volcanic cap of the Young Granodiorite). In the Briingle Basin the sequence was possibly less than 2000 metres thick and comprised andesitic volcaniclastics (Wyangle Formation) overlain by dacitic volcaniclastics (Blowering Formation). In the Bogong Basin the sequence was also probably less than 2000 metres thick and comprised dacitic volcaniclastics (Blowering Formation) . The presence of limestone pods and conglomerates in the Jackalass Slate, and rapid facies changes in the Blowering Formation, suggest that the depth of the Tumut Trough was never great. On both the Wagga Arch (Ellerslie Granodiorite: I/S-type; Wondalga Granodiorite: I-type; source possibly related to the Molong Volcanic arc and the arc basement) and the Yass-Canberra Rise (Young Granodiorite: S-type; source-Mblong Volcanic arc basement), foliated, contact aureole / siibvolcanic granites were intruded, consolidation occurring during the Late Silurian. On the Yass-Canberra Rise the igneous activity reached the surface and S-type dacitic volcanics (Goobarragandra Volcanics) were deposited in a marine shelf environment. Deformation of the Tumut Trough sequence occiirred during the Bowning Deformational Event (-415 Ma) in the earliest Devonian. The north-northwest trending cleavage, hingelines and major faults are consistent with east-west shortening of the trough sequence (cover) and the Jindalee Group (basement) . As part of this shortening of the trough and its component basins (Gundagai, Brungle and Bogong), the Coolac ophiolite suite was obducted along the eastern margin, partly onto the Yass-Canberra High, and the Long Tunnel, Snowball and Valley View Metabasic Igneous Complexes were upthrust into their present positions. During the Early Devonian the deformed T m u t Trough sequence formed part of the Bogan Gate Terrace. The trough sequence and the Yass-Canberra Rise rocks were intruded by massive, contact aureole/subvolcanic I-type granites (Gocup, Bogong, Killimicat and Argalong Granites, derived from high-potassium gabbroic bodies intruded into the lower crust during the Middle to Late Ordovician) and was overlain by associated I-type acid volcanics and shallow marine sediments (Minjary Volcanics, Gatelee Ignimbrite). Intrusion was controlled by the major faults and shear zones. Based on mesoscale observations, four distinct structural units can be delimited. First, the Jindalee Group, characterised by at least two and usually three periods of penetrative deformation. Second, the Coolac ophiolite suite comprising a •knocker-terrane• melange of blocks of Honeysuckle Metabasic Igneous Complex and Blowering Formation in a Coolac Serpentinite matrix; the tectonized harzburgite (Coolac Serpentinite) is multiply deformed whereas the Honeysuckle Metabasic Igneous Conplex displays complex age relationships between veins and mesoscale shear zones. Third, the Bxombolee Creek Fomation, characterised by

Figure 2

MAJOR LINEAMENTS OF THE TUMUT TROUGH AND ADJACENT TERRANES

Comoiled from LANOSAT C images ALS 91-84 and 91-85 Approximata

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mutually interfering conjugate (?) fold systems imposed on a cleavage-producing event. Fourth, the remainder of the trough, characterised by a penetrative, cleavage-producing event, overprinted in some places by minor kink folds. This second event fold is far more intense in the region of the Lacmalac and Killimicat Faults. The third event in the Jindalee Group is tentatively correlated with the first event in the Bumbolee Creek Formation and the remaining Silurian Sequence throughout the trough.

Published with permission of the Secretary, New South Wales Department of Mineral Resources.

COLLISIONAL TECTONICS IN THE EASTERN MEDITERRANEAN Zvi Bea-Avraham Department of Geophysics and Planetary Sciences T e l Aviv University Israel

A large number of oceanic plateaus including prominent seamounts and ridges is apparent in the floor of the eastern Mediterranean; several of these are at present far from the zone of plate boundary between Africa and Eurasia. Many others however, crowd segments of this boundary, particularly in the central and eastern parts of the Hellenic arc. Portions of the oceanic plateaus which arrived at the subduction plate boundary were not able to subduct because of their buoyancy and prominent topography. Instead, they have been accreted to the continental plate while active subduction has shifted outward, generating a new trench. Hence, in this region the complex bathymetry in general, and the multi subduction trench system in particular, are the consequence of the shifting consumption plate boundary following accretion of oceanic plateaus. Similarly, the complex seismicity pattern in the plate boundary zone is a natural result of the same process. In particular, the collision of the Erastosthenes and the Anaximander Seamounts w i t h the Cyprus arc result in a unique tectonic setting due to the small size of the eastern Mediterranean basin. As a result of the collision, some of the motion between the African plate and the Eurasian plate is taking place by thrust faulting along the north African passive margin. It seems that the stress at the plate boundary here may be transmitted southward to cause the reactivation of the pre-existing fatilt zone along the passive margin.

SIGNIFICANCE OF METAMORPHIC TECTONITES IN TERRANE ACCRETION M. C. Blake, Jr. U.S. Geological Survey, Menlo Park,

U.S.A.

Introduction Metamorphic tectonite is defined as a rock whose fabric contains penetrative structures such as schistosity and lineation that attest to solidstate flow during formation (modified slightly from Turner, 1981, p. 213). Metamorphic tectonite belts are linear zones of indefinite extent characterized by metamorphic tectonites. The origin of metamorphic tectonite belts is controversial and obscure, but two main types have been distinguished on the basis of metamorphic mineral assemblages: high T/P (characterized by high inferred temperature/pressure ratios at formation) and high P/T (characterized by high pressure/temperature — or low temperature/pressure—ratios). Furthermore, most early workers agreed that moderately high geothermal gradients were needed to form the high-T/P belts whereas extremely low geothermal gradients were necessary to produce the high-P/T blueschists; however, a tectonic model linking these two different kinds of metamorphic conditions was lacking. With the advent of plate tectonics, such a model became available for testing. Perhaps best championed by W. G. Ernst ( 1 9 8 3 ) , this model was built on the paired-metamorphic-belt concept of Miyashiro (1961) and postulates that the high-P/T and high-T/P belts form simultaneously in an arc-trench environment ( f i g . 1).

<J=3DDDD volcanic front

trench (1) (5)

(3)

1) ZNlite (laumontite) 2) prehnite-pump«llyit9 3)blueschist (lawsonite)

f l T T - f T T

(6)

(4)

4)«clogit»

5)greenschl»t (chlorite) 6)low-ronkamphibolite (biotite) (7) high-rank amphibolite (8)granuiite, hornfels

(1) (23

(3) ^C//^ V M 2—1

.16) (4)

\ :

Il l • -I w ;

.• '

dsthenosphere 50 km.

Fig.

v.;-.

1. Schematic distribution of metamorphic facies types in an arc-trench environment (from Ernst, 1983).

::Vn

More recently, the concept of accretionary tectonics (Coney and others, 1980) has provided a tectonic framework that not only better describes the complex collage of suspect terranes that characterizes the entire Circumpacific belt but also provides a compelling model for the formation of metamorphic tectonites. I propose that many of the high-T/P metamorphic tectonite belts in North America formed during the crustal thickening caused by accretion of both exotic and native fragments and that, although, the driving mechanism for this accretion was plate tectonics, the belts did not form in simple arc-trench systemso By analogy, high-T/P tectonite belts elsewhere in the world may have had similar origins. The high-P/T blueschists, on the other hand, probably did form in subduction zones, but were preserved because subsequent collision events thrust them back to the surface. Examples of metamorphic tectonites related to terrane accretion The best documented example of regional metamorphism related to terrane accretion is in British Columbia, where Monger and others (1982) used this interpretation to explain the presence of two large metamorphic-plutonic complexes. The eastern complex, the Omineca Crystalline Belt, resulted from the accretion of four terranes, which amalgamated by the end of the Triassic and accreted to North America probably in Middle Jurassic time. A second event occurred in the Coast Plutonic Complex to the west probably in middle Cretaceous time, when another composite terrane was accreted onto the earlieraccreted rocks of the continental margin. Both accretion events were accompanied by metamorphism as high as granulite facies and by the emplacement of synkinematic granitic plutons presumably formed by anatexis. A similar accretionary origin has recently been proposed for metamorphic tectonites in the Appalachian orogen of eastern North America (Williams and Hatcher, 1982). Stratigraphic and sedimentologic analyses indicate that the Appalachian orogen formed during three successive Paleozoic accretionary events. The earliest of these, the Taconic orogeny, took place mainly during Middle Ordovician time, when the westernmost terranes were obducted westward onto the North American miogeocline. The resulting metamorphic overprinting is attributed to a profound thickening of the crust, which also caused local melting that produced granitic plutons. The subsequent Acadian (Late Devonian) and Alleghanian (Carboniferous) orogenies involved accretion of additional terranes farther east and the areas affected by regional deformation and metamorphism stepped progressively eastward. In addition, early Taconic-age blueschists that survived the later Acadian metamorphic overprinting provide evidence for an early high-P/T metamorphic event (evidence summarized by Zen, 1983) that does not support the paired metamorphic belt concept. In an attempt to identify further examples of tectonite belts related to terrane accretion, I have compiled a greatly simplified metamorphic-facies map of the U.Se Cordillera (to be presented in poster session at the 3rd CircumPacific terrane conference). This map (1:2,500,000) shows metamorphic, structural, and isotopic data superposed on the new map of tectonostratigraphic terranes (Silberling and Jones, 1984); these data suggest that there may be several very different modes for formation of metamorphic tectonite belts. One group of belts, characterized largely by high-T/P conditions, can be seen to overprint two or more terranes. Examples include the Late Cretaceous North Cascades belt of Washington (Tabor and others, in press), the middle Jurassic Klamath belt of northern California and southwest Oregon (Mortimer, 1985), and the composite Peninsular Ranges belt of Middle Cretaceous age in southern

California (Todd and Shaw, 1985). In addition, another widespread metamorphic event occurred during the Late Jurassic Nevadan orogeny in northern California and southwestern Oregon and affected many previously accreted terranes. Other high-T/P occurrences, the so called metamorphic core complexes, lie to the east of the accreted terranes in the U.S. Cordillera, even though they contain metamorphic rocks and associated penetrative structures that are identical to those found in the accreted terranes to the west. Although the timing and kinematics of these core complexes remain controversial (for a discussion of this controversy see Compton and others, 1977; Coney and Harms, 1984; Gans and others, 1985), the metamorphic core complexes contain minerals such as kyanite and sillimanite that require pressures in excess of the known stratigraphic thickness suggesting that there has been structural thickening, similar to that proposed to the west in the accreted terranes. A second type of metamorphic tectonite belt consists of a single terrane localized along a major terrane boundary. Examples include the Devonian Central metamorphic terrane of the Klamath Mountains, and the Early Cretaceous(?) blueschist-bearing Pickett Peak, Shuksan, Condrey Mountain, and Baldy terranes of California, Oregon, and Washington (Silberling and Jones, 1984). All of these occurrences are characterized by high-P/T tectonites that probably formed in a subduction-zone environment. The problem is how such rocks are returned to the surface quickly enough to preserve the high-P/T minerals, including metamorphic aragonite. The only mechanism that seems to fit the data is one in which a large terrane such as an island arc, oceanic plateau, or continental fragment is carried into the subduction zone and the resulting collision leads to thrusting of the blueschists back to the surface. Conclusions Although some belts of metamorphic tectonites in North America overprint terranes, many appear to be related to terrane boundaries. Some of these belts, in particular those containing high-T/P mineral assemblages and anatectic granitic rocks, appear to have formed by crustal thickening during the piling up of two or more terranes along the continental margin. The metamorphic core complexes of the U.S. Cordillera remain an enigma; however, the timing of metamorphism and the structures and mineral assemblages found in these rocks are remarkably similar to high-T/P belts in the accreted terranes, and these features suggest that during the accretion of the terranes the continental crust of the adjacent margin was also imbricated and structurally thickened. Other belts of metamorphic tectonites are characterized by highP/T mineral assemblages, including the blueschists, and appear to have formed in collision zones along major terrane boundaries. References Compton, R.R., Todd, V.R., Zartman, R.E., and Naeser, C.W., 1977, Oligocene and Miocene metamorphism, folding, and low-angle faulting in northwestern Utah: Geological Society of America Bulletin, v. 88, p. 1237-1250. Coney, P.J., Jones, D.L., and Monger, J.W.H., 1980, Cordilleran suspect terranes: Nature, v. 288, p. 329-333. Coney, P.J., and Harms, T.A., 1984, Cordilleran metamorphic core complexes: Cenozoic extensional relics of Mesozoic compression: Geology, v. 12, p. 550554.

Ernst, W^G,, 1983, Phanerozoic continental accretion and the metamorphic evolution of northern and central California: Tectonophysics, v. 100, p. 287320. Gans, P.B., Miller, E«L., McCarthy, J., and Ouldcott, M.L., 1985, Tertiary extensional faulting and evolving ductile-brittle transition zones in the northern Snake Range and vicinity: new insights from seimic data: Geology, Ve 13, p. 189-193. Miyashiro, A., 1961, Evolution of metamorphic belts: V. 2, p. 277-311.

Journal of Petrology,

Monger, J.W.H,, Price, R.A., and Tempelman-Kluit, D.J., 1982, Tectonic accretion and the origin of the two major metamorphic and plutonic welts in the Canadian Cordillera: Geology, v. 10, p. 70-75. Mortimer, N , 1985, Structural and metamorphic aspects of terrane amalgamation: a case study from the northeast Klamath Mountains of California, Howell, DcGe, ed., Tectonostratigraphic terranes of the Circum-Pacific region: Earth Science Series, v. 1, Circumpacific Council for Energy and Mineral Resources (in press). Silberling, N.J., and Jones, D.L., 1984, Lithotectonic terrane map of the North American Cordillera: U.S. Geological Survey Open-File Report 84-523, 100 p., 4 map sheets, scale 1:2,500,000. Tabor, R«W., Zartman, R.E., and Frizzell, V.A., Jr., in press. Possible tectonostratigraphic terranes in the North Cascades crystalline core, Washington, ^ S c h u s t e r , E., ed., Geology of Washington Symposium Volume: Washington State Department of Natural Resources. Todd, V.R., and Shaw, S.E., 1985, S-type granitoids and an I-S line in the Peninsular Ranges batholith, southern California: Geology, v. 13, p. 231-233. Turner, F.J., 1981, Metamorphic petrology: McGraw-Hill Book Company, 524 p. Williams, Harold, and Hatcher, RcD., Jr., 1982, Suspect terranes and accretionary history of the Appalachian orogen: Geology, v. 10, p. 530-536. Zen, E-an, 1983, Exotic terranes in the New England Appalachians—limits, candidates, and ages: a speculative essay, Geological Society of America Memoir 158, p. 55-81.

CAINOZOIC BASINS AND PETROLEUM POTENTIAL OF PAPUA NEW GUINEA E.A. Bowen Robertson Research (Australia) Pty. Limited Tectonic Setting Papua New Guinea is situated in one of the most tectonically complex regions of the southwest Pacific. For most of the Cainozoic the region has been within a microplate complex, at times up to 1000 km in width, which broadly constitutes the leading edge of the Indo-Australian Plate. Present plate motions are such that the Pacific Plate is moving generally west-northwest at about 10 cm/year and the Indo-Australian Plate north-northeast at about 7 cm/year. In the region of the New Guinea Trench. Taylor (1979) has calculated the convergence across the boundary as about 12 cm/year on azimuth 068® (ENE). This east-northeasterly convergence has probably prevailed in the New Guinea region throughout most of the Cainozoic. It has resulted in the development of major compressive and left lateral strike slip fault structures such as the Papuan Fold Belt and the Sorong Fault. Earlier regimes, prior to the rapid separation of Australia and Antarctica along the southeast Indian Ridge (about mid-Eocene, 45 Ma), are not easily resolved but are considered to be primarily translational and/or extensional in nature. Currently, the boundary between the Pacific and Indo-Australian Plates passes north of the western part of New Guinea. In the east, it bifurcates to enclose the South Bismarck, Solomon Sea and Trobriand Plates (Figure 1). Ripper and McCue (1983) envisage double subduction of the Solomon Sea Plate along the New Britain and Trobriand Trenches. Subduction is driven by back-arc spreading in the Woodlark Basin, and in the New Guinea and Manus Basins of the Bismarck Sea. Almost immediately this led to collision of the South Bismarck and Indo-Australian Plates along the Ramu-Markham suture. Collision occurred initially in the northwest and has migrated progressively towards the southeast. The nature and location of the boundary on the western side of the Trobriand Plate is open to debate. It may exist as a broad zone of deformation corresponding with a rather diffuse pattern of seismicity. Field evidence (Davies et al., 1984) and interpretation of magnetic and gravity data suggests the majority of the movement associated with Woodlark Basin spreading has been accommodated along the Gira Fault and offshore. Stratotectonic Provinces As a consequence of its tectonic evolution. New Guinea can be broadly divided into two stratotectonic provinces : - Australian continental crust - oceanic crust and island arcs. The Collision Zone, between these two, is highly deformed and contains elements of both. The continental province has a block-faulted crystalline basement of Palaeozoic age. Basement is overlain by flat-lying to strongly folded Mesozoic clastics. Tertiary carbonates and Pliocene to Recent clastics and volcanics. The sediments of the northern margin have been highly deformed and thrust southwards to form the southern flanks of the central cordillera. The oceanic province has an ophiolitic basement ranging in age from ?Cretaceous to ?Eocene. Sediments in the lower parts of the sequence are primarily volcanilithic, with some lavas. The upper part of the sequence, following a widespread regression in the Miocene, is dominated by parallc sediments and limestone with isolated volcanics.

Hydrocarbon Potential Sedimentary basins, with hydrocarbon potential, are found onshore and in relatively shallow water within each of the three broad provinces (Figure 2) • These are tabulated below. Province

Sedimentary Basin

Discovery Wells

continental

Papuan (onshore/offshore)

Juha (gas/condensate) Puri (light oil) Pasca (gas)

collision

N. New Guinea (onshore)

oceanic

Cape Vogel Bougainville New Ireland (offshore)

The Papuan Basin contains sediments of continental affinity and is located to the south of the central cordillera of New Guinea. Recent drilling success at Juha has sparked a flood of applications for prospecting licences over the vacant areas of the basin. A wide variety of stratigraphic and structural traps exist in both clastic and carbonate facies rocks. The basins of the central, collision zone lie to the north of the cordillera, but inboard of a series of accretionary blocks. These northern margins are primarily the dismembered parts of a Palaeogene volcanic arce The oceanic basins lie to the north and east. They can be further subdivided into two groups. The first comprises those that are within the present-day microplate complex. These include the numerous sub-basins of the Cape Vogel Basin. Seismic, and limited drilling in these areas indicates a thick Miocene-Recent sequence of clastics and carbonates with reasonable potential for hydrocarbon generation at the deeper levels. The second group of oceanic basins are those to the north and east of the microplate complex, on the Pacific Plate. These include the Bougainville and New Ireland Basins. Both are little known, but reasonable exploration potential exists, particularly in the Bougainville Basin. Exploration potential in the New Ireland Basin is limited by the relatively deep water in most of the area. References Davies, H.L., Symonds, P.A. & Ripper, I.D., 1984, Structure and evolution of the southern Solomon Sea region, BMR Journal of Australian Geology & Geophysics, 9, 49-68. Ripper, I.D., & McCue, K.F., 1983, Selsmlclty of the Indo-Australian/Solomon Sea Plate boundary in the southwest Papua region, Tectonophysics, 87, 355-369. Taylor, B., 1979, Bismarck Sea : evolution of a back-arc basin. Geology, 7, 171-174.

INDO-AUSTRALIAN PLATE

LEGEND

Subduction Zone with Polarity Indicated Plate Motion Showing Rates and Direction (m cm/yr)

^ ^

Transform

Faults

Spreading

Centres

M G I O N A L PLATB T I C T O N I C SETTING RGURE 1

PACIFIC

PLATE

REGIONAL PLATE TECTONIC SETTING

BASIN LOCATIONS FIGURE 2

X 29

TERRANE BOUNDARIES AND TERRANE DISPLACEMENT IN NORTHERN LAND, ANTARCTICA

VICTORIA

J D Bradshaw

University

of Canterbury, Christchurch,

New

Zealand

Contrasted northwest trending belts of Early Paleozoic rock have been mapped in northern Victoria Land and recently have been shown to extend for 400km from the Southern Ocean to the Ross Sea (Figure 1). The primitive island arc character of the Cambrian volcanics prompted the suggestion of a minimum of three discrete terranes (Weaver et al 1984). Terrane boundaries were considered to be straight or slightly curved strike slip faults (Figure I). Further field work in 1983 and 1984 has confirmed the tectonic nature of the contacts, but also shown them to be more complex with clear evidence of thrusting in places. The suspect 'Millen terrane" tentatively drawn between the Bowers and Robertson Bay is a schistose deformation zone containing rocks of Bowers and Robertson Bay parentage. Gibson and Wright (1985) have proposed that regional scale thrusting is the main cause of amalgamation and accretion of terranes and infer the existance of a subduction zone between the Bowers and Robertson Bay terranes. Such a subduction zone was considered by Weaver et al (1984, Figure 6) and rejected due to lack of features typical of subductive margins. Whilst the discovery of the thrusts answers one objection and also permits the suggestion that advancing thrust sheets may have buried the evidence of subduction zones, a model involving thrusting alone remains unconvincing. Similarly the case for strike slip alone is unsatisfactory, and there is evidence that both processes had a role in the development of the terrane boundaries. Eastern Boundary

of Bowers

Terrane

In the Millen Range the contact of the Bowers and Robertson Bay rocks is a synmetamorphic thrust (or group of thrusts) lying within a structural distinctive zone of tight, moderately inclined to recumbent folds with weakly to well developed schistosity and mineral lamination (Findlay and Field 1983). All structures in this zone are refolded by northwest trending upright folds of a type extensively developed in both adjacent terranes. The recumbent fold-thrust domain is about 25km wide between Mt McCarthy and Handler Ridge. North of the Millen Range, discontinuous developments of polydeformed schistose rocks and pillow lavas (Wright 1981, Bradshaw et al 1982, Jordan et al 1984) indicate local infolded occurences of the thrust zone and Bowers terrane rocks, but for 200km the effective terrane boundary is commonly the Leap Year Fault. South of the Millen Range, the youngest strand of the Leap Year Fault appears to lie within the thrust zone. Here the boundary between the simple and polydeformed domains is a pair of faults which are regarded as older parts of the Leap Year Fault offset to the west (Figure 2).

The thrusting documents a period of telescoping which emplaced Bowers terrane over Robertson Bay terrane probably soon after the deposition of the youngest rocks of the latter in the Early Ordovician (Wright and Findlay 1984, Adams and Kreuzer 1984, Wright et al in prep.). However the wide difference in age between the youngest Bowers arc volcanics and the thrusting (25+ Ma) militates against subduction related to that particular arc as a cause (cf Gibson and Wright 1985). Upright folding throughout the Bowers and Robertson Bay terranes ensued and the parallel Leap Year Fault was probably initiated at this time. The omission of 3-5km of strata on the east limb of the Camp Ridge Syncline (Figure 2,3), the juxtaposition of contrasted structural domains, and the differences of metamorphic rank are consistent with either a high angle reverse dip slip or reverse oblique slip fault. Consequently the original separation of the thrust zone and simple domain is uncertain. Subsequently the Leap Year Fault was offset about 15km dextrally by a fault in the eastern Evans Neve which may link with the western terrane boundary. Western Terrane Boundary

of Bowers Terrane

The western boundary is the many stranded Lanterman Fault Zone (Figure 2) which juxtaposes schist and gneiss of the amphibolite facies Lanterman terrane with lower grade Bowers terrane rocks. Within the fault zone two deformed conglomerates have proved difficult to interpret. Gibson (1984) has clarified the relationships and it seems logical to regard both conglomerates as part of the Bowers terrane. The Lanterman Fault Zone has an overall trend of 145^ but is in segments ranging from 165^ to 135°. Areas of thrust faulting correspond to segments with more westerly (anti-clockwise) strike than the average. In the region of the Lanterman Range-Reilly Ridge the fault zone is divided into an eastern strike slip subzone and a western thrust subzone. In the strike slip subzone on Reilly Ridge the typical upright folds are accentuated and rotated so that plunges up to 90 occur. The folds are cut by numerous subvertical faults in a pattern of shear lozenges which resembles an imbricate zone turned on edge. Strain in the eastern zone is minor as shown by slight distortion of trilobites. In the western subzone metamorphic rank increases rapidly to greenschist facies as does strain with the development of extreme oblate spheroids in conglomerate (Bradshaw et al 1982, Figure 4, Gibson 1984, Figure 3) and mylonitic zones. Deformation is complex, as indicated by two schistosities and the obvious refolding of flattened pebbles.. A broadly similar pattern is described by Gibson et al 1984 in the southern part of the fault zone. The Lanterman Fault Zone has a long history of movement extending into the Late Mesozoic. Ordovician faulting postdated initiation of folding in the Bowers Terrane and led to modification of trend and style. The reverse faulting is seen as a secondary effect in compressive sectors of a dominantly strike slip system. The complexity of structure within the two deformed conglomerates and the common intercalation of very coarse grained facies in all units of the Bowers terrane along the western margin suggests a history of movement extending back at least to the early Middle Cambrian.

Conclusions C h a n g i n g d y n a m i c s in m a j o r t e c t o n i c s y s t e m s is w e l l e s t a b l i s h e d ( e . g . L a t e C e n o z o i c of C a l i f o r n i a and New Z e a l a n d ) and t h r u s t s c u t by s t r i k e s l i p f a u l t s and s t r i k e s l i p f a u l t s d e v e l o p i n g r e v e r s e or t h r u s t d i s p l a c e m e n t s a r e not u n c o m m o n . The above system might e i t h e r r e f l e c t an a n t i - c l o c k w i s e c h a n g e in c o n v e r g e n c e v e c t o r s t h r o u g h the O r d o v i c i a n or the c h a n g i n g d y n a m i c s w i t h i n an accreting crustal wedge with constant overall oblique convergence vector. No d a t a is yet a v a i l a b l e to a s s e s s the a m o u n t of d i s p l a c e m e n t b e t w e e n the t e r r a n e s , nor the d i p of any s u b d u c t i o n z o n e s a c t i v e at t h a t t i m e . References A d a m s , C . J . D . , K r e u z e r , H . , 1 9 8 4 , P o t a s s i u m - A r g o n a g e s t u d i e s of s l a t e s and p h y l l i t e s f r o m t h e B o w e r s and R o b e r t s o n Bay t e r r a i n s , north Victoria Land, Antarctica, Geologische Jahrbuch B60, 265-288. Bradshaw, J.D., Laird, M.G., Wodzicki, A., 1982, Structural style and t e c t o n i c h i s t o r y in n o r t h e r n V i c t o r i a L a n d , A n t a r c t i c a , Jjn C r a d d o c k , C . ( e d . ) , A n t a r c t i c G e o s c i e n c e , U n i v e r s i t y of W i s c o n s i n Press, 809-816. F i n d l a y , R . H . , F i e l d , B . D . , 1 9 8 3 , T e c t o n i c s i g n i f i c a n c e of d e f o r m a t i o n s a f f e c t i n g the R o b e r t s o n Bay G r o u p and a s s o c i a t e d r o c k s , n o r t h e r n V i c t o r i a L a n d , A n t a r c t i c a , jji O l i v e r , R . L . et al ( e d s ) , A n t a r c t i c E a r t h S c i e n c e , A u s t r a l i a n A c a d e m y of S c i e n c e , Canberra, 107-112. G i b s o n , G . M . , 1 9 8 4 , D e f o r m e d c o n g l o m e r a t e s in t h e e a s t e r n L a n t e r m a n Range, North Victoria Land, A n t a r c t i c a , Geologische Jahrbuch B60, 117-143. Gibson, G., Tessensohn, F., Crawford, A.J., 1984, Bowers Supergroup r o c k s w e s t of t h e M a r i n e G l a c i e r and p o s s i b l e g r e e n s c h i s t f a c i e s equivalents., Geologische Jahrbuch, B60, 289-318. G i b s o n , G . M . , W r i g h t , T . O . , 1 9 8 5 , I m p o r t a n c e of t h r u s t f a u l t i n g in the t e c t o n i c d e v e l o p m e n t of n o r t h e r n V i c t o r i a L a n d , A n t a r c t i c a , Nature, 315, 480-483. Jordan, H., Findlay, R.H., Schmidt-Thome, M., Mortimer, G,, Muller, P . , C r a w f o r d , A . , 1 9 8 4 , G e o l o g y of the n o r t h e r n B o w e r s M o u n t a i n s , North Victoria Land, Geologische, Jahrbuch B60, 57-81. Weaver, S., Bradshaw, J.D., Laird, M.G., 1984, G e o c h e m i s t r y of C a m b r i a n v o l c a n i c s of the B o w e r s S u p e r g r o u p and i m p l i c a t i o n s for t h e E a r l y P a l e o z o i c t e c t o n i c e v o l u t i o n of n o r t h e r n V i c t o r i a L a n d , Antarcti.ca, E a r t h and P l a n e t a r y S c i e n c e L e t t e r s 6 8 , 1 2 8 - 1 4 0 . W r i g h t , T . O . , 1 9 8 1 , S e d i m e n t o l o g y of the R o b e r t s o n Bay G r o u p , N o r t h Victoria Land, Antarctica, Geologische Jahrbuch B41, 127—138. W r i g h t , T . O . , F i n d l a y , R . H . , 1 9 8 4 , R e l a t i o n s h i p s b e t w e e n the R o b e r t s o n Bay G r o u p and the B o w e r s S u p e r g r o u p : new p r o g r e s s and c o m p l i c a t i o n s f r o m the V i c t o r i a M o u n t a i n s , n o r t h e r n V i c t o r i a L a n d , Geologische Jahrbuch B60, 105-166.

MiMd

Robertson ^ .Boy

Figure 1.

Simple terrane map of northern Victoria Land. The thrusted boundary between Robertson Bay and Bowers terrane is too intricate to be shown at this scale, A synoptic terrane relationship diagram is below. (Mr » Millen Range, Hr = Handler Ridge.)

Figure 2.

Major structural features of the Bowers terrane and its bounding tectonic zones.

Figure 3.

Schematic sections representing the development of terrane boundaries. The movements were probably quasi-continuous and mainly Ordovician in age.

NUMBER, EXTENT AND SOURCE OF ASIAN TERRANES

Clive F. Burrett

Geology Dept., University of Tasmania, Hobart, Australia.

Asia, east of the Urals, consists of a large number of Phanerozoic blocks of various sizes that collided mainly during the Late Palaeozoic and Early Mesozoic. The southern margin of the Siberian Block consists of a wide and complex orogenic belt that appears to be a good example of crustal accretion from the Late Precambrian through to the Permian. Small, exotic terranes may be embedded in this belt. The Kazakkstan Block, which collided with the Siberian Block in the Late Carboniferous, also shows evidence of peripheral growth. The North China Block collided with the Siberian Block in the Permian and with the South China Block (along the Qinling Fold Belt) in the Triassic or earliest Jurassic. Tarim, the Tibetan (3 of them), the Shan-Thai (or Sibumasu) and the Indo-China blocks are small and camparable in size with the Western Cordilleran terranes. Many of these blocks (or terranes) such as N. China, Sibumasu, Tarim and the Tibetan Blocks (3 of them) were almost certainly part of Gondwana. In the last 3 years good biogeographic and stratigraphic evidence has been assembled that confirms a Gondwana origin (in the Indian-Australian sector) for these blocks. Available evidence constrains the time of rifting to between the Lower Ordovician (post-Whiterockian) and the Lower Triassic. There is no good evidence for the Palaeozoic placement of the Indo-China and South China Blocks except that the Cambrian palaeolatitude of South China is similar to that of Australia. Final suturing in South East Asia was in the Triassic producing the Indosinian orogeny. A major research aim is to reconstruct the former positions of the Asian blocks relative to Gondwana, their drift trajectories and their fusion dxiring the Late Palaeozoic and Triassic.

GRAVITY ANOMALIES, OPHIOLITES, AND CRUSTAL STRUCTURE: EXAMPLES FROM THE YUKON-KOYUKUK PROVINCE, ALASKA, AND DUNNAGE TERRANE, NEWFOUNDLAND John W, Cady U.S. Geological Survey, Denver, Colorado 80225, USA Known rocks of oceanic affinity correlate with regional Bouguer gravity anomaly highs in interior and western Alaska (Cady, 1983). Regional gravity and aeromagnetic studies are underway in and around the Yukon-Koyukuk province and Yukon Flats (Figure 1) to determine the subsurface configuration and geometry of accretion of igneous rocks of the oceanic Angayucham, Koyukuk, and Tozitna terranes (Howell and others, 1983). This discussion focuses on the Angayucham and Koyukuk terranes of the Yukon-Koyukuk province, but generalizations about the regional gravity field and crustal structure apply to the Tozitna terrane and Yukon Flats as well. Box (1983) and Patton (1984) concluded that the MLssissippian to Jurassic oceanic crustal rocks that rim the Yukon-Koyukuk province (Angayucham terrane, which includes pillow basalt, diabase, gabbro, tuff, chert, graywacke, argillite, minor limestone, and locally abundant ultramafic tectonite) were emplaced during Early Cretaceous time, when an oceanic island arc (Koyukuk terrane) collided with continental rocks that border the province (Ruby geanticline and Brooks Range). Isotopic studies (Arth and others, 1984) of plutons in the Yukon-Koyukuk province show no evidence of older continental crust beneath the province. The Yukon-Koyukuk province forms a "V" open to the southwest. Box (1983) suggested that the Angayucham terrane is a remnant of oceanic crust that remained unsubducted when a relatively straight magmatic arc collided with an irregular continental margin, in the same way that the South China Sea has been caught between Eurasia and the Philippine Islands. Magnetic anomalies caused by igneous rocks of the Koyukuk terrane also form a "V" open to the southwest, suggesting oroclinal bending of the magmatic arc either during collision with a passive continental margin (as in the Banda Sea) or following collision, as the continental margin was bent by the arrival of exotic terranes from the southeast. In any case, the geometry has subsequently been complicated by right-slip faulting (Box, 1983). Bouguer anomaly values are more positive (> -20 mGal) in the YukonKoyukuk province than in the adjoining Brooks Range (-50 to -100 mGal) or Ruby geanticline (-30 to -50 mGal). The highest gravity and magnetic anomalies coincide with the Angayucham terrane along the northern and southeastern margins of the province. These anomalies are asymmetrical, having steeper gradients to the outside of the province, and can be modeled by assuming dense, magnetic sources that dip 30 to 70 degrees inward beneath the province. Steep gravity gradients coincide with the boundaries between continental and oceanic terranes determined by surface geology and by isotopic data (cf. Arth and others, 1984). My interpretation of these data is that the Yukon-Koyukuk province is underlain by oceanic crust that has been thickened by island arc magmatism and sedimentation. Only at the province margins has oceanic crust been thrust over continental crust. No seismic refraction or deep reflection data are available nearby. An unreversed refraction profile south of Fairbanks (Hanson and others, 1968) shows continental crust thinning from a thickness of 48 km in the Alaska Range

to 32 km near Fairbanks, but the model is poorly constrained. Seismic refraction profiles in the accreted terranes of southern Alaska (G. S. Fuis and others, written commun., 1985) are interpreted in terms of multiple layers with oceanic crustal velocity separated by low-velocity zones. The layers dip 3 to 10 degrees north and are inferred to have been emplaced on low angle thrusts. One model for the emplacement of the Angayucham terrane and the nearby Tozitna terrane is accretion of oceanic allochthons from the south on low angle thrusts ( e . g . Coney, 1983). This model is unsatisfying to me because, if oceanic allochthons once covered large areas of central Alaska, their present distribution—'i.e® restricted to narrow, locally steeply dipping b e l t s — i s fortuituous. In my rooted model, however, the oceanic terranes are denser and stand isostatically lower than continental terranes. Denser oceanic rocks are the isostatic cause of the lowlands in which they persist, protected from erosion. Southern Alaska is gravimetrically different from the interior: Bouguer anomalies are generally more negative and more variable in the south, and deep Bouguer anomaly lows are caused by low-density rocks accreted beneath the Alaska Range® I predict that future seismic studies will find the crustal structure of interior Alaska to be more rooted than that of southern Alaska. le^o

64«

Figure 1.--Index map of interior Alaska.

Moores (1982) distinguished Tethyan ophiolites, which are usually intact and thrust over a continental crystalline substrate, from Cordilleran ophiolites, which are often disrupted and lack an identifiable continental substrate. A preliminary literature survey suggests that high-amplitude Bouguer anomalies (100 to 250 mGal peak to trough) occur over Tethyan ophiolites and collisional orogens such as the Alps and Himalyas (Karner and Watts, 1983), where flexure of the lithosphere supports isostatically unbalanced loads. Smaller Bouguer anomaly amplitudes (30 to 100 mGal) are associated with Cordilleran ophiolites. Hence the Cordilleran ophiolites may have been emplaced by gentle docking that did not greatly flex the lithosphere. Bouguer anomaly amplitudes in interior Alaska are typical of Cordilleran ophiolites. The Paleozoic Dunnage terrane of central Newfoundland is a possible analogue for the oceanic terranes of interior Alaska. Moores (1982) called the ophiolites of central Newfoundland Cordilleran, although Coleman (1984) considered all the ophiolites of the margins of lapetus to be Tethyan. Ophiolites of central Newfoundland have gravity anomaly amplitudes of 30 to 80 mGal (Weaver, 1967), characteristic of Cordilleran ophiolites. The Bouguer anomaly map of central Newfoundland has closures of +30 mGal to +40 mGal, about 30 mGal higher than closures in interior Alaska. As much as 20 mGal of this difference is due to the higher mean elevation (by 100 to 200 m) in interior Alaska. Seismic refraction studies (Ewing and others, 1966) interpreted the crust of the Dunnage terrane in Newfoundland to be 41.5 km thick. Layering in this crustal model, from top to bottom, is as follows: velocity 6.01 km/s, thickness 12.3 km; 6.70 km/s, 10.5 km; 7.52 km/s, 18.7 km. These results are consistent with the generalization by Pakiser and Steinhart (1964) that thick crust and high mean crustal density occur together. It is possible, however, that by using modem instrumentation, a close receiver spacing, and modem interpretation methods, seismologists would interpret Newfoundland to be underlain by multiple low velocity layers and subhorizontal faults. In Newfoundland, as in Alaska, the crustal structure is controversial and the availability of seismic refraction data in Newfoundland has not stifled debate about whether the Dunnage terrane is allochthonous or rooted. Karlstrom (1983) argued that rooted oceanic crust, or even a lower density island arc, would cause gravity anomalies higher than observed, and claimed that the gravity data are best satisfied by an allochthonous Dunnage terrane. Miller (1984) countered that the gravity and seismic data could also be satisfied by a rooted Dunnage terrane with thick crust of intermediate density. Integrated interpretations are impossible because the seismic data were collected offshore, 100 km north of the pertinent gravity and geologic data. Seismic refraction studies are planned in interior Alaska as part of the Trans-Alaska Crustal Transect Program. I am compiling new gravity and magnetic maps and calculating models of crustal structure based upon both the rooted and allochthonous hypotheses. The geophysical maps should help to locate seismic lines strategically, and the models will provide hypotheses to be tested by seismic interpretation. I hope that integrated seismic and potential field interpretations will reduce the ambiguity.

References Arth, J- G., Carlson, J. L., Foley, N. K., Friedtnan, I., Patton, W. W., Jr., and Miller, P., 1984, Crustal composition beneath the Yukon-Koyukuk Basin and Ruby Geanticline as reflected in the isotopic composition of Cretaceous plutons. Geological Society of America, Abstracts with Programs, 16, no. 5, 328 Box, Stephen, 1983, Implications of a possibly continuous 4OOO km long Late Early Cretaceous arc-continent collisional belt in NE USSR and NW Alaska for the tectonic development of Alaska, in Howell, Do Go, Jones, D. Lo, Cox, Allan, and Nur, Amos, Proceedings of the Circum-Pacific Terrane Conference, Stanford University, Publications in the Geological Sciences, 13, 33-35. Cady, J. W., 1983, Oceanic terranes of interior and western Alaska—Evidence for thick crust of intermediate density, in Howell, D. G., Jones, D. L., Cox, Allan, and Nur, Amos, Proceedings of the Circum-Pacific Terrane Conference, Stanford University, Publications in the Geological Sciences, 13, 41-43. Coleman, Rc G., 1984, The diversity of ophiolites, Geologie en Mijnbouw, 63, 141-150. Coney, P. J., 1983, Structural and tectonic aspects of accretion in Alaska, ^ Howell, De G., Jones, D. U , Cox, Allan, and Nur, Amos, Proceedings of the Circum-Pacific Terrane Conference, Stanford University, Publications in the Geological Sciences, 13, 68-70. Ewing, G. N., Dainty, Ae M., Blanchard, J. E., and Keen, M. Je, 1966, Seismic studies on the eastern seaboard of Canada; The Appalachian System. I, Canadian Journal of Earth Sciences, 3, 89-109. Hanson, Kenneth, Berg, Eduard, and Gedney, Larry, 1968, A seismic refraction profile and crustal structure in central interior Alaska, Bulletin of the Seismological Society of America, 58, 1657«1665. Howell, D. G., Schermer, E. R., Jones, D. L., Ben-Avraham, Zvi, and Scheibner, Erwin, 1983, Preliminary tectonostratigraphic terrane map of the CircumPacific region, Howell, D. G., Jones, D. L., Cox, Allan, and Nur, Amos, Proceedings of the Circum-Pacific Terrane Conference, Stanford University, Publications in the Geological Sciences, 13, 227-242. Karlstrom, Ke E., 1983, Reinterpretation of Newfoundland gravity data and arguments for an allochthonous Dunnage zone, Geology, 11, 263-266. Karner, G. D., and Watts, A. Be, 1983, Gravity anomalies and flexure of the lithosphere at mountain ranges. Journal of Geophysical Research, 88, 10449-10477. Miller, H. G., 1984, Comment on "Reinterpretation of Newfoundland gravity data and arguments for an allochthonous Dunnage zone," Geology, 12, 60-

61.

Moores, E. M., 1982, Origin and emplacement of ophiolites. Reviews of Geophysics and Space Physics, 20, 735-760. Pakiser, L. C., and Steinhart, J. S., 1964, Explosion seismology in the western hemisphere, Odishaw, Hugh, ed.. Research in Geophysics, 2, Massachusetts Institute of Technology Press, Cambridge, p. 123-147. Patton, W. W., Jr., 1984, Timing of arc collision and emplacement of oceanic crustal rocks on the margins of the Yukon-Koyukuk Basin, western Alaska, Geological Society of America, Abstracts with Programs, 16, no. 5, 328. Weaver, D. F., 1967, A geological interpretation of the Bouguer anomaly field of Newfoundland, Dominion Observatory of Canada Publication, 35, 223-251.

SOME PROBLEMS ABOUT THE TERRANES AROUND THE CARIBBEAN REGION Maria Fernanda Campa Uranga Petroleos Mexicanos, Mexico, D.F. The tectonic complexity of the Caribbean Region is due to the interaction of two ancient cratons and two present oceans (1). The eastern Paleozoic Appalachian and the western Paleozoic Andean features mingle with the earliest geometry of the Proterozoic Cratons (Central North America and eastern South America), and the southern continuation of the Western Cordilleras intrudes into the Caribbean Region as far as the Central Atlantic. In order to understand the tectonic evolution of the Caribbean Region, terrane analysis seems to be a good method, in spite of its inherent methodological limitations. This abstract presents some reflections on Caribbean tectogenesis. The terranes will be referred to by geographical location. Terranes of Mexico The recognition of basement terranes, overlapped by younger Mesozoic and Cenozoic assemblages, was the first step in the process of tectonostratigraphic terrane analysis (2). Early Mesozoic, Paleozoic and late Precambrian assemblages, have been grouped into three major tectonic regions: 1)

An Eastern Region, surrounding the Gulf of Mexico and limited to the west by the thrust belt front. This Region is mainly formed by upper Paleozoic basement rocks, similar to those of the Appalachian-Ouachita-Marathon Mountains that are overlapped by gently folded and faulted Late Mesozoic and Cenozoic sedimentary assemblages.

A Central Thrust Belt Region placed between the eastern thrust belt front and the western suspect accreted front. This belt is divided into two terranes because of differences in basement rocks: a) the northern terrane, which is the direct southward continuation into Mexico of the North American Precambrian basement and its Paleozoic cover, and b) some scattered terrane fragments mainly formed by Paleozoic and Grenville assemblages; though heterogeneous in character all are considered to have a common origin with the Appalachian-Ouachita-Marathon belt, and were later involved in the Late Mesozoic-Early Cenozoic nappes of the Thrust Belt.

A Western Region, covering one half of Mexican territory and surrounding the Pacific margin, is formed by a composite group of submarine volcanic and sedimentary assemblages and fragmented ophiolites. These terranes involve some scattered blocks similar to the Cordilleran miogeoclinal, Appalachian and Grenvillian rocks of North America.

Terranes of Colombia and Venezuela The rocks underlying Cenozoic volcanic and sedimentary strata of the NW South American territory have been divided into three major tectonic regions (3): 1)

A Southeastern Region formed by early Precambrian rocks and their Paleozoic cover, considered to be part of the Guyana and Amazon shields of the South American craton. This basement is overlapped by the Cenozoic sediments of the Orinoco and Amazon Basins.

A Central-Eastern Thrust Belt Region (Cordillera Oriental, Serranias Perija and Macarena in Colombia, and Andes Merida in Venezuela), which is thrust over the South American craton. This belt is formed by tectonized blocks of Late Precambrian, Paleozoic (Permo-Triassic is absent), Late Triassic, Early Jurassic protorift rocks, and a very thick sequence of late Jurassic, Cretaceous and Cenozoic terrigeneous rocks interstratified

with some limestone beds. 3)

A Central Region (Cordillera Central and Sierra Santa Marta) characterized by fragmented blocks made up of Late Precambrian to Cambro-Ordovician metamorphic and plutonic rocks. Late Paleozoic (Devonian to Permian) to Early Jurassic granitic rocks, and Late.Cretaceous metasediments and granitic plutons.

A Central-Western Region (Cordillera Occidental in Colombia) and Northern (Cordillera Costera, Peninsulas Paraguama, Guajira and Araya-Paria in Venezuela) mainly formed by Cretaceous island arc and ophiolitic assemblages similar to those of the northern Trinidad and Tobago Caribbean islands.

A Northwestern Region (Sierra Baudo) formed by turbidites and island arc assembalges, Cenozoic in age, which continue to the Panama Isthmus.

Terranes of Central America Isthmus The rocks underlying the Cenozoic volcanic.and sedimentary strata of the Central America Territory have been divided into three regions (4): 1)

The Northeastern Thrust Belt (Sierra Cuhumatanes in Guatemala and Maya Mountains in Belize), which is the southward continuation of the Mexican Central Thrust Belt.

The Western and Northern Region (El Tambor in Guatemala and Peninsulas Santa Elena in Nicaragua, Nicoya, Osa and Burlaca in Costa Rica, and Sona Azuero, Choco and Darien in Panama) formed by fragments of island arc and ophiolitic assemblages (5) similar to those of the Western Region in Mexico and the Western, Central-Western and Northern Region in Colombia and Venezuela.

The Eastern Region (Honduras-Nicaragua and nearby oceanic rise) formed by Late Precambrian and Paleozoic metamorphic rocks underlying Cretaceous sediments similar to those of the Oaxaca and Mixteca terranes in Mexican territory.

Terranes of the Caribbean Islands and Oceanic Floor The Northeast and Southeast islands are formed by fragmented island arc and ophiolitic assemblages of Late Mesozoic age, similar to those of El Tambor in Guatemala and Northern Venezuela. These terranes are overlapped by post-accreted Oligocene-Pliocene limestones, like those in Puerto Rico and Jamaica (6). At the Organos-Rasario in Pinar el Rio, Cuba, there is a fragment of the Central Mexican Thrust Belt. The oceanic floor of the Caribbean Sea (Cayman Ridge, Colombian Basin, Beata Ridge and Venezuelan Basin) seems to be formed by Cretaceous to Late Jurassic (?) oceanic and island arc assemblages (1) very similar to those of the islands. The exception is the Nicaragua Rise which seems to be similar to the metamorphic rocks of the Honduras-Nicaragua terrane (called the Chorthis Block). CONCLUSIONS This brief description of the major terranes known in the Caribbean Region indicates that this puzzle is at present a collage very similar to those of the Central Eastern Pacific margin and very different from the Central Western Atlantic passive margin.

The geological boundaries between the North and South American continents are broken in scattered fragments and originated in the Pacific Region. The Santa Marta block in Colombia and the Chortis block in Central American are some of the fragments derived from the southwestern end of the North America Appalachian and Grenvillian rocks. These were kept suspended like exotic blocks within island arc assemblages and ophiolites fragmented during the Late Mesozoic and Cenozoic. The oldest terranes in the Caribbean oceanic floor and islands are late Jurassic. There are no middle Jurassic, Triassic or Permian rocks. The Late Mesozoic western depositional margin in the Eastern Region of Mexico, and in the Centraleastern Region of Colombia, was a passive margin, which was folded and thrust between 80 and 40 ma. It plays the role of a tectonized belt between the Eastern cratonic terranes and the Western accretionary collage. It has not continuity in the Caribbean region. This Continental Thrust Belt is not present between the Polochic Motagua and Bocono-Oca transform fault systems, and it seems to be genetically related to the Paleozoic Appalachian System belt formed between the North American, African and South American Proterozoic cratons. REFERENCES Case, J., Holocombe, T.C., & Martin, R.G., 1984, Map of the geologic provinces in the Caribbean .Region, Geological Society of America, Memoir 162. Coney, P.J. & Campa, M.F., 1984 in Silbering NJ and Jones, D.L., Lithotectonic terrane maps of the North American Cordillera, open file report 84-523 U.S. Geological Survey part D. Campa, M.F., Cediel, F., Barrera, D.,in press, Mapa de Terrenos Tectonoestratigraficos de Colombia. Dengo, G., in press. Mid- America: Tectonic setting for the Pacific Margin from Southern Mexico to North Western Colombia. Bourgois, J., et al, 1982, Ages et structures des complexes basiques et ultrabasiques de la fagade Pacifique entre 3®N et 12®N (Colombie, Panama et Costa Rica), Bulletin societe Geologique du France (7), t. XXIV, No. 3. Wadge, G., Draper, G. and Leuis, J.F., 1984, Ophiolites of the Northern Caribbean: A reappraisal of their roles in the evolution of the Caribbean plate boundary. Geological Society London, Special Publication No. 13.

TERRANES OF THE CARIBBEAN REGION A.NORTH AMERICAN ARCHEOZOIC CRATON,BiGUYANA ARCHEOZOIC CRATON, OGREENVILLIAN PROTEROZOIC BELT.D) APPALACHIAN PALEOZOIC COLLISION COLLAGE AND BELT, E)CORDILLERA MESOZOIC PACIFIC COLLAGE, F)THRUST MESOZOIC-CENOZOIC BELT.

ACCRETION TECTONICS AND CRUSTAL STRUCTURE IN ALASKA:

A TECTONOSTRATIGRAPHIC

TERRANE-TECTONIC ASSEMBLAGE MAP AND STRUCTURE SECTION ACROSS EASTERN ALASKA P . J . Coney^ and D . L Jones^ ^Dept. of Geosciences, Univ. of Arizona, Tucson, Arizona, U.S.A. ^U. S . Geological Survey, Menlo Park, California, U . S . A . A preliminary tectonostratigraphic terrane-tectonic assemblage map and accompanying crustal structure section across eastern Alaska is presented. Here the entire width of the North American Cordillera is made up of "suspect terranes". Since it cannot be demonstrated that continental crust lay where Alaska is today in pre-Cretaceous time, and since a large percentage of present day Alaska is made up of terranes whose protoliths were of "oceanic" rather than "continental" character, the implication is that Alaska is some combination of oceanic materials and shuffled continental fragments of uncertain origins converted into North American continental crust since Early Cretaceous time. The Chugach and Prince William terranes extend for more than 2,000 k m . along the southern margin of Alaska. They represent mainly Late Cretaceous to Recent telescoping and collapse of much of the northeastern Pacific Ocean now swept into southern Alaska. The most northern part of the belt in the Chugach Mountains appears to be a giant twisted duplex of stacked nappes stuffed beneath the Border Ranges fault along the upturned edge of the amalgamated Peninsular-WrangelliaAlexander super terrane. At the great structural elbow near Anchorage the Bouguer gravity is less than -100 milligals which suggests a crustal thickness of telescoped deep marine flysch and other ocean floor materials of 50 k m . or m o r e . In southern Alaska Wrangellia appears to be a nappe thrust over the Alexander terrane, while the Peninsular terrane is known to be thrust northward over Wrangellia. The inboard edge of the super terrane is the Denali fault system, a major active right transpressive structure which arcs across southern Alaska through the Alaska Range. In the eastern Alaska Range Wrangellia lies just across the Denali fault from the Yukon Tanana terrane with intensely deformed flysch and slices of micro-terranes streamed along the narrow vertical contact zone. To the northwest the boundary zone opens up into a westward widening flysch-filled, micro-terrane laden suture zone over 100 k m . w i d e . The thick and highly deformed flysch is known to be of at least Late Jurassic to Cenomanian in a g e . Swimming in the flysch matrix are an astonishing variety of micro-terranes displayed today as long slivers and rootless nappes. The Bouguer gravity over the suture zone is less than -100 milligals. This suggests the crust here is over 50 k m . thick and must be for the most part made up of telescoped Late Mesozoic deep marine flysch. Most of interior eastern Alaska is underlain by the Yukon Tanana terrane, a vast metamorphic assemblage entirely bounded by major fault systems. The terrane itself is a complex terrane specific metamorphic-plutonic assemblage with subhorizontal polymetamorphic fabric exposed over thousands of square m i l e s . Flat ductile thrusts seem to separate many of the diverse lithologies. For such a large intensely deformed metamorphic-plutonic mass in the midst of the North American Cordillera the gravity is remarkably high. Near Fairbanks the crust is probably not more than 35 k m . thick. This may suggest the Yukon Tanana terrane is a thin sheet, perhaps a crustal scale nappe. The northeastern boundary of the Yukon Tanana terrane is the Tintina fault, a structure which extends far into Canada along which hundreds of kilometers of right strike-slip has been recognized for years. The Tintina fault somehow

transforms into a complex zone of northwest vergent thrust faults and right strike-slip faults along the northwest margin of the Yukon Tanana terrane in a zone marked by much highly d e f o m e d Mesozoic flysch and slices of diverse microterranes. The north side of the flysch basin bounding the northwest side of the Yukon Tanana terrane is marked by an important northeast trending steep fault zone which separates southern and interior Alaska from Arctic Alaska to the north. North of the fault zone is the northeast trending Ruby terrane, a very large metamorphicplutonic complex which extends for over 500 kmc from southwest Alaska to its juncture with the eastern Brooks Rangee Structurally overlying the Ruby terrane is the unmetamorphosed mafic volcanic rocks, mafic intrusive rocks, and cherts of the Tozitna terrane which is clearly of oceanic affinity. Since chert ages range from Mississippian to Triassic implacement of the mafic nappe is post-Triassic. Rocks similar to the Tozitna terrane can be traced northward across the Ruby terrane where they plunge northwestward beneath the lowlands of the Koyukuk Basin and are termed the Angayucham terrane. The Koyukuk Basin is a vast triangular shaped lowland which lies northwest of the Ruby Terrane and south of the central and western Brooks Range. Since the Angayucham terrane is last seen dipping northwest beneath the southeastern margin of the basin to reappear dipping south on the north side, it is assumed the entire basin is floored by the mafic oceanic assemblage. A high Bouguer gravity supports this notion since in places the anomaly is near zero. Similarly, the Yukon Flats lowland to the east are apparently underlain by similar mafic assemblages since they outcrop around the margins. The boundary between the Ruby terrane and the south margin of the eastern Brooks Range is marked by a system of major anastomosing right strike-slip faults. The south flank of the eastern Brooks Range is an enormous stack of north vergent nappes. The oceanic Angayucham-Tozitna terrane is thrust northward over the Coldfoot terrane, an undated but thick, probably Paleozoic, graywackeargillite assemblage. The Coldfoot terrane is in turn thrust northward over the Hammond terrane on a gently south-dipping near flat fault. The diverse Hammond terrane includes lower Paleozoic marble, calc-schists, metavolcanic rocks, graywacke and occasional granitoids. Metamorphism increases northward from the unmetamorphosed Angayucham terrane into the Hammond terrane. North of the Hammond terrane is the Endicott terrane, a very thick sequence of Upper Devonian to Lower Mississippian shallow marine to non-marine clastic strata overlain by Carboniferous carbonate platform rocks. The Endicott terrane is clearly thrust over the North Slope terrane which underlies the north slope of the Brooks R a n g e . The Endicott terrane also seems to be structurally overlying the Hammond terrane. In the eastern Brooks Range, where the Hammond terrane plunges eastward from view, the telescoped Endicott terrane wraps around the plunge to the east to pass beneath thrust sheets which bring terranes northward that can be traced to the Yukon flats south of the Brooks Range. Here the Tozitna oceanic mafic sheets rise up from beneath the Yukon Flats to pass northward on thrusts over the Venetie terrane. The Venetie terrane is similar in age range to the Tozitna, but is graywacke, argillite and chert. The Venetie terrane in turn is thrust northward over the Endicott terrane. A l l of these terranes overlap in age, but are of vastly differing facies. The sequence of nappes is oceanic to the south to more continental to the northo The telescoping implied is enormous. Klippen of the roof thrust-like Angayucham-Tozitna and Venetie terranes are scattered across the Brooks Range and in some places lie close to the northern mountain front. The Bouguer gravity along the axis of the Brooks Range is quite low, near -100 milligals o r less. This suggests a large north vergent crustal scale duplex of stacked nappes forms the tectonically thickened core of the Brooks Range.

NEW ZEALAND TERRANES D.S. Coombs Geology Department, U n i v e r s i t y of Otago Dunedin^ New Zealand

J u x t a p o s i t i o n s r e s u l t i n g from 480 km of l a t e Cenozoic d i s p l a c e m e n t on t h e Alpine F a u l t p r o v i d e s t r i k i n g a c t u a l i s t i c e v i d e n c e f o r t h e r e l e v a n c e of suspect terrane concepts. Coombs e t a l . (1976) proposed t h a t t h e E a s t e r n P r o v i n c e of New Zealand can be c o n s i d e r e d i n terms of a b o u t s i x l i t h o t e c t o n i c t e r r a n e s of d i s t i n c t i v e t e c t o n i c and p a l e o g e o g r a p h i c s i g n i f i c a n c e . I t was s u g g e s t e d t h a t one of t h e s e , t h e T o r l e s s e t e r r a n e , o r i g i n a t e d some thousands of k i l o m e t r e s from o t h e r t e r r a n e s w i t h which i t i s now i n c l o s e p r o x i m i t y . Howell (1980) grouped t h e Permian t o C r e t a c e o u s s t r a t a of t h e E a s t e r n Province i n t o f o u r t e c t o n o s t r a t i g r a p h i c t e r r a n e s . With minor m o d i f i c a t i o n s t o t h e p r o v i s i o n a l t e r r a n e map of Bishop, Bradshaw and Landis ( i n p r e s s ) t h r e e t e r r a n e s i n t h e Western P r o v i n c e and about seven i n t h e E a s t e r n P r o v i n c e of t h e South I s l a n d may be proposed as " s u s p e c t " i n t h a t t h e y may once have o c c u p i e d s i g n i f i c a n t l y d i f f e r e n t p o s i t i o n s w i t h r e s p e c t t o t h e i r neighbours t h a n i s now t h e c a s e . The two p r o v i n c e s a r e sepaurated by t h e Median T e c t o n i c L i n e , t h e p r e c i s e p o s i t i o n and s i g n i f i c a n c e of which remains c o n t r o v e r s i a l . T e r r a n e s of t h e Western P r o v i n c e s a r e : 1. Karamea t e r r a n e (Western Sedimentary B e l t of Cooper 1979). A quartzr i c h , l a r g e l y t u r b i d i t e sequence of s a n d s t o n e and mudstone, and g r a p t o l i t i c b l a c k s h a l e s of Late Cambrian t o Late O r d o v i c i a n a g e . 2o Golden Bay t e r r a n e ( C e n t r a l and E a s t e r n Sedimentary B e l t s of Cooper 1979). S e v e r a l t e c t o n i c s l i c e s of a r c v o l c a n i c s , v o l c a n o g e n i c sediments and c a r b o n a t e s of Vendian o r E a r l y Cambrian t o L a t e Ordovician age i n t h e C e n t r a l B e l t , and c a r b o n a t e s f o l l o w e d by q u a r t z - r i c h c l a s t i c s of E a r l y Ordovician t o E a r l y Devonian age i n t h e E a s t e r n B e l t . The two t e r r a n e s a r e s e p a r a t e d by t h e Anatoki T h r u s t which a p p a r e n t l y b r o u g h t them i n t o c o n t a c t by t h e e a r l i e s t Devonian. They were s u b s e q u e n t l y invaded by g r a n i t o i d s of Devonian (Tuhuan) a g e . Extensive granite enplacement f o l l o w e d i n t h e Mesozoic, e s p e c i a l l y i n t h e E a r l y C r e t a c e o u s . Devonian and q u a r t z o s e l a t e Peinaian s t r a t a l o c a l l y unconformably o v e r l i e t h e e a r l i e r Paleozoic sediments. 3. Western F i o r d l a n d . A c r y s t a l l i n e conplex of o r t h o - ^ d p a r a g n e i s s e s i s invaded i n a n o r t h - e a s t t r e n d i n g b e l t 170 km l o n g , by t h e Western Fiordland Orthogneiss. This i s mainly of two pyroxene d i o r i t i c c o i r ^ o s i t i o n , geochemically c o n ^ a t i b l e w i t h a r c v o l c a n i s m , and has a maximum age of enplacement of ^>^120 t o 130 Ma (Mattinson e t a l . , 1984) . By about 116 Ma Western F i o r d l a n d had s u f f e r e d a high p r e s s u r e metamorphic e v e n t (Bradshaw 1984) s u g g e s t i n g o v e r t h r o i s t i n g by a cover more than 15 km thick. Near M i l f o r d Sound rocks w i t h h i g h - p r e s s u r e h i s t o r y a r e j u x t a p o s e d a g a i n s t unmetamorphosed g a b b r o n o r i t e s t o t h e e a s t , d a t e d a t '>^136 Ma, and E a s t e r n F i o r d l a n d g e n e r a l l y shows no evidence of a high p r e s s u r e o v e r p r i n t . This s u g g e s t s t h a t Western F i o r d l a n d , n o r t h of Dusky Sound, i s an a r c r e l a t e d s u s p e c t t e r r a n e t h a t was i n v o l v e d i n a C r e t a c e o u s c o l l i s i o n e v e n t and was s u b s e q u e n t l y j u x t a p o s e d , s t i l l i n C r e t a c e o u s t i m e , a g a i n s t t h e eastern b e l t .

Terranes in the Eastern Province of the South Island from west to east include: 1, Drumduan terrane. A fault sliver of probable Jurassic age immediately east of the Median Tectonic Line north of the Alpine Fault. It locally contains abundant lawsonite. Other enigmatic fault slivers consisting of mostly terrestrial andesitic to rhyolitic volcanics occur in the zone of the Median Tectonic Line on the south side of the Alpine Fault in eastern Fiordland and Stewart Island. 2. Brook Street terrane. Permian marine volcaniclastic sediments, pillow lavas and shallow intrusives up to 15 km thick invaded by Permian and early Mesozoic plutons ranging from layered dunite-wehrlite-eucrite«gabbronorite complexes to granitoids. The terrane is interpreted as an elongate sliver of a volcanic arc coit?)lex which was once substantially wider. The mainly sedimentary sequence is locally overlain by richly fossiliferous Late Permian limestones of the Productus Creek Group, which is heavily melanged. This in turn is overlain by a Jurassic or Cretaceous conglomerate (Landis et al., 1984) which contains granite boulders in excess of 3 m in diameter which were presumably deposited close to source, but for which a potential source area is no longer nearby. 3a. Murihiku terrane. Moderately fossiliferous Triassic and Jurassic arcderived marine siltstones, sandstones and conglomerates, shoaling in the Jurassic, and containing innumerable thin beds of andesitic to rhyolitic vitric and crystal tuffs. The sequence probably results from forearc basin sedimentation, but it is in probable thrust-fault contact with the Brook Street terrane (C.A. Landis, pers. comm.) and is not demonstrably derived from it. Sparce detrital quartz causes the sandstones to plot in the field of island arc source rocks on a QFL diagram (e.g. MacKinnon, 1983), but a '\^1300-1500 Ma zircon component indicates the presence of Proterozoic continental basement beneath the volcanic arc that provided the volcanic ashes and the main bulk of sediment (Kimbrough et alo, 1984) . Fragmentary plant remains are interpreted by Retallack (1985) as of Gondwana continent affinities. 3b. Dun Mountain-Maitai terrane. The Dun Mountain ophiolite belt, dated at about 270-280 Ma (Kimbrough and Coombs, 1983), is overlain by the upper Permian Maitai Groi:^) of largely arc-derived redeposited sediments, partly preserved in zeolite facies, but elsewhere raised to lawsonite-albite-chlorite facies metamorphic conditions by a Cretaceous collision event. Faulting of Maitai-Murihiku contacts is widespread but continuous sedimentation from the Maitai Group to Murihiku Supergroup is not precluded. It is uncertain whether the Murihiku and Dun Mountain-Maitai terranes should be regarded as one or two terranes. 4. Caples terrane. This is a belt of almost unfossiliferous greywacke-type metasediments of low metamorphic grade and probable Permian-Triassic age, in major fault contact with the base of the Dun Mountain ophiolite belt and grading eastwards into the Haast schist. Provenance was substantially volcanogenic but the innumerable thin ash beds, so conspicuous in the Murihiku terrane, are rare or absent in spite of the fact that the two terranes are now only about 15 km apart. Fault slivers of low grade metasediments and some volcanics up to a few kilometres in thickness in the melange zone between the main ophiolites of the Dun Mountain ophiolite belt and Caples terrane, differ subtly in lithology from the belts either side, and may be fragments of other suspect terranes. Detrital lawsonite has been recognized in these and implies a source region, now disappeared, that had undergone lawsonite-albite-chlorite metamorphism long before the probable Cretaceous event that is believed to have produced lawsonite in the Maitai and Drumduan terranes.

5 Older Torlesse (Rakaia) terrane. Comprises the larger part of the South Island Torlesse and is dominated by quartzofeldspathic greywackes and argillites of Permian and Middle and Late Triassic age, probably derived from an active continental margin. Structure is complex with steeply plunging folds and largely unresolved lithostratigraphy• These rocks grade metairorphically westwards into the Haast schists where their original contact with the Caples terrane must be. 6a. Kakahu terrane. A small, suspect terrane, 15 km^ in area, of melanged greywacke, argillite, conglomerate, chert, basic volcanics and limestone, the latter yielding late Carboniferous conodents. ^ 6b. Akatarawa terrane. An exotic mass in the Waitaki Valley, 5 km in area, of basaltic hyaloclastites, chert, highly deformed fusuline limestone of Tethyan affinities also containing corals, crinoids and ammonoids. Associated sandstones are more quartzose than others east of the Median Tectonic Line. 7. Younger Torlesse (Pahau) terrane. Late Jurassic to Early Cretaceous lithic to quartzofeldspathic sandstones and mudstones with some volcanic detritus and pillow lavas, very similar to the older Torlesse. It is separated from the Older Torlesse by the Esk Head Melange and is interpreted as an accretionary complex with progressively younger packets of strata to the east. In spite of exotic masses with Tethyan faunal affinities incorporated into the TOrlesse terranes, no paleomagnetic or other clear evidence for low latitude terranes has so far emerged. Displacements between terranes have probably been the result of opening and closing of marginal basins, collision-subduction processes, and strike slip movements in a zone dominated by plate convergence along the late Paleozoic-Mesozoic Gondwana borderland. A possible model has been provided by MacKinnon (1983). References Bishop, D.G., Bradshaw, J.D., & Landis, C.A., (in press). Provisional terrane map of South Island, New Zealand. In Tectonostratigraphic terranes of the Circum Pacific, D.G. Howell, Ed. Bradshaw, J.Y., 1984, Contrasting rock belts in northern Fiordland: metamorphic evidence for a major collisional event in western New Zealand, Geological Society of New Zealand, Miscellaneous Publication 31A. Coombs, D.S., Landis, C.A., Norris, R.J., Sinton, J.M., Borns, D.W., ^ Craw, D., 1976, The D\in Mountain ophiolite belt. New Zealand, its tectonic setting, constitution, and origin, with special reference to the southern portion, American Journal of Science, 276, 561-603. Cooper, R.A., 1979, Lower Palaeozoic rocks of New Zealand; Journal of the Royal Society of New Zealand, 9, 29-84. Howell, D.G., 1980, Mesozoic accretion of exotic terranes along the New Zealand segment of Gondwanaland, Geology, 8, 487-491. Kimbrough, D.L., & Coombs, D.S., 1983, Uranium-lead ages from the Dun Mountain ophiolite belt. South Island, New Zealand (Abstract), Geological Society of New Zealand, Miscellaneous Publication 30A. Kimbrough, D.L., Mattinson, J.M., & Campbell, J.D., 1984, Zircon U-Pb age constraints on Middle and Upper Triassic biostratigraphic zones in the Murihiku Supergroup, Southland, New Zealand (Abstract), Geological Society of New Zealand,Miscellaneous Publication 31A.

L a n d i s , C . A . , Kimbrough, D . L . , Cawood, & P i l l a i , D.D.L., 1984, Newly r e c o g n i z e d Mesozoic g r a n i t e - b e a r i n g conglomerate - i n p l i c a t i o n s f o r s t r u c t u r e , s t r a t i g r a p h y and g e o l o g i c a l h i s t o r y of t h e P r o d u c t u s Creek r e g i o n . S o u t h l a n d ( A b s t r a c t ) , G e o l o g i c a l S o c i e t y of New Z e a l a n d , M i s c e l l a n e o u s P u b l i c a t i o n 31A. MacKinnon, T . C , , 1983, O r i g i n of t h e T o r l e s s e t e r r a n e and c o e v a l r o c k s , South I s l a n d , New Z e a l a n d , G e o l o g i c a l S o c i e t y of America B u l l e t i n , 94, 967-985. M a t t i n s o n , J . M . , Kimbrough, D . L . , & Bradshaw, J » Y , , 1984, Zircon and a p a t i t e U-Pb age c o n s t r a i n t s and i n i t i a l Pb and Sr i s o t o p i c c o n p o s i t i o n s of Cretaceous g r a n u l i t e s and g a b b r o n o r i t e s from F i o r d l a n d , s o u t h w e s t New Zealand ( A b s t r a c t ) , G e o l o g i c a l S o c i e t y of New Z e a l a n d , M i s c e l l a n e o u s P u b l i c a t i o n 31A. R e t a l l a c k , G . J . , 1985, T r i a s s i c f o s s i l p l a n t f r a g m e n t s from s h a l l o w marine rocks of t h e Murihiku Supergroup, New Z e a l a n d , J o u r n a l of t h e Royal S o c i e t y of New Z e a l a n d , 15, 1 - 2 6 .

DOCKING HISTORY OF A MODERN ALLOCHTHONOUS TERRANE IN PAPUA NEW GUINEA TRACED BY ALONG-SUTURE MIGRATION OF SEDIMENTARY FACIES Keith A W Crook Department of Geology, Australian National University, Canberra, ACT 2601.

In northeastern Papua New Guinea the southwestern margin of the South Bismarck Plate (SBP), which includes the allochthonous AdelbertFinisterre-Huon Terrane, has already docked with the Australian Plate (AP) along the Ramu-Markham Fault Zone in the Markham Valley. Further east, to the south of New Britain, the southern margin of the South Bismarck Plate forms a convergent plate margin in contact with the Solomon Sea Plate (SSP) which is being subducted northwards at the New Britain Trench. The AP-SBP-SSP triple junction is located in the Huon Gulf south of the Huon Peninsula. Seismic profiles recorded by M/S 'Natusushima' in 1983 from this region show typical accretionary prism morphology. Plate vectors in this region are poorly constrained. However, plate geometries and contemporary re-arrangements of plate boundaries indicate that the triple junction is migrating eastwards along the New Britain Trench. Docking of allochthonous terranes along the southern margin of the South Bismarck Plate is occurring progressively. Eventually New Britain will dock against the Cape Vogel Basin with concomitant elision of the oceanic lithosphere in the Solomon Sea Basin. Along the southern margin of the South Bismark Plate modern sedimentary f a d e s vary concomitantly with changes in the character of the plate boundary, as follows: Already docked (Markham Valley): fluvial molasse (alluvial fan) Presently docking (Huon Gulf): marine molasse (inner shelf/mangrove swamp). Future docking (New Britain Trench): flysch (conglomeratic turbidites). Along the northern flank of the Markham Valley, where the allochthonous terrane has already docked, modern alluvial fan sediments are underlain unconformably by the Leron Formation which appears to be entirely of Quaternary age. The vertical f a d e s sequence in the Leron Formation commences with coarse-grained flysch, which passes up into fossiliferous marine molasse and thence into conglomeratic fluvial molasse. This fades sequence reflects progressive docking in the Markham Valley region. When docking is completed, with closure of the Huon Gulf and elision of the Solomon Sea Basin, this f a d e s sequence will occur along the length of the suture between the South Bismarck Plate, the Australian Plate and the successor to the Solomon Sea Plate. Continuity of the f a d e s sequence along the suture could readily be interpreted as evidence for orthogonal collision during docking of the allochthonous terranes. However, because the AP-SBP-SSP triple junction is migrating eastward, the sedimentary f a d e s are also

becoming younger eastwards. They record eastwards migration of the triple junction at a rate of the order of 20cm/yr, oblique collision, and the docking history of the allochthonous terrane. The presence along the length of a suture of a marine-tosubaerial fiysch-to~molasse facies sequence need not indicate orthogonal collision with simultaneous docking of the allochthonous terrane along the length of the suture. The record of oblique collision and progressive docking will be preserved in the timetransgressive character of facies transitions along the suture. Where a triple junction has migrated rapidly along a suture, recognition of the time-transgressive nature of facies transitions will require detailed chronostratigraphic studies, probably using a variety of techniques. Provided such studies have sufficient precision, the rate of triple junction migration can be determined. This may contribute crucial quantitative data for the determination of the plate vectors which operated during the docking of the allochthonous terrane.

POLWHASE TEBBANE ACCBOTON IN THE SCANniNAVIAN CALEDONIDES R.D. DalLne3rer Department of Geology, University of Georgia, Athens, Georgia 30602 U.S.A. The Scandinavian Caledonides are characterized by a sequence of large-scale, far-travelled nappes which were transported variable ^stances eas^ard across autochthonous rocks of the Baltoscandian platfonn during Early to Middle Paleozoic closure of the lapetus Ocean. The nappes f fPi^^,® ^^^^ unifom tectonostratigraphy, and have been subdivided into lower. Middle, Ujpper and Uppermost composite Allochthons ( e . g . . Gee and Roberts, 1983). The lower and Middle Allochthons are largely represented by generally low-grad^ metasedimentary successions of late Proterozoic to lower Silurian age. These originated along the Baltoscandian continental margin both during separation of Baltica and laurentia, and during subsequent closure of the lapetus Oceanic tract. late Proterozoic deposition along the Baltoscandian miogeocline occurred within basins wMch developed during initial attenuation and n f t i n g of Baltica from laurentia (Fig. lA). These largely fluvital successions were overlain by Vendian tillites related to the widespread Varanger glaciation. The tillites are generally succeeded by shallow marine sandstones and subordinate shales which thin eastward and overlap onto crystalline rocks of the Baltoscandian platform. Outboard portions of the miogeocline were pervasively intruded by a system of rift-facies, dolerite dikes. In westernmost, distal portions of the miogeocline these appear to have been related to extrusion of basaltic sequences which are now represented by amphibolite within lower portions of the Upper Allochthon ( e . g . , Seve Nappe Complex). Higher structural levels of the Upper Allochthon contain sequences of oceanic character, including ensimatic arc sequences ( e . g . , portions of the KSli Nappe Complex) and structurally overlying, variably fragnented ophiolite sequences which, at least in part, appear to have been initially obducted westward into a regime dominated by laurentian fauna and considerably removed from the Baltoscandian margin. The I^permost Allochthon contains variably metamorphosed felsic gneisses and imbricated platformal sedimentarry successions which may represent a portiont;of the Laurentian miogeocline. A polyphase tectonothemal evolution appears to characterize most of the Scandinavian allochthons. Initial, Late Cambrian consumption of lapetus oceanic crust occurred outboard of the Baltoscandian margin with development of an ensimatic island arc (Virisen Terrane of Stephens and Gee, 1985).above a west-dipping subduction complex. Imbrication, polydeformation, and variably high-grade metamorphism (including eclogite formation) of Baltoscandian miogeoclinal rocks ( e . g . , SSrv and Seve Nappe Ctanplexes) and underl3ang, Precambrian Baltic crystalline basement ( e . g . , Offerdal Nappe Ccanplex) occurred within an accretionary prism which developed in the subduction complex (Fig. IB), This resulted in eastward younging, diachronous metamorphism throughout the Early and Middle Ordovician. Cooling during subsequent uplift was regionally variable and is reflected by the report of a wide range of isotopic ages recorded by the more refractory radiometric systans ( e . g . , Rb-Sr whole-rock and 40Ar/39Ar hornblende). This early Caledonian activity has been termed the Finnmarkian Orogeny, however it is best viewed as a complex, regionally diachronous series of tectonothemal events which variably affected the Baltoscandian margin from the Late Cambrian through the Middle Ordovician. Extensive erosion of both the accretionary prism and associated island arc terrane occurred during the Late Ordovician (Fig, ID), This was likely caused by uplift which resulted fran the attempted westerly subduction of outboard, transitional Baltic crust. This ultimately terminated subduction, and the Ashgi 11-Llandovery interval was apparently a relatively quiescent tectonic interval during which shallow marine sandstone, shale, and coral lime-

EARLY SLOPE

CAMBRIAN

LEGEND PLATFORM

RISE

Late and \

EARLY

7 \/

ORDOVICIAN ACCRETIONARY

Qrdovician shales (m east) lifflestxines ( m west)

Early-Mid Ordovician and conglomerates

greywacke

E a r l y - M i d Ordovician limestones Camdnan black shales CLASTIC

PRISM

WEDGE Accretionory

Passive and

I 7 / 1

prism

margin

Riphaian

Vendian

sst

and tillites

Sosic volcanites and high level intr\jsions (rifting-related)

Oceanic

LATE

E23

ORDOVICIAN PRE-SCANOIAN

crust

Continental

crust

IMBRICATION Horizontal

scale

100 50

100 km

Verticai scale

Figure !• Cambrian through Ordovician tectonotheiinal evolution of the Baltoscandian margin (from Dallmeyer and Gee^ 1985). stone were deposited both west and east of the eroding accretionary prisma Deepening of this depositional basin occurred during the Middle Llandovery with a rapid transition into a black shale facies. In the Late Llandovery and Early Wenlock a thick sequence of westerly-derived graywacke was deposited as a result of outboard tectonic instability. Nearly complete closure of the lapetus Ocean appears to have been accomplished by the late Ordovician when eugeoclinal terranes which had developed along the laurentian margin were juxtaposed with volcanic arc sequences of the Virisen terrane (e«g«, lower portions of the KSli Nappe Complex). After relative tectonic quiescence throughout the Late Ordovician and Early Silurian, orogenesis commenced in the late Llandovery with thrusting of Laurentian eugeoclinal sequences onto the extensively eroded Virisen arc terrane. These were subsequently imbricated and infolded with predeformed portions of the Baltoscandian accretionary prism. The resultant composite allochthon was then transported eastward over the Ordovician clastic wedge and onto Silurian sedimentary successions. Sequential transport of the nappe ccanplex onto the Baltoscandian platfom appears to have'occurred throughout the Middle and Late Silurian, with significant east-west diachronous post-metamorphic cooling. In more southern areas ( e . g . , Oslo region) this transport continued into the Early Devonian. This sequence of Middle Silurian through Early Devonian tectonic events has been collectively termed the Scandian Orogeny, however they are regionally diachronous both along and across the orogen and therefore should be bracketed with the local controls available® Details of this tectonothetmal evolution have been discussed by Gee (1975) and Hbssack et al. (1985). Bnplacement of composite Scandian allochthons onto the Precambrian crystalline basonent of western Scandinavia apparently resulted in its

SCANDINAVIAN

CALEDONIDES

TERRANE MAP

Scale in km

Save Nappes BALTOSCANOIAN PLATFORM-MIOGEOCLINE (Allochthon) Sedimentary Cover Basement

BALTOSCANOIAN PLATFORM (AutochthonParautochthon)

Figure 2. Simplified terrane map of the Scandinavian Caledonides (frran Dallmeyer and Gee, 1985), depression to depths appropriate for fotmation of eclogite assemblages which record I^Pb zircon and Sn-Nd mineral and whole-rock crystallization ages of Co 425 Ma (Krogh et a l . , 1974; Griffin and Brueckner, 1980)o Scandian 40Ar/39Ar mica dates within easternmost portions of the orogen range from Co 420 to 430 Ma and likely closely date initial translation of allochthons to higher crustal levels across pre-imbricated Finnmarkian nappes which had been previously emplaced onto the Baltoscandian margin. Scandian 40Ar/39Ar mica dates range frOTi Co 410 Ma in central to Co 385 Ma in western portions of the orogeno Similar dates are recorded by a variety of isotopic systems and these likely date rapid cooling during uplift which resulted from crustal rebound following the eastward translation of overriding allochthons. This rapid isostatic uplift led to deposition of neoautochthonous, Devonian mol asse facies in

intemontaine, fault-controlled basins. The polyorogenic evolution of the Scandinavian Caledonides has produced a complex terrane array. Stratigraphic correlations between individual tectonic units are uncertain within most nappe units of the Vppev and Uppermost Allochthons. Indeed, most nappe units are composite, and have undergone a complex internal imbrication history. Many internal tectonic contacts record a polygenetic evolution which includes ductile strains associated with Early Ordovician metamorphism and a distinctly l a t e r suite of Scandian strains which l o c a l l y imbricate previously metamorphosed Finnmarkian basement with Silurian sedimentary successions. Detailed structural mapping with coordinated p e t r o l o g i c and geochronologic investigations have not been carried out across most major tectonic boundaries® This i s clearly required before a comprehensive terrane analysis may be undertaken. Therefore, until the l o c a l details of linnmarkian v s . Scandian tectonotheimal a c t i v i t y i s more clearly resolved, i t i s perhaps most reasonable to portray the various terranes within the Scandinavian Caledonides as outlined in Figure 2 where the variably imbricated and metamorphosed Baltoscandxan and Laurentian miogeoclinal sequences are distinguished from various eugebclinal tracts of uncertain palinspastic relationship t o e i t h e r continental margin. Beferences Dallmeyer, & Gee, DeG., 1985 Cin p r e s s ) , Polyphase Caledonian orogenesis within the Baltoscandian miogeoclinei Evidence from 40Ar/39Ar mineral dates from retrogressed eclogitess^ Bulletin of the Geological Society o f America. Gee, D.G», 1975, A tectonic model f o r the central part of the Scandinavian Caledonides: American Journal of Science, v . 275A, p. 468-515. Gee, D.G., & Roberts, D., 1983, Timing of deformation in the Scandinavian Caleuouidesi Itegional Trends In The Geology Of The Appalachian-CaledonianHercynian-Mauritanide Orogen, P.E. Schenk, e d . . ; New York, D. Peidel Pub. Co., p . 279-292. feiffin, W.L., & Brueckner, H.K., 1980, Caledonian Sm-Nd ages and a crustal origin f o r Norwegian eclogites: Nature, v . 285, p. 319-321. Hossack, J . R . , Garton, M.R., & Nickelsen, S.P®, 1985 (in press). The geological section from the foreland up to the Jotun thrust sheet in the Valdres area, south Norwayg i n . The Caledonide Orogen - Scandinavia And Belated Areas, Gee, D.Gc, & Sturt, B.Ae, eds«| Chichester, England, Wiley & Sons. Rrogh, T.E., Mysen, B»0., & Davies, G.L., 1974, A Paleozoic age f o r the primary minerals o f a Norwegian e c l o g i t e : Annual Report o f the Geophysical laboratory, Carnegie I n s t i t u t e , Washington, B.C., v . 73, p. 575-576. Stephens, M.3., & Gee, D.G., 1985 (in press), A tectonic model f o r the evolution o f the eugeoclinal terranes in the central Scandinavian Caledonides; i n . The Caledonide Orogen - Scandinavia And Related Areas, Gee, D.G., & Sturt, B.A., eds«; Chichester, England, Wiley & Sons.

Terranes In The Circum-Atlantic Paleozoic Orogens

JOINT UK-US WEST ANTARCTIC

TECTONICS

Ian W . D . Lamont-Doherty

PROJECT

- PRELIMINARY

RESULTS

Dalziel

Geological Observatory, Columbia P a l i s a d e s , New York, U.S.A-

University

T h e r e l a t i o n s h i p of W e s t (lesser) A n t a r c t i c a to t h e P r e c a m b r i a n c r a t o n o f E a s t ( G r e a t e r ) A n t a r c t i c a is t h e l o n g e s t s t a n d i n g t e c t o n i c p r o b l e m in A n t a r c t i c g e o l o g y . It h a s a b e a r i n g on G o n d w a n a l a n d reconstruction, but has even more important geotectonic, paleoenviromental, and paleobiogeographic implications. A joint UK-US p r o j e c t w a s i n i t i a t e d in 1 9 8 0 - 8 1 to s t u d y c e r t a i n a s p e c t s of the overall problem. M a j o r f i e l d p r o g r a m s w e r e u n d e r t a k e n in the 1983-84 and 1984-85 Antarctic seasons. A d d i t i o n a l efforts w i l l be m a d e in 1 9 8 5 - 8 6 a n d 1 9 8 7 - 8 8 . A n t a r c t i c a h a s l o n g b e e n k n o w n as the " k e y " to G o n d w a n a l a n d . Alex du T o i t in his c l a s s i c book "Our W a n d e r i n g C o n t i n e n t s " , i n c o r p o r a t e d in h i s b a s i c r e c o n s t r u c t i o n of the s u p e r c o n t i n e n t the r e s u l t s of then r e c e n t d i s c o v e r i e s by s c i e n t i s t s of B r u c e , Scott and Shackleton's Antarctic expeditions. In so d o i n g h e a c c e p t e d the d i s t i n c t i o n b e t w e e n the P r e c a m b r i a n craton of East (Greater) A n t a r c t i c a and the p o r t i o n of the c i r c u m - P a c i f i c m o b i l e b e l t k n o w n as W e s t ( L e s s e r ) A n t a r c t i c a . The latter p o r t i o n of the continent h e p o r t r a y e d as a t e c t o n i c z o n e b r o a d l y c o m p a r a b l e to v o l c a n i c arcs fringing the eastern m a r g i n of the Asian c o n t i n e n t . The tectonic h i s t o r y of W e s t A n t a r c t i c a and its r e l a t i o n s h i p to t h e E a s t A n t a r c t i c c r a t o n r e m a i n o n e o f t h e o u t s t a n d i n g p r o b l e m s not only of Antarctic g e o l o g y but indeed of G o n d w a n a l a n d g e o l o g y . U n a c c e p t a b l e "overlap" of the A n t a r c t i c P e n i n s u l a with the F a l k l a n d Plateau of South A m e r i c a in some of the m o s t m o d e r n r e c o n s t r u c t i o n s o f t h e s u p e r c o n t i n e n t , is b u t t h e m o s t o b v i o u s m a n i f e s t a t i o n of a p r o b l e m t o u c h i n g on g l o b a l p l a t e i n t e r a c t i o n s , the d e v e l o p m e n t of c i r c u l a t i o n in the S o u t h e r n O c e a n , p a l e o c l i m a t e ( i n c l u d i n g the causes of g l a c i a t i o n ) , and p a l e o b i o g e o g r a p h y . C u r r e n t analyses of the c i r c u m - P a c i f i c m o b i l e b e l t in terms of p o t e n t i a l l y d i s p l a c e d "terranes" have emphasized the long recognized possibility that p a r t s of W e s t A n t a r c t i c a m a y be e x o t i c to t h a t c o n t i n e n t , a l t h o u g h p u b l i s h e d p a l e o m a g n e t i c results make that u n l i k e l y (for review see D a n i e l and G r u n o w , 1 9 8 5 , P r o c e e d i n g s of 2nd C i r c u m - P a c i f i c Terranes Conference, Stanford, 1982, AAPG). W i t h these p r o b l e m s in m i n d a joint p r o j e c t h a s b e e n i n i t i a t e d b y t h e U n i t e d K i n g d o m (UK) a n d t h e U n i t e d S t a t e s (US) to e l u c i d a t e the t e c t o n i c h i s t o r y of W e s t A n t a r c t i c a a n d its r e l a t i o n to E a s t Antarctica. The U K i n s t i t u t i o n i n v o l v e d is the B r i t i s h A n t a r c t i c Survey (BAS), a c o m p o n e n t of the N a t u r a l E n v i r o n m e n t R e s e a r c h Council. The US e f f o r t was m o u n t e d t h r o u g h L a m o n t - D o h e r t y G e o l o g i c a l O b s e r v a t o r y of C o l u m b i a U n i v e r s i t y (LDGO) s u p p o r t e d by the D i v i s i o n of P o l a r P r o g r a m s (DPP) of the N a t i o n a l S c i e n c e F o u n d a t i o n (NSF).

The UK has p r o v i d e d a i r c r a f t ("Twin O t t e r " ) s u p p o r t , food camping equipment. The US h a s s u p p l i e d fuel^ s n o w m o b i l e s sledges. The scientific planning, field work, laboratory and a n a l y s i s have been u n d e r t a k e n by s c i e n t i s t s from both working cooperatively.

and and studies countries

It w a s r e c o g n i z e d at the o u t s e t t h a t the b a c k g r o u n d and e x p e r i e n c e of the s c i e n t i s t s i n v o l v e d , t o g e t h e r w i t h l o g i s t i c c o n s t r a i n t s , m e a n t t h a t t h e p r o j e c t s h o u l d i n i t i a l l y b e c o n f i n e d to t h a t p a r t of W e s t A n t a r c t i c a c l o s e s t to S o u t h A m e r i c a a n d the Weddell Sea. S p e c i f i c a l l y it w a s d e c i d e d to c o n c e n t r a t e on t h e t e c t o n i c h i s t o r y and r e l a t i o n s h i p s of the A n t a r c t i c P e n i n s u l a , Ellsworth Mountains-Whitmore Mountains crustal b l o c k , and Thurston Isiand-Eights Coast crustal block. Xt was also d e c i d e d t h a t this s h o u l d i n c l u d e s t u d i e s of the a d j a c e n t m a r g i n of the E a s t A n t a r c t i c craton between the Thiel M o u n t a i n s and the P e n s a c o l a M o u n t a i n s . W h i l e at l e a s t r e c o n n a i s s a n c e g e o l o g i c s t u d i e s had b e e n u n d e r t a k e n in a l l t h e s e a r e a s , n o s i n g l e g r o u p o f s c i e n t i s t s h a d t r i e d to r e l a t e t h e i r g e o l o g i c h i s t o r y and s t r u c t u r e in any d e t a i l . In a d d i t i o n to b a s i c f i e l d g e o l o g y , s t r u c t u r a l g e o l o g y , a n d p e t r o l o g y / g e o c h e m i s t r y , the p r o j e c t has involved extensive c o l l e c t i o n for p a l e o m a g n e t i c s t u d i e s , and for i s o t o p e g e o c h e m i s t r y / g e o c h r o n o l o g y , It w a s r e c o g n i z e d t h a t the p r o j e c t s h o u l d also i n v o l v e a i r b o r n e geophysics. This has involved ice radio-echosounding and airborne m a g n e t i c s u r v e y s e x t e n d i n g and i n t e n s i f y i n g the c o v e r a g e of the recent Scott Polar Research Institute-National Science FoundationT e c h n i c a l U n i v e r s i t y of D e n m a r k P r o g r a m . Some of the i n i t i a l r e s u l t s o f t h e p r o j e c t o b t a i n e d in 1 9 8 0 - 8 1 w i t h s o m e a i r c r a f t f u e l m a d e a v a i l a b l e to B A S by DPP f o l l o w i n g t h e E l l s w o r t h M o u n t a i n s f i e l d p r o g r a m of 1 9 7 9 - 8 0 h a v e a l r e a d y been p u b l i s h e d (Doake et a l e , 1 9 8 3 , A n t a r c t i c G e o s c i e n c e , A u s t r a l i a n A c a d m e y of S c i e n c e , C a n b e r r a ) . The g e o p h y s i c a l f l i g h t s h a v e b e e n flown by t e a m s of BAS g e o p h y s i c i s t s along lines planned jointly with LDGO scientists. Following the i n i t i a l a i r b o r n e g e o p h y s i c a l p r o g r a m in 1 9 8 0 - 8 1 t h e j o i n t U K - U S p r o j e c t has involved a combined g e o l o g i c a l and g e o p h y s i c a l p r o g r a m in t h e E l l s w o r t h M o u n t a i n s - W h i t m o r e M o u n t a i n s - T h i e l M o u n t a i n s r e g i o n ( 1 9 8 3 - 8 4 ) , a n d a g e o l o g i c a l s e a s o n in t h e T h u r s t o n I s l a n d - J o n e s Mountains area (1984-85). F u t u r e f i e l d w o r k is p l a n n e d f o r t h e area between the Antarctic Peninsula, Ellsworth M o u n t a i n s , Thurston Island and M a r i e Byrd Land (airborne g e o p h y s i c s , 1 9 8 5 - 8 6 ) , and for the P e n s a c o l a M o u n t a i n s (geology, 1987—88). A l l t h e a v a i l a b l e r e s u l t s of t h e s t u d y w i l l be d i s c u s s e d at the 3rd C i r c u m - P a c i f i c Terranes C o n f e r e n c e . At the d e a d l i n e for a b s t r a c t s u b m i s s i o n t h e p r e l i m i n a r y r e s u l t s h a v e s t i l l to be discussed amongst the scientists involved, namely the speaker, Bryan Storey (BAS), Steve Garrett (BAS), Anne Grunow (LDGO), Bruce Herrod (BAS), Bob P a n k h u r s t (BAS), and Walt V e n n u m (California State University Sonoma). Papers summarizing these results will b e p u b l i s h e d in t h e P r o c e e d i n g s o f t h e S i x t h G o n d w a n a C o n f e r e n c e (Ohio S t a t e U n i v e r s i t y , C o l u m b u s , O h i o , A u g u s t 1 9 8 5 ; A m e r i c a n Geophysical Union).

T E C T O N I C MAP OF THE S C O T I A ARC Ian W . D . D a l z i e l and

(1:3,000,000) others

L a m o n t - D o h e r t y G e o l o g i c a l O b s e r v a t o r y of C o l u m b i a U n i v e r s i t y , P a l i s a d e s , New Y o r k , U . S . A . A new t e c t o n i c m a p of the Scotia Arc r e g i o n h a s b e e n p r e p a r e d by scientists from the United Kingdom (British Antarctic Survey and the U n i v e r s i t y of B i r m i n g h a m ) and the U n i t e d States ( L a m o n t - D o h e r t y G e o l o g i c a l O b s e r v a t o r y of C o l u m b i a U n i v e r s i t y ) . It w i l l b e p u b l i s h e d d u r i n g 1 9 8 5 b y t h e B r i t i s h A n t a r c t i c S u r v e y , C a m b r i d g e , U . K . , as m a p " B A S ( M i s c ) 3 " . T h e m a p w i l l b e p r e s e n t e d as a p o s t e r s e s s i o n a n d o n e o r m o r e o f t h e c o m p i l e r s w i l l be p r e s e n t to d i s c u s s i t . T h e c o m p i l e r s a r e (in a l p h a b e t i c a l o r d e r ) : P e t e r B a r k e r , Ian D a l z i e l , Bryan S t o r e y , J a n e t T h o m s o n , and M i c h a e l Thomson^ C h a r l e s S w i t h i n b a n k w a s c h i e f a d v i s o r to t h e p r o j e c t t h a t was funded by both the N a t u r a l E n v i r o n m e n t Research C o u n c i l and the N a t i o n a l Science Foundation..

TERRANE TRAJECTORY ANALYSIS FOR THE NORTHEAST PACIFIC Michel Debiche and Allan Cox Department of Geophysics, Stanford University, Stanford, CA 94305 David Engebretson Department of Geology, Western Washington University, Bellingham, WA 98225 In this report we explore the links between three independent fields of contemporary research that are closely related. The f i r s t is terrane analys i s , which has established that many tectonostratigraphic units along the continental margin of North America are allochthonous (Coney et a l . , 1980)• The second is paleomagnetism, which establishes that some of these terranes moved large distances from the south (Beck, 1980, 1976; Hillhouse and Gromme, 1984; Champion et a l . , 1984). The third is plate tectonics, s p e c i f i c a l l y , models for the evolution of oceanic plates in the northern Pacific basin. These plates compose the transport system that moved terranes to their present locations from their points of origin. The goal of terrane trajectory analysis is to test the mutual consistency of results from these three fields of research. Terrane trajectory analysis for the northeast Pacific begins with specif i c plate tectonic models describing the motion relative to North America of adjacent oceanic plates. For each plate model, a set of terrane trajectories is found which shows the position of terranes as a function of time as the terranes move with oceanic plates or are driven by them tangentially along the continental margin. Elements used to define a trajectory are (1) the stage poles describing the motion of the oceanic plates relative to the continent, (2) the sequence of plates carrying the terrane, (3) the time of docking of the terrane and (4) the coordinates of the point of docking. Additional constraints are that terranes are not permitted to migrate across ridges and that the oceanic plate carrying a terrane cannot be younger than the terrane. The plate model of Engebretson et a l . (1985) and several of its variants were used in our analysis. The set of trajectories shown in Figure 1 describes the motion of terranes all arriving at the same time at docking points distributed along the western margin of North America. For each arrival time a reconstruction was made to determine which oceanic plate was adjacent to the docking points. Stage poles for the appropriate plates were then used to calculate the trajectories using backward modeling. These trajectories were compared with maps of plate reconstructions to ensure that the carrier plate was older than the terrane and to determine points at which the terrane moved from one oceanic plate to another. Segments of trajectories on different plates are displayed by different symbols (Figure 1) a l l spaced at 5 m.y. intervals along the trajectories. The oldest points on the trajectories for the 90 Ma arrival time are a l l 180 Ma, the oldest time for which even approximate oceanic plate motions are known (Engebretson et a l . , 1984b). The oldest points on the trajectories for arrival times of 60 and 30 Ma are marked with a star and a number giving the maximum age at which a terrane could have started the trajectory shown (Figure 1). These starting ages were determined as follows. Plate reconstructions were f i r s t made for the indicated arrival times. The age of oceanic crust being subducted at each of the docking points was then rounded downward to the

nearest 5 m.y. to avoid creating a false impression of precision. For trajectories for which we were unable to determine from a plate reconstruction the age of the oceanic crust at the docking point and docking time, the trajectory was superimposed upon a sequence of ridge reconstructions and cut o f f where the trajectory intersected a ridge. The starting ages found by either of these methods, which are shown in Figure 1 as numbers adjacent to stars, are thus not only the ages of oceanic crust being subducted beneath the docking points at the indicated time of arrival, but also the maximum ages at which terranes docking at the times and places shown could have started their trajectories. Arrival time of 30 Ma: The most striking feature of these trajectories is their great variation in length, which in part is strongly model dependent. Trajectory AP is long because the arriving terrane was on oceanic crust that is now part of the Pacific plate but originally was on the Kula side of the Kula-Pacific ridge, as modeled from observed anomalies east of the Emperor trough. QC is short because i t is on crust that formed on the Kula side of the Kula-Farallon ridge around 50 Ma, becoming part of the Pacific plate at the death of the Kula Pacific ridge at 43 Ma. Trajectory CM is short because the Farallon-Pacific ridge is close to the continental margin. Arrival time of 60 Ma: The most striking feature of these trajectories is their northerly trend and great length. Trajectory AP begins on the Farallon plate adjacent to the Farallon-Izanagi ridge, the location of which is poorly known. For trajectory QC, the docking point is located on anomaly M29 (165 Ma) reflected onto the Farallon plate assuming symmetrical FarallonPacific spreading. The starting time of 165 Ma is thus fairly well determined. For trajectory CM, the docking point is located between anomalies M11 and Ml6 reflected onto the Farallon plate assuming symmetrical FarallonPacific spreading. The corresponding time of origin is 140 Ma. Trajectory VP is strikingly different from trajectories CM and QC because the latter were north of the Kula-Farallon ridge at the time of i t s initiation whereas the VP trajectory was near the ridge but south of i t . Comparison of trajectories VP and CM demonstrates that the trajectories of points a short distance apart may diverge and carry once adjacent segments of ocean floor to widely different docking points. Arrival time of 90 Ma: Calculation of these trajectories was straightforward because none are close to known ridges. The 180 Ma starting points shown for the trajectories are not maximum terrane ages, as was true for the younger trajectories, but are simply the oldest time for which information is available about plate motions. The most striking feature of these trajectories is the presence of a strong easterly component of motion not present in the trajectories terminating at later times. In terms of our model, faunas from the western Pacific would be expected to arrive in the Early Cretaceous or early Late Cretaceous rather than at a later time. Each point on the trajectories shown in Figure 1 has an accompanying paleolatitude appropriate for every age. The paleolatitudes are derived from paleopoles of the North American apparent polar wander path such as that of Irving and Irving (1982). Pleolatitude vs. time curves for these trajectories have been calculated and are compared to observed paleomagnetic results.

References Beck, M.E., Jr., 1976, Discordant paleomagnetic pole positions as evidence of regional shear in the western Cordillera of North America: American Journal of Science, v. 276, p. 694-712. 1980, Paleomagnetic record of plate-margin tectonic processes along the western edge of North America: Journal of Geophysical Research, v. 85, Pc 7115-713I0 Champion, D.E., Howell, D.G., and Gromme, C.S., 1984, Paleomagnetic and geologic data indicating 2500 km of northward displacement for the Salinian and related terranes, California: Journal of Geophysical Research, v. 89, p. 7736-7752. Coney, P.J., Jones, D.L., and Monger, J.W.H., 1980, Cordilleran suspect terranes: Nature, v. 288, p. 329-333. Engebretson, D«C., Cox, A., and Gordon, R.G., 1984a, Relative motions between oceanic plates of the Pacific basin: Journal of Geophysical Research, v. 89, p. 1 0 , 2 9 1 - 1 0 , 3 1 0 . Engebretson, Dc, Debiche, Mo, and Cox, A«, 1984b, Plate motions, paleomagnetism, and terrane displacement histories: Stanford University Publications in the Geological Sciences, v. 18, p. 83-85. Engebretson, D., Cox, A., and Gordon, R., 1985, Relative motions between oceanic and continental plates in the Pacific basin: Geological Society of America Special Paper, no. 206, in presso Hillhouse, J.W., and Gromme, C.S., 1980, Paleomagnetism of the Triassic Hound Island Volcanics, Alexander terrane, southeastern Alaska: Journal of Geophysical Research, v. 85, p. 2594-2602. Irving, E., and Irving, G.A., 1982, Apparent polar wander paths. Carboniferous through Cenozoic and the assembly of Gondwana: Geophysical Surveys, v. 5, p. 141-188.

50 Ma

70 Ma

BCE 240-E 180-E

30 Ma R r r i v a l

125 Ma^ 125 Ma«

60 Ma flrr i va I

90 Ma

flrrIvaI

Figure 1. Trajectories of oceanic plates in fixed North America coordinates determined by backward modeling from the arrival times shown. Symbols plotted at 5 m.y. intervals along trajectories are as follows. Circles; Pacific plate; Squares: Farallon plate; Triangles: Kula plate; Pentagon; docking point on North America; Star; oldest point on a trajectory that began at a spreading center at the time indicated by the adjacent number. Eurasia is reconstructed to its position in fixed North America coordinates at the arrival times shown for each plot.

ACCRETED TERRANES AND M I N E R A L RESOURCES OF E A S T E R N Pow-foong

CHINA

Fan

H a w a i i I n s t i t u t e of G e o p h y s i c s , U n i v e r s i t y of Honolulu, Hawaii, U.S.Ac

Hawaii

ABSTRACT The terranes of e a s t e r n China were formed m o s t l y during late Paleozoic and early Mesozoic t i m e . The H u a b e i terrane c o a l e s c e d w i t h t h e Y i s h a n d u r i n g t h e m i d d l e to l a t e C a r b o n i f e r o u s . T h e I n n e r M o n g o l i a t e r r a n e a c c r e t e d to t h e S i b e r i a t e r r a n e d u r i n g the late Paleozoic and collided with the H u a b e i - Y i s h a n terrane d u r i n g t h e T r i a s s i c . T h e S o n g l i a o t e r r a n e p r o b a b l y is a c o n t i n e n t a l fragment that caught between the Inner Mongolia and D o n g b e i terranes and coalesced with the H u a b e i terrane during the T r i a s s i c . The Yangzi terrane coalesced with the Huanan terrane during Silurian t i m e . During the late T r i a s s i c , the Y a n g z i - H u a n a n terrane c o l l i d e d w i t h t h e Q i n b a t e r r a n e to t h e n o r t h , a n d c o a l e s c e d w i t h t h e H u a b e i Yishan t e r r a n e . The Southeast Maritime terrane accreted against the Y a n g z i - H u a n a n terrane during the late T r i a s s i c . P r e c a m b r i a n m i n e r a l i z e d b e l t s in t h e H u a b e i - Y i s h a n t e r r a n e c o m p r i s e two d i s t r i c t s t n o r t h e r n d i s t r i c t consists of g o l d quartz v e i n , skarn and porphyry molybdenum, vein and skarn lead-zinc deposits, which are associated with the Yanshanian g r a n i t e s . The southern d i s t r i c t consists of P r e c a m b r i a n g o l d quartz v e i n , skarn and p o r p h y r y molybdenum, vein and skarn lead-zinc deposits. In I n n e r M o n g o l i a the V a r i s c a n and M e s o z o i c m i n e r a l i z e d belts are s k a r n , p o r p h y r y , and vein copper, vein lead-zinc, and porphyry molydenum. Dongbei terrane c o n s i s t s of V a r i s c a n m i n e r a l i z e d belts of p o r p h y r y c o p p e r and molybdenum, gold quartz v e i n , skarn tungsten, and vein lead-zinc depositsc T h e M e s o z o i c m i n e r a l i z e d b e l t of Q i n b a t e r r a n e c o n s i s t s o f carbonate-hosted lead-zinc, mercury and antimonyc The lower Paleozoic m i n e r a l i z e d b e l t s in t h e Y a n g z i t e r r a n e a r e c a r b o n a t e - h o s t e d m e r c u r y and antimony, vein wolframite and cassiterite, and disseminated c o p p e r . The m i n e r a l i z e d b e l t s in the H u a n a n t e r r a n e are Y a n s h a n i a n s k a r n and vein wolframite and cassiterite, vein and disseminated copper, a n d s k a r n c o p p e r in t h e l o w e r Y a n g z i V a l l e y . T h e m i n e r a l i z e d b e l t s in t h e S o u t h e a s t M a r i t i m e t e r r a n e r e s u l t e d f r o m t h e Y a n g s h a n i a n orogeny and are vein c a s s i t e r i t e , tin s u l f i d e , p o r p h y r y , skarn and vein molybdenum, and vein and lead-zinc deposits. Mineralization p h a s e s in e a s t e r n C h i n a i n c l u d e P r e c a m b r i a n , l o w e r P a l e o z o i c , u p p e r Paleozoic and M e s o z o i c , (Indosinian and Y a n s h a n i a n ) . Among them the Y a n s h a n i a n m i n e r a l i z a t i o n is m o s t w i d e s p r e a d .

TECTONIC HISTORY OF THE TABBERABBERA BELT, EASTERN VICTORIA AND IMPLICATIONS FOR ORDOVICIAN RECONSTRUCTIONS OF SOUTHEASTERN AUSTRALIA C.L. Fergusson Monash University, Melbourne, Australia Pre-Late Devonian rocks between the Mount Howitt Province in the west and the Kiewa Fault in the east are defined as the Tabberabbera Belt of the Lachlan Fold Belt in eastern Victoria. The Belt is dominated by an undifferentiated Ordovician sequence with less abundant granitoids and a narrow syncline of Early Devonian strata at Tabberabbera. The Ordovician sequence consists of predominantly quartz sandstone and mudstone with minor conglomerate, black shale and chert. Graded bedding and sedimentary structures indicate that most of the sequence was deposited by turbidity currents in a probable submarine fan setting. Sediment movement directions, determined from flute and scour marks, indicate flow from west to east. Graptolite and conodont remains indicate the sequence spans the Early to Late Ordovician interval (I. Stewart pers. comm.) A high angular unconformity separates the Ordovician sequence from the overlying Early Devonian Wentworth Group (Talent, 1963). The Group consists of basal conglomerates overlain by interbedded sandstones and mudstones. The conglomerates contain clasts of quartz sandstone, vein quartz and mudstone derived from the deformed Ordovician sequence. The TabberaQjbera Belt is subdivided into two domains by a southeasttrending boundary named herein the Wonnangatta Line (Fig. 1), which is marked by a zone of tectonic melange up to 3km in width. In the southwestern domain the regional structure is dominated by east-southeast trending, shallowly plunging, upright to steeply inclined Fi folds with wavelengths of up to 10km. These folds are open to the southwest and gradually tighten towards the Wonnangatta Line. Hinges are commonly narrow and locally faulted. Fold limbs tend to be planar. Rough cleavage is developed parallel to axial surfaces of Fi folds although local non-axial planar Si cleavage is present. In the northeastern domain there is a well-developed bedding^parallel cleavage that is folded around the F^ fold hinges. Close to isoclinal F^ folds are abundant adjacent to the Wonnangatta Line. Farther northeast the major Fi folds are close to tight with narrow hinge zones and planar limbs. An axial surface crenulation cleavage is sporadically developed. F^ folds are shallowly plunging, upright and easterly trending except in areas of strong D2 deformation where the Fi folds are northwest trending, steeply plunging and locally rec\jmbent. Within both domains the Fi folds are locally downward facing although pre-Fi folds have only been found in the Bulgaback Creek area. These preFi folds may have formed early in the first deformation. A second regional deformation occurs in both domains. The Mitchell Syncline consists of Early Devonian strata and is an F2 fold that has an upright faulted axial surface and an interlimb angle of -10®. Slaty cleavage is developed in mudstones and local refolding is present. In the Ordovician the second regional deformation has formed a steeply dipping north-northwest to north-northeast trending crenulation cleavage and associated open to close folds with typically small amplitude-to-wavelength

ratios. The Fi folding predated deposition of the Early Devonian Wentworth Group and appears, from relationships in the Benambra Subzone to the east, to have formed in the epi-Ordovician to mid-Silurian interval (i.e. Benambran Orogeny). The second deformation was responsible for the Tabberabberan Unconformity and is of Middle Devonian age (i.e. Tabberabberan Orogeny). The Ordovician sequence of the Tabberabbera Belt has been interpreted as part of the Wagga Marginal Sea (Powell, 1983). The major elements of the Ordovician palaeogeography of the Lachlan Fold Belt are considered by Powell to include: (1) a shelf sequence in Tasmania and western New South Wales? (2) a broad realm of quartz flysch in the Wagga Marginal Sea; (3) an andesitic volcanic chain in eastern New South Wales; and (4) a supposed forearc basin and subduction complex on the South Coast of New South Wales. The basement of the Ordovician is known only from the Victorian greenstone belts and less well established Cambrian units in New South Wales. These rocks have oceanic affinities and do not appear to be related to Cambrian sequences on the Australian eraton. Recent reconstructions of the Ordovician in southeastern Australia have ignored the effects of deformation (with the exception of Scheibner, 1974). It is also assumed that there is a linkage between the quartz-'rich flysch of the marginal sea and the shelf sequences to the west. In an alternative interpretation Baillie (1984) suggests that Ordovician rocks east of the Stawell Fault in Victoria and the Gnalta Shelf in New South Wales are part of an allochthonous terrane. One problem with Powell's (1983) reconstruction is the confusing geologic relations in the so-called forearc. The subduction complex is represented by melange and associated rocks of the Wagonga beds. The forearc basin consists of quartz flysch in the coastal greywacke and slate belt and has no major suture contact with the subduction complex (as occurs in nearly all ancient forearc systems). The forearc basin also lacks any detritus derived from the volcanic arc to the west. This is puzzling since arc-derived flysch occurs in the subduction complex. The characteristics of the Ordovician sequence in the Tabberabbera Belt has several implications for these reconstructions. Firstly, the beddingparallel cleavage predates the Fi folding and may be related to extensional tectonics in the Late Ordovician. Seranne et al. (1985) show that beddingparallel fabrics developed due to extension in the Devonian basins of Norway. Extension of the Ordovician package may have caused rupture and soft-sediment deformation in the vicinity of the Wonnangatta Line® Secondly, the first regional folding, and associated melange development along the Wonnangatta Line, reflects approximate north-south compression of the Ordovician sequence. The style of folding, that is tight folds with large amplitude-to-wavelength ratios, indicates there has been significant stratal shortening and that probable intracrustal detachment has occurred between the well-bedded Ordovician and the underlying assemblage of suspect terraneso It is not known how the shortening was accommodated below the level of detachment. Powell relates this deformation to dextral shear accompanying the locking of plates in Chilean-style subduction. In addition to penetrative deformation significant dextral translation probably accompanied this event and this, in part, accounts for the juxtaposition of strongly deformed Ordovician of the Tabberabbera Belt with the Ordovician of the Melbourne Synclinorium (which was not affected by the Benambran Orogeny). This translation may also be

responsible for the anomalous patterns in the forearc of eastern New South Wales. During the Late Silurian to Middle Devonian most of the Lachlan Fold Belt in New South Wales experienced extension and widespread magmatism. Powell relates this "basin and range" style of tectonic activity to regional dextral shear. Farther south, magmatism is more restricted in extent and phases of compression alternated with extension. This tectonic style matches an Andean continental margin where regions of uplift and deformation form at zones of a shallowing dipping subducting slab (Jordan et al., 1983). References Baillie, P., 1984, A Palaeozoic suspect terrane in southeastern Australia and North Victoria Land, Antarctica, Geological Society of Australia, Abstract, 12, 43. Jordan, T.E., Isacks, B.L., Allmendinger, R.W., Brewer, J.A., Ramos, V.A., & Ando, C.J., 1983, Andean tectonics related to geometry of subducted Nazca plate. Bulletin Geological Society of America, 94, 341-361. Powell, C.McA., 1983, Tectonic relationship between the Late Ordovician and Late Silurian palaeogeographies of southeastern Australia, Journal of the Geological Society of Australia, 30, 353-373. Scheibner, B.C., 1974, A plate-tectonic model of the Palaeozoic tectonic history of New South Wales, Journal of the Geological Society of Australia, 20, 405-426. Seranne, M., Seguret, M., & Laiirent, P., 1985, Ductile extensional deformation and low-angle fault/shear zone transition in western Norway Devonian basins. Continental Extension Tectonics (Abstracts), 3-4. Talent, J.A., 1963, The Devonian of the Mitchell and Wentworth Rivers, Memoir Geological Survey of Victoria, 24.

Fig. 1:

Qranitokte

Structural sketch map of the southern Tabberabbera Belt. Key to symbols: Ou - Ordovician undifferentiated, Om - Omeo Metamorphics, De - Wentworth Group (Early Devonian), Dl - Avon River Group (Late Devonian).

THE GWYDIR TERRANE: PALAEOZOIC SUBDUCTION COMPLEX IN THE SOUTHERN NEW ENGLAND FOLD BELT OF EASTERN AUSTRALIA

C.L. Fergusson^, P.G. Flood^ and K.C. Cross^ ^Monash University, Melbourne, Australia ^University of New England, Armidale, Australia ^Western Mining Corporation Limited, Kambalda, Australia The main tectonic elements of the southern New England Fold Belt in the Palaeozoic were: a volcanic chain in the west, a forearc basin in the centre, and a subduction complex in the east (Cawood and Leitch, 1985). The volcanic chain is largely covered by the Sydney Basin but inliers of silicic lavas and ignimbrites at the top of the arc sequence occur at Boggabri and Pokolbin. Remnants of the forearc basin are found in the Tamworth, Hastings and Emu Creek blocksc The subduction complex has been oroclinally folded in the north (Texas - Coffs Harboiir blocks) and disrupted in the south (Armidale Macdonald ~ Manning - Yarrowitch blocks)« Cawood and Leitch (1985) show a pre-Permian reconstmction of the forearc system with removal of all the Permian structural complications» Cawood and Leitch (1985) have identified five tectono-stratigraphic terranes in the southern New England Fold Belt, The Tamworth Terrane consists of the forearc basin sequences and is in fault contact with the remaining four terranes which are part of the sxabduction complex (in both our and their interpretations). However, we suggest that these four terranes are little more than contrasting lithogenetic elements within a disrupted and partly composite terrane (cf. Jones et al.^ 1983, p.297) which we call the Gwydir Terrane. v Following normal usage in tectonized regions the Gwydir Terrane has been subdivided into tectono-stratigraphic units and at least four main types are presents (a) Type 1 (serpentinite-matrix) melanges; (b) Type 2 melanges formed by the tectonic disruption of previously coherent marine strata and associated igneous rocks? (c) Type 3 melanges (deformed olistostromes); and (d) relatively coherent unitsc Type 1 (serpentinite-matrix) melange occurs along the Peel Fault (Figd) and along other faults within the Gwydir Terrane c It has formed by situ intense tectonic disruption of serpentinizedultramafics (ioeo autoclastic melange). The shear foliation in the serpentinite matrix is defined by mmscale anastomising shear fractures that is sub-parallel to the outlines of large massive serpentinite blocks. Especially along the Peel Type 1 melange grades into ophiolitic melange and occasionally contains exotic blocks, including rare blueschists and eclogites. Type 2 melange is widespread throughout the Gwydir Terrane and has been examined in detail in the Woolomin Association by Cawood (1982) and Cross (unpub. data) and in the Coffs Harbour Block by Fergusson (1984a,b). The main lithologies of Type 2 melanges are greywacke, argillite, tuffaceous rocks, chert, jasper and greenstone. Turbidite lithofacies of alternating greywacke and argillite are dominant. The type example of a Type 2 melange is the Gundahl Complex of the central Coffs Harbour Block (Fergusson, 1984a). Structurally, the Gundahl Complex consists of slabs and blocks contained in a relatively deformed matrix. The matrix is comprised of argillite with many slickensided and polished shear fractures that finely divide the rock into mainy small chips. Slabs and blocks within the Gundahl Complex display vairying intensities of internal deformation. Greywacke and greenstone blocks

Fig.l.

Geology of the southern New England Fold Belt, in part after Korsch (1977).

are characterised by 'web structure' (networks of cataclastic veins) and the blocks themselves pinch and swell. Slabs containing well-bedded sequences are disrupted by abundant cryptic bedding-parallel faults and may contain numerous folds. On a map-scale much of the Giindahl Complex has an imbricate stinacture with slabs ranging up to 10 km in length effectively repeating the disrupted pre-melange marine sequence. A tectonic origin for the Gundahl Complex is evident from the abundance of tectonically formed structiires and the lack of any characteristics diagnostic of sedimentary mixing. Type 3 melanges are relatively uncommon and occur in the Wisemans Arm Formation adjacent to the Peel Fault (Leitch and Cawood^ 1980) and in the Texas Block (Fergusson and Floods 1985). In most cases the extent of Type 3 melange is too restricted to identify tectono-stratigraphic units separate from surrounding Type 2 melanges. Relatively coherent units are common in the Gwydir Terrane and the type example is the Coramba beds of the Coffs Harbour Block. Most of this unit consists of thin bedded turbidites with interbedded thick-bedded and very thick-bedded turbidites which were probably deposited in a sediment-filled trench. These rocks are deformed into tight to isoclinal macroscopic foldsr the vergence of which is consistent with the predominant sense of younging to the northeast. Axial surfaces are either steeply dipping to the southwest or vertical, and are typically faulted. In addition to thickening related to macroscopic faulted folds^ the Coramba beds have been tectonically thickened by repetition along cryptic bedding-parallel faults. Another relatively coherent unit is the Silverwood Association of the Texas and Coffs Harboxir blocks. It consists of a monotonous assemblage of fine-grained tuffaceous rocks and altered mafic to intermediate fine-grained igneous rocks. Unlike other relatively coherent units there is no consistent structural grain and the structure is poorly \mderstood. The Silverwood Association was probably deposited in an island-arc related setting (Day et al., 1978). No correlation between the Texas - Coffs Harbour blocks and the Armidale - Macdonald - Manning «= Yarrowitch blocks existed prior to the Carboniferous (Table 1). In the Texas - Coffs Harbour blocks the only known preCarboniferous unit is the Silverwood Association which is regarded as a suspect terrane. The Silverwood Association is distinguished from the Woolomin Sandon associations of the Armidale - Macdonald - Manning - Yarrowitch blocks by the abundance of distinctive blue-grey quartz-albite chlorite rocks in the former. The Woolomin - Sandon associations are Type 2 melanges that formed in the Devonian stibduction complex east of the Peel Fault (Cawood, 1982; Cross, xinpub. data) . The main marker tectono-stratigraphic unit of the Gwydir Terrane is the oolith-bearing greywacke Type 2 melange unit of the Coffs Harbour, Texas, Armidale and Macdonald blocks. Geologists of the Queensland Geological Survey have shown that this unit extends a further 600km to the north of the Coffs Harbour Block (Fleming et al., 1975). Korsch (1977, 1984) has shown that the distinctive greywackes of the Coffs Harbour sequence also occur in the Coffs Harbour Association of the Armidale and Yarrowitch blocks. The Gwydir Terrane formed as a subduction complex above a westward dipping sTobduction zone. The Woolomin Association formed soon after the initiation of subduction, in the Silurian-Devonian interval, by the accretion of abundant pelagic sediments with less common basaltic volcanics and arcderived trench-fill flysch. The volcanic arc prestamably increased in volume with continued sxibduction and this is reflected by the larger abundance of arc-derived flysch that occurs in the Sandon Association and other Type 2 melanges.

Type 2 melanges within the Gwydir Terrane formed by shearing accompanying imbricate stacking of initially coherent sequences of arc-derived flysch overlying a basement of greenstone and pelagic sediments. Disruption of these sequences into slabs and smaller fragments proceeded by bedding-parallel shear. Some blocks within Type 2 melanges may be trench-slope basins kneaded into the subduction complex (e.g., Fergusson, 1984a). This process may account for the distribution of oolith-bearing greywacke blocks in the Sandon Association of the Armidale and Macdonald blocks. During the Middle to Late Devonian the Silverwood Association was accreted to the subduction complex. Day et al. (1978) suggest a marginal sea separated the Silverwood Association from the continental margin prior to the Late Devonian. Relatively coherent units dominate the late Carboniferous part of the subduction complex which presumably indicates a reduced convergence rate and an increased sedimentation rate in the trench. This change in conditions is also partly reflected in the decreased abundance of greenstones, pelagic and hemi-pelagic sediments which suggest that much of the igneous oceanic crust and overlying plate sedimentary sequence was subducted. The increased sedimentation rates in the trench are probably related to infilling of the forearc basin which occxarred in the Late Carboniferous. References Cawood, P.A., 1982, Structural relations in the subduction complex of the Palaeozoic New England Fold Belt, eastern Australia, Journal of Geology, 90, 381-392. Cawood, P.A., & Leitch, E.C., 1985, Accretion and dispersal tectonics of the southern New England Fold Belt, eastern Australia. American Association Petroleum Geologists Bulletin, in press. Day, R.W., Murray, C.G., & Whitaker, W.G., 1978, The eastern part of the Tasman Orogenic Zone, in, E. Scheibner (editor), The Phanerozoic Structure of Australia and Variations in Tectonic Style, Tectonophysics, 48, 327-364. Fergusson, C.L., 1984a, The Gundahl Complex of the New England Fold Belt, eastern Australia: a tectonic melange formed in a Palaeozoic subduction complex. Journal of Structural Geology, 6, 257-271. Fergusson, C.L., 1984b, Tectono-stratigraphy of a Palaeozoic subduction complex in the central Coffs Harbour Block of north-eastern New South Wales, Australian Journal of Earth Sciences, 31, 217-236. Fergusson, C.L., & Flood, P.G., A Late Palaeozoic subduction complex in the Border Rivers area of southeast Queensland, Proceedings Royal Society of Queensland, 95, 47-55. Fleming, P.J.C., Murray, C.G., & Whitaker, W.G., 1975, Late Palaeozoic invertebrate fossils in the Wandilla Formation and the deposition of the Curtis Island Group. Queensland Government Mining Journal, 76, 1-7. Jones, D.L., Howell, D.G., Coney, P.J., & Monger, H.W.H., 1983, Recognition, character and analysis of tectonostratigraphic terranes in western North America, Journal of Geological Education, 31, 295-303. Korsch, R.J., 1977, A framework for the Palaeozoic geology of the southern part of the New England Geosyncline, Journal of the Geological Society of Australia, 23, 339-355. Korsch, R.J., 1984, Sandstone compositions from the New England Orogen, eastern Australia: implications for tectonic setting. Journal of Sedimentary Petrography, 54, 192-211. Leitch, E.C., & Cawood, P.A., Olistoliths and debris flow deposits at ancient consimiing plate margins: an eastern Australian example. Sedimentary Geology, 25, 5-22.

TABLE Is

Tectono-stratigraphic units of the Gwydir Terrane Armidale-Macdonald -Manning-Yarrowitch blocks

Coffs Harbour Block

Texas Block

Coffs Harbour beds Subunits -Coramba beds -Brooklana beds -Moombil beds

Texas beds Subunits - Ct2 - Ct3 - Ct4

Girrakool b e d s , Coffs Harbour Association

Gundahl Complex

Subunit C t l of the Texas beds

Willowie Creek beds

Silverwood Group

(Silverwood Associa tion)

Main features in common

Differences

Late Carboniferous

Greywackes - rich in silicic V R F . Some chert, greenstone in Brooklana b e d s , Ct3

Visean limestone blocks in Type 3 melange of the Texas beds

Oolith-bearing greywackes of the Sandon Association

Early Carboniferous

Greywackes - rich in silicic and intermediate VRF o Type 2 melanges with bluecoloured cherts

Discrete units yet to be fully mapped in the Armidale and Macdonald blocks

Sandon Association Woolomin Association

Devonian and Silurian

Greywackes - rich in mafic VRF

Abundant greenstones andesite and distinctive tuffaceous rocks in Silverwood AssociaSandon tion . Woolomin associations - Type 2 melanges. Woolomins - dominated by chert and siliceous argillite

Proposed age

SUBDUCTION MODE AFTER AND BEFORE CONTINENTAL COLLISION IN THE CASE OF OKHOTSK TERRANE Kimura Gaku Department of Earth Sciences, Faculty of Education, Kagawa University, Takamatsu, 760 Japan We can reveal a convergent history by analysis of ancient subduction complexes. In many cases subduction complexes contain exotic blocks of oceanic material which are different in age from the matrix of the complexes. Then, we can establish when subduction occurred and how old the plate undergoing subduction was. We can also clarify the ancient subduction mode based on the deformation styles of the accretionary complex and volcanic arc. Subduction modes are roughly classified into four types based on the relationship between overriding and subducting plates. Namely, strongly coupled (Chilian type by Uyeda and Kanamori, 1979) normal and oblique subduction modes, and weakly coupled (Mariana type) normal and oblique subduction modes. A transcurrent fault within an island arc as suggested by Fitch (1972) is formed in the case of strongly coupled oblique subduction mode. The weakly coupled oblique subduction mode does not introduce the transcurrent fault but the subduction complex might be deformed oblique to the trend of the trench, or tectonic erosion may occur (Fig. 1). The coupling is determined by the absolute motion of the overriding plate (Uyeda and Kanamori, 1979) or the age of the subducting plate (Molnar and Atwater, 1978; Seno, 1985).

Fig.l

Subduction Modes indicating two kinds of oblique subduction and resulting deformation styles of the island arc. 1) strongly coupled case 2) weakly coupled case

Recently, many researches have suggested that the Asian Continent is mainly composed of accreted continental terranes (McElhinny et al., 1981, Kleimets, 1983; Taira, 1983; Schermer, 1983 and others). Between the terrane, we recognize collision complexes constituted dominatly of ancient subduction complexes before collision. These terranes possibily originated from the lost "Pacifica" continent as suggested by Nur and Ben-Avraham (1979). They also pointed out that "Pacifica" was broken up by ridge-transform system such as the Farallon-KulaPheonix ridges in the past Pacific.

When the c o n t i n e n t a l p i e c e s that o r i g i n a t e d from "Pacifica" collided and accreted against another c o n t i n e n t , the following s u b d u c t i o n h i s t o r y a l o n g t h e t e r r a n e m a r g i n a p p e a r s to h a v e t a k e n p l a c e in t h e s i m p l e s t c a s e ( F i g . 2 ) . In t h e i n i t i a l s t a g e o f s u b d u c t i o n , the s u b d u c t i n g o c e a n i c p l a t e a p p e a r s to h a v e b e e n a r a t h e r o l d o n e f o r m e d s i n c e the b r e a k u p of " P a c i f i c a " . Subsequently a rather younger plate might have been subducted and after that t i m e , a r i d g e - t r e n c h e n c o u n t e r a p p e a r s to h a v e o c c u r r e d . After the ridgetrench e n c o u n t e r , the relative p l a t e m o t i o n changed and rather older plate might have followed^ Nur and Ben A b r a h a m (1979) speculated t h a t b a c k - a r c s p r e a d i n g is f o l l o w e d b y a r i d g e - t r e n c h e n c o u n t e r « B a c k - a r c s p r e a d i n g , h o w e v e r , n e e d n o t to be a s s o c i a t e d w i t h " r i d g e - s u b d u c t i o n " (Delong and F o x , 1 9 7 7 ) . This subduction history is i n t i m a t e l y r e l a t e d t o t h e s u b s e q u e n t c o l l i s i o n - a c c r e t i o n t e c t o n i c s T h e d e t a i l e d h i s t o r y m u s t be r e v e a l e d f r o m t h e a n a l y s i s of the subduction complex inbetween the t e r r a n e s .

Stage I Jiinp of trench Subduction of old plate

Fig. 2

Simplified

subduction

Stage 3 Trap of old plate and jump of trench

Stage 2 Ridge-Trench Encounter followed by the subduction of young plate

history

before

and

after

the

collision.

In t h e f o l l o w i n g , I f o c u s o n t h e c a s e o f t h e O k h o t s k c o n t i n e n t a l t e r r a n e w h i c h w a s a c c r e t e d to t h e K o l y m a a n d S h i k o t e A l i n t e r r a n e s . T h e K o l y m a a n d S h i k o t e A l i n t e r r a n e s a r e c o n s i d e r e d to h a v e a c c r e t e d a g a i n s t t h e S i b e r i a C o n t i n e n t d u r i n g t h e T r i a s s i c to E a r l y J u r a s s i c (Taira, 1983)c T h e O k h o t s k T e r r a n e a p p e a r s to h a v e a c c r e t e d t o t h e m f r o m t h e L a t e C r e t a c e o u s to T e r t i a r y t i m e ( K i m u r a a n d T a m a k i , 1 9 8 5 ) . M e s o z o i c s u b d u c t i o n c o m p l e x e s a r e o b s e r v e d in b e t w e e n t h e t e r r a n e s . H o k k a i d o I s l a n d in J a p a n , w h i c h is s i t u a t e d b e t w e e n t h e O k h o t s k a n d S h i k o t e A l i n t e r r a n e s , is m a i n l y c o m p o s e d o f a J u r a s s i c to C r e t a c e o u s subduction complex. R e c e n t l y , g e o l o g i c a l data on a g e , rock t y p e s , and s t r u c t u r e s h a v e b e e n r a p i d l y a c c u m u l a t e d by m a n y g e o l o g i s t s . The r e s u l t s l e a d u s to t h e f o l l o w i n g s u b d u c t i o n c o l l i s i o n h i s t o r y (Fig 3 ) . i) In M i d d l e J u r a s s i c t i m e , a f t e r t h e a c c r e t i o n o f t h e S h i k o t e A l i n t e r r a n e a g a i n s t the S i b e r i a C o n t i n e n t , the C a r b o n i f e r o u s to Permian oceanic plate was subducting along the eastern margin of S h i k o t e A l i n . ii) T h e m a r g i n w a s a m a i n l y t r a n s f o r m b o u n d a r y in L a t e J u r a s s i c to early Cretaceous time, iii) In t h e E a r l y C r e t a c e o u s ( a b o u t 1 4 0 - 1 3 0 M a ) , a r i d g e - t r e n c h encounter (Farallon-Izangi, Kula?) occurred along the m a r g i n . T h i s e v e n t is s u p p o r t e d b y o p h i o l i t e o b d u c t i o n , i n - s i t u b a s i c magmatism and age reltions between the ophiolite and in-situ sediments. iv) In t h e E a r l y to M i d d l e C r e t a c e o u s a f t e r t h e r i d g e - t r e n c h e n c o u n t e r t h e t r a n s f o r m p l a t e b o u n d a r y w a s c h a n g e d to s u b d u c t i o n z o n e , the rather older P e r m i a n - T r i a s s i c plate was subducted along the marg in.

vi) vii) viii)

The subduction mode was a strongly coupled oblique one during t h e E a r l y to M i d d l e C r e t a c e o u s , w h i c h c h a n g e d to a w e a k l y c o u p l e d o b l i q u e o n e in t h e L a t e C r e t a c e o u s , D u r i n g t h e L a t e C r e t a c e o u s to P a l e g e n e , t h e O k h o t s k t e r r a n e collided and a c c r e t e d a g a i n s t the Kolyma and S h i k p t e Alin terranes. In l a t e C r e t a c e o u s t i m e , t h e K u r i l A r c w a s f o r m e d a l o n g t h e s o u t h e a s t e r n m a r g i n of t h e O k h o t s k T e r r a n e . In P a l e o g e n e t i m e ( a b o u t 6 0 - 5 5 M a ) , t h e s e c o n d r i d g e - t r e n c h encounter (Kula-Pacific) o c c u r r e d along the Kuril Trench (Kimura and T a m a k i , 1 9 8 5 ) .

T h i s s u b d u c t i o n h i s t o r y , r e c o n s t r u c t e d f r o m a n a l y s i s of t h e s u b d u c t i o n complex b e t w e e n the O k h o t s k and Shikote Alin t e r r a n e s , in c o n s i s t e n t w i t h t h e a b o v e m e n t i o n e d h y p o t h e s e s r e l a t e d to a c c r e t i o n tectionics.

(a)

Y^)

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/ f

PLATE

^ m

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IZANAGI

PLATE

140-130 Ma

PLATE

(PERMO-TRIAS)

f f f

^ D H K O T S X

I^ T E R R A N E

-; - V

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PLATE

(d)

(c)

f g

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j j

V'" O K H O T S J < 1^

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Fig.3 Subduction collision history of the northwestern Pacific margin. References Belong,S.E. and Fox,P.J.,1977, Geological consequences of ridge subduction. in "Island A r c s , Deep Sea T r e n c h e s and Back-Arc Basins" eds. M . T a l w a n i and W.D. Pitman III, A m . Geophys. Union, Maurice Ewing Ser. I, 221-228. Fitch,T.J.,1972, Plate c o n v e r g e n c e , t r a n s c u r r e n t f a u l t s , and i n t e r n a l d e f o r m a t i o n adjacent to southeast Asia and w e s t e r n P a c i f i c . J o u r . G e o p h y s . Res., 7 7 , 4 4 3 2 - 4 4 6 0 . K i m u r a , G . and Tamaki,K., 1985, T e c t o n i c f r a m e w o r k of the K u r i l Arc since

its initiation. Proceeding of Oji Seminor, Tokyo, Nov.1983 (in press)o KleimetZjM.P., 1983,Speculations on the Mesozoic plate tectonic evolution of eastern China. Tectonics, 2, 139-166. McElhinny,M.W., Embleton,B.J.J., Ma,X.Ho, and Zhang,E.K., 1981, Fragmentation of Asia in the Permian, Nature, 293, 212-216. Molnar,P. and Atwater,T.,1978, Interarc spreading and Cordilleran tectonics as alternative related to the age of subducted oceanic lithosphere. Earth Planet. Sci. Lett., 41, 330-340. Nur,A. and Ben-Avraham,Z«., 1979, Speculations on mountain building and the lost Pacifica continent. Suppl. Jourc Physo Earth, 21-38. Schermer,EcR., 1983,Tectonostratigraphic terranes of China and Mongolia. Abstc 2nd Terrane Conference. Seno,T., 1985, Age of subducting lithosphere and back-arc tectonics, evolution of the western Pacific since the early Tertiary. Proceeding of Oji Seminor, Tokyo, Nov., 1983(in press). Taira,A., 1983, Plate tectonic evolution of Japan, Abst. 2nd Terrane Conf. Uyeda,S. and Kanamori,H., 1979, Back-arc opening and the mode of subduction. Jour. Geophys. Res.,84, 1049-1062.

74 .

THE BASEMENT TERRANES OF PENINSULA CALIFORNIA AND ADJACENT MEXICO Gordon Gastil, Richard Miller, Gary Girty, Michael Walawender, Melissa Wardlaw, James Crocker, Michael Campbell, John Hoobs, James Reed, Judith Diamond and Charles Knaack San Diego State University, San Diego, California, U.S.A. Peninsular California can be divided into several terranes on the distribution of pre-Cenozoic batholithic and prebatholithic rocks. The adjacent continental borderland and western capes of Baja California are a complex composite terrane. These have been described recently by Moore (1984) and Boles and Landis (1984) and will not be considered here (all place names are shown in Fig. lA)• Inboard of the borderland are at least five lithologic associations (terranes) (Fig. IB), and at least two of these appear to continue into Sonora and Sinaloa. Superimposed upon the five lithobelts are regional differences in tectonic style (Fig. IB), regional differences in isotopic ratios, the magnetite content, heat flow, and gravity are related to batholithic emplacement. The oldest rocks in the lithobelts are miogeoclinal strata, largely clean quartz arenite and metacarbonate rock, with minor metapelite and amphibolite (basalt?). The sequence near San Felipe, Baja California may correlate with the Upper Precambrian-Lower Cambrian strata of northwestern Sonora. On Coyote Mountain, western Imperial County, at least part of the thick carbonate sequence is Early Ordovician in age. Adjacent to the miogeoclinal strata are sequences of bedded chert, thin-bedded carbonate, argillite, poorly sorted lithic sandstone, andesite and basalt, and locally quartz arenite. These are distributed from the Sierra Pintas in the north (Carboniferous) to Arroya Calamujue (Mississippian), and the area south of Puerto Calamujue on the Gulf of California (Devonian). Similar rocks are found east of the Gulf in southern Sonora (Ordovician to Carboniferous), near El Fuerte, Sinaloa (Paleozoic), and San Jose de Garcia, Sinaloa (PennsyIvanian). Further to the west upper amphibolite facies metasedimentary rocks of the Sierra Juarez and Sierra San Petro Martir may be part of this terrane. This belt appears to have been deposited in basinal and base of slope environments, and contains only minor eratonal detritus. The contact relation between these rocks and the Miogeoclinal sequence are unknown. The third terrane consists of Permian and Lower Triassic strata of the 30th Parallel which do not easily correlate with other rocks in the peninsula. They appear to have been deposited in outer shelf and slope environments, and consist of predominantly fine-grained clastic rocks, olistostromes, and minor carbonate rock and bedded chert. Clasts appear to be intraformational. The fourth terrane consists of flysch-like deposits of eratonal derivation occurring between the 3Ist and 33rd parallels. In southern California these deposits contain Triassic and Jurassic fossils, but no fossils have been found in similar rocks south of the border. These rocks form a belt which lies between the Paleozoic terrane to the east, and the volcanic arc terrane to the west.

The f i f t h belt is an arc terrane which is Upper Jurassic in southern C a l i f o r n i a , but is predominantly Cretaceous south of the border. East of the Gulf of California volcanic strata of Aptian to Cenomanian age are found in Sinaloa and western J a l i s c o . Ophiolites of medial Cretaceous age are present in Sinaloa, and ophiolites of uncertain age are found adjacent to arc rock near El Arco, Baja California (28th p a r a l l e l ) . These volcanic-volcanogenic deposits are largely marine, are locally at least six km thick, and contain no detritus of eratonal a f f i n i t y . Immediately east of this arc terrane (resting depositionally on the T r i a s s i c of terrane three) is an equally thick sequence which varies from boulder conglomerate at the base to dominantly fine-grained and c l a s t i c , s i l i c e o u s , and minor carbonate rock, with abundant volcaniclastic deposits locally. Limited f o s s i l evidence indicates that this sequence i s , like the arc to the west, Albian-Aptian. In marked contrast to the non-cratonal arc this sequence contains giant lenses of quartzite boulders near its base and may have formed in a back-arc setting. This "back-arc" section has been observed in three l o c a l i t i e s between the 31st and 29th p a r a l l e l s . At each of these localities the "back-arc" section is structurally separated from arc rocks. The geographic position and cratonal provenance of these rocks is similar to the Triassic-Jurassic flysch terrane described above. Superimposed on these lithoterranes is a boundary between western I-type, relatively shallowly emplaced, only marginally foliated gabbro to granodiorite, and an eastern belt consisting of older foliated S-type (Todd and Shaw) granitic rocks, and younger, largely unfoliated plutonic rocks with properties overlapping the I-S c l a s s i f i c a t i o n boundary. These latter rocks typically form extensive areas of relatively homogeneous leucotonalite, punctuated by large zoned plutons cross-cut by smaller bodies of g a m e t i f e r o u s two-mica rock ( F i g . lA, I B ) . Although a l l granitic rocks in the Peninsular Ranges have been considered until now Cretaceous, a large S-type body in northern Sinaloa has been interpreted as Paleozoic (on structural evidence, Mullen, 1 9 7 8 ) , and some of the foliated S-type rocks in the peninsula may be appreciably older. Speculation that the western belt is underlain by oceanic crust, and that the eastern plutonic belt is underlain by continental crust is heightened by sharp contrasts in i n i t i a l S r 8 7 / 8 6 values (Todd and Shaw, 1 9 8 5 ) , various geochemical parameters ( S i l v e r and others, 1979; Baird and B a i r d , 1 9 8 4 ) , Bouguer and isostatic gravity anomalies ( O l i v e r , 1 9 8 0 ) , a magnetite/ilmenite line (Diamond and others, 1 9 8 5 ) , and marked differences in structural style (see below). Some question must be r a i s e d , however, concerning the proposed oceanic basement for the arc terrane, in view of the high proportion of rhyolite (for example, Begg, 1984) and a Rb/Sr isochron in an ash flow tuff giving an i n i t i a l of 87 Sr/86 Sr value . 7 0 5 8 . The varied structural styles of the peninsula show some correlation with the lithoterranes ( F i g . I B ) . Beginning in the west, the arc consists typically of open folds with nearly horizontal hinge lines. In contrast, rocks in the eastern "back-arc" Albian-Aptian strata are i s o c l i n a l , with a x i a l surfaces dipping steeply to the east, and hinge lines that plunge steeply down the a x i a l surfacesc In places extreme transpositional fabrics grade into pervasive mylonitizationo Analogous structures have been observed from the 29th p a r a l l e l north to San Diego County. East of San Diego highly deformed Upper Jurassic Santiago Peak Volcanics and granitic rocks of unknown age are cross-cut by

unfoLiated granitic rocks of medial Cretaceous age. On the 30th parallel Permian to medial Cretaceous rocks are involved in the deformation. The deformation appears to indicate the upward and southwestward movement of the eastern portion of the peninsula in late medial Cretaceous time. The eastern siliceous terrane is believed to consist of thrusts and recumbent folds (in some areas steepened by Cenozoic tilting). In the northern Sierra Pintas hinge lines trend N80 W and associated foliation is nearhorizontal. South of Puerto Calamujue and in adjacent southern Sonora (Noll, 1981; Poole, 1983) the direction of tectonic transport is to the north or northeast (Fig. IB). The boundary between the miogeoclinal terrane and the deeper water terrane, and the boundary between the deeper water terrane and the arc terrane, as matched across the Gulf of California, seem to tie the rocks east of the Gulf to those of Peninsular California. Comparisons of the lithologies and structures of peninsular California/western Mexico with those now present in the Sierra Nevada and western Nevada are not inconsistent with proposed midJurassic left-lateral movements on the Mojave-Sonora megashear (Anderson and Silver, 1974), but do not really require such displacement. The deeper water terrane may record deposition in a slope-basin setting marginal to the Paleozoic peninsula/west Mexico subcontinent, which was driven northeastward against the North American eraton in Carboniferous time. The Permian to Triassic rocks of the 30th parallel and the Triassic-Jurassic flysch deposits found further north may record continued deposition along the southwestern margin of this landmass. In Jurassic-Cretaceous time a volcanic arc formed to the southwest, near the continent, and both arc derived and continent-derived strata accumulated in the narrow basin separating it from the continent to form a thick back-arc sequence. In late medial Cretaceous time the eratonal margin over-rode the arc, producing a highly tectonized boundary. Magmatic intrusion continued during and following this deformational event.

CONTINENTAL BOROERLANO

30®

Figure lA Plutonic Provincsc of Peninsular California North of the 28th Parallel The «#esternr magnetite seriee rocks are shown in horizontal bar patternt eastern, ilaenite series rocks are shown in dotted pattern. Place nanes in the United States are San Diego (SO)^ and Coyote Mountain (C). In Baja California they are Rancho San Marcos (SM), Sierra Juarez (SJ), Punta China (PC), Sierra Pintas (SP), Sierra San Pedro Martir (PM), Puerto Calamujue (PC), Arroyo Calamujue (AC), El Arco (EA). In Sinaloa they are El Puerto (EF), and San Jose de Gracia (SJG)= Figure IB Terranes of Peninsular California and Adjacent Mainland Mexico Coastlines have been palinspastically reconstructed to reflect 300 km of postMiocene translation between the Pacific and North M e r i c a n plates. Terrane I. Miogeoclinal North America (vertical bar pattern)* II. Devonian to Permian basinal and slope facies (small dot pattern)i III. Mesozoic Flysch (triangular pattern); IV. Jurassic->Cretaceous arc (horizontal/bar pattern); V. Continental Borderland composite ophiolite and arc terranes (large dot pattern). Figure IC Metamorphic Facies of Peninsular California Zeolite facies (large dot pattern), greenschist (horizontal bars) , lower amphibolite facies (vertical bars), upper amphibolite facies (small dots).

LANTERMAN FAULT : BOUNDARY BETWEEN ALLOCHTHONOUS TERRANES, NORTHERN VICTORIA LAND, ANTARCTICA.

G.M. Gibson Darling Downs Institute, Toowoomba, Australia Introduction The Lanterman and Leap Year Faults separate geologically distinct terranes and constitute major tectonic boundaries in northern Victoria Land (Fig. 1). Despite their obvious importance, no agreement has yet been reached about either their origin or the sense of displacement along them. They have been variously interpreted as high-angle normal faults bounding a Bowers graben (Stump e ^ ^ . , 1983) or as major strike-slip faults (Weaver et ale, 1984). New observations on the structure and metamorphism of the Bowers terrane reported here suggest that neither interpretation is appropriate for the Lanterman Fault. The Lanterman Fault is better interpreted in terms of thrust-faulting (cf. Gibson & Wright, 1985) or transpression. It possibly originated as a back-thrust following collision between the Antarctic continent and a west-facing island arc (Fig. 3). Stratigraphy of the Juxtaposed Terranes The Lanterman Fault juxtaposes the Wilson and Bowers terranes (Fig. 1). The Wilson terrane consists mainly of multiply deformed Precambrian amphibolite facies gneisses, intruded by Cambro-Ordovician granitoids (Granite Harbour Intrusives), whereas the Bowers terrane contains a Cambrian-early Ordovician regressive sequence (Bowers Supergroup). At the base of this sequence (Fig. 2) is the Sledgers Group, itself made up of two interdigitating units: the Molar Formation comprising marine volcanogenic sediments and the Glasgow Volcanics consisting of pillow lavas, basaltic and andesitic breccias, and subordinate dacite-rhyolite (Laird & Bradshaw, 1983). Basic rocks within the Glasgow Volcanics have been interpreted (Weaver et , 1984) as primitive island arc tholeiites. Overlying the Sledgers Group are the non-volcanogenic, shallow or marginal marine. Mariner Group and the non-marine, predominantly conglomeratic Leap Year Group (Laird & Bradshaw, 1983). The Lanterman and Husky Conglomerates, both of which occur only in the Lanterman Range as fault-bound slivers along the western margin of the Bowers terrane, are of uncertain stratigraphic affinities although the Husky Conglomerate contains mafic detritus consistent with derivation from the Glasgow Volcanics (Gibson, 1984). The Husky Conglomerate may originally have rested directly upon the Glasgow Volcanics. Structure and Metamorphism of the Bowers Terrane The Bowers terrane was mainly metamorphosed under prehnitepumpellyite or pumpellyite-actinolite facies conditions (Wodzicki et al., 1982) and generally underwent folding about gently plunging, northwesttrending axes (Bradshaw £t , 1982). Multiply deformed greenschist facies rocks with more steeply plunging fold axes (Fig. IB) are restricted to a narrow (2-4km) belt along the western margin of the Bowers terrane. Deformation and metamorphism clearly intensify towards the Lanterman Fault. This intensification is particularly marked within the Lanterman and Husky Conglomerates, both of which are characterised by severely flattened, or sometimes even stretched, pebble fabrics. Similar

Bowers Supergroup greenschist facies with deformed conglomerates l^i

30 km

sub-greenschist facies

Fig. 1 Geological map of the southern Bowers terrane showing the distribution of greenschist facies rocks and orientation of first generation fold axes (dots), stretching lineations (crosses) and the regional foliation (solid line). Contour interval for poles to bedding (two stereoplots only) is 3%, 6%, 9% and 12% per 1% area. Inset (Fig. 1A) is a terrane map of northern Victoria Land.

pebble fabrics have been observed (Stump et al., 1983; Gibson et al., 1984) in conglomerates within the Molar Formation immediately east of the Lanterman Fault in the Mountaineer Range. Stretched vesicles and elongated pyroclastic fragments also occur in deformed Glasgow Volcanics on the Mariner Plateau. These and other lineations in the Bowers terrane are everywhere contained within the regional foliation and plunge downdip, sometimes subparallel to the local fold axes (Fig. IB). The structural and metamorphic relations described here are consistent with the overthrusting of hot Wilson rocks over the Bowers terrane (Gibson & Wright, 1985). This produced higher temperatures (greenschist facies metamorphism) and the intense ductile deformation observed in Bowers rocks immediately east of the Lanterman Fault. A model of regional simple shear (Escher & Watterson, 1974) combined with further localised pure shear flattening best accounts for the minor structures developed in this zone. Thus linear fabrics are transverse extension (stretching) lineations whereas folds now plunging down-dip within the regional foliation (Fig. IB) originally had horizontal or subhorizontal axes and were subsequently rotated into the direction of maximum extension (X direction of finite-strain ellipsoid) during overthrusting. Deformed pebble conglomerates along the western margin of the Bowers terrane are tectonites with their clasts either flattened into the plane of the regional foliation (XY plane of the finite-strain ellipsoid) or else elongated (stretched) parallel to X. The stretched pebbles, together with the other linear fabrics, indicate tectonic transport to the northeast, consistent with crustal shortening at right angles to both the Lanterman Fault and the Bowers terrane. These relations are inconsistent with either normal dip-slip or pure strike-

W1LSON TERRANE

BOWERS TERRANE

i LL obducted & eroded to form Husky Congi.

Wilson Gneisses

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Group

cc CD Molar Glasgow O Formation Volcanics Sledgers Group JL cr III Fig. 2

Fig. 2 1983).

Fig. 3

Stratigraphy of Bowers terrane (modified after Laird & Bradshaw,

Fig. 3 Tectonic model for northern Victoria Land. A - Middle Cambrian; B - Late Cambrian; C - Ordovician. The Lanterman Fault (LF) is shown as a post-collision back-thrust. BT = Bowers Terrane; GV = Glasgow Vole.; HC = Husky Congl.; LYF = Leap Year Fault; MF = Molar Formation; MG = Mariner Group; RBT = Robertson Bay Terrane.

8 1

slip displacement on the Lanterman Fault. The Lanterman Fault is a compressional boundary although compression combined with some component of transcurrent movement (transpression) could possibly explain some of the structural observations reported here. Origin of the Lanterman Fault The Lanterman Fault juxtaposes elements of a Cambrian volcanic arc (Sledgers Group) against continental crust intruded by calc-alkaline granitoids (Granite Harbour Intrusives). Granitoid emplacement took place in an Andean-type setting immediately following a continent-island arc collision (Fig. 3B). The Lanterman Fault originated as a postcollision back-thrust whilst penecontemporaneous erosion of the newly uplifted Glasgow Volcanics led to the deposition of the Husky Conglomerate. The bulk of Bowers sedimentation took place in a fore-arc basin although initially some deposition may have occurred in a back-arc setting prior to the reversal in subduction polarity (cf. Figs 3A & 3B). With continued uplift and erosion of the Wilson terrane along the Lanterman Fault, the arc rocks were buried beneath the Leap Year Group. The Lanterman Fault continued to be active until the Ordovician Ross Orogeny when the Robertson Bay terrane was accreted onto the Antarctic continent (Fig. 3C). I thank Dr F. Tessensohn and the Bundesanstalt fur Geowissenschaften und Rohstoffe, Hannover for the opportunity to join the Ganovex III expedition to northern Victoria Land. References Bradshaw, J.D., Laird, M.G., & Wodzicki, A., 1982, Structural style and tectonic history in northern Victoria Land, In Craddock, C., (Ed.) : Antarctic Geoscience, 809-816, University Wisconsin Press, Madison. Escher, A. & Watterson, J., 1974, Stretching shortening, Tectonophysics, 22, 223-231.

fabrics,

folds and crustal

Gibson, G.M., 1984, Deformed conglomerates Range, Geologisches Jahrbuch, B60, 117-H1.

eastern

the

Lanterman

Gibson, G.M. & Wright, T.O., 1985, Importance of thrust faulting in the tectonic development of northern Victoria Land, Nature (in press). Gibson, G.M., Tessensohn, F. & Crawford, A.J., 1984, Bowers Supergroup rocks west of the Mariner Glacier and possible greenschist facies equivalents, Geologisches Jahrbuch, B60, 289-318. Laird, M.G. & Bradshaw, J.D., 1983, New data on the Lower Palaeozoic Bowers Supergroup, northern Victoria Land, In Oliver, R.L., James, P.R. & Jago, J.B. (Eds.) : Antarctic Earth Science, 123-126, Australian Academy Science, Canberra. Stump, E., Laird, M.G., Bradshaw, J.D., Holloway, J.R., Borg, S.G., & Lapham, K.E., 1983, Bowers graben and associated tectonic features cross northern Victoria Land, Antarctica, Nature, 304, 334-336. Weaver, SoD., Bradshaw, J.D. & Laird, M.Gc, 1984, Geochemistry of Cambrian volcanics of the Bowers Supergroup and implications for the Early Palaeozoic tectonic evolution of northern Victoria Land, Antarctica, Earth Planetary Science Letters, 68, 128-140. Wodzicki, A., Bradshaw, J.D. & Laird, M.G., 1982, Petrology of the Wilson and Robertson Bay Groups and Bowers Supergroup, northern Victoria Land, Antarctica, In Craddock, E. (Ed.) : Antarctic Geoscience, 549-554, University Wisconsin Press, Madison.

PALAEOMAGNETISM OF AUSTRALIA'S ACTIVE NORTHERN MARGIN IN NEW GUINEA John Giddings,^ Chris Klootwijkf Wahyu Sunata^ Charles Loxton^ Chris Pigramland Hugh Davies 1 1) Bureau of Mineral Resources, Canberra, Australia 2) Geological Research and Development Centre, Bandung, Indonesia 3) Geological Survey of Papua New Guinea, Port Moresby, Papua New Guinea On the basis of comparative stratigraphy a large number of terranes have been identified along New Guinea's northern perimeter (Pigram and Davies, this conference). Reliable palaeomagnetic control is essential in tracing their amalgamation in this uniquely complex region of interaction between the Pacific, the South East Asian and the Indian-Australian plate. Apart from some pioneering studies (cf Manwaring 1974, Falvey and Pritchard 1984) such data are virtually nonexistent. The BMR has now developed a program of palaeomagnetic research in conjunction with GRDC under the auspices of the Indonesia-Australia Geological Mapping Program, which has concentrated so far on the Bird's Head of Irian Jaya, and has recently started palaeomagnetic cooperation with the Geological Survey of Papua New Guinea under the auspices of SEATAR in the region of TRANSECT IX. BIRDS HEAD, IRIAN JAYA The Bird's Head region of western Irian Jaya is a salient of continental crust that protrudes into the Pacific Plate. Its tectonic history and origin continue to remain a matter for controversy. Various hypotheses have been put forward: rotation relative to Australia in the late Tertiary of both a clockwise (Hamilton 1977, Robinson and Ratman 1978) and anticlockwise nature (Carter et al 1976); a fixed position relative to Australia (Dow and Sukamto 1984); that it is a composite microcontinent with a separate drift history (Pigram and Panggabean 1984). In the absence of palaeomagnetic information, arguments to date have been based on analysis of geological data with little quantitative evidence on the type and scale of motion, if any, involved. Clearly, palaeomagnetic data are required to provide constraints and help discriminate between hypotheses. We report on preliminary results that we have obtained from the first palaeomagnetic study undertaken on Bird's Head rocks. About 350 samples were taken from nine formations whose ages fall within the range Late Carboniferous to Middle Miocene. The rocks form a sequence of

clastics and carbonates draped over the southwestern margin of the Kemum Block in the vicinity of 132o7E, 1.2S. The preliminary results are based on detailed demagnetization analysis of one specimen per sample; as such, confidence limits on some of the pole positions, at this stage, are large. Nevertheless, the data still provide a broad picture of the drift history. Magnetizations from two of the formations are highly scattered and yield no useful palaeomagnetic information. Results from the remaining formations show that they contain at least 2 components of magnetization. One component is similar in direction for all: it is generally northward-directed with a shallow upward inclination and is interpreted as an overprint acquired in the latest Tertiary; it may be partly of chemical origin. The other component of magnetization is quite different for each of the formations and is interpreted, at this stage, as the primary magnetization. The remanence directions from the Aimau (Late Carboniferous) and Aifat (late Early Permian) Formations are single polarity and reversed, which agrees with the Kiaman Reversed Magnetic Interval. The mixed polarity remanence present in the overlying Ainim Formation magnetically identifies it as no older than late Late Permian (Tartarian) - the end of the Kiaman interval of reversed polarity - confirming the age assigned geologically. Comparison of the Bird's Head poles obtained from the primary magnetizations with an updated apparent polar wander path for Eastern Gondwanaland shows that, overall, they are in gross agreement. We may therefore rule out any large scale relative motion between the Bird's Head and Australia since the Late Carboniferous; we have no palaeomagnetic information for ealier times. However, in detail, the Late Cretaceous and Tertiary poles are off the main path by small amounts suggesting that some relative motion has occurred between the Bird's Head and Australia but that the amount has not been large. The data do at least rule out a large clockwise rotation in the Neogene. Rather, they point to some clockwise rotation occurring between the Late Cretaceous and the Middle to Late Eocene followed by some north-northeast directed motion relative to Australia, which was completed by about the Early to Middle Miocene. A small component of anticlockwise rotation, post- Early to Middle Miocene, may be present in the data and could reflect abutment of the Bird's Head against the margin of the Pacific Plate. WESTERN PAPUA NEW GUINEA Reconnaissance palaeomagnetic sampling was undertaken in late 1984 in order to 1) study the origin and movement of the North Sepik region, 2) detail the relationship between the Kubor massif and the Australian craton and 3) trace the development of the Aure Trough and the Papuan Fold Belt.The program was sponsored by AIMS/BMR as an Australian contribution to lOC-WESTPAC (Margins of Active Plates). In the North Sepik region more than 200 samples were collected from Eocene-Miocene Bliri Volcanics at Wewak, an unnamed 01igo-Miocene limestone at Amanab, and Miocene Puwani Limestone at Imonda and Vanimo. Pilot demagnetization studies indicate rotations whose interpretation in terms of the complex tectonics of the region has to be analyzed further. In the Highlands well over 500 samples were collected from: 1) the Triassic to Miocene sedimentary and volcanic sequence overlying and northwards adjacent to the Kubor massif in the Mount

Hagen-Kerowagi-Kundiawa-Gumine region; 2) Lower to Middle Miocene volcanics and greywacke of the Yaveufa Syncline near Daulo and Lufa; and 3) Eocene to Miocene carbonate, calcareous mudstone and greywacke from the Papuan Fold Belt in the Mendi-Poroma-Ialibu region. Pilot demagnetization studies from the latter region (3) show a primary magnetization indicative for clockwise rotations of minor magnitude. Extensive pilot studies from the former two regions (1,2) show the presence of a pervasive magnetic overprint. This is associated most probably with major igneous activity during the Middle Miocene (12-15Ma, Page 1976), which continued into the Late Miocene (6-7Ma, Mount Michael Diorite, Yaveufa Syncline). No primary magnetization has been identified with certainty so far, except possibly the results from the Middle Miocene volcanics of the Yaveufa Formation near Daulo. These secondary and the single primary magnetization have a general westerly declination and indicate a rather coherent pattern of anticlockwise rotations, varying in magnitude from about 60 degrees to about 110 degrees. Preliminary palaeomagnetic results obtained by Manwaring (1974) from a limited sample collection, taken in 1961, show similar anticlockwise rotations over about 110 degrees for the Bismarck Intrusive and the Yonki Dome intrusive complex east of Kainantu. The general agreement throughout the Central and Eastern Highlands between individual rotation observations suggests bodily rotation of a coherent tectonic unit. Conclusive interpretation of these results must wait until a broader framework of palaeomagnetic data from New Guinea has been established As a matter of discussion it is tentatively proposed that this coherent pattern of anticlockwise rotations is the palaeomagnetic signature of complex movement of a tectonic unit delineated grosso modo by the Central and Eastern Highlands. From an original position which was partly a northern continuation of the East Papuan Composite Terrane (EPCT; Schroeder and Marum Terrane) and partly an extension of the northern edge of the Australian continent (Kainantu, Jimi and Kubor Terrane), this unit has undergone large-scale anticlockwise rotation with draping of the EPCT and reorganisation of the northeastern margin of the Australian continent. This started after the Middle Miocene (probably after 6-7Ma) and probably but not necessarily before accretion of the Finistere terrane (latest Miocene and later or Early Miocene). The Finistere terrane shows no evidence of such a large-scale anti-clockwise rotation with respect to New Britain (Falvey and Pritchard 1984). Accretion of the Finistere terrane further impeded the northward movement of the Australian plate (over more than 2000 km since the Eocene), and contributed to squeezing of the unit in WNW direction and large-scale southward directed overthrusting onto the Australian continental margin. A major overthrust zone may be identified along the Maramuni Fault zone continuing to the south of the Kubor massif. Large-scale telescoping of the northeastern perimeter of the Australian continental margin is presently continuing with southward progradation of the thrust front in the Papuan Fold Belt. REFERENCES Carter, D.J., Audley-Charles, M.G., & Barber, A.J., 1976, Stratigraphical analysis of island arc-continental margin collision in eastern Indonesia, J.Geol.Soc.Lond,, 132, 179-198. Dow, D.B., & Sukamto, R., 1984, Western Irian Jaya: The end- product of oblique plate convergence in the Late Tertiary, Tectonophysics, 106, 109-139. Falvey, D.A., & Pritchard, T., 1984, Preliminary palaeomagnetic results from northern Papua-New Guinea: Evidence for large microplate rotations, in: S.T.Watson (Ed.), Transactions of the third Circum-Pacific Energy and Minerals Resources Conference,593-599.

Hamilton, W., 1977, Subduction in the Indonesian region. Union, Maurice Ewing Series, 1, 15-31.

Am.

Geophys.

Manwaring, E.A., 1974, A palaeomagnetic reconnaisance of Guinea, B.M.R Record 1974/92, 77pp (Unpublished)

Papua-

New

Page, R.W., 1976, Geochronology of Igneous and Metamorphic New Guinea Highlands, B.M.R Bulletin, 162, 117pp

rocks in the

Pigram, C.J., & Pangabbean, H., 1984, Rifting of the northern margin of the Australian continent and the origin of some microcontinents in eastern Indonesia, Tectonophysics, 107, 331-353. Robinson, G.P., & Ratman, N., 1978, The stratigraphic development of the Manokwari area, Irian Jaya, B.M.R J. Geoph., 3, 19-24.

and tectonic Aust. Geol.

ACCRETIONARY TECTONICS. NORTHERN EAST COAST DEFORMED BELT. NEW ZEALAND J. Duane GiiDSon Geology Department University of Auckland Auckland. New Zealand REGIONAL SETTING A portion o£ the northern East Coast Deformed Beit (E,C,D,B.J. North Island, New Zealand, was chosen to investigate the role of accretionary and other types of tectonics. Accretion has occurred repeatedly within this region. The Torlesse Terrane to the west and southwest is recognized as an Early Cretaceous and older accretionary complex (Sporli, 1978K TO the north. the ophiolitic Matakaoa Volcanic Group was oMucted during the latest Oligocene-early Miocene (Brothers and Delaloye, 1982). Off the east coast of the North Island, active plate convergence occurs along the Hikurangi Trench, with westward subduction of the Pacific plate beneath the Indian plate (Crook and Feary, 1982).

Figure 1. Location map for the inlier of middle Cretaceous (Motuan; Mokoiwi Formation, North Island. New Zealand (after Kingma and Speden. 1978).

The Mokoiwi Formation (Speden. 1976) of Motuan (mid-Albian) age forms a tectonic inlier (Fig. 1) of variably disrupted, complexly faulted and folded strata. It consists of three distinct members: 1) a thin-bedded flysch sequence. 2) the Taitai Sandstone, and 3) the Rip Volcanics (Pirajno, 1979). The inlier, of approximately one hundred square kilometers area, is separated by faults from surrounding Upper Cretaceous and lower Tertiary strata. Faults to the southwest and northeast of the inlier have parallel WNW trends but opposite dip directions. The northeast boundary fault dips steeply to the

southwest, and the southwest boundary fault, 30 degrees to the northeast. The northeastern and eastern boundaries of the iniier are apparently folded and faulted. The inlier may onlap onto the Torlesse Terrane along its western margin, as depicted by Kingma and Speden (1978K MAJOR TECTONIC FEATURES Three features dominate the structure of the Mokoiwi Formation: I) the large percentage of relatively undeformed, overturned bedding (b7>. of all bedding measurements with known younging directions;. 2; the occurrence of broken formation throughout the sequence (approximately 2S% of ail measurements), and 3) subhorizontai folds in bedding and broken formation, locally with WNW-ESE and NE-SW trends. Overturned bedding The widespread occurrence of little deformed. overturned bedding within the Mokoiwi Formation is difficult to explain^ It cannot represent an overturned limb of a recumbent fold because zuch overturned limbs are usually highly attenuated,, A combination of two or more separate episodes of folding is required to account for the abundant overturned bedding. A question to be investigated is whether the Taitai Sandstone is equivalent to some portion of the Torlesse Terrane. The Taitai Sandstone is a deformational breccia in which angular fragments of dominantly medium-grained sandstone are separated by anastomosing micr6shears. Comminution of grains is found within these microshears. This phase of brecciation is not present in the Mokoiwi flysch and may therefore predate deposition of the flysch. Locally, the Taitai Sandstone grades into a conglomerate with dominantly well-rounded, silicic volcanic and granitic clasts vSpeden, 1976). Minor greywacke clasts are also found. Broken Formation Broken formation results mainly from stratal disruption, combined with strong layer-parallel extension. The planar fabric within the broken formation is usually subparallel to adjacent bedding and is defined by two distinct elements: 1; fragmentation and separation of sandstone' units, resulting in parallel alignment of elongate and irregular sandstone lozenges or phacoids, and 2) an anastomosing foliation formed by the sheared siltstone matrix. Various stages for development of broken formation, mainly within the Mokoiwi flysch. can be recognized. At low levels of deformation, sandstone beds display minor pinch-and-swell structure and closelyspaced, extensional step faults. The siltstone matrix becomes fragmented and displays mesoscopic ductility. With increased deformation, movement continues along selected step faults, resulting in dominantly monoclinic sandstone lozenges. This deformation leads to complete and unreconstructable separation of the sandstone beds. Intense shearing of the matrix results in polished slip surfaces

within clay-rich intervals• Disruption of bedding increases near some or the faulted margins of the inlier. Along these margins, the broken formation fabric generally parallels the orientation of the adjacent boundary fault. Development of Folds Folds within the Mokoiwi Formation are complex and discontinuous. Coherent bedding and broken formation are commonly folded about the same fold axis, indicating broken formation predates folding. In a few cases, broken formation results from hinge collapse within tight to isoclinal folds. Intervals of disrupted siltstone, with or without sandstone lozenges, locally behave as decollement layers during disharmonic folding. A large domain of WNW trends lies in the northeast portion of the inlier. This domain includes all of the Rip Volcanics. This WNW trend is highly oblique to the N- to NNE-trending structural grain of the Torlesse Terrane to the southwest. Similar WNW trends have been found to the south by Stoneley (1968) and Black (1980) in areas of mid-Tertiary or younger decollement. The maior Taitai Sandstone bodies of Wharekia, Aorangi, and portions of Taitai have dominant NE-trending fold axes. Alternating sequences adjacent to these mountains commonly show complex interference fold patterns with non-cylindrical folding. POSSIBLE AGES OF DEFORMATION Three possible ages of deformation are indicated for the Mokoiwi Formation and surrounding strata: 1) middle Cretaceous (MotuanNgaterianC?): Albian-Cenomaniani?)) (Crook and Feary, 1982: Feary and Pessagno. 1980: Kingraa and Speden, 1978), 2) late(?) Oligocene-early Miocene i.Waitakian(?)-Otaian) (Black. 1980; Stoneley, 1968), and 3) late Tertiary-Quatemary(?) (Kenny, 1984). References Black, R.D., 1980, Upper Cretaceous and Tertiary geology of Mangatu State Forest, Raukumara Peninsula, New Zealand. New Zealand Journal of Geology and Geophysics. 23, 293-312. Brothers. R.N., and Delaloye, M.. 1982, Obducted ophiolites of North Island, New Zealand: origin, age. emplacement and tectonic implications for Tertiary and Quaternary volcanicity. New Zealand Journal of Geology and Geophysics, 25, 257-274. Crook, K.A.W., and Feary, D.A., 1982, Development of New Zealand according to the fore-arc model of crustal evolution. TectonoDhysics. 87, 65-107. Feary, D.A., and Pessagno Jr.. E.A.. 1980, An Early Jurassic age for chert within the Early Cretaceous Oponae Melange (Torlesse

Supergroup), Raukumara Peninsula, New Zealand, New Zealand Journal of Geology and Geophysics. 23, 623-628. Kenny. J.A.. 1984, Stratigraphy, sedimentology and structure or the Ihungia decollement. Raukumara Peninsula, North Island, New Zealand, New Zealand Journal of Geology and Geophysics, 27, 1-19c Kingma, and Speden, I«G., 1978. Cretaceous stratigraphy: eastern North Island, in Suggate, R.P., Stevens, G.R., Te Punga, M.T., (eds.;. The Geology of New Zealand. New Zealand Government printers. Wellington, 368-375. Pirajno, F., 1979, Geology, geochemistry, and mineralisation or a spilite-keratophyre association in Cretaceous flysch. East Cape area. New Zealand, New Zealand Journal of Geology and Geophysics. 22 (3). 307-328. Speden, I.G.. 1976, Geology of Mt. Taitai. Tapuaeroa Valley, Raukumara Peninsula, New Zealand Journal o£ Geology and Geophysics, 19. 71-119. Sporli, K.B.. 1978, Mesozoic tectonics. North Island, New Zealand, Geological Society of America Bulletin, 89, 415-425. Stoneley, R., 1968, A lower Tertiary decollement on the East Coast. North Island, New Zealand, New Zealand Journal of Geology and Geophysics. 11, 128-156.

EVALUATION OF THE I-S LINE IN THE DELEGATE AREA, SOUTHEASTERN AUSTRALIA, AS A POSSIBLE TERRANE BOUNDARY R.A. Glen and A.H.M. VandenBerg Geological Survey of New South Wales, Sydney, Australia and Geological Survey of Victoria, Melbourne, Australia.

The I-S Line in southeastern Australia, separating I-type granitoids on the east from S ± I type granitoids on the west, is a Late Silurian crustal boundary separating granitoids derived from melts of different source rocks (Chappell and White 1977). S-types formed from melts of (meta) sedimentary rocks whereas I-types formed from melts of (meta) igneous rocks (Table 1). Recent work across the I-S Line in the Delegate area (Fig. 1) suggests that this major crustal boundary was already established before granitoid emplacement, and, was manifested by abrupt changes in ages and styles of deformation: fold and thrust belt style of deformation on the west contrasts with an earlier, "more strongly orogenic" style on the east. Upper crustal blocks recognized on either side of this boundary have similar stratigraphic histories and are thus not terranes in the sense of Jones et al. (1983). For this reason they are called structural zones. Stratigraphy The common lithofacies in structural zones on either side of the I-S Line include: 1. Late Ordovician turbidites (quartz rich ± feldspar ± muscovite sandstones) interbedded with horizons of black shales and chert. Further west, in the Molong Structural Zone, these turbidites pass into, and overlie submarine mafic to shoshonitic volcanics with associated volcaniclastics (Owen and Wyborn, 1979). In the Monaro Structural Zone, a shale unit interfingering with this turbidite sequence probably passes up into the earliest Silurian. 2.

Early Silurian quartz-rich sequences. These are turbiditic in the Monaro Structural Zone and pass up into turbidites from shallower water deposits in the Snowy Mountains Structural Zone.

Middle to Late Silurian sequences consisting of shallow-water sediments and felsic volcanics. Local troughs and rifts developed both east and west of the crustal boundary.

Structure * Snowy Mountains Structural Zone Thin-bedded, black shale horizons occur within the Silurian sequence west of the I-S Line. These horizons contain Ordovician (generally Eastonian, but in place Gisbornian to Bolindian) graptolites and are interpreted as fault slices of a single unit, the Warbisco Shale (Fig. 1). These horizons are structurally incompetent: they contain thrust faults, "crumpled" bedding, and the repetition and inversion of, or even gaps in, the faunal succession. We suggest that these black shale horizons are detachment zones, localizing the formation of thrusts which are inferred to be listric on a northwest to westdipping basal thrust. Each thrust sheet passes up from black shale into the Silurian Yalmy Group. Slivers of the Adaminaby beds are also included in some sheets (Fig. 1). Where not folded, these thrusts are moderately to steeply west dipping. The sole thrust in the Warbisco Shale outcrops as the Yalmy Fault and as the McLaughlan Creek Fault Zone (where probable duplex structures occur) and corresponds to the I-S Line.

Structure - Monaro Structural Zone In the Delegate area, the main deformation can be divided into two domains - a right way-up domain, and an overturned domain. The right way-up domain, in the western and eastern parts of the block is characterized by approximately NNE-trending tight, upright folding associated with an axial surface cleavage. The overturned domain, occurring in the central part of the zone, contains overturned F^ folds. Originally meridional in orientation and overturned to the west (westward verging, east dipping axial surface), these folds have been folded around NE trending upright ?F3 folds which postdate deposition of the Tombong Beds. Overturned limbs of these folds contain downward facing and D3 structures. The boundary between the overturned domain and the eastern right way up domain is inferred to be a moderately easterly dipping, NNW to NNE-trending meridional fault (?thrust) syn Dj^ in ageo D2 structures in the overturned domain may correlate with D^ structures in the right way up domains. Timing and Style of Deformation The main deformation in the Snowy Mountains Structural Zone is late Early Silurian, and occurred after the deposition of the Early Silurian Yalmy Group. We see no evidence of any deformation before the Yalmy Group although Owen and Wyborn (1979) recorded a possible 15^ - 20° discordance 100 km to the north, and a higher angle (50°) discordance in the Molong Structural Zone also around 100 km to the north. In the Monaro Structural Zone, the main deformation predates deposition of the Early Silurian Tombong Beds and is earliest Silurian with later deformation in the Middle Silurian. The style of deformation in the Snowy Mountains Structural zone appears to be that of a fold and thrust belt, with east, southeastward and south directed thrusts. In contrast, the main deformation in the Monaro Structural Zone is more strongly orogenic in style and where folds are overturned, they are westwards verging. Tectonic Implications The data described above suggest that although the I-S Line does not represent a boundary between different terranes, it does represent a cryptic crustal boundary which is reflected by different granitoid sources and by different ages and styles of deformation across it® The data also suggest it was already established at least by the earliest Silurian, before granitoid emplacement but not necessarily before granitoid melt generation. We infer that the I-S Line separates more rigid (or stable) crust on the west from more mobile, less rigid crust on the east. This is consistent with the suggestions of White et al. (1976) based on granitoid genesis that the I-S Line marks the eastern boundary of crystalline basement. In speculating further, we can look at three possible tectonic settings for the Delegate area, given general consensus that in the Ordovician it lay oceanwards of a volcanic island arc (the Molong Arc). The first model is non plate tectonic and assumes that since late Precambrian time there has been a west to east transition (since foreshortened) or an abrupt change between sedimentary and igneous middle to lower crust, with the boundary corresponding to the I-S Line. The next two models are plate tectonic and cover divergent and convergent margins with the overlying Ordovician sediments being deposited in a fore-arc basin. In the divergent model, late Precambrian to ?Cambrian rifting is envisaged with the I-S Line corresponding originally to a major normal fault separating thinned crust (?mixed with ocean floor material) to the east from

normal thickness crust to the west. That is, the I-S Line corresponds to a former hinge line. Thinned crust to the east would be more mobile than that to the west. The Ordovician turbidites were thus deposited on thick and thinned continental crust. Generation of intermediate volcanics in response to crustal thinning and melting triggered by a rising asthenosphere east of the I-S Line would provide the sources for I-type granitoids. The convergent margin models incorporate the hinge idea from the divergent margin but attribute the generation of I-type source material either to melting in the lower crust above a downgoing slab, or to the partial tectonic underplating of an exotic Precambrian to Cambrian arc (cf. Scheibner, 1983) which terminated Ordovician subduction when it came into contact with thick crust to the west. Problems exist with the "fixist" divergejit and non collisional convergent margin models and on balance the collisional convergent model may be most likely to explain the different granitoid source rocks, the onset of deformation and perhaps the generation of melt.

References Chappell, B.W., 1984, Source rocks of I- and S-type granites in the Lachlan Fold Belt, southeastern Australia, Philosophical Transactions of the Royal Society London, A.310, 693-707. Chappell, B.W., and White, Pacific Geology, 8, 173-174.

A.J.R.,

1974,

two

contrasting

granite

types.

Jones, D.C., Howell, D.G., Coney, P.J., and Monger, J.W.H., 1983, Recognition, character, and analysis of tectonostratigraphic terranes in western North America, in Hashimoto, M., and Uyeda, S, (eds) Accretion Tectonics in the Circum Pacific Regions, Terra Scientific Publishing, Tokyo. McCulloch, M.T., and Chappell, B.W., 1982, Nd isotopic characteristics of Sand I-type granites. Earth and Planetary Science Letters, 58, 51-64. Owen, M., and Wyborn, D., 1979, Geology and geochemistry of the Tantangara and Brindabella 1:100,000 sheet areas, Bureau of Mineral Resources Bulletin 204, Canberra. White, A.J.R., and Chappell, B.W., 1983, Granitoid distribution in the Lachlan Fold Belt, southeastern Geological Society of America, 159, 21-34.

types and their Australia Memoir

White, A.J.R., Williams, I.S., and Chappell, B.W., 1976. The Jindabyne thrust and its tectonic, physiographic and petrogenetic significance. Journal Geological Society of Australia, 23, 105-112. Scheibner, E., 1983, Suspect terranes in the Tasman Fold Belt System (eastern Australia), Publication Department of Geological Sciences, Stanford University, 18, 170-174.

Table

GRANITOID SOURCE DATA I-types

S-types characteristic minerals

muse ± and ± cd ± sill ± aim ± ilmenite. Several suites recognized

hbl ± muse ± bio ± magnetite. Several suites recognized

source material

from partial melt of (meta) sedimentary rocks in continental crust

from partial melt of (meta) igneous rocks in lower crust

depth of melting

-- 15 km

west of I-S Line, deeper than S-type sources. Not known east of I-S Line.

nature of source

Suite 1 from local melting of surrounding Ordovician sediments (themselves from a PE source). 1 pluton only. Suites 2-4 from pelitic + arenaceous Ca-rich sequence below outcropping sediments. Variations in IcR*s due either to different amounts of weathering or due to incorporation of < 25% mafics in source area.

Suite 1 (to East) from minimum melt of lower crust of recent mantle derivation. Suites 2-3 from non minimum melt of andesitic or dioritic material incorporated in lower crust, and derived by partial melting of (ultra) mafics below old crust.

ages of source

Suite 1 Ordovician Suites 2-4 model ages 1400-1500 Ma « ages when source separated from mantle. Sediment source 1400 to 9 in age (with ?local mixed volcanics).

Suite 1 400 Ma or only few hundred m.yr. older Suites 2-3 model ages 8001400 Ma.

References

White and Chappell (1983), McCulloch and Chappell (1982), Chappell (1984), Chappell (pers. comm. 1985).

wcsr

SNOVsi^ f^0O«4TA<\»4S 5Ta0CTua^U ZO^S

HOW^RO

'^ZOvJC

eoT.

'iKiovAJM K00vjT\va5 ST/eucTOOA-t- Z-OvJC FIGURE !• Legend. 9a - Adaminaby beds (turbidites)r Late Ordovician; 0w Warbisco Shale (black shale), Late Ordovician; 9k- Akuna Formation, (shale)/ Late Ordovician to earliest Silurian; St - Tombong beds (turbidites)# Early Silurian; Sy - Yalmy Group (shallow-water to turbidites), Early Silurian? Ds - Snowy River Volcanics (felsic volcanics), Early Devonian. S-type granitoids crosses I-type granitoidsr diagonal crosses x. Chert beds in 9a — ^ . F^ folds in overturned domainr Monaro Zone F^ folds in right way up domainr Monaro Zone Faults .Sections show possible interpretation.

PALEOMAGNETIC RESULTS FROM UPPER CRETACEOUS VOLCANIC ROCKS IN THE TOGIAK TERRANE, SOUTHWESTERN ALASKA: EVIDENCE FOR ANTICLOCKWISE ROTATION Brian R. Globerman and Robert S. Coe Earth Sciences Board, University oi Cali-fornia Santa Crus, C a l i K 95064 USA INTRODUCTION Compilation of paleofliagnetic data from rocks of Late Cretaceous and early Tertiary age in southern and central Alaska enables us to distinguish two groups of tectonostratigraphic terranes, on the basis of their inferred paleolatitudes with respect to the ''stable" North American continent. Paleolatitudes determined from Group I, which comprises the southern Alaska terranes of Prince William (Plumley et a l , 1983), Chugach (Gromm^ and Hillhouse, 1981), Wrangellia (MacColl Ridge Fm- Panuska, 1983) and northeastern Peninsular (Chickaloon Fm-Stone, 1983) are anomalously shallow. Group II, on the other hand, comprising the Nixon Fork-Ruby-Di11inger-Minchumina (Plumley, 1984), Wrangellia (lower Tertiary volcanic rocks of Talkeetna Mountains- Hillhouse and Gromm^, 1983), and southwestern Peninsular (lower Paleocene lava flows of Lake Clark area- Thrupp and Coe, 1985) terranes, along with the Cantwell Basin in the Mount McKinley area (Hillhouse and Gromm^, 1982) and the Lower Yukon River in western Alaska (Globerman et a l , 1983), give paleolatitudes which are essentially concordant within the 95 percent confidence limits. Strict interpretation of these data implies about 2000 km of relative motion between the two groups, and in places conflicts with the geologic evidence for both interand intra-terrane linkages (e.g. Jones et a l , 1984). This investigation of volcanic rocks of Late Cretaceous to early Paleocene age in the Togiak terrane, informally termed the B r i s t o l Bay v o l c a n i c s e r i e s ^ addresses the first problem by providing tighter constraints on the location of the boundary between the two groups. This is possible since the northern Bristol Bay region, where the paleomagnetic study was undertaken, lies in the broad area between the Koyukuk terrane and Upper Cretaceous post-accretionary deposits to the north, and the Peninsular terrane south of Bristol Bay (Jones et al , 1984). A second question concerns the steeply arcuate topographic pattern of the Alaska Range, Kuskokwim Mountains, and Denali, Hiy^on Fork - Iditarod, and Kaltag faults west of longitude 147 degrees W. Is this pattern an initial paleogeographic feature, or does it represent the effects of a secondary process such as regional oroclinal bending, as suggested by Carey (1955) and Freeland and Dietz (1973)? If bending occurred in post- latest Cretaceous time, it should be detectable paleomagnetical1y in the Bristol Bay volcanic series. GEOLOGY AND REGIONAL SETTING At the southwesternmost extremity of the Kuskokwim Mountains magmatic belt (Wallace and Engebretson, 1984), a thick sequence of basaltic-andesite lava flows and associated pyroclastic deposits are exposed on Hagemeister, Crooked, and Summit Islands (Hoare and Coonrad, 1978). The Kuskokwim Mountains belt overlaps several major tectonostratigraphic terranes such as Goodnews-Togiak-Tikchik, Innoko, Ruby-Nixon Fork-Di1 linger-Minchumina, Nyack, Kilbuck, and Yukon-Tanana; along with the Kuskokwim Group, an Upper Cretaceous post-accretionary clastic sequence (Decker, 1984). Owing to these ties.

paleoinagnetic data from the Bristol Bay volcanic series should be applicable to a broader region of southwestern Alaska. Approximately 2500 meters of volcanic section on Hagemeister Island, and 1750 meters on Crooked Island, were examined in detail. Hagemeister Island is characterized by its steeply dipping lava flows and locally thick volcanogenic sedimentary interbeds, whereas Crooked Island comprises a larger proportion of pyroclastic deposits which are nearly flat-lying. Three samples from Hagemeister Island give an average whole-rock K/Ar age of 67.7+ 3 Ma. Initial Sr isotopic ratios from three samples range from 0.7037 - 0.7041, suggesting that this part o+ the Togiak terrane is not underlain by Precambrian continental basement. Majorand trace-elements were analyzed by automated x-ray fluorescence in 15 and 11 samples, respectively. These lava flows are basaltic to andesitic since Si02 concentrations range from 51 to 57 wt.-/.. All have low Fe0*/I1g0 ratios irrespective of Si02 concentrations, TiQ2 <1.75 wt.7., and K20 <ESiQ2 X 0.145) 5.1353; thus the suite is strongly calcalkaline according to Gill's (1981) criteria. Average ratios of Ba/La= 42, La/Nb= 3.5, and Ba/Ta*(Ta approximated by Nb/16)= 2300 in the Bristol Bay volcanic suite are characteristic of orogenic andesites associated with either convergent margins or continent-continent collision zones (Gill, 1981, 1982). Origin of the suite in a backarc or within-plate (e.g. hotspot) setting is not supported by these geochemical data. Hence the Kuskokwim Mountains belt is interpreted as a Late Cretaceous to early Tertiary volcanic arc. However, several problems arise from this seemingly straight-forward interpretation. For one, the convergence angle between the Kuskokwim belt and the Kula plate (relative to fixed North America) would have been too oblique (<30 degrees, in present coordinates) to have produced the voluminous quantities of orogenic magma (Wallace and Engebretson, 1984). Furthermore, Late Cretaceous to early Tertiary volcanism and plutonic complexes which are probably associated with the Kuskokwim Mountains belt extend over a broad area from the Alaska Range to the Arctic Circle, indicating an arc width of about 900 km. Given the presumed obliquity of convergence, it is difficult to explain this broad, diffuse magmatism as a result of a shallow subduction angle related to high rates of convergence. This hypothesis has been invoked to explain the occurrence of early Tertiary volcanism some 1000 km inboard of the plate boundary in the southwestern USA (e.g. Coney and Reynolds, 1977), but seems less tenable for western Alaska. PALEOMAGNETIC

RESULTS

About 700 samples were collected from 91 subaerial volcanic flows and pyroclastic deposits (collectively termed sites) on Hagemeister, Crooked, and Summit Islands for a detailed paleomagnetic study. Both alternating field (AF) and thermal stepwise demagnetization experiments were analyzed with vector diagrams to isolate a characteristic component. AF and thermal methods were equally successful, except for a small fraction of samples in which thermal demagnetization was necessary to remove a secondary component carried by hematite. Since the characteristic component carried by magnetite and hematite was the same, it is most likely that hematite originated by autooxidation processes during primary cooling. The mean directions of six coarse-grained tuffaceous deposits and one sedimentary interbed were considered unreliable and were discarded. In addition, the results of ten lava flows with strongly discrepant directions were eliminated. These discrepant directions show strong serial correlation and probably represent geomagnetic excursions. The remaining 74 sites all have unambiguous normal polarity. However, the presence of thick well-laminated sedimentary interbeds and the probable geomagnetic excursion within the Hagemeister Island section suggest

that the flows span a p e r i o d s u f f i c i e n t l y long to o b t a i n a r e p r e s e n t i ti ve tiine a v e r a g e of the a n c i e n t f i e l d . T h e s e sedi/nentary i n t e r b e d s , along with b a k e d c o n t a c t s and t u f f a c e o u s i n t e r c a l a t i o n s , p r o v i d e good c o n t r o l on b e d d i n g a t t i tudes. Rotation of the flow (nean d i r e c t i o n s into s t r a t i g r a p h i c coordinates using these attitudes y i e l d s a s t r o n g l y p o s i t i v e fold test (fIcFadden and Jones, 1981) t h a t is s i g n i f i c a n t at 95 p e r c e n t c o n f i d e n c e , s u g g e s t i n g that the c h a r a c t e r i s t i c c o m p o n e n t is p r i m a r y . The i n c l i n a t i o n of the mean c h a r a c teristic d i r e c t i o n of the 74 s i t e s is c l o s e to the e x p e c t e d v a l u e for latest Cretaceous t i m e , but the d e c l i n a t i o n is s i g n i f i c a n t l y r o t a t e d in a counterclockwise senceHence l i t t l e n o r t h w a r d d i s p l a c e m e n t (9+ 7 d e g r e e s ) with respect to N o r t h A m e r i c a is i n d i c a t e d , but t h e r e is a s t r o n g indication of c o u n t e r c l o c k w i s e r o t a t i o n (43+ 23 d e g r e e s ) . B e c a u s e of the o v e r l a p s p r e v i o u s l y d i s c u s s e d , the e s s e n t i a l l y c o n c o r d a n t p a l e o l a t i t u d e of t h e s o u t h e r n part of the Kuskokwim M o u n t a i n s belt p r o b a b l y a p p l i e s to a l a r g e p o r t i o n of western A l a s k a as w e l l . The e n i g m a t i c b o u n d a r y b e t w e e n Group I, whose Late Cretaceous to e a r l y T e r t i a r y p a l e o l a t i t u d e s are s i g n i f i c a n t l y s h a l l o w , and Group 1 1 , w h o s e p a l e o l a t i t u d e s are not a p p r e c i a b l y d i s c o r d a n t , must lie s o m e w h e r e to the south of the s t u d y a r e a , a l t h o u g h g e o l o g i c e x p r e s s i o n of such a major s t r u c t u r e r e m a i n s u n d o c u m e n t e d (Csejtey et a l , 1982). Coastwise translation along one or s e v e r a l s t r i k e slip f a u l t s w o u l d be a p o s s i b l e e x p l a n a t i o n (ecg M o o r e et a l , 1 9 8 3 ) , p a r t i c u l a r l y in light of r e c e n t g e o l o g i c e v i d e n c e for >950 km d e x t r a l displacement on the T i n t i n a T r e n c h and N o r t h e r n Rocky Mountain T r e n c h fault s y s t e m of the n o r t h e r n C a n a d i a n C o r d i l l e r a in L a t e C r e t a c e o u s to early Tertiary time (Gabrielse, 1985). However, if the s o u t h e r n m a r g i n of Alaska had its p r e s e n t a r c u a t e p a t t e r n d u r i n g t h i s t i m e , the kinematics of e m p l a c e m e n t of G r o u p I t e r r a n e s w o u l d h a v e been e x t r e m e l y c o m p l i c a t e d . TECTONIC

INTERPRETATION

The 43+ 23 d e g r e e s of c o u n t e r c l o c k w i s e r o t a t i o n in the B r i s t o l Bay volcanic series may s i m p l y be a l o c a l i z e d r e s p o n s e to d e x t r a l m o t i o n along the western segment of the D e n a l i f a u l t system (Togiak-Tikchik, Holitna, and F a r e w e l l f a u l t s ) , w h i c h t r a n s e c t s the a r e a (Hoare and C o o n r a d , 1978; G r a n t z , 1966). H o w e v e r , s i n c e c l o c k w i s e r o t a t i o n s are m o r e c h a r a c t e r i s t i c of d e x t r a l shear regimes (e.g. B e c k , 1 9 7 6 ) , and c o u n t e r c l o c k w i s e r o t a t i o n s of early Tertiary volcanic s u i t e s are o b s e r v e d e l s e w h e r e in s o u t h w e s t e r n and central A l a s k a (e.g. L a k e C l a r k r e g i o n - T h r u p p and C o e , 1985; Talkeetna MountainsHillhouse and Gromm^, 1983; C a n t w e l l B a s i n - H i l l h o u s e and Gromm^, 1982), regional rotation of w e s t e r n A l a s k a is a m o r e likely tectonic scenario. Opening of the C a n a d a Basin c o u l d not h a v e been the driving mechanism, as suggested in F r e e l a n d and D i e t z ' (1973) m o d e l for the r o t a t i o n history of A l a s k a , s i n c e c o m p l e t i o n of the r i f t i n g e v e n t p r e - d a t e s e r u p t i o n of the volcanic suites. I n s t e a d G r a n t z ' (1966) s u g g e s t i o n that E u r a s i a n - N o r t h A m e r ican p l a t e c o n v e r g e n c e in latest C r e t a c e o u s and e a r l y T e r t i a r y t i m e provided the m o t i v e f o r c e for r o t a t i o n and a s s o c i a t e d d e x t r a l f a u l t i n g is m o r e consistent with t h e s e p a l e o m a g n e t i c r e s u l t s . Geologic evidence supports east-west c o m p r e s s i o n in w e s t e r n A l a s k a (Patton and T a i l l e u r , 1 9 7 7 ) , and p l a t e motion models (Engebretson, 1982; Harbert et a l , 1985) s u g g e s t E u r a s i a n - North A m e r i c a n c o n v e r g e n c e d u r i n g L a t e C r e t a c e o u s t h r o u g h late P a l e o c e n e t i m e . The m a x i m u m c o n v e r g e n c e r a t e o c c u r r e d b e t w e e n a b o u t 70 - 50 Ma b . p . , i ^ e . during or soon a f t e r the p e r i o d in w h i c h the A l a s k a n v o l c a n i c s u i t e s with discordant declinations were e r u p t e d . H e n c e it is s u g g e s t e d that r e g i o n a l c o u n t e r c l o c k wise rotation o c c u r r e d in r e s p o n s e to t h i s convergencec The pre-rotation geometry of the s o u t h e r n A l a s k a n m a r g i n w o u l d h a v e had a s t r a i g h t e r n o r t h w e s t t r e n d , thus involving f e w e r m e c h a n i c a l d i f f i c u l t i e s in e m p l a c i n g Group I

tsrranes along strike slip faultCs) parallel to the coastline. Furtherluore, removal of about 50 degrees of counterclockwise rotation re-orients the axis of the Kuskokwim Mountains magmatic belt such that the convergence angle between Kula and North American plates would have been considerably greater, and consistent with convergence angles noted at contemporary volcanic arc settings (Gill, 1981). Finally, it is suggested that rapid Eurasian - North American convergence as well may have been a significant factor in the widespread occurrence of calcalkaline volcanic and plutonic complexes in western interior Alaska. A possibly analogous setting is the continent-continent collision zone in eastern Turkey, Soviet Armenia and western Iran, where appreciable post-collisional, calcalkaline magmatism is reported (Gill, 1982; Zhou, 1985). REFERENCES Beck, M.E., Jr., 1976, Discordant paleomagnetic pole positions as evidence of regional shear in the western Cordillera of North America: American Journal of Science, 276, 694-712. Carey, S.W., 1955, The orocline concept in geotectonics: Proceedings of the Royal Society of Tasmania, 89, 255-288. Coney, P.J., and Reynolds, S.J., 1977, Cordilleran Benioff zones: Nature, 270, 403-406. Csejtey, B., Jr., Cox, D.P., Evarts, R.C., Strieker, G.D., and Foster, H.L., 1982, The Cenozoic Denali fault system and the Cretaceous accretionary development of southern Alaska: Journal of Geophysical Research, 87, 3741-3754. Decker, J., 1984, The Kuskokwim Group: A post-accretionary successor basin in southwest Alaska: Geological Society of America Abstracts with Programs, 16, 277. Engebretson, D.C., 1982, Relative motions between oceanic and continental plates in the Pacific Basin: CPh.D. thesis! Stanford, Stanford University, 211 PFreeland, G.L., and Dietz, R.S., 1973, Rotation history of Alaska tectonic blocks: Tectonophysics, 18, 379-389. Gabrielse, H, 1985, Major dextral transcurrent displacements along the Northern Rocky Mountain Trench and related lineaments in north-central British Columbia: Geological Society of America Bulletin, 96, 1-14. Gill, J.B., 1981, Grogenic andesites and plate tectonics: New York, SpringerVerlag, 390 p. Gill, J.B., 1982, Mountain building and volcanism, in Hsu, K.J., ed., Mountain building processes: London, Academic Press, 13-17. Globerman, B.R., Coe, R.S., Hoare, J.M., and Decker, J., 1983, Paleomagnetism of Lower Cretaceous tuffs from the Yukon-Kuskokwim delta region, western Alaska: Nature, 305, 516-520.

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G e o l o g i c a l O b s e r v a t o r y of C o l u m b i a P a l i s a d e s , New York U o S . A .

Kent University

West A n t a r c t i c a and New Z e a l a n d are the m o s t d i f f i c u l t p a r t s of G o n d w a n a l a n d to r e c o n s t r u c t . This is p a r t l y due to the e x t e n s i v e t e c t o n i s m that o c c u r r e d along the m a r g i n of the P a c i f i c O c e a n during and since f r a g m e n t a t i o n of the s u p e r c o n t i n e n t . It is p a r t l y a result of the e x t e n s i v e ice cover in A n t a r c t i c a . Yet, as h a s been p o i n t e d out e l s e w h e r e , the t e c t o n i c e v o l u t i o n of this region has i m p o r t a n t i m p l i c a t i o n s for u n d e r s t a n d i n g of g l o b a l p l a t e i n t e r a c t i o n , p a l e o climate and p a l e o b i o g e o g r a p h y . It was with this in mind that the joint U K - U S West A n t a r c t i c T e c t o n i c s P r o j e c t , was i n i t i a t e d (see a b s t r a c t by D a l z i e l , this v o l u m e ) • P a l e o m a g n e t i c s t u d i e s are clearly an e s s e n t i a l part of such a p r o j e c t , e s p e c i a l l y in the light of e v i d e n c e that some g e o l o g i c t e r r a n e s b o r d e r i n g the Pacific O c e a n h a v e been d i s p l a c e d large distances. E x i s t i n g p a l e o m a g n e t i c data suggest that the four m a j o r crustal b l o c k s of West A n t a r c t i c a h a v e been a d j a c e n t to the East A n t a r c t i c c r a t o n at least since the Late J u r a s s i c to Early C r e t a c e o u s (for review see D a l z i e l and G r u n o w , P r o c e e d i n g s of 2nd C i r c u m - P a c i f i c Terranes Conference, Stanford, 1982). The data base is not e x t e n s i v e , h o w e v e r , and there are i n d i c a t i o n s from " o v e r l a p " in G o n d w a n a l a n d r e c o n s t r u c t i o n s and from g e o l o g i c c o r r e l a t i o n as w e l l as from some of the p a l e o m a g n e t i c r e s u l t s , that limited r e l a t i v e m o t i o n of the blocks and of East A n t a r c t i c a h a s o c c u r r e d (for r e v i e w see D a l z i e l and E l l i o t , T e c t o n i c s , V o l . 1, N o . 1 1 9 8 2 ) . Early J u r a s s i c r a d i o m e t r i c ages r e p o r t e d from g r a n i t e s in n u n a t a k s south of the E l l s w o r t h M o u n t a i n s gave cause for o p t i m i s m that p a l e o m a g n e t i c p o l e s m i g h t be o b t a i n e d for a c r i t i c a l time p e r i o d p r i o r to G o n d w a n a l a n d b r e a k - u p . At this time A n t a r c t i c a was in a m i d - l a t i t u d e p o s i t i o n and h e n c e s u b s e q u e n t r o t a t i o n s should be r e s o l v e d m o r e readily than with y o u n g e r p o l e s r e f l e c t i n g a very high l a t i t u d e . During the 1 9 8 3 - 8 4 and 1984-85 field seasons, t h e r e f o r e , two of us (AMG and IWDD) m a d e e x t e n s i v e c o l l e c t i o n s for p a l e o m a g n e t i c studies in the E l l s w o r t h M o u n t a i n s - W h i t m o r e M o u n t a i n s crustal b l o c k , the a d j o i n i n g Thiel M o u n t a i n s (part of the T r a n s a n t a r c t i c M o u n t a i n s ) , and the T h u r s t o n I s l a n d - E i g h t Coast crustal b l o c k . The c o l l e c t i o n sites were c h o s e n on the b a s i s of d e t a i l e d field o b s e r v a t i o n s by Bryan Storey (British A n t a r c t i c S u r v e y ) and Ian D a l z i e l . The s a m p l e s w e r e studied in the p a l e o m a g n e t i c l a b o r a t o r y at L a m o n t - D o h e r t y G e o l o g i c a l O b s e r v a t o r y by G r u n o w and K e n t . A l t h o u g h few of the exposed rocks are ideal for p a l e o m a g n e t i c study, p r i m a r i l y due to a b s e n c e of p a l e o h o r i z o n t a l m a r k e r s , some of the r e s u l t s a l r e a d y o b t a i n e d from our first s e a s o n ' s c o l l e c t i n g in the E l l s w o r t h M o u n t a i n W h i t m o r e M o u n t a i n s b l o c k , do p r o v i d e new i n s i g h t s into the t e c t o n i c e v o l u t i o n of West A n t a r c t i c a and h e n c e of the P a c i f i c m a r g i n of Gondwanaland. It is t h e r e f o r e a p p r o p r i a t e to p r e s e n t at the 3rd C i r c u m - P a c i f i c T e r r a n e s C o n f e r e n c e a s u m m a r y of these r e s u l t s and our joint i n t e r p r e t a t i o n of t h e m .

102

The details will be published elsewhere at a later date. At the time of the abstract deadline the preliminary laboratory results have yet to be fully discussed by the authors. The preliminary results will be outlined in a paper to be published in the Proceedings of the Sixth Gondwana Conference (Ohio State University, Columbus, Ohio, August 1985; American Geophysical Union).

103

DEFORMATION ASSOCIATED WITH THE ACCRETION OF THE GYMPIE TERRANE IN EASTERN AUSTRALIA

H.J. Harrington & R.J. Korsch

Bureau of Mineral Resources, Canberra, Australia

The Gympie terrane lies along the eastern edge of the New England-Yarrol Orogen, in southeastern Queensland (Fig. 1 and BMR, 1976). The whole terrane is probably part of a volcanic arc: its lithology is dominantly volcanogenic, and one major unit consists of probable island arc tholeiitic basalts. It has been interpreted as an exotic terrane (Harrington, 1983, 1985), which possibly arrived from the north, and extended to New Zealand, where it forms the Brook Street, Maitai, and Caples terranes (Fig. 2). It is separated from accretionary wedge assemblages to the west by the Widgee serpentinite belt. The older rocks of the Gympie terrane are unfossiliferous, but possibly Carboniferous, and the younger contain Permian and Early Triassic marine faunas. The time of completion of docking and accretion can be defined within narrow limits as Mid Triassic, (Anisian and Ladinian), about 235-240 Ma on the Harland scale. This close dating is possible because the youngest rock unit, the Brooweena Formation, is Early Triassic and was probably deposited before accretion. The suture and the Widgee serpentinite belt are crossed by a stitching pluton, the Station Creek Adamellite (Day et^ al^-, 1983), which has an isotopic age of 231-236 Ma, late Mid Triassic (Ladinian). In addition, the terrane and the suture are covered near Brisbane by the early Late Triassic Ipswich Basin. The accretion of the Gympie terrane can be correlated with the end of sedimentation in the major coal-bearing basins of eastern Australia, some of which also contain oil and gas fields. One of these, the Sydney-Bowen Basin is 1750 km long and formed in the latest Carboniferous or earliest Permian, possibly as a set of linked transform basins along and near the Mooki Fault System (Harrington & Korsch, 1985). Strangely, it shows very few effects of the major mid Permian (Kungurian) oroclinal bending of the New England-Yarrol Orogen (Korsch & Harrington, 1985). In the Late Permian, the Sydney-Bowen Basin was the site of widespread deposition of coal measures, in which nearly all Australia's black coal mines are now located. Deposition of the overlying red beds of Moolayember Formation and its correlatives continued into the Mid Triassic and ceased with deformation and erosion, which we correlate with the accretion of the Gympie terrane. There is a major unconformity between the Sydney-Bowen Basin and the Late Triassic basal beds of the overlying Great Artesian Basin (Surat Basin). There was an essentially identical history in the Cooper, Galilee, Pedirka and Arckaringa Basins, which extend almost to the middle of Australia. The basins are similar to the North Sea rifts (Fig. 3) and cover a similar area. Deposition in them commenced in the Late Carboniferous, was interrupted locally at the time of the mid Permian oroclinal bending, and then continued until Mid Triassic deformation and erosion. These basins also are covered unconformably by the Great Artesian Basin (Eromanga Basin). It seems that the accretion of the Gympie terrane was associated with deformation and the destruction of major sedimentary basins for a distance of about 1300 km into the Australian craton from the suture. In other words, there seems to be a direct correlation between the accretion of a terrane at the edge

104

Fig. 1. Locations of sedimentary basins that were deformed and terminated when the Gympie terrane accreted to the New England-Yarrol Orogen in the Mid Triassic.

105

Fig. 2. The Australasian region before the opening of the Tasman Sea in the Late Cretaceous. The arrow joins the Carboniferous, Permian and Early Triassic Gytnpie terrane with the similar Brook Street and Maitai terranes in New Zealand. The Ordovician Ballarat terrane in Australia is remarkably similar to the Golden Bay and Buller terranes in New Zealand (cross-hatching). Fyfe's Line in New Zealand separates the Golden Bay and Brook Street terranes and is probably to be •correlated with the Mid Triassic Widgee suture at the western edge of the Gympie terrane. Fyfe's Line is also known as the Median Tectonic Line and Great Tectonic Line. It is crossed in the south by Triassic stitching plutons. The Brook Street terrane has been traced far out to sea by the Stokes Magnetic Anomaly System. Sketch from Harrington (1983).

106

CARTOON

NORTH

COMPARISON

QUEENSLAND

SEA

CAINOZOIC

SUBSIDENCE, EVERSION

CRETACEOUS

SUBSIDENCE

JURASSIC

RIFTS, MARINE

TRIASSIC

RIFTS, SALT

PERMIAN CARBONIF. DEVONIAN

CALEDONIAN FOLD BELTS

COOPER - GALILEE "N/" V

COALS RIFTS, LST. OLD RED SST.

SILURIAN ORDOVICIAN

EROMANGA

RIFTS. RED BEDS V

SUBSIDENCE, EROSION

ADAVALE -DRUMMOND

THOMSON - LACHLAN FOLD BELTS

CAMBRIAN Fig. 3. A schematic comparisoa of the North Sea Basin with the Great Artesian Basin (Eromanga Basin sub-section) and its infrabasins. All these basins contain oil and gas fields. The local hiatus in the middle Permian in the Cooper and Galilee basins can be correlated with an oroclinal folding event in the New England-Yarrol Orogen. The Mid Triassic unconformity is sub-continental in extent and we correlate it with the accretion of the Gympie terrane.

107

of the craton in the Mid Triassic and significant intracratonic faulting and macroscopic folding in a region of sub-continental size. There is a great contrast between the mid Permian and Mid Triassic deformations. The mid Permian oroclines were almost confined to an accretionary wedge. This long narrow wedge was detached from the craton and bent like a crumpled ribbon. Similar detachment and crumpling of fore-arc terranes have been recognised independently in the Mediterranean region by W.J.M. Van der Linden of Utrecht (personal communication: unpublished manuscript) and he uses the term ribbon tectonics for the detachment and bending. The deformation is intense in the New England-Yarrol ribbon, but weak in the adjoining cratonc By contrast, the Gympie terrane was also a long narrow ribbon, but it was a volcanic arc and does not show penetrative mesoscopic deformation or large-scale oroclinal bending. During its accretion the deformation in the craton was significant although not intense. Major basins came to an end in the Mid Triassic and were then eroded until late in the Triassic. An attempt can be made to explain these differences. The Gympie volcanic arc was a thick ribbon with deep roots in the lithosphereo The New England ribbon was a thin semi-viscous or semi-brittle layer consisting almost entirely of an accretionary wedge bounded underneath by the top of the subducted ocean floor, and bounded to the west by the Peel-Yarrol Fault System and serpentinite belt. The top of the subduction zone became a ready-made detachment surface. When relative motion changed from subduction to strike-slip, the New England ribbon could slide over the detachment surface, pull away from the craton, and crumple oroclinally. Similar oroclines can be recognised elsewhere, at a range of scales, as for example in the Torlesse terrane in New Zealand, and pose many new problems. These include the evolution of basins in oroclinal bends (several kinds) and the mechanics of bending around sub-vertical axes of a complex but relatively thin accretionary wedge of duplex stacks. References Bureau of Mineral Resources, 1976, Geological map of Australia, ls2 500 000, Bureau of Mineral Resources, Geology & Geophysics, Canberra, 4 sheets. Day, R.Wo, Whitaker, W.G., Murray, C.G., Wilson, I.H., & Grimes, K.G., 1983, Queensland Geology: A companion volume to the 1:2 500 000 scale geological map (1975), Geological Survey of Queensland Publication 383. Harrington, H.J., 1982, Tectonics and the Sydney Basin, Proceedings of the Sixteenth Symposium on Advances in the Study of the Sydney Basin, University of Newcastle, 15-29. Harrington, HoJ., 1983, Correlation of the Permian and Triassic Gympie terrane of Queensland with the Brook Street and Maitai terranes of New Zealand, Geological Society of Australia, Queensland Division, Permian Geology of Queensland, 431—436. Harrington, H.J., 1985, Terrane analysis and the Sydney Basin, Proceedings of the Nineteenth Symposium on Advances in the Study of the Sydney Basin, University of Newcastle, 1-6o Harrington, H.Jo, & Korsch, R.J., 1985, Tectonic model for the Devonian to middle Permian of the New England Orogen, Australian Journal of Earth Sciences, 32, in press. Korsch, R.J., & Harrington, J.H., 1985, Fragmentation and deformation of terranes in the New England Orogen, eastern Australia, this volume.

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EVOLUTION OF THE BROKEN RIVER PROVINCE

A MID-PALAEOZOIC ACTIVE MARGIN

TERRANE OF THE NORTHERN TASMAN OROGENIC ZONE R.A. Henderson Department of Geology, James Cook University, Townsville, Australia. The Broken River Province is an enigmatic feature of the northern Tasman Orogenic Zone. It contains a thick, heterogeneous, folded sedimentary assemblage of Early Silurian to Early Carboniferous age, occupying a prominent SW-trending indentation into a cratonised terrane of Proterozoic and Early Palaeozoic rocks. Its trend is at variance with the general structural grain of the northern Tasman Orogenic Zone. Two tectonostratigraphic divisions are apparent for it. The Graveyard Creek Subprovince, with an ordered sequence of heterogeneous, generally well-dated units occupies the southwestern part. It is of miogeosynclinal character. The larger Camel Creek Subprovince to the northeast is a Siluro-Devonian flysch terrane for which a proper stratigraphy has yet to be resolved and age control is poor. Its characteristics are that of a eugeosyncline. The province is divided from older terranes to the west and south by the Burdekin River and Clarke River Fault Zones respectively and the two subprovinces are separated by the Gray creek Fault Zone. To the north, the Broken River Province merges with the Hodgkinson Province, a mid-Palaeozoic terrane of like content and inferred history. The miogeosynclinal-eugeosynclinal pair represented by the Graveyard Creek Subprovince and Camel Creek Subprovince lie to the east of SiluroDevonian granitoids which are regarded as the remnants of a contemporaneous magmatic arc. The entire assemblage is interpreted as the product of a Siluro-Devonian oblique-slip active margin similar to that now recognised for the eastern North Island of New Zealand. The Camel Creek Subprovince is regarded as a subduction complex, with accretionary thrusting and rotation of flysch packets in a stress regime of pure shear having prevailed in its outboard parts. A simple shear stress regime, characterised by strike-slip faulting is thought to have prevailed in its inner parts. Tight folding on subvertical axes, associated with a weak slaty cleavage subparallel to bedding, is characteristic of the subprovince and is thought to have been induced under this stress regime. The Gray Creek Fault Zone is interpreted as a major strike-slip fracture of Siluro-Devonian age, dividing the subduction complex from the miogeosynclinal Graveyard Creek Subprovince which is regarded here as a forearc basin. In latest Devonian (Famennian) time the assemblage suffered severe compressional stress, thought to relate to an increased rate of plate motion and a change in its orientation. Failure occurred within the craton and a major thrust transported basement eastwards to cover much of the forearc basin. The southwestern segment of the Burdekin River Fault Zone formed as a tear-fault boiinding the thrust sheet to the south. Development of the Clarke River Fault Zone, regarded as a transform fault offsetting the continental margin in a sinistral sense, is also inferred for Famennian time. Northeasterlytrending folds, characteristic of the Graveyard Creek Subprovince and recognised as a discrete fold phase in the southern part of the Camel Creek Subprovince, are thought to have been induced by simple shear adjacent to the Clarke River Fault Zone and to wedging of the continental margin sedimentary assemblage between basement blocks.

109

Oblique siibduction is thought to have resumed in Early Carboniferous (Tournaisian) time. Two previously separate structures, the thrust sheet - bounding tear fault of Famennian age and part of the strike-slip fault which had formerly separated the accretionary complex and forearc basin, united as a single strike-slip fracture. Movement on this structure, the Burdekin River Fault as currently recognised, sponsored the development of a deep Early Carboniferous pull-apart molasse basin which retained stratigraphic and structural continuity with the forearc basin that preceded it. Extensive shallow Early Carboniferous molasse basins developed in response to vertical movements on the Gray Creek and Clarke River Fault Zones, With sxibsequent vertical movement coupled with erosion, the Burdekin River Fault has come to mark the eastern limit of overthrust basement.

110

GBODYNAMICS OF CONTINENTAL GPCWTH AND TOE NEW CIRCUM-PACIFIC TERRANE MAP

D, G. Hcwell and D. L. Jones U.S. Geological Survey Menlo Park, California 94025

Growth of continental crust through accretion of terranes is a fundamental geodynamic process and is a principal element in the origin of all continental masses. Little is known about Precambrian and early Paleozoic accreticair but major late Paleozoic and Mesozoic accretionary events in the circum-Pacific regiai are reasonably well portrayed on our new 1:17,000,000 scale terrane map. On this map the composition of terranes is generalized into five categories and eadi type is shown either as a modem example (oceanic plateau) or an accreted ancient counterpart. Vie also distinguish docking times as either before or after the breakup of Pangaea. The five types of terrane consist of: (1) oceanic rocks including oceanic crust and seamounts; (2) oceanic rocks mixed with continent-derived sediments; (3) oceanic volcanic island arcs; (4) volcanic arcs with probable post-Precambrian continentalized basement; and (5) continental fragments with Precambrian basement. The oceanic crust of Panthalassa has been subducted since the breakup of Pangaea. But how many other global oceans have been created and destroyed in geologic history? At the present there are 56,000 km of globe-girdling midocean spreading ridges. Given an average spreading half-rate of 5 on/yr, these spreding ridges can replace the Earth's oceanic crust in approximately 115 Ma. Subduction and mid-ocean spreading related to plate tectonics are known to have operated in the Precambrian. Because the kinenatics of the lithosfiiere may have slowed down as the heat flux of the earth decreased, we can speculate that at least 20, but probably more, world oceans have come and gone since Archean time. Compositional and tenporal charactaristics of rock units in the circumPacific region suggest that Proterozoic cratonic regions can be subdivided into orogenic belts representing past episodes of terrane accretion. The "cratons" created by accretion processes throughout the Proterozoic were assanbled in the Permian and Triassic time to form the giant amalgam, Pangaea. In Asia, the sequence of accretion of cratonic blocks indicates that the Siberian platform acted as a backstop against which early and middle Paleozoic

111

accretions produced the Baikalian orogeny. The locus of accretion then stepped southward to include the late Paleozoic accretions of the Sino-Korean craton and the Tarim craton. For areas farther south, we infer the Triassic accretion of the Yangtze craton and the middle Mesozoic accretion of the Malaysian craton. India began accreting in the early Tertiary, but that process is still active today and is evidenced by the rising Himlayas and the crustal dispersions within China. Surrounding the cratonic inassifs on our inap we depict Paleozoic belts of accretiai. These are best demonstrated by the Baikalian (Asia), Lachlan (Australia), and Appalachian (Nbrth America) fold belts. When terranes are grouped into broad belts defined by the age of accretion, a pattern emerges. In eastern Nbrth America, the Archean regions are surrounded by 1.8-1.7 Gaotogenic belts. To the Southeast is the 1.1- to 1.0-Ga Greenville belt, followed by belts representii^ early to middle- and late Paleozoic accretion. In general, locations of subsequent rifts lie within these accretionary belts; in eastern m r t h America, such rifting characterized the opening and closing of the lapatus Ocean (Paleozoic time) and the opening of the Gulf of Mexico (middle Mesozoic time). These successive belts of accretiai, each representing 100-300 Ma, do not parallel the continental margin in western North America. Instead, we see that they are truncated by the northwest-trending younger accretionary terranes of the Cordillera. Geologic evidence points to a major rifting event at 550 to 700 Ma. A minor example of a late Paleozoic accretionary episode is the Antler orogeny that affected a small regiai centered in the State of Nevada. For the most part, however, the entire Cordillera, extending from northern South America to eastern Russia, represents Mesozoic accretions. Besides accretion, dispersive processes also control continental shapes. Paleozoic orogenic bodies that have since becone allochthonous include the Mayan terrane, which rifted away from the Texas area when the Gulf of Mexico spreading canmenced in the Middle-Jurassic, and the Tuhua terrane of New Zealand, which consists of Paleozoic and Mesozoic rocks that accreted onto the Gondwana margin in the middle Mesozoic and subsequently rifted off in the middle Cretaceous. Phanerozoic volcanic arcs incude both the Andean type (formed on sialic crust) and the Mariana type (formed on simatic crust). Oceanic island arcs are known to be mobile, but paleonagnetic data from the Cordillera of NOrth America indicate that Andean-type arcs are also vulnerable to large-scale displaconents: batholithic bodies of western NOrth America have undergone significant latitudinal and (or) rotational post-intrusive displacements. In the Pacific rim as a whole, broad patterns of accretion that shifted through time are perceived. During the Paleozoic, for example, the northeast quadrant was a rifed passive margin, while accretionary processes were active in the northwest and southwest quadrants (the Baikalian and Lachlan/Ross orogenic belts, respectively). In Triassic time, accretionary processes were active in the southeastern Asia and eastern Australia-New Zealand regions, but by Jurassic and Cretaceous time accretion had shifted to the northern and

112

northeastern Pacific. This changing pattern of accretion, interrupted by phases of dispersive rifting, reflects large-scale changes in plate motions. Because so much oceanic crust has vanished forever into the depths of ancient subduction zones, the only ranaining evidence of much past plate motions is to be gleaned from the bits and pieces of accreted material found plastering the Pacific rim. Around the Pacific rim, terrane accretion resulted in the formation of approximately 25 million square kilometers of continental crust. This accretion accompanied the subduction of Panthalassa. Terranes of sialic ccn^xasition represent redistributed older continental crust as well as eroded continental debris repositioned or recycled back to a continental crustal setting. The remaining part (more than 50%?) of the newly formed crust, represents accretion of a variety of oceanic plateaus as well as post accretionary plutonic bodies that intrude the region of accreted terranes. The aggregate volume of the aforementiaied material suggests circum-Pacific sialic crustal expansion at a rate of 2.5 kmVyr for the past 200 Ma. To determine the net growth rate of continents, we must subtract the volume of recycled sediment and repositioned older sialic material from the total volume of accreted material. Sudi volumetric measuranents are now underway in the few areas where adequate detailed terrane maps exist. Better maps for the entire circum-Pacific region should be a Icxig-term goal.

113

TECTONOSTRATIGRAPHY

OF THE G R E E N S T O N E S Masao

IN T H E E A S T E R N

Tokushima University, Tokushima, Introductory

SHIKOKU,JAPAN

Iwasaki Japan

remarks

In t h e M i k a b u G r e e n s t o n e B e l t o f t h e s o u t h - w e s t e r n p a r t of t h e Japanese I s l a n d s , a suite of greenstones occur in a rather n a r r o w b e l t . T h e b e l t in e a s t e r n S h i k o k u is a p p r o x i m a t e l y 2 - 3 k m in w i d t h a n d is s i t u a t e d at t h e b o u n d a r y b e t w e e n t h e S a n b a g a w a a n d C h i c h i b u Z o n e s . ( F i g . 1) T h e b e l t is s e p a r a t e d f r o m t h e S a n b a g a w a Z o n e to t h e n o r t h b y a f a u l t a n d f r o m C h i c h i b u Z o n e to t h e s o u t h b y a c o n t a c t o f conformity. T h e g r e e n s t o n e b e l t is c o m p o s e d of t h r e e t e c t o n i c s l i c e s e a c h t e c t o n i c s l i c e is b o u n d e d b y t h r u s t f a u l t s . ( F i g . 2 ) .

and

T h e g e o l o g i c a l a g e d i f f e r e n c e of t h e i n d i v i d u a l s l i c e s is n o t k n o w n , but the scheme for the e v o l u t i o n of the M i k a b u g r e e n s t o n e c o m p l e x is i n f e r e d f r o m t h e f i e l d i n v e s t i g a t i o n a n d t h e p e t r o g r a p h i c a l p o i n t of v i e w ( I w a s a k i , 1979) (Table 1 ) . K - A r age determinations have been made on metagabbros which b e l o n g to t h e M i k a b u g r e e n s t o n e c o m p l e x . T h e w h o l e m a g m a t i c e p o s i d e h a s b e e n d a t e d a t 1 3 4 - 1 7 6 m y ( m i d d l e to u p p e r J u r a s s i c ) . The data is c o i n c i d e n t w i t h t h e a g e o f r a d i o l a r i a n f o s s i l s f r o m t h e r e d s h a l e . Table 1

SCHEME FOR THE EVOLUTION OF THE MIKABU OPHIOLITIC

SEDIMENTS

E x t r u s i o n of the l o w e r p i l l o w

lava

D e n u d a t i o n of b a s a l t i c , g a b b r o i c a n d u l t r a b a s i c r o c k s . D e v e l o p m e n t of u l t r a m a f i c o l i s t o s t r o m e by gravity sliding ( s l u m p i n g ) . A c c u m u l a t i o n of t h e b a s a l u l t r a m a f i c o l i s t o s t r o m e .

Ocean-floor metamorphism

E x t r u s i o n of t h e u p p e r p i l l o w l a v a . I n t r u s i o n o f t h e f e e d e r d y k e s w a r m f o r t h e u p p e r p i l l o w l a v a ; d e v e l o p m e n t of t h e cumulate rocks.

(greenschist

facies).

Reworking

D e n u d a t i o n of b a s a l t , g a b b r o and p l a g i o g r a n i t e . A sequence of gabbro breccia and red shale with occasional igneous rock debris accumulated by gravity sliding (slumping). The a g e of t h e r e d s h a l e w a s d e t e r m i n e d as u p p e r J u r r a s i c b y t h e study of r a d i o l a r i a n f o s s i l s .

and r e d e p o s i t i o n of the u p p e r p i l l o w lava

D e p o s i t i o n of r e d

chert.

114

formation.

r i f . 1. Ccolofkal map of the KamiywM-^aoafody e«tcm part of Shikoku. The field ae) k occurrence of the gabbroie breccia (ola< 1. Upper pOkm Uva (Unit I) Meta-hvalocIasdiCL 2. Lamtr pilkwr Uva (Unit II) Meta.hyaloda«ii« aeeompanyinc with diabaie dyket and ciumiiauvc intruiivct. 3. Cabbra bm-ua uriiioifmme (Unit III) 3a. Cabbro blocks (olisiolitht) and -ihaie). (Cabbroic^ ndtto 3b. Matns tedimeau of the oU 3 c Sabaarine lUdinc depoMia. 4. Sedimena of the Chichibu Zone (?ennian-TriaiMc). 3. Pditic schiMs of the Sanbagawv Zone.

Fig. 2. Cross scction through ihc green stone complex of the eastern part of Shikoku. The ornaments are same as the Fig. 2. The structural trend lines within the Units I and II are determined by the me.isured bedding plane of tlie hyaloclnstite. In Units I and II, some diabase dykes and cumulative intrusives are shown.

115

D e s c r i p t i o n of o p h i o l i t i c

sediments

T h e t e c t o n i c s l i c e of t h e n o r t h e r n p a r t o f t h e z o n e r e p r e s e n t s the l o w e r m o s t f o r m a t i o n of the g r e e n s t o n e c o m p l e x . The f o r m a t i o n is c o m p o s e d of a n u l t r a m a f i c o l i s t o s t r o m e w h i c h c o n t a i n s o l i s t o l i t h s of p i l l o w l a v a , g a b b r o i c rocks and some a m p h i b o l i t e s . The matrix of the o l i s t o s t r o m e r e p r e s e n t s an u l t r a m a f i c s e d i m e n t a r y rock w h i c h is c o m p l e t e l y r e c r y s t a l l i z e d I n t o a c h l o r i t e - a c t i n o l i t e assemblage w i t h o u t quartz and a l b i t e . The sediment commonly contains d e t r i t a l c h r o m i a n s p i n e l w h i c h is s u p p o s e d to b e d e r i v e d f r o m t h e ultrabasic basement rocks. The u l t r a m a f i c s e d i m e n t c o n t a i n s a lot of c u m u l a t i v e i n t r u s i v e s and diabase d i k e s . The cumulative intrusives and dikes intrude into the sediment with c h i l l e d contacts and c o r r e s p o n d r e s p e c t i v e l y to s m a l l - s c a l e m a g m a - p o o l s a n d f e e d e r c h a n e l s o f t h e u p p e r h o r i z o n of t h e f o r m a t i o n . T h e g a b b r o i c b r e c c i a ( o l i s t o s t r o m e ) is in t h e u p p e r m o s t h o r i z o n of the g r e e n s t o n e f o r m a t i o n . The g a b b r o i c rocks w i t h i n the o l i s t o s t r o m e o c c u r as b l o c k s , c o b b l e s a n d p e b b l e s in s e d i m e n t s of g a b b r o i c c o m p o s i t i o n . The size of the b l o c k s s o m e t i m e s e x c e e d s s e v e r a l h u n d r e d m e t e r s in d i a m e t e r . A l l g a b b r o s a r e t h o l e i i t i c rocks with augite and minor p i g e o n i t e . The surrounding sediments are alternating sandstone and s h a l e , a n d g r a d e d b e d d i n g is o b s e r v e d in t h e m f r e q u e n t l y . They are composed of a n g u l a r f r a g m e n t s o f d e t r i t a l c l i n o p y r o x e n e a n d p l a g i o c l a s e a l o n g with some quartz g r a i n s . Features which represent submarine sliding a r e s e e n f r e q u e n t l y in t h e s e d i m e n t s . T h e c o m p l i c a t e d m i x i n g o f the thin strata of red shale and the g r e e n sandy layers are c o m m o n in t h e u p p e r m o s t h o r i z o n o f t h e g a b b r o b r e c c i a ( o l i s t o s t r o m e ) . R a d i o l a r i a n f o s s i l s of u p p e r J u r a s s i c age h a v e b e e n f o u n d in the red s h a l e . The red shale s o m e t i m e s c o n t a i n s p e b b l e s of p l a g i o g r a n i t e . T h e s t r a t i g r a p h i c s e q u e n c e o f t h e g r e e n s t o n e c o m p l e x is s h o w n i n F i g ..3. T h e t o t a l t h i c k n e s s of t h e g r e e n s t o n e in t h e e a s t e r n p a r t o f S h i k o k u is 1.6 - 2.5 k m . Metamorphism In t h e l o w e r m o s t u l t r a m a f i c o l i s t o s t r o m e f o r m a t i o n , t h e rocks have been entirely recrystallized and rather monotonous r o c k s w e r e f o r m e d . T y p i c a l m i n e r a l a s e m b l a g e s in t h e r e s u l t a n t r o c k s include c h l o r i t e , actinolite and t i t a n i t e . The c h e m i c a l composition o f t h e r o c k s is g i v e n in T a b l e 2 . In s p i t e o f t h e i r h o s t r o c k s ( u l t r a m a f i c o l i s t o s t r o m e ) h a v i n g undergone complete metamorphic r e - e q u i l i b r a t i o n , the dikes and cumulates which are chilled against them have rather fresh c l i n o p y r o x e n e both in p h e n o c r y s t and g r o u n d m a s s . In the A p e n n i n e s , d e f o r m a t i o n and recrystallization of h o s t g a b b r o i c r o c k s h a s b e e n a n t e c e d e n t to t h e i n t r u s i o n o f f e e d e r d i k e s of p i l l o w lava ( B o r t o l o t t i , et a l . , 1 9 7 6 ) . The s e q u e n c e of m e t a m o r p h i c e v e n t s h a d b e e n s o m e w h a t s i m i l a r to t h e M i k a b u g r e e n s t o n e s in t h i s case^

116

The chlorite-actinolite assemblage may be from ocean-floor metamorphism under conditons of low pressure and permeation of an aqueous fluid in the ocean. Concluding

Remarks

Most of the Mikabu greenstones are composed of gravity-sliding deposits and the detrital materials came from o p h i o l i t e . No ophiolitic stratigraphy which corresponds with oceanic crust, however, have been detected in the greenstone c o m p l e x . Occurrence of the ophiolitic sediments in the studied area, could be explained by the accretion tectonics of island arc areas. References Bortolotti, v., Cortesogno/ L., Gianelli, G., Piccardo, G.B., & Serri, G., 1976, I filioni basaltici delle ofioliti dell 'Appennino settentronale e il loro significato nella formazione del bacino oceanico Ligure, Ofioliti, 1, 331-336. Iwasaki M., 1979, Sequence of the Paleozoic in the Mikabu Zone and the neighbouring Chichibu Science (Chikyu Kagadu), 32, 345-351.

igneous events Zone, Earth

Table 2. Bulk chemistxy of the chloritetremolite rock in the lower pillow lava formation. 3142

3315 B

la)

»b)

SiO, 46.73 TiOl 1.90 AUO> 11.69 FeO* 12.22 MnO 0.28 MgO 13.96 CaO 11.24 Na,0 1.83 KiO 0.14

48.15 128 7.12 13.18 0.21 20.45 8.56 0.93 0.12

45.68 2.21 8.50 Fe,0, 5.03 FeO 11.65 18.93 6.97 1.02

47.83 2.20 6.77 4.65 10.61 18.85 7.90 1.19

act chl tit

—

55 40 5

65 30 5

FeO": Total iron as FeO. 3142, 3315 B : Pillow lava of lower pillow lava formation, now converted into chlorite-actinolite-titanite rock, (a), (b): Caluculated chemical composition of chlorite-actinolite-titanite rocks. Ratio of the minerals shown in each column.

117

Red ch«rt nmd ihml* («utoa«in« slidiag d€po«lt) MbbU ^.bbro and pU^io^rmnlf) -Gibbro breccia (9*bbro oli«to«cro3») (KAcrix - b«sie t«dia«Aes loUawU^ Reworked

pp«r pilXov lavm ^BasaI ultrasAfic olistoseroa* Katsix - basic ultrasafic i«dia«ftts OlisMlith pillow lavac cuBulatc rocks aad ultrabasic rocks

Fig. 3.

118

LATE P R O T E R O Z O I C

ACCRETED

A TECTONIC

MODEL

T E R R A N E S AND S U C C E S S O R FOR THE A R A B I A N

P. R. J o h n s o n ^ ,

E. S c h e i b n e r ^ ,

and E. A,

JV A s s o c i a t e s ,

Ltd., W a s h i n g t o n

DC 20024,

Dept, of M i n e r a l and Riofinex

Resources,

Limited,

Sydney,

Jiddah,

Saudi

BASINS:

SHIELD

NSW,

Smith^ USA Australia

Arabia^.

The A r a b i a n Shield evolved by terrane a c c r e t i o n , d e p o s i t i o n of overlap s e q u e n c e s and the i n t r u s i o n of I-, S- and A-type g r a n i t o i d s , during a f o u r - s t a g e late P r o t e r o z o i c tectonic cycle. During the initial p r e c a t o n i c stage of the cycle, i n t r a o c e a n i c to c o n t i n e n t a l - m a r g i n a l v o l c a n i c arcs, a s s o c i a t e d s e d i m e n t a r y basins and o p h i o l i t e c o m p l e x e s were d e p o s i t e d (950-700 Ma) in an e n v i r o m e n t which had p r o b a b l y been formed by e a r l i e r rifting of a middle Proterozoic super-continent. C a l c - a l k a l i c , m a f i c to i n t e r m e d i a t e m a g m a t i s m a c c o m p a n i e d the f o r m a t i o n of these r o c k s . The p r e c r a t o n i c rocks c o n s t i t u t e as many as ten t e r r a n e s ; these accreted b e t w e e n 770 and 620 Ma to form a p r o t o c r a t o n on which y o u n g e r rocks were deposited.. The succeeding t r a n s i t i o n a l - t e c t o n i c stage includes v o l c a n o s e d i m e n t a r y o v e r l a p s e q u e n c e s d e p o s i t e d (760-570 M a ) in e n s i a l i c , s u c c e s s o r b a s i n s c o n s t r u c t e d above the accreted t e r r a n e s . The stage also i n c l u d e s abundant late-to p o s t k i n e m a t i c I - t y p e , and alkali g r a n i t o i d s (680-610 M a ) . The third, r e a c t i v a t e d - t e c t o n i c stage i n c l u d e s S- and A-type g r a n i t o i d s (625-570 Ma)», and v o l c a n o - s e d i m e n t a r y rocks d e p o s i t e d in p u l l - a p a r t b a s i n s . T h e s e rocks were formed during i n t r a p l a t e m o v e m e n t s a s s o c i a t e d with the formation of t r a n s c u r r e n t faults of the Najd s y s t e m . By about 550 Ma, these events had resulted in the f o r m a t i o n of a stable block of c o n t i n e n t a l l i t h o s p h e r e , m a r k i n g the onset of the final, e p i c r a t o n i c stage of tectonic e v o l u t i o n . The c o n t i n e n t a l block is p a r t l y covered by P h a n e r o z o i c s e d i m e n t a r y and C e n o z o i c v o l c a n i c s , and is c u r r e n t l y being d i s p e r s e d by o c e a n - f l o o r spreading in the Red Sea, an event that may mark the onset of a new cycle of crustal g r o w t h s Acknowledgementst The research behind the m o d e l d e s c r i b e d in this paper was m a i n l y carried out u n d e r the a u s p i c e s of the Saudi A r a b i a n Deputy M i n i s t r y for M i n e r a l R e s o u r c e s , and we thank HE G h a z i Sultan, the Deputy M i n i s t r y , for h i s support of our w o r k . Much of the geologic i n f o r m a t i o n u t i l i z e d by us was first a v a i l a b l e in o p e n - f i l e reports of the Deputy M i n i s t r y ; we g r a t e f u l l y a c k n o w l e d g e access to these reports, and wish to thank our many c o l l e a g u e s in Saudi A r a b i a whose work s t i m u l a t e d our i n v e s t i g a t i o n s .

119

COLLISION OF OCEANIC AND CONTINENTAL TERRANES IN THE BROOKS RANGE AND ADJACENT AREAS, NORTHERN ALASKA David L. Jones\

Peter Coney^, and David G. Howell^

U.S. Geological Survey, Menlo Park, California, U.S.Ao^ and Department of Geosciences, University of Arizona, Tuscon, Arizona, U.S.A.^

The growth of new continental crust by accretion of allochthonous terranes is magnificently displayed in Alaska^ where ninety five percent of the State lacks pre-Cretaceous connections to cratonal North America. Instead^ the crust there is made up of a vast collage of separate fragments (=terranes) of mixed continental^ oceanic island arc, and oceanic seamount or plateau origin. The sweeping together of these heterogeneous terranes produced new continental crust during the late Mesozoic of extraordinary complexity with a multitude of contrasting structural, depositional, and tectonic settings. Several of these tectonic settings are of particular interest in that they elucidate some of the processes involved in crustal formation. Such settings include: accretionary prisms of Late Cretceous and early Cenozoic age in southern Alaska; collapsed flysch basins of Cretaceous age in south-central and central Alaska; and continental platform strata buried tectonically by gigantic thrust sheets composed of continental margin and oceanic rocks in northern Alaska. The latter setting is particularly important because it involves the interface between a collision of oceanic and continental material in the hinterland, and the development of a fold and thrust belt in the foreland. Oceanic thrust sheets in northern Alaska Oceanic rocks of the Angayucham, Tozitna, and related terranes of northern and north-central Alaska are composed dominantly of pillow basalt, diabase, and gabbro, with subordinate chert, argillite, volcaniclastic rocks, and serpentinite. Some of the pillow piles grew upward to shallow water, as shown by the presence of basaltic conglomerates, tuffaceous bioclastic limestone, and carbonate-cemented pillow breccias. These basaltic edifices appear to represent oceanic seamounts - not primary oceanic crust, and this interpretation is supported by chemical analyses that show compositions similar to Pacific Ocean seamounts rather than to midocean ridge basalts. Basaltic volcanism appears to have been episodic throughout a long period stretching from at least Late Devonian through the Triassic (more than 150 my). Internal structure within these terranes is very complex and is characterized by imbrication due to thrust faulting followed by disruption along high-angle strike slip faults. Age control is provided mainly by radiolarian cherts which occur in three modes: 1) thick packets of chert in which mafic rocks are absent or scarce; 2) packets of chert intruded by large sills of diabase amd gabbro; and 3) thin lenses and pods of chert and argillite that occur within pillow basalt. These three modes of occurrence are essentially coeval and represent different environments that originally were widely separated within an oceanic setting. Because

120

these different assemblages are now closely juxtaposed, large scale internal dislocations are inferred. Cherts range in age from Late Devonian to Early Jurassic, and major periods of volcanism seem to have occurred in the Late Devonian, mid Carboniferous, and Late Triassic. The original crust upon which the deep marine sediments were deposited and on which the sea mounts grew has not been identified. Because this oceanic assemblage is structurally bounded below throughout its entire known extent of thousands of square kilometers, the absence of basement rocks supports the concept of regional crustal delamination during obduction as an important and fundamental geodynamic process. The subduction zone in which these basement rocks must have disappeared, however, has not been recognized. Continental

rocks beneath oceanic

thrust

sheets

A complex assemblage of continental rocks were tectonically buried by advancing thrust sheets composed of continental margin and oceanic materials. During emplacement of the deep-water allochthons, the continental strata themselves were subjected to strong regional deformation and metamorphism that ranges from zeolite to greenschist facies. Four major terranes have been recognized, two of which (Coldfoot and Venetie) contain continental margin deposits, and the other two (Hammond and Endicott) contain fully continental strata, as follows: Coldfoot terrane- structurally beneath the mafic rocks of the Angayucham terrane in the south central Brooks Range is an assemblage of strongly folded quartzose graywacke and slate with a few scattered blocks of chert. This assemblage, which is intruded by gabbro and diabase, is only weakly metamorphosed on the south near the Angayucham contact, but becomes progressively more deformed and metamorphosed to quartz mica schist northward toward the nearly flat fault contact with the underlying Hammond terrane. Age of the clastic rocks is uncertain, although some shaley parts of the assemblage may be interstratified with radiolarian chert of Mississippian age. Venetie terrane- Another terrane with clastic rocks occurs to the east along the southern flank of the Brooks Ranges structurally below the mafic rocks of the Tozitna terrane and structurally above both the Hammond and Endicott terranes. The Venetie terrane consists of a thick, strongly folded assemblage of graywacke, shale, radiolarian chert, and argillite. Mafic rocks are lacking. The clastic rocks reflect submarine fan depositional settings. They are poorly dated, but some are as old as Devonian. Chert ranges in age from Mississippian to Triassic. Contacts between the clastic rocks and chert appear everywhere to be tectonic (low-angle faults), so their original relations are obscure. Hammond terrane- A highly deformed and regionally metamorphosed assemblage of Precambrian(?) to Upper Devonian strata dominated by thick sequences of lower Paleozoic carbonate rocks, occurs along much of the southern part of the Brooks Range. Characteristic rocks include limestone, dolomite, and marble, with subordinate amounts of fine to coarse-grained clastic rocks and minor silicic volcanic and volcaniclastic rocks. This assemblage is intruded by gneissic granitic bodies of Late Devonian age.

121

Endicott terrane- a well stratified assemblage of Upper Devonian to Lower Mississippian slope, shelf, and paralic clastic strata that may be lithofacies of the Venetie terrane occurs throughout the central and northern parts of the Brooks Range. These rocks are overlain by Carboniferous platform carbonate rocks of the Lisburne Group. This well known assemblage is deformed into a complex foreland fold and thrust belt that locally has been prospected by wildcat drilling. The Endicott terrane structurally overlies the Hammond along a regional major detachment surface whose structural significance and origin are still not fully understood. No depositional contacts have been verified between Endicott and Hammond strata, and their original paleogeographic setting is enigmatic, particularly with respect to the North American craton. Structural relations in the eastern Brooks Range The structurally highest thrust sheet in the southeastern Brooks Range is the Tozitna terrane, consisting of a thick assemblage of pillow basalt, diabase, radiolarian chert, argillite, gabbro, ultramafic rocks, and minor silicic volcaniclastics. Chert ranges in age from Mississippian to Triassic and is similar to the underlying Venetie terrane. In many ways, this volcanic assemblage is comparable to that of the Angayucham terrane to the west, but no direct connection between the two can now be established. The Endicott, Venetie, and Tozitna terranes may represent distinct facies of a single continental margin to deep ocean basin paleogeographic setting. Obduction resulted in a stack of imbricate thrust sheets in which facies patterns crudely descend from distal oceanic rocks (Tozitna terrane) at the top to proximal continental strata at the bottom (Endicott terrane). Structures within the Endicott terrane of the northern Brooks Range are characteristic of a foreland fold and thrust belt with north-verging recumbently folded strata riding on low-angle imbricate thrust faults that merge at depth into a master decollement. Structures within the Hammond terrane are more complex and poorly understood owing to regional metamorphism, stratal disruption, and lack of a well defined stratigraphic sequence; large -scale recumbent folds are prevalent, but their vergence is not clearly established. Major thrust faults have been observed, but their regional extent is unknown. The contact between the Hammond and Endicott terranes is a complex zone characterized by large detached blocks of limestone and marble floating in a matrix of graywacke and shale. In summary, the tectonic elements that make up the eastern Brooks Range consist of two collided terranes of continental origin overlain structurally by terranes of oceanic and continental margin character. The site of origin of these terranes during the Paleozoic and early Mesozoic is uncertain. By Cretaceous time, however, they were being assembled by accretion and attained their present position. The structural style, degree of penetrative deformation, and grade of metamorphism within each terrane differ. The structurally lowest terrane (Hammond) displays the highest degree of metamorphismo The folds and thrusts of the Endicott terrane in the northern Brooks Range appear to be genetically related to the collisional events that occurred in the hinterland to the south, but the mechanical linkage between these different structural domains is obscure. (Research supported by contract from DOE/Morgantown Energy Techology Study, Deep Gas Resources Project)

122

M . P . Klimetz Department of Geological Sciences, State University of New York at Albany, Albany, New Y o r k , U.S.A.

(no title nor abstract received late registrant)

123

PALAEOMAGNETISM OF THE TASMAN FOLD BELT: INDICATION FOR MID-CARBONIFEROUS LARGE-SCALE SOUTHWARD DISPLACEMENT OF THE NEW ENGLAND REGION Chris Klootwijk Bureau of Mineral Resources, Canberra, Australia OVERVIEW Palaeomagnetic results from the Tasman Fold Belt (TFB) and from contemporaneous basins in cratonic Australia have accumulated steadily since inception in the late fifties of palaeomagnetic studies at the ANU by Irving and coworkers and subsequently by McElhinny and coworkers. Early work concentrated on the Silurian and Devonian of the Lachlan Fold Belt (LFB), on the Upper Palaeozoic and Mesozoic of the Sydney Basin, and on the Carboniferous of the Tamworth Trough. With improved techniques higher quality data could be obtained and some of the earlier studies were reinvestigated (cf Schmidt and Embleton). In the early seventies Early and Middle Palaeozoic data from cratonic Australia, ie from the Adelaide Fold Belt, the Amadeus Basin and the Ord, Victoria River and Daly River basins, became available for comparison with data from the TFB. Plate tectonic interpretations of the TFB had been proposed and an apparent disagreement between palaeomagnetic data from SE.Australia and the Australian craton was interpreted by Embleton et al.(1974) in terms of a SE.Australian subplate. There was no fully convincing geological support for this hypothesis which proved later to be based largely on incorrect results from Silurian volcanics of the Canberra region, ie the Ainslie Volcanics and the Mugga Mugga Porphyry which had not been corrected for tilting, and on a palaeomagnetic result of suspect primary origin from the ill-dated Silurian-Devonian Mereenie Sandstone of the Amadeus Basin Schmidt and Morris (1977) subsequently combined the same Silurian results from the LFB with Early and Middle Palaeozoic results from cratonic Australia into a single apparent polar wander path (APWP) by reversing polarity of the Cambrian and Ordovician poles. This single pole path implied absence of palaeomagnetically detectable movement of the LFB with respect to the craton since at least the Middle Palaeozoic. Goleby (1980) made a detailed study of Lower Ordovician to Middle Devonian sediments from the Cowra Trough and the Molong High, and constructed an APWP representative for the studied region only. He was able to fit the Ordovician-Silurian segment of this APWP to the Cambro-Ordovician APWP for cratonic Australia (Embleton 1972, Klootwijk 1980) and reconciled its Devonian segment with Late Palaeozoic results from the craton and the neocratonized part of the TFB, implying absence of palaeomagnetically observable differential movement.

124

Since the mid seventies much needed attention has been paid to removal of secondary magnetization components. Schmidt and Embleton (1981) established the predominant presence of a Late Cretaceous overprint in the Tasman Sea board of the TFB. They attributed this overprint to removal of overburden prior to opening of the Tasman Sea. However, overprint poles from regions as far apart as SW.Tasmania, the Sydney Basin and SE.Queensland coincide or do very nearly so, which suggests a short-lived phase of thermo-chemical activity prior to breakup as a more likely origin. NEW DATA Late Palaeozoic and Late Mesozoic phases of widespread overprinting may have obliterated largely the primary magnetization signature of the TFB. Large-scale palaeomagnetic programs of a regional extent offer best prospects for identification and interpretation of such secondary magnetizations and for isolation of any remaining primary magnetization. The BMR initiated in late 1983 a regionally oriented palaeomagnetic study of the TFB to obtain palaeomagnetic constraints on the tectogenesis of the fold belt. Particular attention is being paid to possible differential movements between suspect or tectonostratigraphic terranes within the TFB itself and with respect to the craton. Work has concentrated so far on two regions, ie the Cowra Trough-Molong High of the LFB and the southern New England region. In the former region a thousand samples were collected from Middle to Late Silurian volcanics and early Late Silurian to late Early Devonian sediments from the Yass, Taemas and Wee Jasper basins, from late Middle to early Late Silurian volcanics from the Canberra region and from Late Silurian to Early Devonian plutons from the Berridale Batholith. This study aims to further detail Goleby's (1980) reconnaissance APWP for the Molong High-Cowra Trough tectonostratigraphic unit and its relation with cratonic Australia, to update and extend on suspect data from Silurian volcanics in the Canberra region, to ascertain the palaeomagnetic potential of plutons and more specifically to test the tectonic aspect of the I-S line which crosses the Berridale Batholith. Results will be reported depending on progress. The original incentive for the New England study was a test of the Permo-Carboniferous orocline hypothesis (Flood and Fergusson 1982), with an additional investigation of the origin of the bend in the Tamworth Belt (Cawood and Leitch 1984), and the possibility of large-scale southward displacement of the New England region with respect to the already cratonized part of the TFB (Harrington pers. comm. 1983). Serious problems in interpretation were foreseen because of the high inclination of the geomagnetic field during the latest Palaeozoic in the New England region which makes declination observations less significant, and the scarcity of Carboniferous and Permian palaeomagnetic data from Australia outside the New England region. These problems were taken for granted, however, because of the potentially far reaching implications of these hypotheses. A reconnaissance collection of 330 samples was taken from turbiditic sequences of probable Late Carboniferous age from the Texas Beds and from the possibly Late Carboniferous Coffs Harbour sequence (Flood and Fergusson 1984), with broad coverage of the continuing variation in strike along the Z-shaped megafold. The magnetization of these rocks proved to be dominated by a steeply west to southwest downward directed magnetization of exclusively reversed polarity which is of post-tectonic

125

origin. The directions of this secondary component do not show any significant change in declination along the megafold. No primary magnetization could be identified and the palaeomagnetic test on oroclinal bending is thus inconclusive. In the Rouchel region of the Tamworth Trough another 130 samples were taken from Lower Carboniferous carbonate and siltstone of the Brushy Hill Limestone and the Woolooma Formation and in particular from ignimbrites of the Visean Isismurra Formation. A secondary magnetization similar to the one observed in the Texas and Coffs Harbour Blocks predominated the magnetization content. However, seven out of ten sites from the Isismurra ignimbrites showed in addition a north to northwest directed component of very low inclination and of both normal and reversed polarity. The equatorial palaeolatitude indicated by this result is in agreement with the low latitude palaeo-environment concluded from the highly diversified brachiopod fauna (Roberts 1984). This magnetization component is interpreted, therefore, as the primary magnetization. No such component was identified in earlier studies of the same formation. There are only few data of roughly comparable age available from (neo-)cratonic Australia, ie new and as yet unpublished results from volcanics and associated sediments of the Newcastle Range Volcanics of the Georgetown Inlier (Giddings and Idnurm, pers. comm. 1985), the Yetholme Adamellite from the LFB, and the Alice Spring Orogeny overprint from the Amadeus Basin. These data extrapolate to an expected 30 degrees or higher southern palaeolatitude for the Rouchel region. So far there is no indication that this discrepancy of about 30 degrees in palaeolatitude is an artefact resulting from an unwarranted comparison of data of different age during a period of fast southward movement of eastern Gondwana. The notion of fast southwards movement during the Middle Carboniferous seems largely based on palaeontological and now superseded palaeomagnetic data from the New England region, whose direct representation for the remainder of Gondwana may be questioned. Analysis of new Indian Lower Gondwana data and African Karroo data suggest in fact a latest Carboniferous or Permo-Carboniferous date for such a Gondwana-wide movement. There is only one result of probable Early Carboniferous age available from Gondwana outside Australia, ie the Dwyka varves of the Karroo System. This result also extrapolates to a similarly moderate southern palaeolatitude for the Rouchel region and supports the discrepancy. Therefore, a large-scale post- or possibly syn-Visean southward displacement of the New England region east of the Mooki Fault, with respect to (neo-)cratonic Australia is concluded tentatively. Late Carboniferous results from the Tamworth Belt (Irving 1966) loosely group with contemporaneous results from other Gondwana continents, including new and unpublished data from the Indian Lower Gondwana. No significant discrepancy is apparent. This confines the displacement to the Middle or possibly Late Carboniferous. Whether this displacement represents accretion of an exotic terrane (Cawood and Leitch 1984) or dextral transcurrent movement within the eastern TFB (Harrington, pers. comm. 1983) cannot be resolved from the present data. The comparable overprints from the Texas Block, the Coffs Harbour Block and the Rouchel region can be dated as Early Permian, possibly Permo-Carboniferous to Middle Permian, from comparison with an updated APWP for Gondwana based largely on Indian and Australian data. Formation of the Texas Block-Coffs Harbour orocline must have resulted from transcurrent movements prior to acquisition of these secondary components and a relation with the southward displacement of the New England region may be surmised. The two phases of overprints observed so far in the TFB, ie Late Cretaceous and probably Early Permian, have some common characteristics. Both overprints represent thermochemical activity in a rift environment prior to breakup (Early Permian: Cawood and Leitch 1984) and were acquired at comparable palaeolatitude ranges, ie 60-70 degrees S in the Late Cretaceous and about 60 degrees S during the Early Permian. Though palaeomagnetism cannot provide palaeolongitude control, which is less

126

essential anyway at these high latitudes, intermittent persistent hot spot (line) may be speculated.

activity of

REFERENCES Cawood,P.A. and Leitch,E.C.,1984, Accretion and dispersal tectonics of the southern New England Fold belt, Eastern Australia, preprint. Embleton,B.J.J.,1972, The palaeomagnetism of some Palaeozoic from Central Australia, J. Proc. R. Soc. N.S.W.,105,86-93.

sediments

Embleton,B.J.J., McElhinny,M.W., Crawford,A.R. and Luck,G.R.,1974, Palaeomagnetism and tectonic evolution of the Tasman orogenic zone, J. Geol. Soc. Aust.,41,187-193. Flood,P.G. and Fergusson,C.L.,1982, Tectono-stratigraphic units and structure of the Texas-Coffs Harbour region, in: P.G.Flood and B.Runnegar (Eds.), New England Geology, UNE,71-78. Flood,P.G. and Fergusson.C.L.,1984, The geological development of the Northern New England province of the New England Fold belt, in: H.K.Herbert and J.M.W.Rynn (Eds.), 1984 Field Conference, Geol. Soc. Aust. Qld. Div. 1-19 Goleby,B.R.,1980, Early Palaeozoic palaeomagnetism Australia, J. Geomag. Geoelectr.,32, Sup 3, 11-21.

South

East

Irving,E.,1966,Palaeomagnetism of some Carboniferous rocks from New South Wales and its relation to geological events, J. Geophys. Res.,71,6025-6051. Klootwijk,C.T.,1980, Early Tectonophys i cs,64,249-332.

Palaeozoic

palaeomagnetism

Australia,

Roberts,J.,1984, The Carboniferous geology of Australia, preprint. Schmidt,P.W. and Morris,W.A.,1977, An alternative view of the Gondwana Palaeozoic apparent polar wander path, Can. J. Earth Sci., 14,2674-2678. Schmidt,P.W. and Embleton,B.J.J.,1981, Magnetic southeastern Australia and the thermal history of its Geophys. Res.,86,3998-4008.

127

overprinting in rifted margin, J.

PALEOMAGNETIC EVIDENCE OF LEFT-LATERAL DISPLACEMENT OF THE MEDIAN TECTONIC LINE IN SOUTHWEST JAPAN Kazuto Kodama Department of Geology, Faculty of Science, Kochi University, Kochi, Japan A paleomagnetic study was carried out on the Upper Cretaceous sedimentary sequences of the Izumi belt in southwest Japan in order to examine paleomagnetically any possible tectonic influence of the Median Tectonic Line (MTL) in the geological past. Extending continuously from northwestern Shikoku to the western Kii Peninsula, the Izumi belt forms a synclinal fold with an inclined fold axis plunging about 30® to 50® to the east. To the north, the Izumi belt is overlain by the Ryoke metamorphic belt with angular unconformity and, to the south, is separated from the Sanbagawa metamorphic belt by the MTL. The studied area is a specific region in the western part of the Asan Mountains, Tokushima prefecture, where there are good representative rock facies of the Izumi group such as sandstone, shale and mudstone, outcropping along the north of the Yoshino River (Fig. 1). The age of the Izumi group in this region has been assigned generally to Campanian to Maastrichtian (Upper Cretaceous) by the occurrence of some ammonites and bivalves. A radiolarian age analysis carried out in this study yielded a few species of radiolaria that can be assigned to the Upper Campanian. Extensive geologic mapping was conducted to determine the orientation of the plunging fold axis in the studied area as precisely as possible, resulting in the estimation of an optimum fold axis plunging 30° to the east. Remanent magnetization of 193 specimens prepared from 16 sites as shown in Fig. 1 were measured with a cryogenic magnetometer. Their rock types are mudstone (9 sites), tuffaceous siltstone (5 sites) and sandstone (2 sites). The correction of bedding tilt to the in situ paleomagnetic directions in this study were carried out by taking the synclinal fold with an eastward plunging axis into consideration. Table 1 summarizes the paleomagnetic results with both the mean directions before and after the tilt corrections, each also being illustrated in Fig. 2(a) and 2(b), respectively. The results show a clear difference in grouping of the mean directions. The uncorrected directions scatter generally from 10® to 80® in declination and 50® to 80® in inclination, whereas those corrected do not show such divergent directions but seem to be divisible into two groups, corresponding to sites located in the northern and southern parts of the studied area. The nine mean directions of the northern sites cluster around 90® to 120® in declination, whereas those of the southern sites are deflected more northerly with declinations from -16® to 40®, although no significant difference in inclination can be found between the two groups. Since these qualitative interpretations can be reinforced by conventional statistical analysis, the difference in declination between the two groups may indicate true relative tectonic rotation around a vertical axis. Although it is ambiguous as to which mean direction can be assigned to the Upper Cretaceous paleomagnetic direction in Southwest Japan, it may be reasonable to attribute the net relative rotation to horizontal drag due to the strike-slip movement of the MTL. That is to say that if we assume the mean direction from the northern sites to be the coeval paleomagnetic direction of Southwest Japan, then the direction from the southern sites suggests counter-clockwise rotation about a vertical axis relative to Southwest Japan. Tectonically, this rotation may mean the left-lateral displacement of the MTL has caused plastic deformation near the fault zone prior to the folding of the Izxami group of this area.

128

Table 1 Site

Bock type

67.3

60.9

0^5

47.9

113.8

105.0

4.3

93.0

379.7

2.3

57.0

65.9

38.5

IIS

68.6

75.9

51.7

115.5

490.1

3.5

52.6

101.1

49.5

5.0

t®

i^lS

72.1

46.2

53.8

54.8

48.0

96.7

131.4

2.7

L-IS

52.2

73.5

40.0

101.3

113.2

4.1

iyiS

71.1

2.7

58.0

111 .2

311.8

2.6

52.8

48.1

43.1

84.9

104.5

4.3

59.5

27.9

55.9

88.3

211.7

2.4

&3S

82.2

325.1

67.2

27.4

344.1

2.0

i^iS

73.9

49.1

41.1

35.0

255.2

2.3

70.7

8.9

52.1

27.0

98.0

6.1

-50.5

337.7

44.5

337.5

78.4

4.9

66.4

46.4

49.5

33.8

404.3

2.4

71.7

17.1

53.7

38.0

23.3

16.2

65.6

42.3

40.6

25.2

281.3

2.6

Rock type; MS = mudstone, SS = sandstone, TS = tuffaceous siltstonk N; niinber of specimens. I, D; in-situ inclination and declinaticai, I^,, inclination and declination after correction for bedding tilt, k, 0^5; Fisher's precision parameter and soniangle of 95% confidence cone.

129

Fig. 1

Generalized geologic map of the Iziami belt in the western Asan Mountains in Shikoku, Southwest Japan, showing sampling localities for paleotnagnetic studies. 1; raudstone, 2; alternations of sandstone and shale, 3; acidic tuff and tuffaceous siltstone, 4; basal conglcmerate, 5; Ryoke metainorE^c belt.

(B)

(A) Fig. 2

Site mean directions before (a) and after (b) the corrections for bedding t i l t . Circles and triangles are those from the northern and southern areas, respectively. Equal-area projection on the lower (solid) and upper (open) hemisphere.

130

FRAGMENTATION AND DEFORMATION OF TERRANES IN THE NEW ENGLAND OROGEN, EASTERN AUSTRALIA

R.J. Korsch and H.J. Harrington Bureau of Mineral Resources, Canberra, Australia Workers have begun to apply the concepts of tectonostratigraphic terrane analysis to the New England Orpgen in eastern Australia (Cawood, 1983; Harrington, 1983; Scheibner, 1983). At least eight discrete major deformational events have been recognised in the New England Orogen (Korsch & Harrington, 1981) and hence it is an ideal region in which to examine the deformational effects of the accretion of various terranes. The effects range from intense penetrative mesoscopic deformation in some cases to macroscopic folding over very large regions in other cases. The Devonian to Carboniferous paleogeography of the New England Orogen consists of essentially three distinctive parallel belts which, from west to east, are interpreted as (I) a volcanic arc, (2) a forearc basin containing a thick sequence of terrestrial to marine volcaniclastic sediments derived from the volcanic arc, and (3) an accretionary prism consisting predominantly of deep-water marine turbidites also derived from the volcanic arc (eg Leitch, 1975; Day et al., 1978). The accretionary prism itself can be subdivided into at least three tectonostratigraphic units which were progressively added to the prism. As defined by Korsch (1977) these are the Woolomin Association (?Silurian to Early Devonian), the Sandon Association (Late Devonian to Early Carboniferous) and the Coffs Harbour Association (?Early to Late Carboniferous). Using sedimentary petrography Korsch (1984) showed that the Tamworth Belt (Fig. 1) has a distinctive petrographic evolution path and that most of the associations in the Tablelands Complex (domains 2-9 on Fig. 1) can be correlated with the Tamworth Belt. Because of this provenance linkage, most of the New England Orogen during time interval A of Figure 1 can be regarded as a single large terrane evolving at a subduction-related plate boundary. The exception is the Beenleigh block in which the quartzose turbidites are distinctly different from those in the rest of the New England Orogen (Korsch, 1984). Hence we interpret the Beenleigh Block as an exotic terrane that was once part of the Curtis Island Group (terrane) which is now located some 800 km to the north (far north of the region shown on Fig. 1). The Curtis Island-Beenleigh terrane was probably accreted to the New England Orogen as a single coherent terrane in postCarboniferous time, and then divided into two terranes by later dispersal. During the Late Carboniferous to Early Permian the tectonic regime ^ changed, with the development of the Mooki transform fault system (Harrington & Korsch, 1985) and the accompanying formation of the Sydney-Bowen transform basin system. The change in tectonism was accompanied by a change in the style of sedimentation in the Tablelands Complex. The widespread, thick, turbidite sequences that were characteristic of the Carboniferous were succeeded by scattered fault-controlled basins in which spectacular thicknesses of marine diamictites were deposited (Korsch, 1982). The basins varied from pull-aparts associated with major strike-slip faults to rifts and small half-grabens associated with normal faults and strike-slip faults. A subduction-related volcanic arc was replaced by rift-related bimodal basalt-rhyolite suites.

131

.STANTHORPE

•248 ' NEW ENGLAND,

V^L

•286

- 300

HILLGROVE.

COFFS HARBOURI

i •380

SANDON

r^ : i

TASMAN

-400

-408

Figure 1.

Time-space diagram for domains in the New England Orogen. During time interval A the orogen consisted of one large terrane. Time interval B saw the fragmentation of this terrane and development of individual schollen. Pluton stitching during time interval C amalgamated the schollen.

The strike-slip faulting and rifting had the effect of fragmenting the central and southern part of the New England terrane into a series of crustal slivers or schollen (Dewey & Sengor^ 1979). In the Early Permian, the schollen were probably sandwiched between two major strike-slip faults, The Mooki Fault was to the westc In the east there was a narrow network of faults near the western margins of the present-day Curtis Island and Gympie terranes and the present-day Esk Troughs The region between the two major fault systems was up to 250 km wide« Within it there were both tensional and compressional regimes, and deformation occurred between individual schollen. Jones et al. 0 9 8 2 ) , Crowell (1983) and others have shown how crustal blocks (schollen) can develop and rotate between major strike-slip faults. Rotation of the schollen has implications for the geological processes operating between adjacent schollen.

132

The model in Figure 2A examines the behaviour of a scholle (plate A) that is surrounded by two larger plates. The Eulerian pole RP^g moves along a great circle defined by Kulerian pole EPg^t and plates A and C are rotating sinistrally relative to plate B (fixed). The point X within plate A traces out a prolate trochoid path during ISO^ rotation of plate A and 30° rotation of plate C relative to plate B. This prolate trochoid path determines the nature of the boundary segments for plate A. For example, segment 1 is initially of dextral strike-slip character at X^ but it evolves into a dextral rift. Then the character of the segment changes again rapidly to normal rifting, sinistral rifting, sinistral strike-slip, sinistral convergence and normal convergence. There follows prolonged dextral convergence and finally dextral strike-slip at X|. A similar modified trochoid movement path for a fixed point X can be produced when an Eulerian pole migrates across a single plate boundary (Fig. 2B). Here, dextral rotation of plate B occurs about the Eulerian pole relative to a fixed point A. Progressively different conditions exist along the boundary ab. When the Eulerian pole is at EPQ the character of the boundary at points A, B and C is represented by the solid arrows, whereas when the Eulerian pole is at E P p the character of the boundary is represented by the dashed arrows. Hence rapidly changing processes can be associated with the same plate boundary segment or with a scholle rotating between two major plates. The situation becomes extremely complicated when the zone between the major plates consists of several schollen such as in the model of Jones et al. (1982), or as existed in the central and southern New England Orogen during the Permian. North of the schollen in the northern part of the New England Orogen, Lucas (1960) recognised that the geological units formed an arc that was concave southwards. A regional-scale syncline was recognised in the Coffs Harbour Block by Korsch (1975). Both structures were considered to be parts of the same large Z-shaped megafold by Flood & Ferguson (1982) As almost all tectonostratigraphic units of the New England Orogen are involved in the megafold, it can be regarded as a double orocline, here termed the Texas and Coffs Harbour oroclines respectively. The timing of formation of the oroclines is constrained between the D^ folding of the Coffs Harbour province in the Late Carboniferous and the intrusion of the New England Batholith in the Late Permian. Murray et al. (in prep.) considered that the oroclinal bending occurred at about 300 Ma (very late in the Carboniferous). However the Early Permian fault-bounded sedimentary slivers in the Texas-Inverell region are deformed parallel to the curve of the orocline, and we infer that this deformation occurred at the same time as the formation of the oroclines. A major tectonic event which we consider might have been associated with the formation of the oroclines is the dispersal of the Beenleigh terrane from the Curtis Island terrane but much more work is needed on that problem. In the southern part of the New England Orogen, the schollen which were developed by strike-slip faulting and rifting in the Early Permian were compressed later in the Permian and there was intense penetrative mesoscopic deformation of some sub-regions. There was particularly intense tectonic shredding and deformation in the Nambucca Block which is located immediately south of the developing Coffs Harbour Orocline

133

Boundary ab

/^PUVTE B ^ Segment

J JaBJC^

Rifting

PLATE A

PLATE

(fixed)

TEir-

Transform: •

QJI

EPi Mfgrationn^ZJZlA ^fon path of ^p

EPo

"EPBC

Xfinai

Subduction

'ABJC.

Path of EPtg rotating on EPgc

Figure 2.

A. Theoretical model for the rotation of a scholle (plate A) between two larger plates (B and C). The Eulerian pole EP^g for plates A and B moves southwards relative to B around EPg^,. A point X on plate A traces out a prolate trochoid from position Xo to position X . There are different patterns of oblique convergence, divergence and strike-slip on segments 1, 2 and 3 of the boundary of plate A. B. Migration path of a fixed point X within a moving plate B (relative to a fixed plate A) due to migration of the Eulerian pole for plates A and B across the plate boundary ab.

(Fig. 1). Leitch & McDougall (1979) showed that sedimentation in the Nambucca Block was occurring in the interval 285-270 Ma and they also obtained K-Ar ages of 268-249 Ma from slates. They considered that these ages date the metamorphism which accompanied the deformation of the sedimentary sequence. We consider that the orocline also developed at that time. Hence in the Late Permian, the eastern portion of the New England Orogen saw intense tectonic activity, including the arrival of the Beenleigh terrane,the formation of the Texas and Coffs Harbour oroclines, and widespread mesoscopic deformation which was most severe in the Nambucca Block. On the coast at Nambucca Heads and Valla the sediments were metamorphosed to greenschists and were intensely crenulated and sliced into a stack of thin thrust sheets. This Permian penetrative mesoscopic deformation was very different from the Triassic macroscopic deformation that accompanied the arrival of the Gympie terrane as described by the authors in another paper in this symposium. References Cawood, P.A., 1983, Accretionary tectonics and terrane dispersal within the New England Fold Belt, Eastern Australia, Stanford University Publications in the Geological Sciences, 18, 50-52.

134

Crowell, J . C . , 1983, The recognition of terrane dispersal in transform belts, Stanford University Publications in the Geological Sciences, 18. 74-78. Day, R . W . , Murray, C.G. & Whitaker, W . G . , 1978, The eastern part of the Tasman Orogenic Zone, Tectonophysics, 4 8 , 327-364. Dewey, J . F . & Sengor, A . M . C . , 1979, Aegean and surrounding regions: complex multiplate and continuum tectonics in a convergent zone. Geological Society of America Bulletin Part I , 90, 84-92. Flood, P . C . & Fergusson, C . L . , 1982, Tectono-stratigraphic units and structure of the Texas-Coffs Harbour region. I n : Flood, P . C . & Runnegar, B.N. New England Geology. Proceedings of a Symposium on the Geology of the New England region. University of New England, Armidale, 71-78. Harrington, H . J . , 1983, Correlation of the Permian and Triassic Gympie Terrane of Queensland with the Brook Street and Maitai Terranes of New Zealand. I n : Permian Geology of Queensland, Geological Society of Australia Queensland Division, 431-436. Harrington, H . J . & Korsch, R . J . , 1985, Tectonic model for the Devonian to middle Permian of the New England Orogen, Australian Journal of Earth Sciences, 32, in press. Jones, D . L . , Cox, A . , Coney, P. & Beck, M . , North America, Scientific American, 2 4 7 ( 5 ) ,

1982, The Growth of Western 70-84.

Korsch, R . J . , 1975, Structural analysis and geological evolution of the Rockvale-Coffs Harbour region, northern New South Wales, Unpublished Ph.D. thesis. University of New England. Korsch, R . J . , 1977, A framework for the Palaeozoic geology of the southern part of the New England Geosyncline, Journal of the Geological Society of Australia, 25, 339-355. Korsch, R . J . , 1982, Early Permian tectonic events in the New England Orogen, I n : Flood, P.G. & Runnegar, B.N. New England Geology, Proceedings of a Symposium on the Geology of the New England region. University of New England, Armidale, 35-42. Korsch, R . J . , 1984, Sandstone compositions from the New England Orogen, eastern Australia: Implications for tectonic setting. Journal of Sedimentary Petrology, 5 4 , 192-211. Korsch, R . J . & Harrington, H . J . , 1981, Stratigraphic and structural synthesis of the New England Orogen, Journal of the Geological Society of Australia, 28, 205-226. Leitch, E . C . , 1975, Plate tectonic interpretation of the Paleozoic history of the New England Fold Belt, Geological Society of America Bulletin, 8 6 , 14 1-144. Leitch, E . C . & McDougall, I . , 1979. The age of orogenesis in the Nambucca Slate Belt: A K-Ar study of low-grade regional metamorphic rocks. Journal of the Geological Society of Australia, 26, 111-119. Lucas, K . G . , 1960, Border Rivers, Journal of the Geological Society of Australia, 7 , 139-140. Scheibner, E . , 1983, Suspect terranes in the Tasman Fold Belt System (Eastern Australia), Stanford University Publications in the Geological Sciences, 18, 170-174. Published with permission of the Director, Bureau of Mineral Resources.

135

A TASMAN FOLD BELT PERSPECTIVE ON TERRANE ANALYSIS E . G . Leitch^ and E . Scheibner^ University of Sydney^ and Geological Suirvey of New South Wales^, Sydney, Australia Terrane analysis in orogenic belts focuses attention on structural and stratigraphic discontinuitiesr mainly faults, and uses these as boundaries to crustal elements which are considered to have evolved independently of adjacent elements u n t i l the opposite can be demonstrated. Thus many of the boundaries on a terrane map of eastern Australia are familiar (Fig. 1 ) . They coincide with the margins of masses variously referred to as highs, troughs, blocks, platforms, geanticlines, zones etc. The important feature of interpreting the elements as terranes is that we admit the possibility that they are collectively a collage, rather than a pattern that has maintained an essential coherency during the development of the orogenic b e l t . The methodology of terrane analysis requires that, where major rock masses now juxtaposed show contrasting geological development over much of their histories the possibility, or even likelihood, that they evolved in widely separate regions be considered. Conversely, the possibility that masses now far apart, but which show marked similarities in their geological histories and faunal elements, evolved contiguously and were only parted towards the end of their development,.must be admitted. In defining terranes an attempt is made to identify large crustal blocks that show a common geological history. By definition their boundaries are either tectonic, o r obscured by later rocks which provide linkages constraining the age of terrane amalgamation. Disruption of amalgamated terranes is common. This can occur along old sutures o r in part o r entirely transverse to these structures. Movements on a scale of kilometres to perhaps a few hundred kilometres may not destroy the overall pattern of terranes within an orogenic belt, but the accretion of an exotic terrane formed in part of cratonic rocks within one b e l t , necessarily requires the prior dispersion of elements from an earlier b e l t . On our map we show combined within some individual terranes a number of structural features considered independent in different contexts. F o r example, the three belts of Ordovician volcanics in the central west region of N e w South Wales, those of the Goonumbla, Molong and Gapertee 'highs', and the intervening Gowra and H i l l End troughs, are a l l included within the Molong - Monaro Terrane. The highs comprise part of the same Ordovician feature, and the troughs are products of later dispersion, which though involving at least 10's of kilometres of movement has not destroyed its unity. Parameters important in the definition and interpretation of the terranes are summarised in Table 1 . Clearly the divisions we postulate w i l l undergo modification: many are discussed in detail elsewhere in this volume. At present biogeographic and palaeomagnetic data, so useful in recognising largescale terrane displacements, are just beginning to emerge for the Tasman Fold Belt and accompanying papers include the first to discuss these topics within a terrane framework. Only when systematic work on many fossil groups is much more advanced, and a much greater array of well-dated pole positions determined, w i l l we be able to fully trace the development of this large composite orogen. Thus our attempt to define terranes inevitably encountered problems, but it also provided insights into the development of the Tasman Fold B e l t . Examples of both are outlined in the following paragraphs.

136

Tasmanian terranes The Tamar Fracture System clearly separates redeposited quartzose sandstone and siltstone of Early Ordovician - Early Devonian age grouped in the Mathinna beds in the east of Tasmania from a complex of Early Palaeozoic grabens ('troughs') and intervening Precambrian horsts ('geanticlines') further west. Three belts of Precambrian rocks, those of the Rocky Creek 'Geanticline', the Tyennan and Forth 'Geanticlines', and the Badger Road 'Geanticline' and the Jubilee Block differ in lithological makeup, metamorphic character, structural style and age of deformation, and form the basis for three terranes. Indeed at least the Rocky Creek might be composite, for the Burnie Formation and correlative rocks of the eastern part of this geanticline are separated from lithologically distinct sequences further west by the metamorphic rocks of the Arthur Lineament the eastern side of which may make a major suture. The status of the intervening troughs is less clear. The Eocambrian Cambrian sequence of the Dundas trough lies unconformably on the Rocky Creek Anticline, and the Cambrian strata interfinger eastward with rocks of the Mt. Read Volcanic Belt that in turn rest unconformably on the Tyennan Geanticline. Thus at least the upper part of the Dundas Trough succession provides an overlap sequence and a minimum age for the amalgamation of the two terranes. Mafic and ultramafic bodies found within the Dundas Trough are indicative of rifting and apparently do not define a continuous suture. Cambrian rocks of the Adamsfield Trough appear to overlap the Tyennan and Jubilee Terranes in a similar fashion. Thus the western two-thirds of Tasmania probably comprised a single composite terrane by the Middle Cambrian. The presence of distinctly different Precambrian rocks on either side of the troughs suggests that rifting was preceded (or accompanied) by major strikeslip movements. Deformation of the troughs, and of the Kanmantoo - Glenelg Terrane probably involved the inversion of a passive margin, possibly in a back-arc setting. Terranes in Central Victoria^ Considerable problems are encountered in defining terranes within the Lachlan Fold Belt in central Victoria. The system outlined takes as terrane boundaries a number of major faults that separate areas of seemingly different structural history. The Stawell - Bendigo Terrane is bounded by the fault system along the eastern side of the Heathcote greenstone axis. The Melbourne Terrane with which the Stawell - Bendigo Terrane is juxtaposed is bounded to the east by the Mansfield Fault, west of the Mt. Wellington greenstone axis. The Howqua - Tabberaberra Terrane extends from the Mansfield Fault to the Kiewa Fault with the Wagga-Omeo Terrane lying east of the latter structure. Each terrane contains Cambrian rocks and Ordovician sequences with many characters in common. The oldest rocks in the western three include altered mafic/intermediate volcanics, volcaniclastic sedimentary rocks, and shale, of Cambrian age. Each belt is characterised by a thick Ordovician sequence of redeposited eraton-derived quartzose sandstone, siltstone and black shale lacking calcareous rocks and intercalated volcanics. Linkages are also suggested by the mapping of the same unit within adjacent terranes (the Early Ordovician Howqua Shale and the Middle-Late Ordovician Mount Easton Shale in the Melbourne and Howqua - Tabberaberra Terranes), and the likelihood that the Wagga - Omeo Terrane metamorphics were derived from rocks similar to those of the eastern Howqua - Tabberaberra Terrane. The duration of Ordovician sedimentation varies between the terranes and their subsequent histories diverge. We thus consider it prudent not to combine them, but clearly their independent status remains to be established. Amongst differences we note are the possibility of Late Cambrian (Delamerian)

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deformation in the western part of the Stawell - Bendigo Terrane, the absence of Early Silurian (Benambran) deformation from the Melbourne Terrane despite its presence in both the Stawell - Bendigo and Howqua - Tabberaberra Terranes, and the restriction of exposed medium-high grade metamorphics to the Wagga - Omeo Terrane. Although the eastern part of the Kanmantoo - Glenelg Terrane is characterised by quartzose sandstone and siltstone similar to that in terranes further east, a Delamerian age for the deformation of this region is well established. Exotic terranes in the Tasman Fold Belt System By the end of the 1970's it had become clear to many of those working in the Tasman Fold Belt System that all its elements might not be autochthonous. Amongst masses that have been variously considered independent elements accreted within the major fold belts are the Hastings and Silverwood blocks of the New England Fold Belt, the Gympie Terrane, microcontinental blocks considered to form the basement of the Molong - Monaro and Melbourne Terranes (Molong and Victorian Microcontinents), and various fragments within the Woolomin, Cockburn and Narooma Terranes. It has yet to be demonstrated that the whole of the New England Fold Belt is not grossly allochthonous with respect to those parts of the Tasman Fold Belt System further west» At least a tenuous linkage of the Lachlan terranes to a cratonic mass is indicated by the widespread quartzose sandstones that characterise many Ordovician sequences. Although this linkage is assumed to be with shield areas of Australia this has yet to be conclusively established. On the other hand it is of considerable significance that despite subduction of the oceanic lithosphere of the Ur-Pacific for a total period in excess of 200 million years during the evolution of the Tasman Fold Belt System no major continental collision is recognised. In terms of east Gondwana tectonics this period can be extended by perhaps a further 130 million years by considering the history of New Zealand, still without convincing evidence of a major continental collision. The main exotic elements carried to the convergent eastern margin of Gondwana throughout this period were arcs and microcontinental blocks. Accretionary

subduction complex terranes of New England

Three of the terranes recognised in the New England Fold Belt comprise accretionary subduction complexes. Each is composite, being broken by faults and/or melange zones most of which are surfaces along which the upper levels of the crust on the subducting plate were stripped from the downgoing lithospheric slab. A strict reading of the definition of tectono-stratigraphic terranes requires that each fault-bounded slice be considered a terrane, for the history of each differs from that of the adjacent slice. However such division seems, at least at the moment, to be of little value. Instead we have divided the complex into three terranes based on age, internal structure and the relative proportions of basaltic ocean floor, pelagic sediment, and trench turbidite sandstones, in the slices. Differences in the latter two characters reflect important changes in both accumulative and tectonic history and are sensibly related to the inferred ages of off-scraping. The boundaries of the terranes, and important divisions within the Texas Terrane, provide valuable markers for removing the effects of rifting, strikeslip faulting and oroclinal bending that disrupted the Fold Belt, mainly in the Permian. Using the resulting reconstruction the pattern of sedimentation which gave rise to the distinctive Early Permian overlap sequence is accounted for, as is the nature of the accompanying igneous activity. Amalgamation and accretion history of the Tasman Fold Belt System The southern part of the Tasman Fold Belt System can be viewed as comprising three 'super-terranes' coincident with the New England Fold Belt, the Lachlan

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Fold Belt and an extended Kanmantoo Fold Belt. Terranes within the Kanmantoo Fold Belt had been amalgamated by the end of the Late Cambrian - Early Ordovician Delamerian Orogeny. We treat the Silurian and Devonian sequences of the Lachlan Fold Belt as overlap assemblages and consider that the basic pattern of terranes recognised in this element had emerged by the end of the Late Ordovician - Early Silurian Benambran Orogeny. Subsequent dispersal could have involved movements aggregating a few hundred kilometres. Although we recognise provenance linkages between terranes in the New England Fold Belt that are as old as Early Devonian^ con^lete amalgamation is indicated by overlap and stitching rocks of Early Permian age. Accretion of these 'super-terranes' to the.eratonic mass of south-central Australia occurred sequentially. The Kanmantoo Fold Belt evolved close to the craton margin and can be linked to it by rocks at least as old as Middle Cambrian. Overlap sequences clearly tie the Lachlan Fold Belt to the Kanmantoo by the Early Devonian. Provenance linking of the New England to the Lachlan Fold Belt is first convincingly recognised in the Late Carboniferous but transcurrent movement parallel to the contact could have continued until the Early Permian. The mechanisms of amalgamation of the separate terranes constituting the fold belts, and the accretion of these composite terranes to the craton, have engendered wide discussion, mainly in a plate-tectonic framework. In the 1970* s siibduction was seen as the dominant process but more recently the role of transcurrent faulting has been increatsingly recognised. Published with permission of the Secretary, New South Wales Department of Mineral Resources.

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Figure 1

/LOLWORTH^ /^RAVENSWOO^^^

/^Nambucca k PT MACQUARIE )LOMIN

Is, '

Okm

^4narooma

W.TASMAN \y

r 1

/ / TfJUBILEE-AOAMSRELO

DUNOAS-TYENNAN

Preliminary schematic map of suspect terranes in the Tasmanides (modified from Cawood and Leitch, 1985; Scheibner, 1983). C Clark River Fault; G - Gilmore Suture; GC - Gray Creek Fault Zone; GT - Gambier-Tamar Fracture Zone; H - Heathcote Greenstone Belt and Mount William Fault; HMG - Hunter-Mooki-Goondiwindi Thrust System; K - Kiewa Thrust (Suture); LI - Long Plain-Indi Fault Zone; P - Peel Fault System (Thrust); W - Mount Wellington Greenstone Belt; WO - Woorndoo Fault; Y - Yarrol Fault System (Thrust); rift and rifting denotes zones of Meso-Cenozoic riftingc

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TABLE 1: East Australian Terranes

Terrane

Age of pre-accretion strata

Bounding elements

Terrane Type

Inferred tectonic character

West Tasmania

ProterozoicM. Cambrian

Fault beneath Dundas Trough

? Composite

Passive margin basement fragment

Tyennan

Precambrian

Faults beneath Dundas § AdamsfieId Troughs

Metamorphic

Passive margin basement fragment

Jubilee

Precambrian

Fault beneath Adamsfield Trough. Tamar Fractures.

? Composite

Passive margin basement fragment

East Tasmania

E.OrdovicianE. Devonian

Tamar Fracture System

L. Proterozoic

Woomdoo F.

Metamorphic

Passive margin sequence r?back-arc basin)

Wonominta

ProterozoicE. Cambrian

Faults beneath Bancannia Trough § Darling Basin

Composite

Passive margin basement and cover (in part)

Cooper

? PrecambrianE. Palaeozoic

Cork-Diamantina River -Lake Blanche F.

LolworthRavenswood

Precambrian? Ordovician

Clarke River F.

? Composite

?Arc back-arc basin

StawellBendigo

E. CambrianM. Ordovician

Woomdoo F. Mt IdaMclvor-Mt WilliamDjerriwarh F.

Stratigraphic

?Marginal Sea (foreland basin)

Melbourne

E. CambrianL. Ordovician

Mt Ida etc. Mansfield F.

Stratigraphic

Marginal plateau, microcontinent

HowquaTabberabbera

?CambrianOrdovician

Mansfield F. Kiewa F.

WaggaOmeo

E. OrdovicianL. Ordovician

Kiewa F. GilmoreLong Plain-Indi F.

Metamorphic

Girilambone

?L. ProterozoicCambrian

Gilmore F. Parkes F.

Metamorphic

AnakieNebine

?L. ProterozoicCambrian

Obscured by overlap sequences

Metamorphic

MolongMonaro

?L. CambrianL. Ordovician

Parkes-Gilmore-Long Plain-Indi F.,unestab.

Stratigraphic

Island arc (in part)

Narooma

Ordovician

Unestablished

?Disrupted

?Accretionary subduction complex

HodgkinsonBroken River

?ProterozoicCarboniferous

Palmerville-Burdekin -Clarke River F.

?Composite

TamworthYarrol

CambrianE. Carboniferous

Fault beneath SydneyBowen Basin. Peel Fault- Stratigraphic Yarrol-Philpott-Mimosa F.

Arc fringe-forearc basin

Wisemans Arm

Early Devonian

Peel Fault System Stratigraphic Mt Abundance-Goat Mt F.

Outer fore-arc basin

Woolomin

Early Palaeozoic

Peel Fault System Mt Abundance-Goat Mt F. Disrupted Spring Ck F.

Accretionary subduct ion complex

Cockbum

Devonian

Spring Ck F.Bundarra Granites

Disrupted

Accretionary subduct ion complex

Texas

Carboniferous

Bundarra granites

Disrupted (Composite)

Accretionary subduct ion complex

Wandilla

Carboniferous

Gympie

Carboniferous E. Triassic

KanmantooGlenelg

Yarrol-Philpott-Mimosa F. Disrupted "Widgee serpentinite (?Composite) belt" "Widgee serpentinite Stratigraphic belt" (Harrington § Korsch)

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??back-arc basin

?Marginal sea (foreland basin) Marginal Sea

Accretionary subduct ion complex Arc-fore-arc

TERRANES OF THE WONOMINTA BLOCK, FAR WESTERN NEW SOUTH WALES E . G . Leitch, B . D . W e b b y , K . J . Mills and P . Kolbe Department of Geology & Geophysics, University of Sydney, Sydney, N S W , Australia The Wonominta Block encompasses little known Precambrian rocks, and associated Early Palaeozoic strata, that emerge from beneath a cover of Devonian and younger sediments in the Wilcannia-Tibooburra-Scopes Range region of northwestern New South Wales. To the west the older rocks disappear beneath the thick Devonian fill of the Bancannia Trough and to the east they are buried by rocks of similar age that comprise the Darling Basin. Cretaceous units surround inliers of the Block around Tibooburra and completely cover it further north, whereas to the south it is obscured by Quaternary materials of the Darling River flood plain and Tertiary strata of the Murray Basin. Previously the Wonominta Block has been treated as a single terrane but it is suggested herein as representing a composite mass of four different terranes. The distinctive characteristics of these divisions and their tectonic significance is s\:iinmarised below: Mt Wright terrane; The Mt Wright terrane comprises rocks of Early to Middle Cambrian age exposed to the west of the Mt Wright Fault. In ascending order the sequence is mafic-intermediate volcanics (Mount Wright Volcanics) containing lenses of archaeocyathan limestone of Early Cambrian (? Atdabanian) a g e , silicic ash-fall tuff and sandstone with lenses of archaeocyathan limestone (Cymbric Vale Formation) of Late Atdabanian-Early Lenian a g e , silicic ash flow tuff, breccia and conglomerate (unnamed), and fine quartzose sandstone, siltstone and limestone (Coonigan Formation) of Middle Cambrian (Ordian-Templetonian) age. Although published maps and sections indicate that the Cambrian rocks rest on older metamorphosed 'Wonominta Beds' we have found no evidence for such a relationship, although close to the Mt Wright Fault a narrow strip of limestone, dolomite and tuff may be interpreted as stratigraphically underlying the Mount Wright Volcanics. The rocks of the Mt Wright terrane disappear to the west beneath unconformably overlying latest Cambrian-Early Ordovician and Devonian sequences. They extend west of the Lawrence Fault and are presumed to be bounded by faults beneath the Bancannia Trough. Wertago terrane; Metamorphic rocks belonging to at least two discrete units, that outcrop between the Koonenberry Fault and the Mt Wright Fault and its covered extension to the north-northwest, make up the Wertago terrane. One u n i t , assumed to be the older and characterised by multiply deformed foliated phyllite, schist and amphibolitic greenschist, crop out in zones a few kilometres wide adjacent to the bounding faults. Between these zones, in fault contact with the multiply deformed rocks, is a very thick unit of simply folded and cleaved slate, slaty sandstone, dolomite, limestone and quartzite that contains a prominent metabasalt horizon. Near Nundora Station the stratigraphically highest part of this unit consists of interbedded quartzrich lithic sandstone and shale. Both units are intruded by altered dolerite dykes, and metarhyolite sills are present locally in the simply deformed u n i t . The mass of mafic volcanics at Mt Arrowsmith occurs along strike from the metabasalt horizon and is included in the Wertago terrane, but the significance of Middle Cambrian sedimentary rocks, exposed in a fault-bounded syncline just to the w e s t , is yet to be established.

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Rocks of the terrane are unconformably overlain by Early Ordovician and Devonian quartzose sandstone dominated sequences, but otherwise there is little direct evidence of their age. Lithological comparisons suggest correlation with the Early Proterozoic Willyama Complex and the Late Proterozoic-(?)Cambrian Torrowangee Supergroup of the Broken Hill region. Kayrunnera terrane; Most of the Wonominta Block exposed east of the Koonenberry Fault comprises redeposited quartz-rich lithic sandstone and slate which have yielded rare trace fossils and siliceous sponge spicules suggesting an Early Cambrian age. Lithological correlation with the sandstone and shale around Nundora may be indicated. However despite this, aeromagnetic maps show a great change in the overall magnetic character of the crustal rocks across the Koonenberry Fault. To the west the Wertago terrane is characterised by a strong northnorthwest trending magnetic grain, with well-defined laterally continuous high amplitude anomalies, whereas to the east magnetic contours are much more widely spaced, the magnetic grain runs east-west, and there is an absence of alongate high-amplitude anomalies. This indicates the Koonenberry Fault is a major boundary separating areas of contrasting basement rocks and hence it is interpreted as the boundary between the Kayrunnera and Wertago terranes. Late Cambrian fluvial-shallow marine strata unconformably overlie the older rocks of the Kayrunnera terrane, and in turn are overlain by Devonian quartz sandstone sequences. Early Ordovician carbonates (Kandie Tank Limestone) are also unconformably overlain by the Devonian sequence. Tibooburra terrane; Quartz-rich sandstone, slate and mafic extrusive and/or shallow level intrusive rocks, regionally metamorphosed to low greenschist facies assemblages, emerge from beneath Mesozoic cover around and southwest of Tibooburra. The rocks have been intruded by latest Silurian-earliest Devonian (410Ma) granite and show simple trace fossils which, as they occur in rocks of relatively deep-water facies, probably indicate an Early Palaeozoic age. The oldest strata overlying these rocks are Cretaceous, and although they may be of similar age to the older exposed rocks of the Kayrunnera terrane they might equally be significantly younger, perhaps even Ordovician. Discussion; On the basis of the lithological correlation of the units of the Wertago terrane with those of the Late Proterozoic-earliest Palaeozoic cratonic margin of Australia in the Broken Hill district it is suggested that this terrane is essentially autochthonous. The younger unit in the terrane probalby comprises slope and rise sediments deposited on basement blocks (older unit) that foundered during formation of a passive continental margin in Middle or Late Proterozoic times. By the start of the Palaeozoic the basement rocks of the Kayrunnera terrane may have been jiixtaposed along the eastern edge of the Wertago terrane with sedimentation across the suture which is an early manifestation of the Koonenberry Fault. There is no evidence to suggest that subduction caused amalgamation of these terranes, and strike-slip faulting is considered more likely. Similar movements may have carried the Tibooburra terrane into its present position, or it may be autochthonous or very nearly so, and expose distal passive margin rocks of similar age to the fluvial-deltaic shelf sequences (Gnalta Shelf) of Late Cambrian-Early Ordovician age preserved on the terranes to the south.

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The present position of the Mt Wright terrane appears anomalous, sitting as it does with its volcanic dominated Early Cambrian sequence, between the cratonic mass of Australia and Early Cambrian rocks of the Kayrunnera terrane, and possibly the Wertago terrane, which contain abundant cratonic detritus but none of volcanic derivationc Although it is possible that these sequences are of earliest Cambrian (Tommotian) age, predating the Mount Wright volcanism, their thickness and the presence of Atdabanian faunas in the upper part of the Mount Wright Volcanics makes this unlikely. Thus the Mt Wright terrane may be allochthonous. Craton derived detritus is first recognized in the Middle Cambrian (OrdianTempletonian) Coonigan Formation, and hence by this time the terrane was close to the continental margin. Overlap sequences clearly tying it to the Wertago and Kayrunnera terranes are no older than latest Cambrian. Until the accretionary path followed by the Mt Wright terrane has been traced, and the magmatic affinity of its volcanic rocks established, schemes for the tectonic development of this part of the Tasman Fold Belt System, like the terrane, remain suspect.

HIGH-GRADE GNEISS TERRANES IN NEW ZEALAND Brian Mason Smithsonian Institution, Washington, D.C., U.S.A. In the South Island of New Zealand high-grade gneisses of Precambrian age are widespread in Westland and adjacent parts of Nelson province, west of the Alpine Fault (Fig* 1)• Associated with them are intrusive rocks of granitoid compositions, and rare qua£tz-free variants (Table 1). Most occurrences are as isolated hills completely surrounded by Quaternary fluvioglacial deposits, and almost all are covered with dense rain forest; hence, in spite of considerable relief, exposure is often poor and contacts with other formations are lacking. The limited amount of isotopic dating has given ages of around 650 Ma for the metamorphism, which makes them the oldest rocks known in New Zealand. The northernmost occurrence on Fig. 1 is a narrow strip along the coast at Charleston and a much larger area along the crest of the Paparoa Range to the east (Hume, 1977). Hume distinguished two major lithologies. Banded Gneiss and Leucocratic Gneiss. He described the Banded Gneiss as a complexly deformed highgrade metamorphic rock, ramging from granitic to dioritic composition. Sillimanite occurs rarely, but v^ere seen is coarsely crystalline, suggesting metamorphism in the upper part of the sillimanite zone. He interprets the Banded Gneiss as consisting of metamorphosed and partly melted sediments. The Leucocratic Gneiss is described as highly siliceous, with only a minor mafic component, and is interpreted as of magmatic origin. Rb-Sr whole rock analyses yielded an age of 680±21Ma for the gneiss at Charleston (Adams, 1975). The remaining occurrences are immediately west of the Alpine Fault, and are present intermittently over a distance of some 300 km, from the Grey River to the Haast River. Between the Grey and Ahaura Rivers Mt. Elliot consists largely of a banded series of paragneisses, well exposed in high cliffs on the south face of the mountain, overlooking the Ahaura valley. These paragneisses consist of quartz + andesine + biotite ± almandine ± hornblende; minor sillimanite is present in some specimens, and kyanite in one; accessory minerals include apatite, titanite, rutile, ilmenite, and small rounded zircon grains. Analyses of a kyanite-bearing specimen (llAA) and a sillimanite-bearing specimen (12AA) are given in Table 1. Orthogneiss of granodioritic composition (3AA, Table 1) is quarried near the southern end of Mt. Elliot; it consists of quartz + andesine + microcline + biotite, with accessory apatite, muscovite, almandine, zircon, allanite, and scapolite. The scapolite is Ca-rich (CaO 16.2%, Na20 3.9%) and sulphate-rich (SO3 3.8%); chlorine was not detected, and the calculated CO2 content for stoichiometry is 2.5%. At the extreme southern end of Mt. Elliot and abandoned quarry exposes a moderately deformed hornblende diorite (20AA, Table 1). On the south side of the Ahaura River, a small inlier about 1 km square, surrounded by alluvium, consists of quartz-rich gneisses, some of which resemble in hand specimen those on Mt. Elliot. However, neither scapolite nor almandine has been found in them. The rock in a small quarry has been analyzed (6HP, Table 1); it is a gneissose tonalite consisting of quartz + andesine + biotite + muscovite.

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Granite Hill is a subcircular massif, about 5 km in diameter, rising some 3000* above the surrounding lowland. Outcrop is poor because of dense forest, but float indicates that the hill consists almost entirely of gneiss, with little or no true granite. Most of the gneisses are tonalitic in composition (4CR, SCR, Table 1), consisting of quartz + andesine + biotite ± almandine ± hornblende ± sillimanite; accessory zircon (rounded) and apatite are present, and a little K-feldspar in some; both ortho- and paragneisses are probably presento Some outcrops are rich in almandine and hornblende and hence less silicic (6CR, Table 1). Similar gneisses {12R0, Table 1) make up most of a small inlier (2 x 1 km) 3 km SW of Granite Hill. At the NE end a quarry exposes a gneissose granodiorite, possibly an augen mylonite (5R0, Table 1); it has orthoclase porphyroclasts up to 2 cm long/ and the quartz is granulated. Along the SW margin of this inlier the gneisses cure intruded by a metanorite (8R0, Table 1) , consisting of bytownite + hornblende (sometimes with hypersthene cores) + cummingtonite + biotite + serpentinized olivine; it is undeformed and is probably much younger tham the gneisses. It is probably a sill injected along a bounding fault. Feldspathic gneisses, similar to some of those on Granite Hill, are present on the SE side of Mt. Turiwhate, but have not yet been investigated. Farther to the SW, Mt. Upright, at the head of Lake Kaniere, consists largely of hornblende-rich gneiss - quartz + andesine + hornblende ± biotite ± almandine. Between the Hokitika and Mikonui Rivers is a belt of biotite-rich gneisses with minor areas of gneissic granite, 13 km long and up to 5 km wide. Young (1968) described these rocks, noted extensive mylonitization, and introduced the name Eraser Fromation . c . "to bring into one grouping the cataclastically deformed rocks and the associated gneissic granites, not significantly cataclastically deformed, that lie between the Fraser Fault and the Alpine Fault." Not all the gneisses are cataclastically deformed; Hattori (1967) described two boulders from streams draining to the Hokitika River as relatively undeformed sillimanite-almandine-biotite gneisses, and I have collected similar rocks in streams draining to the Mikonui River. Gneissic grauiodiorite is excellently exposed along a 1 km gorge in the Hokitika River. The rock (2HG, Table 1) consists of quartz + orthoclase + andesine -H biotite (partly chloritized) + muscovite; it is not noticeably gneissic in thin section ^ but the quartz is partially granulated and shows undulose extinction. Hornfelsic enclaves (3HG, Table), consist of quartz + labradorite (largely altered to zoisite) + biotite + muscovite. The gneisses continue as a naorrow strip immediately west of the Alpine Fault from the Mikonui to the Waitaha River, and then expand to form the Mt. Bonar massif. Mt. Bonar consists almost entirely of gneisses similar to those already described; their composition is quartz + andesine + biotite ± almandine ± K-feldspar ± sillimamite. Aplitic granite is present on the north end of Mt. Bonar, exposed in a small quarry on the road along the south side of the Waitaha River; it consists of quartz + oligoclase + orthoclase + almamdine + muscovite (traces). Southwest from Mt. Bonar there is a gap of some 35 km before the next occurrence of gneisses at Whataroa. A quarry 1.5 km S of Whataroa has been opened in an orthogneiss of granitic composition (5WH, Table 1); the rock has been cataclastically deformed, the

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quartz and feldspar being extensively granulated. Other outcrops are of paragneisses and migmatites, consisting of quartz + andesine + biotite ± almandine ± K-feldspar (3WH, Table 1); no sillimanite was seen in these specimens. Beyond Whataroa to the south no gneisses west of the Alpine Fault have been recorded for some 120 km, until the Haast River is reached. Small areas of granitoid rocks are present, but they are undeformed and intruded into Greenland Group (Ordovician) greywackes. One of these is the pyroxene-quartz diorite of Canavan Knob (2CK, Table 1); it consists of quartz + andesine + orthoclase + biotite + hypersthene (Wo3En5o) + augite {^O^^ETI/^Q) + ilmenite. The Haast River bisects an area of gneisses forming Mosquito Hill on the north side and Zillian Hill on the south. A quarry on Zillian Hill exposes a medium-grained biotite-rich gneiss (6HA, Table 1), consisting of quartz + amdesine + biotite + muscovite orthoclase, with accessory apatite, zircon, and epidote. A small area of similar rock forms the north abutment of the Haast River bridge; this rock is massive, equigranular (average grain size 0.3 ram), quartz-rich with labradorite, biotite, and muscovite, and is probably a non-gneissic variant of the Zillian Hill rock. Possible hypotheses for these gneiss terranes are: (a) they are isolated outcrops of a basement complex underlying the New Zealand microcontinent; (b) they represent a sliver derived from the West Antarctic basement of Marie Byrd Land; (c) they are independent terraines derived from widely separated sources and brought to their present position during long-continued movements along the PacificAustralian plate boundary. References Adams, C.J., 1975, Discovery of Precambrian rocks in New Zealand: age relations of the Greenland Group and the Constant Gneiss, West Coast, South Island, Earth and Planetary Science Letters, 28, 98-104. Hattori, H., 1967, Occurrence of sillimanite-garnet-biotite gneisses and their significance in metamorphic zoning in the South Island, New Zealand, New Zealand Journal of Geology and Geophysics, 10, 269-299. Hume, B.J., 1977, The relationship between the Charleston Metamorphic Group and the Greenland Group in the central Paparoa Range, South Island, New Zealand, Journal of the Royal Society of New Zealand, 7, 379-392. Young, D.J., 1968, The Fraser Formation in central Westland, New Zealand, New Zealand Journal of Geology and Geophysics, 11, 291-311. Key to Table 1 (grid references in parentheses)

8R0: Metanorite. Rocomanu (S52/043623); 6CR: Hornblende-garnet gneiss. Crooked R., Rotomanu (S52/105642); 20AA: Hornblende diorite. Ahaura R. (S52/293783); 2CK: Pyroxene-quartz diorite. Canavan Knob, Waiho (S71/8157AO); 3WH: Garnet gneiss, Waitangi-taona R.. Whataroa (S71/9478S2); 12R0: SiUiaanitegarnet gneiss, Rotomanu (SS2/05562S); 6HP: Gneissose tonalite, Haupiri (SS2/300765); SRO: Gneissose granodiorite, Rotomanu (S52/061682); SCR: Gneissose tonalite. Crooked R., Rotomanu (S52/075647); 12AA: Sillimanite-garnet gneiss, Troulands Creek, Ahaura R. (SA5/281826); 2HC: Gneissose granodiorite, Hokitika Gorge (S58/565264); 5WH: Gneissose granite, Whataroa (S71/992866); 6HA: Biotite gneiss, Zillian Hill. Haast R. (S87/847107); 3AA: Gneissose granodiorite, Ahaura R., (S52/287793); 4CR: Garnet-hornblende gneiss. Crooked R.. Rotomanu (S52/100642); llAA: Kyanite-garnet gneiss, Troulands Creek, Ahaura R. (S45/281826); 3HC: Hornfels enclave in migmatite, Hokitika Gorge (SS8/565264).

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THE

DISSECTION OF OLD TRAPPED OCEANIC CRUST IN SOUTHEAST ASIA NEOCENE TERRANE TRANSLATIONS AND ISLAND ARC GROWTH.

Robert McCabe and Chao Shing Lee Both at Department of Geophysics and Geodynamics Research Texas A&iM University, College Station, Texas, U.S.A. 77843

Program,

INTRODUCTION The Banda Sea, Celebes Sea, and Sulu Sea are three poorly understood marginal seas located between the South China Sea and Australia. Marine geophysical investigations from the Celebes and Sulu Basins (Murauchi et al., 1973) and the Banda Sea (Bowin et al., 1980) show that each of these basins are underlain by oceanic crust. The complex geological arrangement of islands in this region, imcomplete data sets from each of these three marginal seas, and complex late Cenozoic tectonics of the region has complicated any tectonic interpretation of this region. On the bases of magnetic anomalie data, measured heat flow data and bathymetry, we speculate that the three southern seas, Banda Sea, Celebes Sea, and Sula Sea are the once continuous remanents of a trapped Mesozoic and early Tertiary ocean basin. Bowin et al., (1980) suggested a similar scenario for the Banda Sea which he suggested was related to the late Jurassic Argo Plain off northwest Australia. Here we extend Bowin et al.'s, trapped basin to include the Celebes, Sulu and perhaps the southwestern portions of the South China Sea. On the bases of marine geophysical data we suggest that the Sulu Sea, Celebes Sea, and Banda Sea formed a continous ocean basin during the Cretaceous and early Tertiary Times. This conclusion is supported by the fact that the various islands that currently fragment this older basin arrived at their present location either by Neogene tectonic movements or were built in place by late Neogene subduction. This late Neogene subduction resulted in the shorting of this old basin by subducting portions of this basin under the Banda Arc, North Arm of Sulawesi, and Sulu Archipelago. Below we present a speculative model to explain the evolution of this region. Late Jurassic to early Cretaceous Similar trending magnetic anomalies in the Banda Sea and Argo Plain suggest that rifting begins off of the North Australian Margin during this period. Timor forms a portion of the southern rifted margin. We have not attempted to identify the continental fragment which was rifted off of this margin. We predict that this fragment likely forms portions of Sunda or Indochina. Cretaceous to Eocene The fact that our identified anomalies are only one limb of the spreading system suggest that the other limb was being subducted beneath the Asian Continental Margin during this period. The presence of Cretaceous and Eocene granitiod batholiths along the coast of southeastern China (Jahn et al., 1976) and Eocene volcanic

149

rocks in the Ryukyu Islands (Sasajima, 1977) are likely the products of this subduction. Since v/e only recognize one limb of this spreading system, we suggest that spreading was terminated in the Eocene when the spreading ridge was subducted. We further suggest that the subduction of this ridge resulted in SE extension (possibly accompanied by back-arc spreading ) in southern China. As a result of this extension of the Dangerous Ground-Reed Bank region migrated southward. We suggest that the ENE trending anomalies in the western portion of the South China Sea (Taylor and Hayes, 1980; Bowin et al., 1978), north of the Dangerous Grounds, are the products of this extension episode. Eocene to middle Miocene Subduction began along the western margin of the Philippines (DeBoar et al., 1980; McCabe et al., 1982; Tamasis, 1982; Uyeda and McCabe, 1983) Paleoinclination data from the Philippine Arc and Philippine Sea suggest that the Philippine Arc formed the western edge of the Philippine Sea Plate during this time (McCabe et al., recently submitted manuscript). As a result of this eastward dipping suduction the Philippine Islands and the Philippine Sea translated to the northwest over the Sulu-Celebes-Banda ocean basin. Middle Tertiary (Oligocene?) extension along the eastern margin of the Sunda Shelf (Bishop, 1980) and subduction under Sulawesi migrates the island eastward (Hamilton, 1979) . Oblique convergence in the equatorial Indo-Pacific region due to the westerly motion of the Pacific and Caroline Plates relative to Australia slivers off portions of northern Australia (Sula Microcontinent, Buru, Ceram) and translates them westward tov/ard Sulawesi into the northern Banda Basin (Hamilton, 1979, Silver and Smith, 1983). Rifting of the Palawan Microcontinent soutxhward during Oligocene to Miocene opening of the eastern South China Sea.

the

Middle Miocene to Pleistocene Collision in central Philippines (Hamilton, 1979; McCabe et al., 1982) reverses the subduction polarity of the Philippine Arc to the present Philippine Trench Collision of the Sula Microcontinent with Sulawesi results in reversal of subduction polarity and clockwise rotation of the North Arm (Silver et al., 1983 a,b). As a result of this rotation, the old Sulu-Celebes-Banda Basin begins subduction under the North Arm of Sulawesi. Subduction results in the development of the Sulu Island Arc. This subduction begins consuming portions of the Sulu-Celebes-Banda Basin in this region. Initiation of subduction along the southern portion of the Banda Sea begins in late Miocene (Hamilton, 1979) . This subduction results

150

in the islands of inner volcanic arc north of the southern Banda Sea. As a result of this convergence, portions of the older oceanic basin are subducted and the rifted basement of Timor is thrust southward over continental Australia (Hamilton, 1979) .

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GEODYNAMICS AT ACTIVE PLATE MARGINS Mike M c E l h i n n y Bureau of M i n e r a l R e s o u r c e s , Canberra The relative motions of c o n t i n e n t a l and oceanic crust at an active plate margin can be considered in terms of the evolution of triple j u n c t i o n s . Using the northeast Pacific as an e x a m p l e , it is shown how the motion leading to subduction at the c o n t i n e n t a l margin of western North America is converted into strike-slip motion along the m a r g i n . The motion that ultimately produces the elongation of accreted terranes is initially a direct consequence of plate tectonic t h e o r y . The precise mechanism(s) of elongation are effects superimposed upon this basic m o t i o n . The value of p a l e o m a g n e t i c studies in determining the motion of accreted terranes depends upon the geometry of the s i t u a t i o n . W h e n considering studies of the eastern margin of the Australian c o n t i n e n t , the geometry of the situation in SiluroDevonian times appears especially f a v o u r a b l e , but this is not the case in Jurassic-Cretaceous t i m e s .

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TECTONIC HISTORY OF THE NORTH PACIFIC BORDERLANDS AND SEVERAL RELATED NORTH AMERICAN HYDROCARBON FIELDS J.R.H. McWhae Petro-Canada Resources, Calgary, Canada Tectonic History A distance of 800 km separates the stable shelf of the Canadian craton at the Laramide thrust belt from the subduction zone off Vancouver Island (Figure 1). Three tectonic processes extended the North American Plate westward since mid-Jurassic time: 1) Accretion of exotic terranes from the south, 2) westerly jumping subduction zones, and 3) northwesterly trending right-lateral faults also shifting oceanward. The Queen Charlotte-Fairweather transform fault extends from the Aleutian subduction zone in the north to the relics of the Farallon Plate at the subduction zone off Vancouver Island. This forms the present boundary of the Pacific and North American Plates. The Fairweather Transform terminates in a series of western splay faults connecting the Aleutian subduction zone and the Boundary Ranges Fault. The latter curves westward to pass along the southeastern side of the Cook Inlet forearc basin. The shoreward extension of the active Aleutian volcanic arc passes along the northwestern side of Cook Inlet. The Aleutian subduction zone was initiated by a southward jump from Alaska and northeast Siberia with the accretian of the exotic Okhotsk Terrane in Late Eocene time. This was coeval with the end of the main Laramide Orogeny and the elbow in the hotspot-induced Emperor Seamounts-Hawaiian Island Chain about 43 Ma ago. The subsequent more westerly component of Pacific spreading, following the Hawaiian Islands trend, then contributed to the right-lateral movement along the Kuril Arc subduction zone that was transmitted to the Kaltag Fault in post-Eocene time. The Kaltag Fault now extends northeastward from the Kuril Arc across the Bering Sea and Alaska. After a northerly deflection and splaying near the Mackenzie Delta adjacent to the stable Canadian Shield, it continues northeastward along the outer Canadian Arctic Islands. The Kaltag Fault appears to have been active in Late Cretaceous ^ e n the transtensional Norton Basin developed between 85 and 70 Ma ago immediately north of the Yukon Delta where this fault passes into the Bering Sea. Early Cretaceous tectonic history was dominated by continent-to-continent collision of the noses of the North American and Eurasian Plates when the Kolyma or the Omolin exotic terrane rafted between them in Hauterivian time. Subsequently, the Canada Basin seems to have spread about a pole southeast of the Mackenzie Delta between 125 and 85 Ma ago. This new oceanic segment and the adjacent continental Alaskan Plate were then ruptured from the North American Plate along the Kaltag Fault zone and becaune bonded to the northeasterly moving Eurasian Plate since 85 Ma. The Kaltag Fault has continued to act as the suture between the Eurasian and North American Plates until today with very different types of structures developed on either side including Neogene folds on the northwest side near the Alaskan-Yukon border. Hydrocarbon Fields and Prospective Basins British Columbia, West Coast One well out of thirty wildcats located in the West Coast insular belt encountered a weak oil show. This was the Shell Sockeye BIO well in the Queen Charlotte Basin that penetrated 4500 m of Plio-Miocene continental to restricted

153

marine clastics with the oil show at 1000 m. Poor reservoirs are typical of arc-related basins seaward of accreted terranes composed mainly of oceanic sediments and basic volcanics. The reservoirs tend to improve upwards (become more quartzose) in the Tertiary with the unroofing of shoreward granite plutons formed by collision of Cretaceous terranes with the North American Plate. Porosity should be best developed in the Upper Cretaceous and younger marine sandstones and turbidites deposited in nonvolcanic phases. Fracture porosity may be important and it probably accounts for the bitumen shows in the Lower Miocene volcanics of the Queen Charlotte Islandso Abundant marine source sediments are present from the Upper Triassic to Lower Jurassic Kunga shales and limestones of the Queen Charlotte Islands. These approach oil shale richness in organic carbon and are followed by almost continuous deposition of marine shales on the ocean side of the islands. These source beds compare very favorably with those of Cook Inlet. No simple anticlines have been identified as in Cook Inlet. Pinchout traps and complex small fold-fault traps associated with transform faulting may be expected. Cook Inlet Basin Continental Tertiary sandstone, shale and coal beds more than 5 tan thick were deposited in the inner part of Cook Inletc The basin is bounded by major converging faults that were probably initially strike-slip following the Late Eocene inception of subduction but changed to extensional faults in Oligocene to Late Tertiary time when thick arkosic sandstone and conglomerate reservoirs were deposited® Unmetamorphosed Cretaceous and Jurassic sediments unconformably underlie the Tertiary clastics, including Middle Jurassic marine source shales and Cretaceous sandstone reservoirs. Renewed strike-slip faulting in Late Pliocene formed transpressional anticlines in the converging inner basin. These anticlines contain both oil and gas. The oil is believed to have migrated from the Middle Jurassic shales when Tertiary sedimentation exceeded the migration thickness-heat threshold. Most of the gas was sourced by the Tertiary coals. The reservoirs vary from 8 to 20 percent in porosity, including secondary porosity from the solution of feldspars in the moderately arkosic sandstones. The provenance area lies to the west and north where granites and older Palaeozoic sediments are exposed in a belt 200 to 300 km wide. The first Cook Inlet oil was produced in 1959 and exceeded 200,000 bbls/day from 1969, but it is now declining; gas production reached 400 MMcf/d in 1969 and has doubled since. Prudhoe Bay The supergiant Prudhoe Bay oil field straddles the west-northwesterly trending Barrow Arch which has collapsed on the north side, parallel with the edge of the passive margin type Canada Basin. In place heavy oil reserves of 20 to 40 billion barrels are present in uppermost Cretaceous-Paleocene sands of the western part of the field. These are underlain by more than a billion barrels of recoverable oil in the Lower Cretaceous rift sandstones. The main Prudhoe Bay field consists of recoverable reserves of 10 billion barrels in the Triassic-Permian cherty sandstones and in excess of 1 billion barrels in Mississippian limestones and basic clastics. Upper Jurassic and Cretaceous shales are the main source beds. Mackenzie Delta and Adjacent Beaufort Sea Rift and delta front sandstones and turbidites of Lower Cretaceous, Eocene, and Oligocene age lie undeveloped in this area with proven recoverable reserves in the order of 10 TCP of gas and over 1 billion barrels of oil—slightly below the economic threshold. The proximity to the Kaltag Fault suture induces a large

154

variety of structural traps (fault traps, folds and mud diapirs) changing from compressional to tensional as the ste±)le Canadian Shield is approached. Source rocks range in age from Late Jurassic to Tertiary. Reference McWhae, J.R.H., 1985, Ms, Tectonic history of the north Pacific, Arctic Canada and Spitsbergen region since Early Cretaceous time. AAPG?

COO INLET

PACIFIC PLATE

QUEEN CHARLOTTE ISLANDS J FARALLON PLATE Rslics Figure 1 inaecurat* map projaetlon aceentuatas tha band In tha Kaltag Fault. Mid Juraaalc adga of North Amarlcan Plata - JM.

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TECTONOSTRATIGRAPHIC TERRANES IN THE JAPANESE ISLANDS AND THEIR ENVIRONS IN EAST ASIA Shinj iro Mizutani Department of Earth Sciences, Nagoya University, Nagoya, Japan Since the first report on radiolarian fossils was published in Japan by Yehara (1926), about 250 papers have been written by Japanese paleontologists and biostratigraphers in the succeeding sixty years. Of these, two-thirds have been published in the 1980's. Dealing with radiolarian skeletons extracted from shale, tuffaceous shale, tuff, siliceous shale and chert, most of the papers discuss the age, assignment and biostratigraphy of late Paleozoic and Mesozoic groups. Published data, rapidly acciamulated, are simmarized together with unpublished information, and twenty radiolarian assemblages, ranging in age from Permian to Cretaceous identified in Southwest Japan (Nakaseko et al, 1983; Mizutani, 1984). The results have forced us to reappraise our ideas about all the older sedimentary complexes in the Japanese Islands. It is revealed that almost all of the sedimentary complexes which were believed to be of Permian or of Permian to Triassic age are composed of olistostrome complexes made up of Permian limestone and Triassic chert included in Jurassic shale and sandstone (e.g., Mizutani et al., 1981; Ishiga, 1983; Yamamoto, 1985). A Permian olistostrome complex consisting of Permian limestone, tuff and chert in Upper Permian shale is also confirmed to be distributed in Southwest Japan (Miyake, 1985). Based on these lines of new evidence, structural divisions of the Japanese Islands have been re-investigated. The tectonostratigraphic terranes recognised in the Japanese Islands are, from Hokkaido to Kyushus Nemuro (Cretaceous to Paleogene sedimentary), Tokoro (Jurassic olistostrome), Hitaka (Cretaceous olistostrome), Sorachi (Jurassic ophiolitic), Kamuikotan (Cretaceous metamorphic and ophiolitic), North Kitakami (Jurassic olistostrome), South Kitakami (Carboniferous - Cretaceous sedimentary), Abukuma (Cretaceous metamorphic), Hida (metamorphic with Jurassic cover), Maizuru (Triassic ophiolitic), Yamaguchi (Permian olistostrome), Tamba-Mino-Ashio (Jurassic olistostrome), Chichibu (Jurassic olistostrome), and Shimanto (Cretaceous to Tertiary accreted sediments). The Sangun, Ryoke and Sambagawa belts are regarded as the metamorphic equivalent of the Yamaguchi, Tamba-Mino-Ashio and Chichibu Terranes, respectively, although much older isolated terranes are supposed to be interleaved within these belts. The structural trend and spatial distribution of the Jurassic olistostrome complex of the Tamba-Mino-Ashio Terrane (Fig. 1) suggest the northern extension of this terrane is probably located in the Sikhote Alin region. Mizutani et al (1984) discussed that a Jurassic olistostrome complex similar to the Tamba-Mino-Ashio Terrane is traceable to Krasnorechensk in the Iman River region of Sikhote Alin, where Triassic limestone blocks are embedded in clastic facies of intercalated limestone breccia, siltstone and tuffaceous shale which yield radiolarian fossils of Upper Jurassic type (Zhamoida, 1972). In the eastern part of Heilongjiang Province of northeast China, it is reported by Li et al (1979) that limestone contains Permian fusulinacean faunas, and radiolarian fossils of Jurassic type are found in shale, as displayed in the plate of their paper. The Nadanhata Terrane, so described by Mizutani and Zhang (1985, MS), and the Tamba-Mino-Ashio Terrane have much in common in their lithologic and biostratigraphic associations. Undoubtedly, the Nadanhata Terrane is composed of an olistostrome complex of Jurassic age, probably having been joined to or occupying a part of the Tamba-Mino-AshioSikhote Alin Terrane. Feng and Yang (1984) pointed out the chaotic nature of geologic formations in the Nadanhata Range zone, and stated that from a tectonic viewpoint it represents a mobile belt not connected to the mainland of China, but related to the circum-Pacific region.

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Paleogeographic reconstruction of the Japanese Islands prior to the opening of the Sea of Japan demonstrated by Otofuji and Matsuda (1983) on the basis of their paleomagnetic investigation is shown in Fig. 2 , in which all the Jurassic olistostrome complexes mentioned above are denoted. This interterrane relationship, consistent with paleogeographic discussions on the Jurassic shallow marine sediments in the Japanese Islands by Yu (1983) , or on the Mesozoic tectonics concerning the Mino Terrane by Hattori (1982) , leads to the conclusion that these areas originally formed a big terrane in the continental margin of East Asia.

References Feng, Y . and Yang, Y . , 1984, Chaotic geological bodies in China (in Chinese Chinese with English abstract), Bull. Clin. Acad. Geol. Sci., 10, 77-92. Hattori, I., 1982, The Mesozoic evolution of the Mino terrane, central Japan: A geologic and paleomagnetic synthesis, Tectonophysics, 85, 313-340. Ishiga, H., 1983, Two suites of stratigraphic succession within the Tamba Group in the western part of the Tamba Belt, Southwest Japan (in Japanese with English abstract). Jour. Geol. Soc. Japan, 89, 443-454. Li, W . , Han, J . , Zhang, S . and Meng, F . , 1979, The main characteristics of the upper Palaeozoic stratigraphy at the north Nadanhada Range, Heilongjiang Province, China (in Chinese with English abstract), Bull. Clin. Acad. Geol. Sci., 1 , 104-120. Miyake, K., 1985, Permian olistostrome complex in the Katsuyama area, Okayama Prefectoire, Southwest Japan (in Japanese with English abstract), Jour. Geol. Soc. Japan, 9 1 (in press). Mizutani, S., 1984, Co-operative research on radiolarian biostratigraphy of Mesozoic and Paleozoic groups in Japan, Rept. Co-Op. Res. Rad. Biostrat. Mesozoic - Paleozoic Groups in Japan, 1-12. Mizutani, S., Hattori, I., Adachi, M . , Wakita, K . , Okamura, Y . , Kido, S., Kawaguchi, I. and Kojima, S., 1981, Jurassic formations in the Mino area, central Japan, Proc. Japan Acad., 57 (B), 194-199. Mizutani, S., Uemura, T . and Yamamoto, H . , 1984, Jurassic formations in Niigata Prefecture and the Ashio Belt (in Japanese), Jyoetsu Belt and Ashio Belt, ed. by Chihara, K . , 1 , 44-50. Nakaseki, K . , Mizutani, S. and Yao, A . , 1983, Radiolarian fossils and Mesozoic geology of the Japanese Islands (in Japanese), Kagaku (Science), 53, 177-183. Otofuji, Y . and Matsuda, T . , 1983, Paleomagnetic evidence for the clockwise rotation of Southwest Japan, Earth Plant. Sci. Let., 62, 349-359. Yamamoto, H., 1985, Geology of the late Paleozoic - Mesozoic sedimentary complex of the Mino Terrane in the southern Neo area, Gifu Prefecture and the Mt. Ibuki area, Shiga Prefecture, central Japan (in Japanese with English abstract). Jour. Geol. Soc. Japan, 91, 353-369.

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Yehara, S., 1926, On the Monobegawa - and Shimantogawa-Series in southern Shikoku, Jour. Geogr., 38, 1-10. Yu

K M . , 1983, Sedimentological study on the early Jurassic shallow marine f a d e s in Southwest Japan and the comparison with Daedong Supergroup in south Korea, Mem. Fac. Sci., Kyoto Univ., Ser. Geol. Mineral., 49, 1-62.

Zhamoida, A.I., 1972, Biostratigraphy of the Mesozoic siliceous strata of the east of the USSR (as based on the study of radiolarians), Min. Geol., USSR, Trans. Geol. Sci. Res. Inst., (VSEGEI), New Ser., 183 (NEDRA), 1-199.

36«N

UO'E Upper Paieezoie MMdlt Jurassic Upper Paleozoic Upper Jurassic

Fig

1 - Tamba(Tm)-Mino(M)-Ashio(A) Terrane in central Japan. Dotted areas show the exposed sedimentary complex of Upper Paleozoic to Jurassic age.

Fig. 2 - Reconstructed paleogeographic relation of Tamba-Mino-Ashio-Sikhote Alin Terrane. "N": Nadanhata Range in Heilongjiang Province of China and K : Krasnorechensk of Sikhote Alin.

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PALAEOMAGNETISM AND TECTONICS OF MALAITA, SOLOMON ISLANDS: A CAUTIONARY TALE FOR TERRANE SEEKERS Robert J. Musgrave University of Sydney, N.S.W. 2006, Australia The island of Malaita is a part of the Pacific geological province of the Solomon Islands (Figure 1) (Coleman, 1966; Hackman, 1973). This province is generally believed to be a terrane accreted onto the Central province as a result of overthrusting of the southern margin of the Ontong Java Plateau, a region of thickened Pacific plate (Packham, 1973; Coleman & Packham, 1976). This is thought to have occurred at the close of a period of subduction of the Pacific plate below the Australian plate. The Central province represents the island arc associated with this process. An arc polarity reversal then followed along both the Solomon Islands and Vitiaz arcs. The Solomon Islands now lie on the Pacific plate. As part of a larger palaeomagnetic project in the Solomon Islands, 59 sites were sampled in Malaita. A Late Cretaceous to Pleistocene sequence, composed of calcareous sediments - the 'Are'are Limestones, subdivided into the Apuloto Limestones, the Haruta Calcisiltites and the Hada Calcisiltites - with intercalated Eocene volcanics - the Basalts" - was sampled throughout. Initial AF demagnetization of the limestones suffered from the effects of rotational remanent magnetization. Duplicate specimens of these limestones (together with the volcanics) were demagnetized by thermal means. The primary magnetization in the sedimentary specimens was very strongly overprinted by a viscous remanent magnetization (VRM) acquired over the Brunhes Normal polarity epoch. Konigsberger ratios were very low. Short relaxation time VRM (acquired in the laboratory) and super-paramagnetic effects were also severe. Distinction of the stability spectra of the primary component, and the long- and short-relaxation time VRM components, was more readily made under thermal than AF demagnetization. The presence in carbonates of VRMs which are relatively hard under AF demagnetization, but substantially more easy to remove by thermal means, has been noted by other workers (e.g., Achache et al., 1982). Despite these difficulties, the primary magnetization was resolved in sufficient sites to allow palaeomagnetic poles to be determined; in many sites vrtiere the primary component could not be completely isolated, its polarity could be recognized, and a contribution made to the construction of a magnetostratigraphic column. Such a column was produced for large parts of the sequence. The age of two transitional boundaries within the 'Are'are Limestones has been determined thereby (Figure 2). In particular, a notable facies change indicating marked uplift has been dated as occurring during anomaly 3 time. This suggests that the arc polarity reversal occurred in the Solomon Islands arc at approximately 5 Ma, an age very similar to that accepted for the equivalent event in the Vitiaz-New Hebrides arc (Falvey, 1978). The near simultaneity of this event over such a long arc segment has implications for the process of polarity reversal. Three palaeomagnetic poles derived from the Malaitan sequence lie surprisingly close to positions of corresponding age on the Australian apparent polar wander path, and are quite distinct from any acceptable Pacific path (Figure 3). The agreement is made even closer when the poles are rotated to account for five Ma of Pacific-Australia relative motion (Figure 4). Palaeolatitudes calculated from datable sites within the sequence do not fit the

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palaeolatitude curve derived from a DSDP site on the Ontong Java Plateau (Hammond et al., 1975). A strong case can hence be made that Malaita is not a distinct terrane accreted to the rest of the Solomon Islands arc from the Pacific plate, but is rather an integral part of the arc, having undergone arc polarity reversal and translation from the Australian to the Pacific plate along with the Central province. Sedimentological evidence also supports the view of the Solomon Islands arc as a single entity throughout its development (Turner & Hughes, 1982). The supposed similarities between the sequence exposed on Malaita and the cores recovered from DSDP holes drilled in the Ontong Java Plateau (Coleman et al., 1978) have served to deeply entrench the notion that the Pacific province is a derivation of the Plateau. It is questionable, however, whether the similarities are any greater than would be expected for two shallow marine, Tertiary, predominantly pelagic sequences, whether they had developed in juxtaposition or not. A number of differences in sedimentary style have largely been ignored. The fixity of the prevailing model for the Solomons has caused even those authors who have recognized the consanguineous origin of the Pacific and Central provinces to still insist on a link between the Pacific province and Ontong Java (e.g., Ramsay, 1982). This in turn required that the arc polarity reversal be regarded as a fiction; i.e., subduction had always been down a Benioff zone dipping north-east (Turner & Hughes, 1982), and consequently the Solomon Islands arc must have originated somewhere in the Pacific. All of these complexities can be eliminated if the Pacific province is viewed as the fore-arc zone (prior to arc polarity reversal) of the original north-east facing arc, with Pacific plate being subducted below the Australian plate. In this view the original trench is now represented by the largely compressed and infilled Ulawa Deep - North Solomons Trough. References Achache, J., Cox, A., & O'Hare, S. 0., 1982, Palaeomagnetism of the Devonian Kennett Limestone and the rotation of the eastern Klamath Mountains, California, Earth and Planetary Science Letters, 61, 365-380. Coleman, P. J., 1966, The Solomon Islands as an island arc. Nature, 211, 1249-1251. Coleman, P. J., McGowran, B., & Ramsay, R. W., 1978, New, Early Tertiary ages for basal pelagites, northeast Santa Isabel, Solomon Islands (central southwest flank, Ontong Java Plateau), Bulletin of the Australian Society of Exploration Geophysicists, 9(3), 110-114. Coleman, P. Jc, & Packham, G. H., 1976. The Melanesian borderlands and the India-Pacific plates boundary. Earth Science Reviews, 12, 197-233. Falvey, D. A., 1978, Analysis of palaeomagnetic data from the New Hebrides, Bulletin of the Australian Society of Exploration Geophysicists, 9(3), 117-123. Hackman, B. D., 1973, The Solomon Islands fractured arc, in Coleman, P. J. (ed.). The Western Pacific: Island Arcs, Marginal Seas, Geochemistry, University of Western Australia Press, Nedlands, Western Australia. Hammond, S. R., Kroenke, L. W., & Theyer, F., 1975, Northward motion of the Ontong-Java Plateau between -110 and -30 M.Y.: a paleomagnetic investigation of DSDP Site 289, in Andrews, J. E., Packham, G. H., et ai.. Initial Reports of the Deep Sea Drilling Project, 30, 415-418, U.S. Govt Printing Office.

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Packham, G. H., 1973, A speculative Phanerozoic history of the south-west Pacific, in Coleman, P. J. (ed.), The Western Pacific: Island Arcs, Marginal Seas, Geochemistry, University of Western Australia Press, Nedlands, Western Australia. Ramsay, W. R. H., 1978, Field, mineralogical, and structural observations on some basement rocks, south-east Choiseul, Solomon Islands, Bulletin of the Australian Society of Exploration Geophysicists, 9(3), 107-110. Turner, C. C., & Hughes, G. W., 1982, Distribution and tectonic implications of Cretaceous-Quaternary sedimentary facies in Solomon Islands, Tectonophysics, 87, 127-146.

"TT" 167 S

Figure 1:

The geological provinces of the Solomon Islands (after Hackman, 1973)

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Figure 2: Magnetostratigraphic column of the 'Are'are Limestones and intercalated basalts. Black segments of the column are of normal polarity, white reversed. Hatched sections indicate portions of the sequence where polarity could not be recognized. Position of sites within the sequence is indicated to the left of the magnetostratigraphic column. A series of sites arranged vertically indicates a section of the sequence where stratigraphic relationships were clearly observable in the field. Swung dashes indicate inferred relationships. Question marks indicate a degree of doubt about the assigned stratigraphic position, while arrows indicate that the position is known only within a broad range. Asterisks are used to represent positions for sites Sl-105 and Sl-106 as they would be interpreted from the available geological map; the author's preferred position for these sites is shown.

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Figure 5: Three palaeomagnetic poles derived from Maiaita, shown on the southern hemisphere in comparison with the Australian apparent polar wander path (to the left) and an equivalent path for the Pacific plate, chosen from a number of such paths from a variety of sources as the "worst case" (i.e., that which is closest to the Australian path, and so most difficult to distinguish from the latter). Ages are given in Ma; the Australian path has indicated 95-6 confidence intervals for its track and for the position of the Hada pole (approximately 3 Ma), the square represents the Haruta pole (approx. 23 Ma) while the large dot represents a combined Apuloto - 62 Basalts pole (approx. 48 Ma). Circles around the poles are their 9S% confidence limits.

Figure 4: The Haruta and Apuloto - Sz Basalts poles rotated to correct for 5 Ma of Pacific - Australia relative motion, and plotted on the same projection and reference curves as Figure 3.

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COMPARATIVE EVOLUTION OF FAULT SYSTEMS IN THE YARROL FOLD BELT AND CANADIAN CORDILLERA - A LANDSAT STUDY C.R. Nash Hunting Geology and Geophysics, Canberra, Australia Structural interpretation of Landsat imagery over the deeply dissected terrains of the Canadian Cordillera and Eastern Highlands of Australia provides an excellent basis for the analysis of regional fault and fracture systemsThis paper examines the tectonic significance of interpreted structural trends in the Yarrol Fold Belt (New England Orogen) of southeastern Queensland and the Cordillera of southern British Columbia, Canadian Cordillera. Relative motions of the Kula and Farallon Plates with regard to North America suggest a general northerly to northeasterly convergence during the past 100 my (Riddihough, 1982). This period encompasses the late Mesozoic to early Cainozoic 'Laramide' orogeny of western North America, during which the present tectonic framework of the Cordillera was formed (Monger and Price, 1979). Lithotectonic terrane analysis provides evidence of massive continental accretion during this period, involving a collage of exotic blocks (Monger et al., 1982)o Other manifestations of the Laramide orogeny were the emplacement of the Coast Plutonic Complex, widespread Eocene continental volcanism and late-stage plutonism, and the evolution of the Palaeocene Cordilleran fold-thrust belt and foreland basin. The prominent regional fault systems which dominate the geology of the Canadian Cordillera were also formed during late Me^zoic - early Tertiary times, when cumulative displacements approaching 10 km took place along a system of dominantly dextral transcurrent dislocations between the North American craton and accreted terranes to the west (Gabrielse, 1985). The subsequent late Tertiary tectonic evolution of the Cordilleran region has been dominated by vertical Basin-Range movements attributable to uplift possibly associated with incipient back-arc spreading (Stewart, 1978). These normal faults tend to follow earlier fault trends and thus enhance rather than obscure the latter. Interpretation of Landsat imagery over southern British Columbia (Figure 1) reveals the presence of three distinctive structural provinces, separated by the dextral Takla-Fraser transcurrent fault system and the southern Rocky Mountain Trench lineament respectively. The eastern domain is characterised by NW-trending lineaments which appear to reflect lateTertiary normal faulting parallel to early Tertiary foreland thrust belt trends. The central domain however coincides with the main belt of dextral strain between craton and younger accreted terranes to the west. Structural trends in this domain are distinctively northwest to meridionally oriented, reflecting an anastomosing pattern of thrust and strike-slip faults (Gabrielse, 1985). Lineaments in the western accreted domain are generally oriented in a northwesterly direction, corresponding in part to northeasterly-verging thrust slices attributable to plate convergence. The NNE and NE orientation maxima correspond closely to theoretical synthetic and antithetic strikeslip fault directions (Figure 1). The prominent NNW-trending dextral transcurrent faults of the central domain are thought to offset earlier NW-trending thrust faults and to be due to changes in orientation of the principal compressive stress affecting the Cordillera (Riddihough, 1982), although both directions may be accommodated with the parameters of the same model.

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UNITED S T A T E S 0

250 Km

Figure 1,

Major structural domains in southern British Columbia defined by orientation of Landsat lineaments.

Yarrol Fold Belt. A regional penetrative lineament fabric seen throughout the Yarrol Fold Belt of eastern Queensland may be interpreted in terms of a simple compressive shear model (Nash, 1984). Detailed photogeological studies however reveal that most of the observed structures display extensional components, corresponding to Mesozoic rifting and dyke emplacement and to Cainozoic grabens (Nash, in prep.). Resolution of this dilemma requires an understanding of Mesozoic plate interactions along the northeastern margins of the Australian continent: this record is fragmentary however, and depends largely upon interpretation of surviving lithotectonic assemblages (Veevers, 1984). Structural trends in the Yarrol Fold Belt are therefore considered in terms of the late Palaeozoic and Mesozoic tectonic evolution of the New England Orogen. Orogenesis and crustal shortening in the Yarrol Fold Belt of southeastern Queensland appears to have terminated by the close of the Palaeozoic era. This episode of westerly-directed compressive stress resulted in penetrative deformation of Palaeozoic flysch, folding of Palaeozoic shelf sediments, regional emplacement of granodioritic batholiths and tectonic emplacement of ultramafic bodies along regional NNW-trending thrust faults (Day et al., 1978).

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Figure 2..

Major structural features of the Yarrol Fold Belt in southeastern Queensland, attributed to Palaeozoic crustal shortening (left), and Mesozoic extension (right)c

To the east, late Permian sedimentation in the adjacent Bowen foreland basin is related to the terminal phases of this episode, during which 5km of paralic-paludal coal measures accumulated between about 260 and 248 Ma (Veevers, 1984), foreland basin subsidence being largely coeval with magmatism and uplift in the adjoining New England Orogen, Palaeozoic crustal shortening in the Yarrol Fold Belt may be attributed to a convergent margin with a principal compressive stress vector oriented at approximately N80°E (Figure 2a), which would also explain the inferred sinistral dislocation along NW-oriented lineaments revealed by photogeological interpretation (Nash, 1984). The prominent Rockhampton lineament zone coincides with a major deflection in the orientation of the New England Orogen and a depocentre in the Bowen foreland basin (Veevers, op cit,). These lineaments are now expressed as normal faults caused by Mesozoic and Cainozoic reactivation.

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Continued uplift, rifting and graben development in early Triassic times (Day et al., 1974) marks a transition from horizontal to vertical movements in the Yarrol Fold Belt. The NNW-trending grabens are spatially associated with Triassic post-tectonic plutons (Figure 2b), which appear to be linked to a regional pattern of dykes, faults and fractures (Nash, in prep.). Triassic subsidence of the Taroom Trough and westerly-verging folding and thrust faulting in the northern Bowen Basin are apparently related to the same event (Staines and Koppe, 1980). The extrusion of flat-lying late Triassic ignimbrite sheets appears to post-date the peak of Triassic vertical tectonism. Tertiary rift trends closely follow NNWoriented Triassic structures such as the Mount Perry lineament zone (Figure 2b). References Day, R.W., Cranfield, L.C., and Schwarzbdck, H., 1974, Stratigraphy and structural setting of Mesozoic basins in southeastern Queensland and northeastern New South Wales. In: A.K. Denmead, G.W. Tweedale and A.F. Wilson (eds.). The Tasman Geosyncline - a Symposium. Brisbane, Geological Society of Australia (Queensland Division), 319-362. Day, R.W., Murray, C.G., and Whitaker, W.G., 1978, Tasman Orogenic Zone, Tectonophysics, 48, 327-364.

The eastern part of the

Gabrielse, H., 1985, Major dextral transcurrent displacements along the Northern Rocky Mountain Trench and related lineaments in north-central British Columbia, Geological Society of America Bulletin, 96, 1-14. Monger, J.W.H., and Price, R.A., 1979, Geodynamic evolution of the Canadian Cordillera - progress and problems, Canadian Journal of Earth Sciences, 16, 770-791. Monger, J.W.H., Price, R.A., and Tempelman-Kluit, D.J., 1982, Tectonic accretion and the origin of the two major metamorphic and plutonic welts in the Canadian Cordillera, Geology, 10, 70-75. Nash, C.R., 1984, Tectonic interpretation of the northern Tasman Orogenic Zone from Landsat structural data. Proceedings Third Australasian Remote Sensing Conference, Gold Coast, 1984, 433-441. Nash, C.R., (in prep.), Permo-Triassic tectonic evolution and metallogenesis of the Rockhampton-Maryborough area, Queensland - a photogeological investigation. Ore Geology Reviews. Riddihough, R.P., 1982, One hundred million years of plate tectonics in western Canada, Geoscience Canada, 9, 28-34. Staines, H.R.E., and Koppe, W.H., 1980, The geology of the north Bowen Basin. In: R.A. Henderson and P.J. Stephenson (eds.). The Geology and Geophysics of Northeastern Australia. Brisbane, Geological Society of Australia (Queensland Division), 279-298. Stewart, J.H., 1978, Basin-range structure in western North America - a review. Geological Society of America Memoir, 152, 1-31. Veevers, J.J., (ed.), 1984, Phanerozoic Earth History of Australia, Oxford.

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THE ASPIRING LITHOLOGIC ASSOCIATION, AN OCEANIC LITHOLOGIC SUITE BETWEEN THE CAPLES AND TORLESSE TERRANES, SOUTH ISLAND, NEW ZEALAND R.J. Norris and D. Craw University of Otago, Dunedin, New Zealand

The Alpine Assemblage of the New Zealand Rangitata Orogen represents a complex of accreted sedimentary units of late Paleozoic and Mesozoic age. Two major sedimentary suites may be recognised, the Caples terrane comprising mainly flysch sequences of intermediate volcanic provenance, and the Torlesse terrane composed mainly of quartzofeldspathic flysch from a continental or ensialic arc source. The junction between these two contrasting terranes is obscured by the complex deformation and metamorphism within the Haast Schists, tho metamorphosed equivalents of the lower grade rocks (Coombs ^ , 1976; Carter et , 1978). Field work in NW Otago has revealed that a third major lithologic suite, the Aspiring lithologic association, occurs in the region of the supposed terrane junction. This association is composed dominantly of pelitic schist, in contrast to the mainly psammitic schists derived from Caples and Torlesse rocks, and also includes a relatively high proportion of basic metavolcanic material (greenschists) and cherts (Craw, 1984). The greenschists range from thin (<1 m) actinolite-rich horizons within pelitic schist that may have originated as tuffs, to larger more variable bodies within greenschist/chert dominated zones hundreds of metres thick. Some of the epidote-rich varieties clearly originated as pillow lavas and breccias. The cherts include manganese-rich varieties containing spessartine or piemontite, and metalliferous cherts with lenses of magnetite and stilpnomelane in addition to the manganese minerals. Rare serpentine pods also occur within this unit (Cooper, 1976). The lithologies suggest that the Aspiring lithologic association represents a suite of oceanic material accreted between the Torlesse and Caples terranes. Metamorphic grade is mainly greenschist facies but rare cores of blue amphibole within the barroisitic actinolites suggest an initially higher pressure metamorphism (Yardley, 1982). Structural relationships are complex but indications are that the Aspiring rocks are overthrust in opposite directions by second phase nappes of both Torlesse and Caples psammitic schists. The lower limbs of the nappes are high strain zones in which minor structures have been rotated towards the stretching direction and refolded (Craw, 1985). To the southeast, the Aspiring rocks appear to be buried by these nappes, appearing in tectonic windows beneath them. It is possible that a melange complex containing abundant chert and pelite with rare basic volcanic bodies that outcrops on the coast south of Dunedin (Nelson, 1982) may represent a southeastward continuation of the Aspiring lithologic association. The age relationships between the Aspiring rocks and those of the Torlesse and Caples terranes are unknown, so it is difficult to justify defining them as a separate allochthonous terrane. However their presence, as a probable accretionary complex of oceanic material at the junction of the two major clastic rock suites, provides further evidence for treating the Torlesse and Caples units as separate terranes subsequently juxtaposed by accretion during the Mesozoic. References Bishop, D.G., Bradshaw, J.D., Landis, C.A. & Turnbull, I.M., 1976, Lithostratigraphy and structure of the Caples terrane of the Humboldt Mountains, New Zealand, New Zealand Journal of Geology and Geophysics, 19, 827-848. Carter, R.M., Hicks, M.D., Norris, R.J., & Turnbull, I.M., 1978, Sedimentation patterns in an ancient arc-trench-ocean basin complex: Carboniferous to Jurassic Rangitata Orogen, New Zealand, in Stanley, D.J. and Kelling, G. (editors) "Sedimentation in Submarine Canyons, Fans and Trenches", Dowden, Hutchinson and Ross, Inc pp.340-361.

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Coombs, D.S., Landis, C.A., Norris, R.J., Sinton, J.M,, Borns, D.J., & Craw, D., 1976, The Dun Mountain Ophiolite Belt, New Zealand; Its tectonic setting, constitution and origin, with special reference to the southern portion, American Journal of Science, 276, 561-603. Cooper, A.F., 1976, Concentrically zoned ultramafic pods from the Haast Schist Zone, South Island, New Zealand, New Zealand Journal of Geology and Geophysics, 19, 603-623. Craw, D., 1984, Lithologic variations in Otago Schist, Mt Aspiring area, northwest Otago, New Zealand, New Zealand Journal of Geology and Geophysics, 27, 151-166. Craw, D., 1985, Structure of schist in the Mt Aspiring region, northwestern Otago, New Zealand, New Zealand Journal of Geology and Geophysics, 28, 55-75. Nelson, K.D., 1982, A suggestion for the origin of mesoscopic fabric observed in accretionary melange, based on features observed in the Chrystalls Beach Complex, South Island, New Zealand, Geological Society of America Bulletin, 93, 625-634. Turnbull, I.M., 1980, Structure and interpretation of the Caples Terrane in the Thomson Mountains, northern Southland, New Zealand, New Zealand Journal of Geology and Geophysics, 23, 43-62. Yardley, B.W.D., 1982, The early metamorphic history of the Haast Schists and related rocks of New Zealand, Contributions to Mineralogy and Petrology, 81, 317-327.

169

FROM OCEANIC PLATEAUS TO ACTIVE MARGIN AND ACCRETED TERRANES Amos Nur^ and Zvi Ben-Avraham^ Department of Geophysics, Stanford University, Stanford, California, U.S.A.^ and Department of Geophysics and Planetary Science, T e l Aviv University, T e l Aviv, Israel^ Many anomalous rises in today's oceans may be submerged continental crusts detached from previous continents, ancient island arcs, or basaltic piles formed by hot spots and spreading centerso These rises are embedded in their respective moving oceanic plates and are fated to be consumed at active margins. Many past rises, including numerous continental fragments, have been recognized within mountain belts as allochthonous terranes. They constitute a large portion of the orogenic belts in the North Pacific from Mexico through western North America, Alaska, east Siberia, Japan and in New Zealand. The orogenic deformation in these belts is possibly the result of the accretion of allochthonous terranes. Many terranes have been accreted with sxibstantial deformation also in the Alpine chain, well before major continent-continent collisionsc It is suggested, therefore, that the accretion of fragments may be the common process of the deformation phase of mountain building. Subduction of normal oceanic crust may be insufficient for deformation, whereas full continent-continent depends critically on whether allochthonous terranes caused orogenic deformation in the Andes or not. Most of the accreted fragments with continental affinities in the Mesozoic-Cenozoic orogenic belts of the world can be traced back to the breakup of Gondwana, beginning with a Pacific domain in the Permian, through a larger Indian domain in the early Mesozoic, and continuing through the separation of the Somalia plate in the near future. The reasons for this 250 million year breakup process are not known, but some kind of thermal process, possibly of mantle-wide scale, is implied. Where oceanic rises are being consumed at present, e.gc, the Nazca Ridge, they disrupt the motion of the downgoing slab, cause shifts in plate boundary configurations and modify the seismicity pattern. Most significantly, these rises disrupt volcanism. Most of the volcanic gaps at active margins are associated with oceanic rises, and may be caused by the reduction of water supply via the downgoing slab.

170

EVIDENCE FOR SINISTRAL MOVEMENT ON "mE PEEL FAULT SYSTEM IN SERPENTINITES, GLENROCK STATION, N.S.W. R. O f f l e r and A.J. Williams University of Newcastle, Newcastle, A u s t r a l i a and Dehl1-Petroleum Pty. Ltd. Adelaide, A u s t r a l i a

The Peel Fault System (PFS) Is a major t e c t o n i c s t r u c t u r e In the Palaeozoic New England Fold B e l t . Several I n t e r p r e t a t i o n s of the h i s t o r y of t h i s zone have been made but there Is s t i l l l i t t l e agreement on the sense and t i m i n g of movement, and the amount of displacement. In the Glenrock Station area, the PFS i s delineated by cleaved d i a m i c t i t e s , t e c t o n i c breccia and two major zones of l e n t i c u l a r bodies of cleaved and massive s e r p e n t i n i t e s , trending 140 . Within these s e r p e n t i n i t e s , several f o l i a t i o n s can be recognised on the basis of o v e r p r i n t i n g r e l a t i o n s h i p s . The e a r l i e s t f o l i a t i o n t o develop i s a c l o s e l y spaced, f i n e , penetrative s c h i s t o s i t y (S^) defined by a mixture of l i z a r d i t e and orthochrysotile, elongate magnetite aggregates, and flattened serperttinised u l t r a m a f i c lenses. Microscopic evidence suggests t h a t t h i s f o l i a t i o n i s a plane of f l a t t e n i n g and corresponds t o the XY plane of the f i n i t e s t r a i n e l l i p s o i d . I t has a meridional s t r i k e and dips steeply east or west. S^^ is overprinted by a set of microshears (S2 i S ^ ) which in some outcrops resemble c r e n u l a t i o n cleavage. More commonly, however, these planes l i e at an angle of 45° or less t o S^. They are axial plane t o mesoscopic, open f o l d s and are more widely spaced (<0.5 30cm) than S^^. S 2 i s the dominant shear plane and shows a s i n i s t r a l sense of movement; S£ is dextral. The age r e l a t i o n s h i p s between these two planes i s not known. M i c r o s c o p i c a l l y , they are defined by long f i b r e c h r y s o t i l e suggesting t h a t they are t r u e planes of shear. Evidence for shearing i s also shown by the presence of serpentine " f i s h " , bent and f r a c t u r e d " b a s t i t e s " and dragging of S^ i n t o S2. In the southern s e r p e n t i n i t e b e l t , S2 has a south-easterly trend and a steep n o r t h - e a s t e r l y dip but in the northern b e l t i t s o r i e n t a t i o n i s variable. This i s a t t r i b u t e d t o the more Intense f o l d i n g associated with the formation of S3 t h a t has occurred in t h i s b e l t (Table). The o r i e n t a t i o n of S^ i s v a r i a b l e , however, in areas less a f f e c t e d by later folding, it trends approximately in a north-north-east direction. S^ and S2 i n t e r s e c t over a wide zone but most plunge steeply about north, south-east and south-west axes (Table). A second set of microshears (S^) displaces and r e f o l d s both S^, S and S 2 • These planes are more widely spaced than the e a r l i e r shear sets (1 t o 50cm) and show a consistent s i n i s t r a l sense of movement. Mesoscopic f o l d s developed during t h i s deformation have t h e i r axial planes oriented a t 45° or less t o S3. Microscopically, these shear sets show features s i m i l a r t o S^ and S ^ . S-pole diagrams suggest t h a t S planes are s i m i l a r i n o r i e n t a t i o n t o S 2 (Table).

171

However, the plots in this case are misleading because an angular discordance is always noted between the earlier and later shear sets. This feature is borne out by the detailed analysis done in a small area within the north-western part of the northern belt (Table). The near co-incidence in orientation of S2 and S3 in the plots is attributed to the folding that occurred during and subsequent to the formation of S 3 . Textural evidence suggest that the formation of these planes took place at different crustal depths. S^ appears to have developed under ductile conditions and the shear sets under brittle - ductile conditions (Williams, 1979). The meso- and microscopic features shown by the foliated serpentinites are similar to those observed in Type I S-C mylonites (Lister & Snoke, 1985) in that an earlier foliation (S^ = S) anastomoses in and out of zones of relatively high strain (S2 » S^ = C where "C" stands for 'cisaillement' or shear; terminology from Berthe et al, 1979). Such S-C relationships are used to determine sense of shear in mylonites (Berthe et al, 1979). Similar relationships in the Glenrock Station area suggest a left-lateral movement on the PFS and the geometry of these planes indicates that this movement has been essentially strike-slip. This accords with the movement pattern proposed by Cawood (1980) on strike faults adjacent to the PFS. The deformation that produced these structures appears to have taken place between 252 Ma and 270 Ma since the PFS is cut by the Inlet Monzonite (252 Ma (corrected). Cooper et al, 1963) near Tamworth and tilts rocks containing Permian Fauna II (Sakmarian-Artinskian boundary) near Woodsreef (Price, 1970).

Berthe, D., Choukroune, P., & Jegouzo, P., 1979, Orthogneiss, mylonite and non coaxial deformation of granites:

the example of the South

Amorican Shear-Zone, Journal of Structural Geology, 1, 31-42. Cawood, P.A., 1980, The geological development of the New England Fold Belt

in the Woolomin

- Nemingha

and Wisemans Arm Regions:

The

evolution of a Palaeozoic fore-arc terrain. Unpublished Ph.D. thesis. University of Sydney. Cooper, J.R., Richards, J.R., & Webb, A.W., 1963, Some potassium-argon ages in New England, N.S.W., Journal

of the Geological

Australia, 10, 313-316.

172

Society of

Lister,

G.S.,

Snoke,

1985,

S-C

Mylonites,

Journal

Structural Geology, 6, 617-638, Price, I., 1970, The setting of an Alpine-type serpentinite intrusion near Woodsreef.

Unpublished B.Sc. (Hons.) Thesis, University of New

England. Williams,

A.J.,

1979,

Foliation

Glenrock, New South Wales,

development

Tectonophysics, 58, 81-95.

TABLE

Structural Element

Average Orientation

North Belt

South Belt

Small area. North Belt

76/272

76/86

60/284

77/261

76/40

70/250

81/229 78/68 (3 maxima) variable

variable

77/256

75/135

60/304

72/358 80/220

72/45

173

85/24

serpentinites,

TRANSFORHIMG THE EASTERN LACHLAH FOLD BELT Gordon H. Packham The University of Sydney,

Australia

The Ordovician volcanic arc of the eastern Lachlan Fold Belt can be mapped using outcrop, gravity and aeromagnetic data (Figure 1). Using the arc as a datum a major sinistral fault (the Tumut-Marromine Transform) can be identified. Within the outcrop area east of the Melbourne Trough eleven other sinistral dislocations in the fold belt disrupt the arc on a smaller scale or bring together unlike sequences (Figure 1). Rest of the Lachlan arc is an extensive Ordovician quartz-rich turbidite facies generally interpreted as back arc basin fill derived from a southwesterly continental mass» In the area south of Bathurst and Orange these two facies are intermixed and are hence parts of the same terrane. The Mew England arc containing Lower Palaeozoic elements was east facing. The western part of this arc that can be traced up to 27 degrees south on the gravity map may be part of the same arc as that recognised in the Lachlan, with a postulated major dislocation along its western side (the Eden-Horee Transform). Basic metavolcanics and serpentinites dredged from the lower slope off Eden possibly lie on this line. Reconstruction of the arc as a single linear feature indicates a total displacement of 1400 km between late Late Ordovician (445 my) and mid Early Devonian (400 my) at an average rate of 35 mm per year. Brief outline of tectonic evolution. Late Ordovician - A trench-arc system faced east with narrow marginal sea behind. The S-I line lies behind and parallels the arc. Benambran Orogeny 1. Late Ordovician - There was subduction of a spreading ridge trending southeast. If the ridge was intersected by transform fault this could have been achieved in a few million years. The triple point moved south along the trench with sinistral transform motion north of it and subduction of young crust south (Figure 2). First evidence of tectonism in the Hagga Belt, eg gravity sliding east of Canberra. 2. Early Silurian (Early Llandovery) - The arc-marginal sea complex became a zone of strike-slip tectonics and the uplift and deformation of the Hagga belt commenced by transform motion along the Gilmore Fault (a failure along the western side of the arc). High temperature/low pressure metamorphism was accompanied by the generation of granites and their emplacement in an en echelon pattern in the Hagga Metamorphics. The granite melts had high water content and formed at low temperature. They solidified before reaching the surface hence the lack of volcanics. Cooling ages of granites are as young as Ludlow. Less intense deformation took place on southern extension of belt (present Southern Highlands) where sediment thicknesses may have not been so great and therefore temperatures were lower in the lower crust. Quartz- rich sediment deposited in the Kiandra and Parkes areas, volcanogenic sediments deposited near Orange . Quidongian Orogeny 1. Early Silurian (Late Llandovery) - Deformation continued as the metamorphic belt stiffened. Rupturing of the arc occurred along the TumutNarromine Transform and basic volcanism and the minor emplacement of mantle melts in the eastern Tumut Trough in the developing transpressional

17a

transtenslonal zone. To the east of this line the Hagga Trough rocks Rere brought up against the transform. Subsidence of the CoHra Trough commenced. Granite generation started in the Southern Highlands (Berrridale, Murrumbidgee and Kosciusko Batholiths). Quartz-rich turbidites were deposited unconformabiy on the Ordovician in the Southern Highlands (then located much further south). By the end of the Early Silurian almost all of the southern region was above sea level and the hiatus continued into the Middle Silurian. Deposition did not commence in some parts until the Late Silurian. 2. Middle Silurian to Late Silurian (Henlock-Ludlow) - An outburst of acid/intermediate vulcanism that was strongly concentrated in the CoRra Trough and its extension through Yass and Cooma mainly along the eastern side of the Tumut-Marromine transform zone followed shortly after transform movement started. Upwelling of hot mantle and crustal stretching along the transform may have triggered the volcanic episode by partial melting of the sediment pile on basement rocks. Deposition of these sediments was in shallow water and thicknesses are some kilometres. Two other zones of strong volcanic activity developed, in the Captains Flat-Hoodlawn and the Braidwood areas where deposition did not commence until Late Silurian. After the development of the western Cowra-Yass volcanic belt transform motion probably transferred to the east to produce the second major disruption to the arc (the Eden-Moree Transform) along the western edge of the New England arc segment. A minor dislocation that was formed at the same time was responsible for the formation of the Hill End Trough. It lay just west of the Hiagdon Thrust and possibly along the line of the highly elongated Rologorong Granite. A significant amount of crustal extension must have occurred beneath the Hill End Trough so that the seven kilometre thick Siluro-Devonian turbidite sequence could accumulate. During the Henlock and Ludlow some of the major plutonic bodies were emplaced, notably the Berridale, Kosciusko and Murrumbidgee Batholiths. Only the last has a possible volcanic equivalent. Contemporaneously nonvolcanic sequences were deposited at Quidong and Coolamin Plains. Monvolcanic quartz-rich sedimentation commenced in the early Late Silurian eg the Bogan Gate Platform, the Hill End Trough, the Cowra Trough, the Taralga-Goulburn area and Yass,and in some localities extended to the end of the Silurian eg Yass. Elsewhere sedimentation was interrupted or terminated by renewed acid volcanism. Granite bodies related to the volcanics were emplaced later. 3. Late Silurian (Pridoli) - Quartz-rich sediments, mainly turbidite facies, were deposited at Taralga, Braidwood, Hoodlawn and in the Cowra Trough. Apart from slope reversal in the eastern Hill End Trough and the deposition of shallow water carbonates and shales at Bungonia that may have accompanied movement on the suggested eastern transform, the relatively quiescent interval in the Pridoli in the east was probably when deformation was once more switching back to the west and major structures were reactivated. The Cheesmans Creek olistotrome on the Molong High was formed. Bowning Orogeny 1. Basal Devonian - Following the stiffening of the crust in the Southern Highlands and the Hagga belt by metamorphism and intrusion: (a) movement along the Gilmore Fault resulted in the commencement of formation of the Cobar Trough on the northwest margin of the Hagga Metamorphic Belt, (b) displacement on the Long Plaint Fault accompanied by the emplacement of the Young Granite brought the Coolamin sequence against the Goobragandra Volcanics and uplift and erosion unroofed the Kosciusko Batholith^ (c) extension took place in the Taemas area forming the Early Devonian Basin

175

and northwards there was further extension in the Cowra Trough^ (d) slip on the Lake George fault moved the Molong High northwards and displacement on the Columbine Thrust moved the Cheesemans Creek olistostrome north and the Hyangala Batholith Mas emplaced in the zone thus extended south of the Molong High, (e) slip occurred on the Copperhannia-Lake George Fault and the Lake Bathurst-Taralga structure and the Boro Granite was emplaced^ (The volcanics that accompanied these movements are mostly unconformable on the underlying Ordoviciaa or Silurian formations)^ and (f) Northwest trending wrench faults developed in the more cratonised region of the Southern Highlands intersecting the Berridale and Hurrumbidgee Batholiths and the Jerangle Granite Co«plex« (2) Early Devonian (Gedinnian-Siegenian) - Shortlived deep water quartz rich sedimentation in the axial region of the Cobar Trough transgressed outwards to shallow water deposits to the west and east (onto the Bogan Gate Platform). Dacitic volcanics were extruded in the southern Cobar Trough at Ht Hope and Mymagee and at Mineral Hill probably on fault lines. They are also found at Albury, Jemba in thw Hagga belt and on Parkes platform. These volcanics are all presumably crustally derived and reflect the persistence of high mantle temperatures with melting occurring on a local scale in regions of higher strain. k last burst of strong volcanic activity in the Cowra Trough and limited activity on the Molong High is recorded by deposits conformable on Silurian beds. Siegenian carbonates follow on Molong High. Volcanics and minor quartz-rich sediments were deposited unconformably on Silurian rocks in the Tumut Trough. Acid volcanics were extruded unconformable on granite in eastern Victoria and Silurian strata in southern MSH. The most intense explosive activity is found in the northeast part of the fold belt in the Hill End Trough (Merrions and C r u d i n e ) ^ Rockley-Burraga area and the Hollondilly Valley (Bindook Volcanics etc). Although the strike^slip faulting was followed by widespread acid volcanism plutonism was restricted. The Bega batholith magma was probably generated at this time since it has a cooling age of 380 to 390 m. a. The Yeoval, Bugowra and Bogong masses may also be of this date. Interlude Lower Devonian (Siegenian-Emsian) - Sedimentation was predominantly nonvolcanic, carbonates and shales at Buchan and Taemas but quartz felspatholithic turbidites with an easterly source in the Hill End Trough. This flood of clastic debris contrasts with the small clastic supply and the subdued topopgraphy that must have existed in the vicinity of the Taemas,Buchan and Garra areas indicating the limited affect of the Bowning Orogeny as a mountain building episode in the Southern Highlands. Carbonates muds and sands were deposited on the Parkes Platform after the volcanics. Clastic sediments were deposited in the Cobar and Melbourne Troughs. The Hinburn Tuff on the Capertee High may be an exception to the volcanic quiet. Tabberabberan Orogeny and aftermath Middle Devonian - By the late early Devonian the Cobar Trough, the Melbourne Trough, the Buchan and Taemas Basins and the Hill End Trough had not yet been through a major deformational cycle. Kith the exception of the Cobar Trough these were deformed in the Tabberabberan episode. The fold belt became a sediment source area. The folds and faults developed are suggestive of orthogonal compression. There is no evidence that the transform tectonic regime was still operative at this time. It is likely that this ceased at the time of the recommencement of subduction in the New England arc in the Early Devonian probably indicating a new plate tectonic regime.

176

Late Devonian - Onset of volcanic activity with granitic intrusions in the former Melbourne Trough. Bimodal volcanism in the NSH coastal belt. Uplifted Tabberabberan belt subsided and was eroded becoming an area of sedimentation for essentially fluvial deposits most of which are quartz rich. Kanimblan Orogeny Early Carboniferous - The terminal tectonic event of the fold belt. Open folds and steep limbs near faults in general reflect the strength of much of the underlying basement resulting from previous deformations. Movements took place on some of the old suture lines and were of greater intensity in the northeast of the belt. Like the Tabberabberan Orogeny this deformation does not fit into the transform model. It affected a region that extended from Broken Hill in the west to the western margin of the Sydney Basin. There is no evidence of a significant deformation in the stratigraphy of the Mew England arc nor is there any sign of the flood of quartz-rich detritus characteristic of the Late Devonian of the Lachlan Fold Belt. The New England arc was perhaps like the little man upon the stair

177

Tosmon Sea

PRESENT

-42

Figure 1

K m (kIOOI

Figure 1. Present day tectonic elements. Hajor faults and sutures. (1) Keiiia Fault (2) Gilmore Fault (3)Tumut> Marrowine Transfer* (4) Long Plains-Indi Fault (5) Murru«bidgee Fault (6) Coluiibine Thrust (7) "Bell River- Fault (8) Copperhannia-Lake Qeorge Fault (9) Deakin Fault (10) Hiagdon Thrust (11) -Lake Bathurst-Tarago- Structure (12) Eden-Moree Transforw Snaller subdivisions of the Lachlan Fold Belt. CBT Cobar Trough, QZ airilaiibone Zone, MHT Mineral Hill Trough. BOP Bogan Gate Platforw, TT Tufiut Trough, CT Cosra Trough, YC2 Yass Canberra Zone. HET Bill

178

Figure 2. R.coB.tructioa of the e . e t e m L.ehl.n Fold Belt at the end of the Ordovician.

179

ANATOMY OF THE KAIKOURA TERRANE IN THE OBLIQUELY CONVERGENT AUSTRALIA-PACIFIC PLATE BOUNDARY, NORTH ISLAND, NEW ZEALAND

Jarg R. Pettinga^ and Keith B. Lewis^ Geology Department, University of Canterbury, Christchurch, New Zealand^, and 2 N.Z. Oceanographic Institute, DSIR, Wellington, New Zealand

Introduction The New Zealand continental block straddles the boundary between the Australia and Pacific plates (Fig 1), and relative motion of these plates has controlled the Upper Cenozoic evolution and present shape of New Zealand. Deformation is distributed over a wide boundary zone. The most complex part of the boundary is where it transects the continental edge (Fig IC) and is called the Hikurangi Margin. Here an accretionary prism has developed in response to oblique plate convergence. To the south this prism is replaced by the Marlborough transpressional zone. The definition of the Kaikoura terrane as essentially the Miocene-Holocene accretionary prism (Ben-Avraham and Scheibner 1983) raises three topics of interest in relation to terrane geology:- 1) the history and structure of the terrane; 2) the recognition of similar terranes in the geological record; and 3) the refinement of terrane concepts and terminology. History and Structure of the Kaikoura

Terrane

The Australia-Pacific plate boundary was propagated through the New Zealand continent about 40 My B.P., with the Australia plate moving more rapidly northward than the Pacific plate. Within the New Zealand region the boundary is a complex shear zone with movement in excess of 500km, showing transcurrent, transtensional and transpressional tectonics at different times as it evolved. The development of the Hikurangi Margin as an accretionary prism, in response to oblique plate convergence, extends back to at least the beginning of the Neogene (Pettinga 1982). Substantial clockwise rotation and realignment of the structural trends from NW-SE (early Miocene) to NE-SW (present) has occurred (Fig IB); and the compressional component is interpreted to have markedly increased at about anomaly 5 (10 My) (Walcott 1984). Deformation along the plate boundary is being accommodated on a wide zone (up to 200km) within the Hikurangi Margin. The compressional vector of relative plate motion is taken up within the accretionary slope, with the highest accretionary ridge being exposed onland in the Hawke Bay-Cape Palliser sector. The strike-slip component is seen at the back of the accretionary prism, within the axial ranges. The intervening Forearc Basin represents a transition zone with both high-angle reverse faulting and strike-slip faulting.

180

A three-fold s t r a t i g r a p h i c s u b d i v i s i o n w i t h i n the H i k u r a n g i Margin r e f l e c t s d i s t i n c t c o n s t r u c t i o n a l e l e m e n t s of the a c c r e t i o n a r y prism; 1) the oldest rocks are tectonized slices of T o r l e s s e terrane g r e y w a c k e (Pahau s u b - t e r r a n e ) , and r e p r e s e n t part of the " b a s e m e n t " of the a c c r e t i o n a r y prism. The Pahau s u b - t e r r a n e was part of the M e s o z o i c active margin of G o n d w a n a , prior to b r e a k - u p , and i s o l a t i o n of New Z e a l a n d ; 2) the Upper C r e t a c e o u s - P a l e o g e n e s u c c e s s i o n s of shelf and slope a f f i n i t y represent a dormant ( " p a s s i v e " ) m a r g i n s e t t i n g , and also form part of s t r a t i g r a p h i c " b a s e m e n t " of the K a i k o u r a t e r r a n e ; and 3) the actual a c c r e t i o n a r y phase ( " c o v e r " ) s e d i m e n t a t i o n of M i o c e n e to H o l o c e n e age; o n s h o r e this is r e p r e s e n t e d by m a r i n e s e q u e n c e s of p r o g r e s s i v e l y shallowing o r i g i n , exposed w i t h i n the highest a c c r e t i o n a r y ridge; and m a r i n e to n o n - m a r i n e s e q u e n c e s in the Forearc Basin. Slope basin ( f l y s c h ) and slope (massive m u d s t o n e ) s e d i m e n t s p r e d o m i n a t e , recording Neogene e v o l u t i o n of the a c c r e t i o n a r y slope (Pettinga 1982). O n s h o r e , w i t h i n the h i g h e s t a c c r e t i o n a r y r i d g e , c o m p l e x l y deformed ofter faulted Upper C r e t a c e o u s to P a l e o g e n e s u c c e s s i o n s occur in narrow NE-SW trending s t r u c t u r a l h i g h s . W i t h i n these highs d e f o r m a t i o n styles include tight r e c u m b e n t and i s o c l i n a l folding and thrust f a u l t i n g , the latter often being a c c o m p a n i e d by major tectonic m e l a n g e and broken f o r m a t i o n . A n a l y s i s of s m a l l - s c a l e s t r u c t u r e s has revealed three or more phases of d e f o r m a t i o n are o v e r p r i n t e d , not all are of the same style; and v e r g e n c e and sliding d i r e c t i o n a l i n d i c a t o r s are v a r i a b l e (Pettinga 1982). M i o c e n e to H o l o c e n e a c c r e t i o n a r y prism s e d i m e n t s of slope basin and slope type are exposed onland in broad often s y n c l i n a l structures. The dynamic i n t e r a c t i o n of slope basin s e d i m e n t a t i o n and s t r u c t u r a l high e v o l u t i o n in the s u b d u c t i o n complex is recorded onshore and has allowed detailed i n t e r p r e t a t i o n s of the a c c r e t i o n a r y slope since early M i o c e n e (Pettinga 1982). I n t e r p r e t a t i o n s from the highest a c c r e t i o n a r y ridge o n s h o r e are matched e x c e l l e n t l y by a n a l o g o u s o f f s h o r e s e d i m e n t a r y and s t r u c t u r a l features of the inner trench slope. I n t e r n a l l y the narrow elongated ridges are s t r u c t u r a l l y c o m p l e x , dominated by thrust faulting and a s s o c i a t e d folding (Lewis in press). The i n t e r v e n i n g basins display thick s e d i m e n t a r y s e q u e n c e s which are p e r s i s t e n t l y tilted l a n d w a r d , and show greater d e f o r m a t i o n (growth thrust faulting and f o l d i n g ) at depth. Rapid lateral t h i n n i n g , minor m a r g i n a l u n c o n f o r m i t i e s and onlap r e l a t i o n s h i p s of basin fill are also clear and have their e q u i v a l e n t exposed o n s h o r e . Implications

for R e c o g n i t i o n

of Similar

Terranes

in Ancient

Margins

The H i k u r a n g i Margin a c c r e t i o n a r y prism is not " f l o o r e d " by oceanic crust, as is considered the case with classic c o n v e r g e n t m a r g i n s ; no o p h i o l i t e s are k n o w n , and the only e v i d e n c e for seafloor i n c o r p o r a t i o n into the i m b r i c a t i n g stack is close to the d e f o r m a t i o n front, above the trench axis. Instead the " b a s e m e n t " to the a c c r e t i o n a r y prism i n c l u d e s s u c c e s s i o n s from two older c o n t i n e n t a l m a r g i n s , the Pahau s u b - t e r r a n e of M e s o z o i c age, and the o n l a p p i n g Upper C r e t a c e o u s - P a l e o g e n e s e q u e n c e . P r o p a g a t i o n of the C e n o z o i c plate boundary through the r e g i o n , at h i g h - a n g l e to the c o n t i n e n t a l m a r g i n , caused the older " b a s e m e n t " s u c c e s s i o n s to become gradually more d e f o r m e d , in r e s p o n s e to the o b l i q u e l y c o m p r e s s i v e regime. N e o g e n e "cover" merely r e f l e c t s that a c t i v e c o n t i n e n t a l margin s e d i m e n t a t i o n continued on the g r a d u a l l y deforming (imbricating and d i s p e r s i n g ) basement w e d g e . This process c o n t i n u e s to the present day.

181

CO ro

The c o n t i n e n t a l margin " b a s e m e n t " rocks have been broken up into n u m e r o u s f a u l t - b o u n d e d blocks and tectonized s l i c e s and in part are now dispersed by s t r i k e - s l i p faulting along the realigned (active) continental margin. Similar p a t t e r n s of a c t i v e margin e v o l u t i o n are likely to have occurred e l s e w h e r e in the past and could show similar complex h i s t o r i e s . Implications

for T e r r a n e

Concepts

and

Terminology

T e c t o n o s t r a t i g r a p h i c t e r r a n e s are defined as " f a u l t - b o u n d e d g e o l o g i c a l e n t i t i e s of r e g i o n a l e x t e n t , each c h a r a c t e r i z e d by a g e o l o g i c a l history that is d i f f e r e n t from the h i s t o r i e s of c o n t i g u o u s t e r r a n e s " (Howell and J o n e s 1983). D i f f i c u l t i e s with respect to this d e f i n i t i o n are clearly a p p a r e n t in the case of the K a i k o u r a terrane which was o r i g i n a l l y defined as the "early M i o c e n e - R e c e n t t u r b i d i t e s of the a c c r e t i o n a r y prism" (Ben-Avraham and S c h e i b n e r 1983). D e s p i t e e x t e n s i v e d e f o r m a t i o n there are still s e c t i o n s showing s e d i m e n t a r y c o n t a c t s between rocks of the N e o g e n e a c c r e t i o n a r y prism and the s e d i m e n t s of the C r e t a c e o u s - P a l e o g e n e "passive-margin". Equally there are s e d i m e n t a r y c o n t a c t s between C r e t a c e o u s rocks of the " p a s s i v e m a r g i n " and r e l i c t s of the rocks of the M e s o z o i c a c t i v e m a r g i n . The K a i k o u r a terrane c o m p l i e s with the d e f i n i t i o n in terms of a u n i q u e g e o l o g i c a l history and tectonic b o u n d a r i e s but cannot be d e f i n e d , as a b o v e , in terms of a p a r t i c u l a r suite of rocks of a p a r t i c u l a r age. It clearly e m b r a c e s relics of older t e r r a n e s . Are these relics to be regarded as subsumed w i t h i n the K a i k o u r a terrane and do they loose their old terrane identity and names? The older t e r r a n e s in New Zealand and e l s e w h e r e appear to have rock a s s e m b l a g e s which are mutually e x c l u s i v e , probably due to high d e g r e e s of terrane d i s p l a c e m e n t . The K a i k o u r a terrane shows an active margin in an early stage of d e v e l o p m e n t and a s i t u a t i o n not likely to be unique. It i l l u s t r a t e s the p o s s i b i l i t y of using " p r e - t e c t o n i c " basement rocks as a means of tracking terrane displacement. It also i l l u s t r a t e s the p o s s i b i l i t y of d i s c r i m i n a t i n g between a c t i v e m a r g i n s close to c o n t i n e n t a l edges and a c t i v e m a r g i n s in which the fore-arc basement is o c e a n i c . The former are perhaps i n d i c a t i v e of s i t u a t i o n s when an obliquely c o n v e r g e n t plate boundary is d i s r u p t i n g a c o n t i n e n t a l f r a g m e n t . References B e n - A v r a h a m , Z., S c h e i b n e r , E., 1983, Preliminary T e c t o n o s t r a t i g r a p h i c T e r r a n e Map of the C i r c u m - P a c i f i c Proc. C i r c u m - P a c i f i c T e r r a n e C o n f . , S t a n f o r d , Stanford Univ. P u b l i c a t i o n , Geol. Sci. 15, 227-242.

Region.

H o w e l l , D.G., J o n e s , D . L . , 1983, T e c t o n o s t r a t i g r a p h i c t e r r a n e a n a l y s i s and some terrane v e r n a c u l a r . Proc. C i r c u m - P a c i f i c T e r r a n e C o n f . , S t a n f o r d , S t a n f o r d , Univ. P u b l . , G e o l . Sci., 15, 6 - 9 . Lewis, K.B. (ed) (in p r e s s ) , New seismic p r o f i l e s , cores and rocks from the H i k u r a n g i M a r g i n . N. Z. O c e a n o g r a p h i c Inst. Report. P e t t i n g a , J.R. 1982, Upper C e n o z o i c Southern H a w k e ' s Bay, New Z e a l a n d . 149-191.

dated Field

structural history, coastal N.Z. Jnl Geol. G e o p h y s . , 25,

W a l c o t t , R.I., 1984, R e c o n s t r u c t i o n s of the New Zealand region for the N e o g e n e . Palaeogeography, Palaeoclimatology, Palaeoecology, 46, 2 1 7 - 2 3 1 .

183

REVIEW OF FAUNAL EVIDENCE FOR SILURIAN TERRANES IN AUSTRALIA J.W. Pickett Geological Survey of N.S.Wo, Sydney, Australia Strata of Silurian age in Australia are practically restricted to the Tasman Fold Belt System- West of this, confirmed Silurian sediments are known only from the Canning and Carnarvon Basins of Western Australia. In central Australia a Silurian age has been suggested for unfossiliferous strata in the Amadeus and Georgina Basins, but evidence is only circumstantial and it is quite probable that the Silurian is not represented there. Within the Tasman Fold Belt System, rocks of established Silurian age occur within the Melbourne Trough, the Lachlan Fold Belt, in the Wisemans Arm Formation along the Peel Thrust, and in the Graveyard Creek Formation of the Broken River Basin. In Queensland there is also a number of poorly documented occurrences, at places chiefly along the coast, e.g. near Rockhamptono Areas along the N.SoW. ~ Queensland border, earlier ascribed a Silurian age (e.g. Talent, 1975), are now considered younger (Fergusson & Flood, 1985). Conodonts Conodonts have been reported from Yarrangobilly (Cooper, 1977), the Yass Basin (Link & Druce, 1972), Long Plain (Nicoll & Rexroad, 1974), Borenore (Bischoff in Talent et al., 1975), near Bathurst (De Deckker, 1978; Bischoff & Fergusson, 1982) and the central west (Pickett, 1978). None of the assemblages is in any way remarkable, the only observation possible being that of Nicoll & Rexroad (1974): "The fauna(s) appear to have European rather than American midcontinent affinities." Brachiopods The composition of this group is best known in the Yass-Canberra area, documented in a number of works, chiefly Strusz (1982, 1984), and Mitchell (1921, 1923). Other significant contributions are those of Talent (1965, 1967 and references therein) from Victoria, and McKellar (1969) from Rockhampton. These encompass for the most part later Silurian assemblages, in which the genera Howellella, Molongia, Atrypoidea and Aegiria are among the most characteristic. Molongia occurs in all areas from which Ludlovian faunas are known, and seems to be the only element of probable Australian affinities, the only other recorded occurrences being in China (fide Strusz, 1984). Aliconchldlum is the only genus uniquely Australian but is restricted to a single occurrence. Offering a balance to these apparently local taxa, however, is the presence of forms apparently indistinguishable from European species {Protochonetes cf. minimus, Atrypa cf. dzwinogrodensis; Strusz 1984). dpik's (1953) documentation of the assemblages from the Illaenus band at Heathcote remains the only work on Llandovery forms» Trilobites Sherwin {in Pickett, 1982) provided a summary of Silurian trilobite occurrences in N.S.W. In Victoria this is supplemented by the Australian form Thomastus of the Llandoverian Illaenus-band at Heathcote ('Opik, 1953), and Wenlockian/Ludlovian forms such as Gravicalymene, Trimerus (DipleuraJ , and Encrinurus (Talent 1965, 1967). Since Sherwin's summary there has

184

been a number of larger works documenting important assemblages. Strusz (1980), considering Australian encrinurids in a world context, particularly in relation to phylogeny, made no remarks indicating regional pecularities within Australia. Of possible significance is his genus Batocara, known so far only from Australia. Chatterton & Campbell (1980) introduced a number of genera known so far only from Australia, but concluded that the overall affinities of their Wenlockian assemblage were with eastern Asia. Reports from Queensland are restricted to a solitary occurrence of an encrinurid from near Rockhampton, considered by Strusz to be possibly similar to E, civicae, Corals Possibly the best known groups in the Australian Silurian are the rugose and tabulate corals, due chiefly to the works of Hill, Jones and McLean (numerous references). These groups have also formed the basis of palaeogeographic analyses (Hill, 1959; Pickett, 1975; McLean 1977). McLean concluded that the Llandoverian rugose of N.S.W. show greatest similarity with those from the Siberian Platform. Pickett suggested similarities with Japan and the eastern Asian area on the basis of genera such as Schedohalysites, Falsicatenipora, Mucophyllum, Yassia, Holmophyllia and Eattonia. Of these, possibly Hattonla remains the most reliable. Corals offer the most reliable basis for detection of suspect terranes, because, in general, the shallow water assemblages show greatest provincialism, and because we have better knowledge of this group for New England and northern Queensland. Thus, for the Jack Limestone (Ludlovian), Munson (unpubl.) reports an assemblage of 15 rugosan species, of genera unremarkable in comparison with the Yass fauna, and of which at least 5 species are common to both areas (only 9 are identified to species). The next most southerly occurrence, in the Peel Fault zone near Tamworth, and 1700km to the south,lies in a position which renders an allochthonous origin likely, and indeed, it has already been suggested that Ordovician and Silurian limestones along the Peel Fault could be cappings of guyots (e.g. Scheibner, 1973), or more recently, olistoliths in the Wisemans Arm Formation (Leitch & Cawood, 1980), intimately related to rocks interpreted as deformed oceanic crust. Silurian corals reported from these limestones by Hall (1978) comprise thirteen species, six of which are new, five of which are known from elsewhere in N.S.W., and one of which iPlasmoporella) may be mislocated. The genera include no remarkable forms; the only rugosan, although a new species, belongs to the genus Mazaphyllum, known from a number of localities elsewhere in N.S.W. However it must be conceded that the significance of differences of this kind are at present incapable of reliable interpretation. For years it has been a rule of thumb that halysitids were absent from younger Silurian faunas in Eastern Australia. This has now been confounded by unequivocal evidence of a Ludlovian age (Cooper, 1977) for Halysites yarrangobillyensis from the Yarrangobilly Limestone. This limestone lies between the Tumut Pond Serpentinite belt and the southward continuation of the Coolac Serpentinite belt, a zone about 15km wide at this point. From a tectonic point of view it might well be regarded as suspect, but it seems unlikely, in view of present views of the relative positions of the blocks on either side, that the separation could have been great. Summary Convincing conclusions regarding the occurrence of terranes cannot be reached for the following reasons: a) knowledge of the faunas themselves is too scant; we have barely scratched the surface; b) reliable information on ranges and both local and international distribution is too poor.

185

References Bischoff, G.C.A., & Fergussorir C.L., 1982, Conodont distributions, and ages of Silurian and Devonian limestones in the Palmers Oakey district, N.S.W., Journal of the Geological Society of Australia, 29, 469-476. Chatterton, B.DoEo, & Campbell, K.S.W., 1980, Silurian trilobites from near Canberra and some related forms from the Yass Basin, Palaeontographica A167, 77-119» Cooper, B.J., 1977, Upper Silurian conodonts from the Yarrangobilly Limestone, southeastern New South Wales, Proceedings of the Royal Society of Victoria, 89, 183-194. De Deckker, P., 1976, Late Silurian (Late Ludlovian) conodonts from the Kildrummie Formation, south of Rockley, New South Wales, Journal and Proceedings of the Royal Society of New South Wales, 109, 59-69. Fergusson, C.L. & Flood, P.G., 1985, A late Palaeozoic subduction complex in the Border Rivers area of southeast Queensland, Proceedings of the Royal Society of Queensland, 95, 47-55. Hall, R.L., 1978, A Silurian (Upper Llandovery) coral fauna from the Woolomin Beds near Attunga, New South Wales, Proceedings of the Linnean Society of New South Wales, 102, 85-108. Hill, D., 1959, Distribution and sequence of Silurian coral faunas. Journal and Proceedings of the Royal Society of New South Wales, 92, 151-173. Leitch, E.C. & Cawood, P.A., 1980, Olistoliths and debris flow deposits at ancient consuming plate margins: an eastern Australian example. Sedimentary Geology, 25, 5-22. Link, A.G., & Druce, E.C., 1972, Ludlovian and Gedinnian conodont stratigraphy of the Yass Basin, New South Wales, Bulletin of the Bureau of Mineral Resources, Geology and Geophysics Australia, 134, 1-136. McKellar, R.G., 1969, Brachiopods and trilobites from Siluro-Devonian strata in the Rockhampton district, Queensland, Publications of the Geological Survey of Queensland, 337, Palaeontological Papers 11, 1-13. McLean, R.A., 1977, Biostratigraphy and zoogeographic affinities of the Lower Silurian rugose corals of New South Wales, Australia, Memoire du Bureau de Recherches geologiques et minieres, 89, 102-107. Mitchell, J., 1921, Some new brachiopods from the Middle Palaeozoic rocks of New South Wales, Proceedings of the Linnean Society of New South Wales, 45, 543-551. Mitchell, J., 1923, The Strophomenidae from the fossiliferous beds of Bowning, New South Wales, Part 1, Stropheodonta, Proceedings of the Linnean Society of New South Wales, 48, 465-474. Nicoll, R.S. & Rexroad, C.B., 1974, Llandovery (Silurian) conodonts from southern New South Wales, Geological Society of America, North Central Section, 8th Annual Meeting, Abstracts, 6(6), 534-535. 'dpik, A.Ac, 1953, Lower Silurian fossils from the "Illaenus band". Heathcote, Victoria, Memoirs of the Geological Survey of Victoria, 19, 1-42. Pickett, J.W., 1975, Continental reconstructions and the distribution of coral faunas during the Silurian, Journal and Proceedings of the Royal Society of New South Wales, 108, 147-156. Pickett, J.W., 1978, Silurian conodonts from Blowclear and Liscombe Pools, New South Wales, Journal and Proceedings of the Royal Society of New South Wales, 111, 35-39.

186

Pickett, J-W., ed., 1982, The Silurian System in New South Wales, Bulletin of the Geological Survey of New South Wales, 29, 1-264. Scheibner, E., 1973, A plate tectonic model of the Palaeozoic history of New South Wales, Journal of the Geological Society of Australia, 20, 405-426. Strusz, D.L., 1980, The Encrinuridae and related trilobite families, with a description of Silurian species from southeastern Australia, Palaeontographica, A168, 1-68. Strusz, D.L. 1982, Wenlock brachiopods from Canberra, Australia, Alcheringa, 6, 105-142. Strusz, D.L., 1984, Brachiopods of the Yarralumla Formation (Ludlovian), Canberra, Australia, Alcheringa, 8, 123-150. Talent, J.A., 1965, The Silurian and Early Devonian faunas of the Heathcote district, Victoria, Geological Survey of Victoria Memoir, 26, 1-55. Talent, J.A., 1967, Silurian, sedimentary petrology and palaeontology, in Geology of the Melbourne district. Geological Survey of Victoria Bulletin, 59, 24-29. Talent, J.A., 1975, in Talent, J.A., et al.. Correlation of the Silurian rocks of Australia, New Zealand and New Guinea, Geological Society of America Special Paper, 150, 1-108.

187

NEW GUINEA TERRANES C.J. Pigram and H.L. Davies Bureau of Mineral Resources, Canberra, Australia. The completion of 1:250,000 reconnaissance mapping of Papua New Guinea and new data from similar mapping in Irian Jaya between 1978-1982 provide a base from which it is possible, firstly, to identify the Mesozoic margin of the Australia craton in New Guinea (Pigram and Panggabean, 1984) and secondly to identify the tectonostratigraphic terranes that lay outboard of itc Using comparative stratigraphy we have identified ^thirty two terranes in the New Guinea orogen. The terranes of New Guinea each record unique stratigraphic and structural histories that reflect widely varying depositional environments including terranes with continental basements and well layered inner shelf sedimentary strata such as the Kemum terrane, deep water chert and carbonate rich terranes formed far from any source of terrigenous detritus such as the Port Moresby terrane, and island arc terranes with ophiolite basement such as the Waigeo terrane. The terrane boundaries are either faulted or concealed by sedimentary basins. The nature of the faults separating terranes is commonly not clear. Some such as the Sorong Fault Zone are transcurrent, others such as the Owen Stanley Fault are thrusts. Many of the long linear faults that separate terranes have long been suspected of transcurrent movement (Bain, 1973) but evidence is equivocal. A feature of the New Guinea orogen is the large number of basins that have developed over terrane boundaries, particularly between composite terranes. These basins include the North New Guinea, Bintuni and oil-bearing Salawati Basins of Irian Jaya and the Aitape, Lumi and Wewak Basins and Aure Trough of Papua New Guinea. All of these basins are Miocene or younger in age, formed rapidly, and contain 3-7 km of turbiditic sediments (Visser and Hermes, 1962; Pieters et al., 1983; Williams and Amiruddin, 1983; Hutchison and Norvick, 1980; Brown et al., 1975). Their mode of formation and subsidence histories are poorly known, but they may be modern analogs of the thick Mesozoic flysch, now highly deformed, that separates some of the terranes in the North American cordillera (Coney et al., 1980). Orogens elsewhere in the world typically contain terranes that are predominantly of oceanic affinity. In the New Guinea orogen a large proportion of the terranes are of continental affinity. Some of these were formerly microcontinents detached from eastern Gondwana in the Mesozoic, that have subsequently re-joined the Australian craton by docking in the mid-Tertiary (Pigram and Panggabean, 1984; Giddings and Sunata, this conference). The docking of terranes with the Australian craton began in middle to late Oligocene time and appears to involve a number of composite terranes (eg Western Irian Jaya, Eastern Papua). These composite terranes formed in the Palaeogene. Tectonic events that previously were thought to mark the beginning of orogenesis on the northern edge of the Australian craton, were in fact a consequence of the amalgamation processes that formed the composite terranes and which took place in an ocean basin at some distance from the Australian craton. The western North American orogen developed by accretion of terranes from a plate which was being subducted beneath the craton. This contrasts with the New Guinea orogen, where there is no evidence for subduction beneath the

188

craton prior to the early to middle Miocene so that the initial docking in the Oligocene took place without subduction beneath the craton. This can be explained if it is remembered that in the Late Cretaceous when Australia separated from Antarctica (Cande and Mutter, 1982) the northern edge of the Australian craton faced an ocean basin that had developed during the Jurassic and Cretaceous (Pigram and Panggabean, 1984). We postulate that this basin was consumed along a north dipping subduction zone as Australia moved northward. The arrival of the leading edge of the Australian craton at this subduction zone in the mid to late Oligocene initiated the development of the New Guinea orogen and caused the docking of a number of terranes that had already assembled at this subduction zone. References Bain, J.H.C., 1973, A Summary of the main structural elements of Papua New Guinea ji Coleman, Patrick J. (Ed) The Western Pacific: Island Arcs, Marginal Seas, Geochemistry, University of Western Australia Press, 147-161. Brown, C.M., Pieters, P.E. & Robinson, G.P., 1975, Stratigraphic and Structural development of the Aure Trough and adjacent shelf and slope areas, The APEA Journal, 61-71. Cande, S. and Mutter, J., 1982, A revised identification of the oldest sea-floor spreading anomales between Australia and Antarctica, Earth and Planetary Science Letters, 58, 151-160. Coney, Peter, J., Jones, David, L. and Monger, James, W.H., 1980, Cordilleran suspect terranes. Nature, 288, 329-333. Hutchison, D.S. and Norvick, M.S., 1980, Geology of the north Sepik Region, Papua New Guinea, Bureau Mineral Resources Record, 1980/24. Pieters, P.E., Pigram, C.J., Trail, D.S., Dow, D.B., Ratman, N., and Sukamto, R., 1983, Stratigraphy of Western Irian Jaya, Proceedings Indonesian Petroleum Association 12th Annual Convention. Pigram, C.J. and Panggabean, H., 1984, Rifting of the Northern Margin of Australia and the origin of some microcontinents in East Indonesia, Tectonophysics, 107, 331-353. Visser, W. and Hermes, J.J., 1962, Geological results of the exploration for oil in Netherlands New Guinea. Government Printing Office, The Hague, 20, 265. Williams, P.R. and Amiruddin, 1983, Diapirism and deformation east of the Mamberambo River, Northern Irian Jaya, Proceedings Indonesian Petroleum Association 12th Annual Convention.

189

p THE LOWER PALEOZOIC LOCKETT CONGLOMERATE OF NORTHWEST NELSON, NEW ZEALAND: A MARINE DEPOSIT K. S. Pound Geology Department, University of Otago, Dunedin, New Zealand Introduction The Cambro-Ordovician sequence of northwest Nelson and Westland, New Zealand (Figure 1) represents one segment of the now widely-dispersed Lower Paleozoic margin of Gondwanaland (Cooper, 1973). This paper reports the results of preliminary work in the middle to upper Cambrian Lockett Conglomerate (Grindley 1980) of northwest Nelson. Regional Geology The Lower Paleozoic rocks of northwest Nelson and Westland form three lithologically- and structurally-distinct belts (Western, Central and Eastern Belts), presently separated by two major faults, the Anatoki and Devil River Thrusts (Figure 1). Original paleogeographic relations between the belts are the subject of some discussion (Cooper, 1979; Grindley, 1980). The Western Belt has been called the Buller Terrane by Bishop et al. (1984), (Karamea Terrane of Cooper, 1984), and the Central and Eastern Belts the Golden Bay Terrane by Bishop et al., (1984) (Takaka Terrane of Cooper, 1984). Lower Paleozoic rocks are also found in the Fiordland region of New Zealand (Figure 1). Fiordland rocks are more highly metamorphosed (greenschist to amphibolite facies) than their northwest Nelson and Westland counterparts, from which they are now separated by 480 km of dextral transcurrent movement on the Alpine Fault (Figure 1). The graptolite succession and stratigraphy of the Preservation Inlet area (Figure 1) of Fiordland is very similar to that of the Western Belt of northwest Nelson (Cooper, 1974); both sequences are included in the Buller (or Karamea) Terrane of Bishop et aL (1984). Work by Ward (1984) in the Dusky Sound area of Fiordland suggests that the False Edgecumbe Formation conglomerate of that area is a lithostratigraphic equivalent of the Lockett Conglomerate. Both conglomerates are included within the Golden Bay (or Takaka) Terrane of Bishop et al. (1984). Haupiri Group The Lockett Conglomerate is part of the Haupiri Group, which forms the lower portion of the structurally-complex Central Belt (Figures 1 and 2). Locally, the Lockett Conglomerate conformably overlies the Tasman Formation. Elsewhere the Lockett Conglomerate lies unconformably on the Devil River Volcanics and the Balloon Formation (Figure 2). For the most part, the Tasman Formation represents volcaniclastic, siliciclastic and calcareous material deposited on the slope or base of slope from "low density" or ''distal" turbidity currents and debris flows. Pebbly debris flows in the top of the Tasman Formation mark the

190

transition to the Lockett Conglomerate. These debris flows contain clasts of both the underlying Tasman Formation and the Devil River Volcanics (Figure 2).

LOCATION MAP

NORTH

)

M A P AREA NW NELSOw

200km

Preservat^ion Inlet

ISPALKOZOIC

ROCKS

(intruded late Paleozoic and Cretaceous-Tertiary intrusions)

•MOTUEKA

MAP OF NW NELSON NEW ZEALAND

WESTERN BELT

SHOWING THE D I S T R L B U T T O N OF W E S T E R N , C E N T R A L AND EASTERN B E L T S . POST-DEVONIAN S E D I M E N T A R Y ROCKS ARE NOT SHOWN. (AFTER COOPER, 1979)

t: .; [CENTRAL BELr ^ ^ E A S T E R N BELT I

I INTRUSIONS

Figure 1

Typical Lockett Conglomerate comprises 10 cm to 2 m thick beds of poorly sorted, chaotic to normally or inversely graded pebbles, cobbles and boulders of chert, sandstone, limestone and assorted basic to intermediate volcanic and plutonic rocks, all set in a poorly sorted sandstone matrix. Detrital chromite in the Lockett Conglomerate matrix may be derived from the mafic-ultramafic Cobb Igneous Complex, now exposed in fault-bounded slivers and blocks within the Central Belt (Grindley, 1980). Estimates of the stratigraphic thickness of the Lockett Conglomerate vary from 300 m (Cooper, 1979) to over 1000 m (Grindley, 1980). The Lockett Conglomerate grades laterally and vertically up into the Anatoki Formation, a marine sandstone containing occasional Lockett-type conglomerate lenses. The fact that the Lockett occurs within an otherwisedemonstrably marine sequence, is matrix-supported, and contains no terrestrial indicators, suggests that the conglomerate was deposited rapidly in a submarine fan-type environment and is not a subaerial fanglomerate, as suggested by Grindley (1980). Work in progress will provide more information regarding the provenance of the Lockett Conglomerate, and provide constraints for the tectonostratigraphic evolution of the northwest Nelson region.

191

SUMMIT LIMESTONE sandy limestone & dolomitized limestone min. 370m thick PATRIARCH FORMATION •volcaniclastic & calcareous • siltstone & sandstone with _ {Limestone lenses 200-400m thick

UJ $

(flu <

•H

QC UJ Q. a. D

Figure 2

ANATOKI FORMATION volcanogeaic sandstone with-* conglomerate lenses

600-1300m thick

LOCKETT CONGLOMERATE : (see text) 650-1000+ m thick

E- -H X Qc n j-i ^

a; o ^

age-diagnostic f o s s i l s

Z OJ

conformable contact

s 1

_ TASMAN FORMATION siliceous, calcareous and volcanogenic siltstone and sandstone; limestone lenses 200-600m thick O DEVIL RIVER VOLCANICS basic intermediate volcanics, flows, s i l l s , xtal & vitric tuff:

unconformable contact

COBB IGNEOUS COMPLEX maficultramafic conplex

gc Q. < X

c. 2500m thick

50Q-20Q0m thicfe ^

lateral

fault

gradation

BALLOON FORMATION arkosic sandstone and siltstone

contact

300-1500m thick

SCHEMATIC COLUMN SHOWING LITHOSTRATIGRAPHIC UNITS OF THE HAUPIRI GROUP OF THE CENTRAL SEDIMENTARY BELT (after Cooper,1984; Formation names from Grindley, 19 80) References Bishop, D, G., Bradshaw, J. D., and Landis, C. A., Provinsional Terrane map of south Island, New Zealand, Proceedings of the Circum-Pacific Terrane Conference, D. G. Howell, D. L, Jones, A. Cox, and A. Nur eds., Stanford University Publications, Geological Sciences, Volume XVIII, 24-31. Cooper, R. A., 1973, New Zealand and southeast Australia in the early Paleozoic, New Zealand Journal of Geology and Geophysics, 18(1), 1-20. Cooper, Ro A., 1979, Lower Paleozoic rocks of New Zealand, Journal of the Royal Society of New Zealand, 9(1), 29-84. Cooper, R. A., 1984, Field trip notes for Otago University field trip. May, 1984. Grindley, G. W., 1980, Sheet S13 Cobb (1st ed.) Geological Map of New Zealand 1:63360. Map (1 sheet) and notes (48 p.), N.Z. Department of Scientific and Industrial Research, WellingtonWard, C. M. 1984, Geology of the Dusky Sound area, Fiordland, New Zealand, Ph.D. Thesis, University of Otago, Dunedin, New Zealand.

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POSSIBLE GEOSUTURES IN THE WAGGA WAGGA TO BATEMAN'S BAY TRANSECT OF THE LACHLAN OROGEN M.J. Rickard Department of Geology, Australian National University, Canberra, Australia. 1:250,000 - scale structural and geophysical profiles (Duff, et al., 1985) will be presented as a poster, and a simplified version is included in this abstract. At first sight the time-space plots indicate a coherence in the structural pattern and geological history of the various fault-bounded blocks across this sector of the Lachlan orogen that precludes the interpretation of major geosutures. In detail, however, there are differences that provide 'suspicions' of lateral allochthoneity. At the western end of the profile complexly folded Ordovician turbidites and metamorphics, trending generally north-westwards, are intruded by mid Silurian S-type granites. The junction with the western side of the T\imut Trough marks a regional boundary between N.W. and N trends and there is a wide belt of shearing and complex multiphase cleavage development along the Gilmore Fault Zone. In places to the north and south of this transect, lenses of ultramafics occur. The Tumut Trough itself is characterised by Silurian flysch and bimodal volcanics followed by Early Devonian ignimbrites and I-type granite. The basal unit of mafic amphibolites and volcanics (Silurian or older?) abuts an extensive linear ultramafic belt defining the eastern edge of the trough. Ophiolite sequences occur in places. The unique occxirrence of Silurian flysch and ultramafics makes this location a prime suspect for a major geosuture. There are magnetic and gravity highs over the trough mcurgins, probably reflecting the ultramafics, but the Bouguer values over the trough itself are not anomalous. Most likely a small ocean-basin rift collapsed with minor obduction of mantle material, and the closure of a lozenge-shaped trough might explain the disjunct trends across the western margin. Eastwards the meridional trending horsts and grabens of the CanberraYass Zone expose widespread felsic volcanics and shallow-water sediments of Silurian (S-type) and Devonian (I-type) age overlying Ordovician quartzrich flysch. The Eastern boundary of this zone is the S-I line (Chappell & White, 1974); here the Silurian volcanics and sediments are of deeper water facies in the Captains Flat Trough and the margin of the Murrumbidgee batholith is strongly sheaured. This shearing appears to be related to dipslip movements although mega boudins of limestone and late kinks indicate some strike slip movements. Along narrow Cooma-type metamorphic complex in the Cullerin Horst culminates at the latitude of the profile line. None of these features points clearly to the existence of a major geosuture. The Monaro Zone to the east is similar to the Canberra-Yass Zone except that all of the felsic magmatic rocks have I-type chemistry (Silurian volcanics and Early Devonian granites). Moreover, east of the Comerong rift there are no Silurian or Early Devonian deposits. The Comerong rift itself is a major feature possibly situated on a geosuture. The structures in the adjacent Ordovician turbidites overturn to downward facing in places against the rift margins. Basal bimodal volcanics flank the rift and small A-type granite plutons occur; these cause marked magnetic anomalies on each side. All the structural evidence (volcanics, faults and folds) points to E-W lateral movements - extension followed by compression; there is no evidence for longitudinal movements.

193

In conclusion^ the Lachlan orogen at the latitude of this profile is probably essentially autochthonous - considerable E-W shortening has occurred but major strike slip movements have not been identified; the Tumut Trough, the S-I line and possibly the Comerong rift may well repay further investigation. References Duff^ B.A., Ward^ Po, Crooks KoA^We & Rickard, MoJ., 1985, Tasman orogen profile - Wagga Wagga-Batemans Bay, N.SoW. Geological Surveyc Chappell, B.We, & White, A.J.R., 1974, Pacific Geol., 8, 173-174.

194

Two contrasting granite types.

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CARBONIFEROUS FAUNAS OF THE TASMAN BELT, EASTERN AUSTRALIA John Roberts School of Applied Geology, University of Nev/ South V7ales Two distinct marine invertebrate assemblages are present within the Tasman Belt of eastern Australia, an older (Early Carboniferous) Cosmopolitan Assemblage and a younger (Middle-Late Carboniferous) Gondwanan Assemblage (Fig, 1). The Cosmopolitan Assemblage contains representatives of virtually all preservable groups and is diverse in species and genera. Groups with high levels of diversity include brachiopods, bryozoans, trilobites and rugose and tabulate corals; those with a medicd rate of diversity include the bivalved molluscs, gastropods, echinoderms, particularly crinoids, and conodonts; and those with low rates of diversity include the amnraonoids and foraminifera.The assemblage has a low rate of eodenism (Roberts, 1981). The Gondwanan Assemblage, on the other hand, has an extremely low level of diversity, and fewer of the major groups of organisms are represented. Brachiopods, bryozoans, bivalves and gastropods are the most abundant of the marine organisms, but their level of diversity is far BRACHiOPOO aRACHlQPOO lower than that of the previous EUROPC ASSEMBLAGES ZONES assemblage (Roberts, 1981). AmrietiU»0m9 Conulariids, trilobites and conodonts g are poorly represented, and the rugose M99Mpirit9r t eam^fHl and tabulate corals and amttonoids are I I abseit. I STEPHAMIAN SONOWAMAN Affinities of the Cosmopolitan ASSEMBLAGE I Assetiblages IweSTPHAUAN

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A. Australia. Early Carboniferous marine invertebrate faunas in the Bonaparte, Canning and Carnarvon Basins of Western Australia are also cosmopolitan in nature, but differ from those of the Tasman Belt at the species level. Roberts (1971) maintained that this was caused by isolatiai rather than hy climatic contrast, and Runnegar & Campbell (1976) suggested that environmental factors, particularly the type of sediment, could also have been responsible for faunal differences, successions in the west being doninated by limestones and quartz sandstones vAiereas those in the Tasman Belt were largely volcanogenic« The virtual absence of formaninifera from eastern Australia has been ascribed to the abundance of volcanogenic detritus

196

(B.L. Mamet^ pers. conm.). The only area in vAiich eastern and western faunas occur together is in the Yarrol Qrogen, where faunas from Roberts' (1971) Punctospirifer pauciplicatus Zone (Bonaparte Basin, W,A.) are found with those of the Marginirugus barringtonensis Zone in the Killala Creek Limestone (Nazer, 1977). B. Overseas. At a generic level there are affinities with Early Carboniferous faunas in western Europe, central and western North America, and Asia, particularly with Japan (Campbell & McKellar, 1969; Jull, 1969; Roberts, 1975; Campbell et al., 1983; see Roberts et al., 1985 for information on brachiopods, trilobites, bryozoans, conodonts, amnnonoids, spores and pollen and plant fossils). Affinities of the Gondvanan Assemblage A. Australia. Gondwanan Assemblages are not recognised in Western Australia because most bsisins underwent uplift and regression late in the Visean. Marine sedimmts of Middle to Late Carboniferous age are present in the subsurface of the offshore Bonaparte Basin and possibly the Carnarvon Basin, but macrofaunas have not been collected. B. Overseas. The Gondwanan Assemblage is represented by the Levipustula levis Zone in Argentina (Amos et al., 1973). At least 8 species ccmpai^ble with those in the L. levis assonblage, including a species of Levipustulat are present in a fauna in the Lake Baikal region of USSR (Kbtlyar & Popeko, 1967; Roberts et al., 1976), but are acccmpanied by other forms not represented in either Australia or South America. One or tvio elements of the assemblage are also present in Vfestphalian faunas in northwestern Europe (Boger & Fiebig, 1963) and Spain (Winkler Prins, 1968). Uie Change fron Cosmopolitan to Gonc3wanan Assemblages The change between the two assanblages was relatively abrupt and took place during the latest Visean to earliest Namurian within the Marginirugus barringtc^iensis Zone. CamjiDell & McKellar (1969) and RdDerts (1981) believed the faunal change was caused by a deterioratiai in the climate due to rapid southward movement of the Australian continent. Ihe change coincides with the ccramencement of glaciation and postulated generation of a widespread ice cap (Powell & Veevers, 1984). Estimates on the amount of southerly movement vary from 40^ (Irving, 1966), to 3(P (Smith et al. (1973), to 25^ (Ziegler et al. (1979). Ihe abrupt nature of the change was highlighted by Roberts' (1981) analysis of the brachiopod faunas which denonstrated that 94% of all brachipod species became extinct at the end of the Rhipidomella fortimuscula Zone. The change frcm Cosmoplitan to Gondwanan assenblage took place abruptly in the southern part of the Tasman Belt, but in the north some of the cosmopolitan genera lingering on into the I4arginirugus barringtonensis Zone; they had all disappeared by the Levipustula levis Zoie. Location of Carboniferous Faxanas Shelly faunas were virtually caifined to a shelf area which became

197

CARBONIFEROUS CENTRAL

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divided tectonically into the Yarrol and Tainworth Shelves of the Yarrol and New England Qrogens (Fig. 2). An Andean volcanic chain flanked the shelf to the west, and in the east the shelf edge appears to have been defined by the Yarrol-Peel Fault System. Sediments deposited on the shelf consisted of volcanogenic lithic sandstone, siltstone and mudstone; oolitic limestone wcis more widespread in the Yarrol Shelf than in the Tamworth Shelf.

East of the Yarrol-Peel Fault Syston deep water sediments (pelagic sediments and turbidites) as well as melanges (Fer gusson, 1984) accumulated within a slope and basin province or as part of an accretionary prism in a subduction complex. Sane of these sediments contain astings Sh«lf s«dlm«nts allochthonous eoliths and poorly Block •••pwat«r s«diin«nta preserved fossil detritus m«l«ng« (Fleming et al., 1974; Floning et Major fauita al«, 1975). Whilst the fossils Qaoiogieal boundariaa are too fragmentary to give a precise age, the shell debris has Carboniferous affinities, and one of the elliptical crinoid stens Fig. 2. Carboniferous rocks of the has the same morphology as stems central eastern Tasman Belt. restricted to the Schellwienella cf. burlinqtonensis Zone in N.S.W. These fragments tentatively suggest that the Wandilla and Wbolonin Slope and Basin provinces were adjacent to the Yarrol and Tamworth Shelves during the Early Carboniferous (oolitic limestone deposition was restricted to this time) and were part of a single depositional system on the convergent eastern margin of Australia. Possible Allochthonous Fossils at Murgon Other parts of the slope and basin area may not have the same relationship with the shelf. A sliver of Middle Carboniferous rocks located on the eastern edge of the Yarraman Block at Murgon, Queensland, appears to be different to both shelf and slope and basin successions. Limestone lenses interbedded with andesitic volcanics and mudstone contain conodonts, fish, annelids and foramanifera (Palmieri, 1969). Hiis fauna is regarded as warm water in origin and is contanporaneous with cold water Levipustula levis Zone faunas on the Yarrol Shelf. In addition to the faianal differences, limestone is absent from shelf areas at this time, and volcanism, vrfiich had been essentially dacitic, had ceased in the Yarrol Qrogen at the end of the Visean. The rocks from

198

Murgon therefore appear to be allochthonous. Neighbouring rocks in the Yarraman Block have been referred to the Maronghi Greek Beds (lyiurphey et al., 1976) and consist of chert, mudstone, greyv^cke and minor basalt. Allochthonous ooliths found in the western part of the block (Fleming et al., 1975) suggest that the Yarraman Block was part of the original slope and basin successiai in the Yarrol Qrogen. Because of its structural location on the eastern edge of the Yarraman Block, and its position west of the Carboniferous D'Aguillar Block (from which it is separated by the Triassic Esk Trough), the allochthonous block at Murgon must have arrived at the eastern edge of Australia prior to an episode of dextral wrenching and defomnation at the end of the Carboniferous. Ihis event moved the Yarrol Qrogen an estimated 550 km southeastwards relative to the New England Qrogen (Flood & Fergusson, 1984), metamorphosed the southeastern part of the Yarrol Qrogen and generated a m e ^ o l d in the northern New England Qrogen. H^tings Block In a tectonic sense, the Hastings Block in the eastern part of the New England Qrogen presently appears to be out of place. The block has features similar to the Tamworth Shelf, containing a succession of madLnly shelfal sediments with sparse marine faunas, and having suffered relatively mild folding and faiiLting. Faunas within the block belong to both the Cosmopolitan and Gondwanan Assemblages. Its present position can be explained by sinistral movenent during the Late Permian in a manner similar to that proposed by Cawood (1982). References Amos, A.J., Antelo, B., Gonzalez, C.R., Marinelarena, M.P. & Sabattini, N., 1973. Sintesis sobre el conocimiaito bioestratigratico del Carbonico y Permico de Argentina. Actas del Quinto Congresso Geologioo Argentino, 3, 3-20. Boger, H. & Fiebig, 1963. Die Faiona des W^stdeutchen Qberkarbons II. Die articulatai brachiopodoi des Westdeutchen Qberkarbcxis. Palaecxitographica A 122, 111-165. Campbell, K.S.W. & McKellar, R.G., 1969. Carboniferous invertebrates: sequence and affinities. In Stratigraphy and Palaeontology, Essays in Honour of Dorothy Hill, 79-119. K.S.W. Campbell (ed.). Australian National IMiversity Press, Canberra. Cawood, P.A., 1982. Tectonic reconstruction of the New England Fold Belt in the Early Permian: an example of developinent at an oblique slip margin. In New England Geology. 25-34. P.G. Flood & B. Runnegar (eds). Department of Geology, University of New England and AH7 Club, Armidale. Fergxasson, C.L., 1984. Tectaio-stratigraEiiy of a Palaeozoic subduction ccnplex in the central Coffs H^bour Block of north-eastern New South Wales. Australian Journal of Earth Sciences, 31, 217-236.

199

Flaning, P.J.G., Day, R.W., Murray, C.G. & Whitaker, W.G., 1974. Late Palaeozoic invertebrate macrofossils in the Neranleigh-Femvale Beds. Queensland Government Mining Journal, 75, 105-107. Fleniing, P.J.G., Murray, C.G. & Whitaker, W.G., 1975. Late Palaeozoic invertebrate fossils in the Wandilla Formation, and the deposition of the Curtis Island Group. Queensland Government Mining Journal, 76, 416-422. Flood, P.G. & Fergusson, C.L., 1984. The geological development of the northern rtew England province of the Nev/ England Fold Belt. In 1984 Field Conferoice. Volcanics> granites and mineralisation of the Stanthorpe-Brmaville-Drake region. H.K. Herbert & J.M.W. Rynn (eds). Geological Society of Australia, Queensland Division, Brisbane. Irving, E., 1966. Palecroagnetism of sane Carboniferous rocks from New South Wales and its relation to geological events. Journal of Geophysical Research, 71, 6025-6051. Jull, R.K., 1969. The Lower Carboniferous corals of eastern Australia: a review. In Stratigrafiiy and Palaeontology, Essays in Honour of Dorothy Hill, 120-139. K.S.W.Campbell (ed.). Australian National University Press. Kbtlyar, G.V. & Popeko, L.I., 1967. Biostratigrafia, Mshanki i Brakhiopodi verchnogo paleozoya Zabaikalya. (Biostratigraphy, bryozoa and brachiopods of the Upper Palaeozoic of the Zabaikal region. Zap. Zabaikal. Fil., Geogr. 0-va, S.S.S.R., 28, 1-323. Murphy, P.R., Schwarzbock, H., Cranfield, L.C., Withnall, I.W. & Murray, C.G., 1976. Geology of the Gympie 1:250,000 sheet area. Geological Survey of Queoisland Report, 96, 1-157. Nazer, R., 1977. Late Visean brachiopods with Western Australian affinities from the Yarrol Shelf. Queenland Government Mining Journal, 78, 126-131. Palmieri, V., 1969. Upper Carboniferous conodonts from limestones near Murgon, south-east Queensland. Geological Survey of Queensland Publication 341, Palaeontological Papers 17, 1-13. Powell, C.McA. & Veevers, J.J., 1984. Termination of the Uluru Regime: the mid-Garbcaiiferous lacuna. In Phanerozic Ecu±h History of Australia. 348-350. J.J. Veevers (ed.). Oxford Geological Sciences Series, 2. Clarendon Press, Oxford. Roberts, J., 1971. Devonian and Carboniferous brachiopods from the Bonaparte Gulf Basin, northwestern Australia. Bureau of Mineral Resources, Geology & Geofiiysics Bulletin, 122, 1-319. Roberts, J., 1975. Early Carboniferous brachiopod zones in eastern Australia. Journal of the Geological Society of Australia, 22, 1-32. Roberts, J., 1981. Control mechanisms of Carboniferous brachiopod zones in eastern Australia. Lethaia, 14, 123-134.

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Roberts, J., Hunt, J.W. & Thcnpson, D.M., 1976. Late Carboniferous marine invertebrate zones of eastern Australia. ALcheringa, 1, 197225. Roberts J., et al., 1985. Australia. In Carboniferous of the World. lUGS Publicaticxi 16, Instuto Geologico y Minero de Espana and Ehipcessa Nacional Adaro de Investigaciones Mineras S.A., Spain. Runnegar, B. and Campbell, K.S.W., 1976. Late Palaeozoic faunas of Australia. Earth-Science Reviews, 12, 235-257. Staiith, A.G., Briden, J.C. & Drewry, G.E., 1973. Phanerozoic Vforld Maps. In Organisms and Continents Hirough Time. N.F. Hughes (ed.). Special Papers in Palaeaitology, 12, 1-42. Winkler Prins, C.F., 1968. Carboniferous Productidina and Oioietidina of the Cantabrian Mountains (NW Spain): systenatics, stratigraphy and palaeoeoology. Leid. geol. Meded., 43, 41-126. Ziegler, A.M., et al., 1977. Paleozoic biogeography of continents bordering the lapetus (pre-Qiledonian) and Rheic (pre-Ifercynian) oceans. In Paleontology and Plate TectCTiics. R.M. West (ed.). Milwaukee Public Musajm Special Publications in Biology and Geology, 2, 1-23.

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A MECHANISM FOR THE COLLAPSE OF MARGINAL SEAS W.D. Roots Macquarie University, Sydney, Australia The opening and closing of marginal seas can be seen as two events in a sequence of events that results from the presence of an evolving hierarchy of convection cells in the mantle. Even though continental and oceanic lithospheres have similar surface heat flows, the rate of heat removal from the mantle under oceans is twice that beneath continents (Fig. 1, data from Pollack and Chapman, 1977), and this difference initiates rising mantle convection cells under the centres of super-continents. Continental rifting follows in the long run, and a new ocean develops flanked by passive margins (Fig. 2a).

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Fig. 1

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Both fragments of the super-continent drift from their original positions over the convection cell, exposing the cell core to oceanicrate cooling. Both fragments move to cover areas that were previously covered by oceanic lithosphere, halving the cooling rate there and leading to the start of two new convection cells (Fig. 2b). Both new convection cells are established within the domain of the original convection cell, and they expand at its expense. Between two cells (one old, one new) there is an interface between converging flows, and this interface moves as the competing cells change in relative power. The new converging-flow will expand until rifting occurs, the old cell is shrinking, so the converging-flow interface between them moves outward from the centre of the continent, towards the last-formed passive margin (Fig. 2c). The interface dips toward the continent centre, as interface- movement occurs when the increasing-velocity (top) flows of the new cell over-ride the falling-velocity flows of the old waning cell.

202

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The convergence of mantle flows from two cells to a common place applies converging tractive forces to, and compresses, the lithosphere above. The turning of both flows downward into the mantle plucks at the lithosphere above. The lithosphere would be subducted if buoyancy permitted, trapped between and pulled down by the two flows. Once the convergent flow zone (C-zone) passes seaward beneath a (passive) continental margin, subduction of oceanic lithosphere can begin (Fig.3). Further seaward movement of the C-zone is temporarily halted, with the start of subduction, as such movement now requires the subducting slab to be moved through the mantle orthogonal to its strike. The two competing cells still continue to change in power, however, until the increasing velocity differential between them truncates the slab at the top (Fig.4a). Freed from the inhibiting influence of the slab (which continues to sink, opening a window to the convection flows), the competing flows re-equilibrate at a point seaward of the continental margin (Fig.4b), and subduction begins at this new location. A C-zone located away from a continental margin can cause the development of a nappe in oceanic lithosphere (Fig.4b), and entrap either a single (Fig.4c) or a double thickness (Fig.4d) of lithosphere in the C-zone interface if the landward lithosphere can move seaward. Tensile stress is applied to the landward lithosphere by any attempt at double-slab subduction, and tensile failure (at the last active volcanic arc) will result in the development of a spreading marginal sea (Fig.4d,e). Double-slab subduction will consume the oceanic lithosphere landward of the subduction site, up to the point where the (unsubductable) half-arc arrives at the subduction zone. The landward side slab will then rift free of the half-arc, which will remain in position above the C-zone (Fig.4e). Marginal sea spreading will then cease, and subduction will continue at that site, until the next slab truncation occurs.

203

Fig. 4

New marginal seas will continue to develop in this way, each seaward of the prior one, until the adjacent (waxing) convection cell rifts the continent above. Rifting will begin the collapse of the marginal sea complex, as the near half-continent begins movement, under the traction of the seaward moving convection flow, toward the subduction zone (Fig.5a). The marginal sea complex will be compressed between the leading continental edge and the subduction zone (Fig.5b). Of the material arriving at the subduction zone, denser components may be subducted but the rest will remain unsubducted as a set of contrasting terranes accreted to the continental margin. A balance will be struck between seaward-directed tractive forces pushing the continent (and accreted terranes) across the slab (Fig.5c), and landward-directed tractive forces tending to prevent this happening. The result will be that the point of departure of the slab from the base of the lithosphere will be landward of the trench site, explaining the structure of the Pacific-American margins, and their contrast to western Pacific arcs.

20^

(a)

(b)

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The terranes resulting from the collapse of marginal seas will be interpretable from the geology of western Pacific marginal seas. In particiaar, the direction of sediment transport in relation to the sediment source should match the present patterns. Thus one should expect a continental terrane flanked by a marginal sea terrane that can be divided into sub-terranes showing transport of continental sediment from the landward side (on the landward side) and continental plus volcanic sediment from the seaward side (on the seaward side). Between these two should occur a zone withshore-parallel transport of intermixed sediment from both sources. An arc that includes a continental fragment should outlie these terranes (equivalent to the Japanese Islands). Next outboard should occur another marginal sea (with sediments from both directions as before), but showing a reduction, in or complete absence of, continental material in the seaward source. A marine terrane should outlie the last marginal sea set. The arc splitting process outlined above will move melange deposits always to the seaward edge, so these will characterise the final marine terrane. Non-orthogonal marginal sea closure will move terranes along a margin, as long as this (Andean) form of subduction continues, and may upset the original order to some extent. References Pollack, H.N. and Chapman, D.S., 1977. Mantle heat flow. Earth Planet. Sci. Lett. 34, 174-184.

205

MULTIDISCIPLINARY STUDIES OF CONTINENTAL SEISMICITY AND THE SUPPOSED PASSIVE MARGIN OF QUEENSLAND AND NORTHEASTERN NEW SOUTH WALES J.M.W. Rynn Department of Geology and Mineralogy, University of Queensland, Brisbane, Australia.

(no abstract received)

206

GEOTECTONIC UNITS OF THE JAPANESE ISLANDS AND THEIR ORIGIN Yasuji Saito Department of Geology, National Science Museum,

Tokyo,

Japan

The increasing and consistent paleomagnetic data from Tertiary rocks of the Japanese Islands indicate that the present-day island arc was formed by back-arc spreading during Middle Miocene time, and modified by later collision with other island arcs. Before the opening of the Sea of Japan, the Japanese Islands occupied a marginal part of the East Asia continent, where pre-Neogene rocks were arranged in enechelon subparallel belts. The pre-Neogene rocks of the Japanese Islands consist mainly of four collective units. They include 1) "older" rocks with continental characteristics, 2) Permian to Triassic (?), 3) Jurassic, 4) Cretaceous to Paleogene subduction complexes. 1) The older rocks are "exotic" to the subduction complexes and include two types: Precambrian to Paleozoic metamorphic rocks and Siluro-Devonian sedimentary and volcanic rocks. The former comprises a terrane on the Japan Sea side of central Japan, and is considered to be derived from the Sino-Korea massif. Intermediate-pressure type metamorphic rocks developed in the eastern margin of the terrane are similar to those of the Okcheon zone of the Korea Peninsula in metamorphic facies and age. Carboniferous foraminifers recently found in the eastern zone of the terrane are also close to those of the SinoKorea massif. The latter partly including Ordovician and Early Carboniferous material, is limited in distribution to tectonic mobile zones. Most of the older rocks occur as tectonic blocks associated with serpentinite. Recent paleomagnetic data for Silurian welded tuff and limestone suggest that they were formed in an equatorial region and have travelled far northwards. Much paleontologic evidence therefrom demonstrates similarities between the older Japanese rocks and those of the Yantze massif and eastern Australia. Such older rocks may have formed a continent together with parts of China and Australia during Siluro-Devonian time. 2) The Permian to Triassic (?) subduction complex is distributed in the northwestern part of the Inner Zone of southwest Japan. In the late Permian clastic matrix of the complex are embedded exotic mafic volcanics, limestone, and chert ranging in age from late Early Carboniferous to Middle Permian. Metamorphic rocks of high-pressure type on the Japan Sea side of southwest Japan are considered to be a part of this complex. 3) The Jurassic subduction complex is most predominant and constructs the framework of the Japanese Islands. It is stratigraphically subdivided into two subunits. The older one is composed of Early to Middle Jurassic clastic facies containing exotic blocks of Late Carboniferous to Triassic chert, limestone, and mafic volcanics. The younger one consists of Late Jurassic clastic facies with Triassic to Early Jurassic chert blocks. In some cases, pelagic shale intercalations in chert tend to increase upwards to grade into siliceous shale and finally into coarse-grained clastic sediments. Paleolatitudes of those blocks having an oceanic affinities, such as Late Paleozoic to Triassic mafic volcanics and chert, are very low and the chaotic sediments can be explained by mingling formations in a subduction zone at the continental margin. Reconstructed stratigraphic sequences based on those blocks of oceanic affinities are considered to represent oceanic plate sequences developed on the subducting slab. 4) The Cretaceous to Paleogene subduction complex is developed in the Pacific Ocean side of the Japanese Islands, lying outboard of the Jurassic subduction complex. The results of paleomagnetic investigations on the complex indicate that clastic matrix and acidic volcanics were deposited near the present latitude, while mafic volcanics and chert accumulated in a low latitude area. This complex has obviously been formed by accretion processes related to subduction.

207

From the Korea Peninsula and Sikhote Alin to the Pacific side, the older rocks, STibduction complexes, their metamorphic equivalents, and acid igneous rocks, forming subparallel belts, are repeatedly arranged en echelon. Such juxtaposition of geotectonic units of different origin is representative of a convergent boundary with siibduction.

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ECONOMIC, MAINLY METALLOGENIC, ASPECTS OF TERRANE ACCRETION Erwin Scheibner Geological Survey of New South Wales, Sydney, 2000, Australia Soon after the advancement of the concept of tectonostratigraphic terranes in western North America, the economic, mainly metallogenic, aspects of terrane accretion have been investigated. The early proponents were: Albers (1981, 1983), Berg (1979), Berg et al. (1981), Campa and Coney (1983), Damon et al. (1983), Dawson (1984) and Tooker (1983). Berg (1979) observed that fundamental differences among terranes imply differences in origin, distribution, types, controls and history of mineral deposits that occur in them. A major factor in testing and applying the terrane concept in metallogenic analysis and resource appraisal is that it is capable of predicting basic differences in mineral deposits that originated in different terranes before they were assembled and accreted at continental margins (Berg, 1979). It also predicts potentially important modifications to pre-existing mineral deposits that originated during and after the final terrane accretion. Mineral deposits can be classified in relation to terrane accretion as: pre-accretionary, (syn-) accretionary and post-accretionary. This classification applies well in regions which had simple accretionary history and adequate data are available on the age of mineral deposits, e.g. western North America (ops. cit.). However, most orogenic belts which developed at active plate margins had a polyphase terrane accretion history (cf. Dallmeyer, this Abstracts) and the Tasman Fold Belt System is a good example of this. During the Late Proterozoic to Triassic time the region of the Tasmanides experienced about three major episodes of terrane dispersal and six terrane accretionary events. The relatively straight forward relationships of mineral deposits to terranes described from the North American Cordillera have been telescoped in fold belts with multiple terrane accretionary events. The terranes which accreted during the early episodes became the neocratonic couples for younger terranes, and this has led to superposition of cratonic deposits over the terrane related ones, and resulted in further con^lications. To obtain a useful insight into the metallogenesis of orogenic belts characterized by multiple accretionary events, it is necessary to carefully decipher the relationships for each episode of terrane accretion. Such an analysis of the Tasman Fold Belt System will take a considerable time, and here only a few preliminary thoughts are presented. Degeling, Gilligan, Scheibner and Suppel (1984, in prep.) discussed the metallogenic development of New South Wales in relation to terrane dispersion and accretion, and these ideas are not repeated here. Tasmania is divided by the Tamar Fracture Zone into contrasting regions, which appear to have experienced only two Palaeozoic episodes of terrane accretion, one during the Cambrian affecting the western region and the other during the Middle Devonian, affecting the whole island. Tasmania can illustrate the mentioned spatial relationships of metal deposits and terrane accretion. A recent synthesis of tectonics and mineral deposits in Tasmania by Collins and Williams (1984, in press) is particularly valuable. Pre-accretionary deposits west of the Tamar Fracture Zone Few deposits occur in pre-dispersal rocks which are the Precambrian of the Rocky Cape, Tyennan and other basement blocks. The best known is the Savage River volcanogenic magnetite-pyrite deposit. Minor native copper occurs in Eocambrian to Early Cambrian tholeiitic basalts possibly associated with terrane dispersal. To a similar category belong the 'osmiridium', nickel, and chromite deposits associated with ultramafic/ mafic bodies and the copper-nickel sulphides in gabbroic intrusives, all emplaced in zones of extension. However, the most typical pre-accretionary

209

deposits in Tasmania are associated with the Mt Read Volcanics representing remains of a volcanic chain of uncertain affinity. Volcanogenic deposits include massive sphalerite-galena-chalcopyrite-pyrite with gold and silver. Such deposits include Rosebery and Que River and disseminated pyrite-chalcopyrite, with minor gold and silver, represented by the Mt Lyell deposit. As yet there are no known syn-accretionary deposits west of the Tamar Fracture Zone associated with the late Middle to Late Cambrian terrane accretion which caused inversion of extended areas between the basement blocks. Post-accretionary deposits west of the Tamar Fracture Zone During the Late Cambrian to Early Devonian time the western region of Tasmania became the neocratonic couple for the East Tasmania or Mathinna Terrane which is east of the Tamar Fracture Zone. The post-accretionary deposits include stratabound, disseminated and vein sphalerite-galena-silver mineralization in the Middle to Late Ordovician Gordon Limestone. There are as yet no known pre-accretionary deposits in the East Tasmania or Mathinna Terrauie. Accretionary and post-accretionary deposits associated with the Middle Devonian terrane accretion During the Tabberabberan Orogeny (Middle Devonian) the East Tasmania Terrane was accreted and also the western region was affected. Stitching plutons were emplaced throughout Tasmania. Some of the auriferous gold, sphalerite and auriferous galena vein deposits could be syn-accretionary, but most of the metal deposits in Tasmania are associated with these stitching plutons. They include various deposit-types of tin, wolfram, molybdenite and bithmuth. Late Carboniferous and younger rocks represent overlap complexes and have platform cover character. They contain coal, oil shale and alluvial placer deposits. The above brief description of the Tasmanian deposits shows telescoping of groups of deposits related to two episodes of terrane accretion. Deposits typical of cratonic margin were superposed on terrane related ones, and in turn these were followed by superposed new terrane accretion (stitching plutons) related ones. Comparison of distribution of metal deposits in the Tasmanides and western North America This comparison is based mainly on Albers (1981, 1983, and pers. communication) and Dawson (1984, and pers. communication). Manganese chert deposits as in western North America are present in Eastern Australia, are restricted to accreted oceanic terranesjand have pre-accretionary character. Some occur in the Jindalee Beds in the Lachlan Fold Belt and mainly in accretionary prism rocks in the New England Fold Belt. Similar is the distribution of Cyprus-type massive sulphide deposits in the Woolomin Beds and Besshi-type deposits in the Girilambone Group in New South Wales. Volcanogenic massive sulphide of Kuroko-type occur in accreted arc terranes as in the North American Cordillera. In Australia these deposits are mostly related to volcanic rifting during episodes of terrane dispersal and hence are pre-accretionary. Other volcanogenic deposits are related to volcanic arcs which could be either pre- or post-accretionary. Mercury, as in the North American Cordillera is known only from accreted oceanic and arc terranes, perhaps closely related to faults. There is some indication that they are mostly post-accretionary. The present erosional surface strongly distorts the original distribution of mercury deposits.

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Iron-rich skarns (Paddys River, Mt Jackson, Pigna Barney, Tallawang, Cadia in New South Wales) also occur in accreted terranes as do the tungsten skarns (Attunga, New England). These deposits are post-accretionary where the accreted terranes behaved as neocratons. Porphyry-type deposits (post-accretionary) in the paratectonic zone are rare in Eastern Australia, examples are Cu/Mo porphyry mineralization associated with Ordovician granites in South Australia (Anabama, Netley and Bendigo) and others in the Georgetown Inlier. Cu/Au, Cu & Mo porphyry deposits (Cu/Au porphyries of Devonian age around Parkes, Cu-porphyry at Copper Hill, Mo-porphyry at Glen Eden in New England, a l l in New South Wales) occur in accreted terranes. These are post-accretionary as such deposits are in North America. In some cases it is difficult to prove their setting as being in w e l l defined magmatic arcs. Other porphyry deposits in Queensland may be related to Andean-type magmatic arcs (Lacy, 1980). Lineament control appears to be important. Post-accretionary antimony mineralization is restricted in its occurrence, with the New England Fold Belt being particularly rich. Lindsay Gilligan (pers. communication) pointed out that some genetic relationship exists between the eastern part of the Lachlan Fold Belt and the New England Fold Belt since post-Kanimblan Orogeny (intra-Carboniferous) time. Both regions contain antimony deposits. This could have resulted from igneous or tectonic underplating with antimony enriched source during the Middle Devonian and/or Carboniferous accretion of the New England terranes. Tin is more common in Eastern Australia than western North America, and a l l the individual tin belts here are post-accretionary and related to crustal melts either of leuco S-type and A-type, or A-type granitoids. Pb/Zn carbonate replacement deposits occur in the paratectonic zone in South Australia (Lower Cambrian rocks) and this is similar to the North American situation. Such deposits are also in post-accretionary settings in the allochthonous terranes when they have become neocratons (e.g. Queenstown in Tasmania, Wyelba district south of Y a s s , and Cooleman in southeastern New South W a l e s ) . Silver deposits are typically post-accretionary and include the epithermal Au/Ag deposits at Drake and the high grade granitoid-related vein deposits, both in New England. This is to some extent similar to western North America. Gold is omnipresent, but the largest accumulations occur in the accreted mainly arc terranes and this is identical to western North America. The main Victorian deposits are possibly related to a foreland fold and thrust belt structure of the Stawell-Bendigo Belt (Cox et al., 1983). The hydrothermal solutions scavenging gold from Cambrian greenstones were focussed by thrust and reverse faults resulting in these highly productive deposits. A rich province in New South Wales is related to the Gilmore Suture and the intrusives closely associated with this suture. A l l sutures in E . Australia should be scrutinized for syn- and post-accretionary gold deposits. S m a l l accumulations of metals (Cr, N i , platinum group) which have affinity for oceanic crust occur in the accreted oceanic terranes also in Eastern Australia. O i l and gas The hydrocarbon deposits in Eastern Australia are not related to Palaezoic terrane accretion, with the exception of some productive horizons in overlap complexes of Late Palaeozic to Early Mesozoic a g e . In North America the main deposits are confined to the craton, but important accumulations.

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some ultra-rich, occur in the pull-apart or California-type basins of Bally and Snelson (1980). Translation along transform boundaries or megashears causes terrane dispersal. High rates of sedimentation (700-4000 m/Ma) and high heat flow in pull-apart basins contributed to the formation of relatively young hydrocarbon deposits in Santa Maria, San Joaquin, Ventura, Los Angeles Basins in California, Taranaki in New Zealand and similar ones around the Pacific. The Great Basin-type basins of Bally and Snelson (1980) are also related to transtension, but are unimportant for hydrocarbon accumulations. The difference between the Great Basin-type and California-type basins being in substrate, old continental versus orogenic (accreted terranes) The Chinese-type basins (Bally and Snelson, 1980) are related to transfer of forces during continental collisons. Such processes lead to terrane dispersal and hence are relevant to this discussion. China-type basins contain hydrocarbon accumulations. Conclusion It appears that the terrane concept offers a new insight into metallogeny of orogenic beltse It is important during metallogenic analyses to classify deposits into pre-, syn- and post-accretionary settings, especially in orogenic belts which experienced multiple accretionary events. It is important to follow a tectonic approach to untangle the metallogenic complexities. It is hoped that in this way we w i l l be able to delineate highly prospective belts. Acknowledgements Permission to publish this paper was given by the Secretary of the Department of Mineral Resources of New South W a l e s . I greatly benefited from discussions of metallogenic problems with ray colleagues David S u p p e l and Lindsay Gilligan, and I am thankful for their comments and also comments by Ms Jeannette Adrian and Richard G l e n . References Albers, J . , 1981. A lithologic-tectonic framework for the metallogenic provinces of California. Economic Geology, 76, 765-790. Albers, J . P o , 1983. Distribution of mineral deposits in tectonostratigraphic terranes and cratonal rocks in western United States. Canadian J o u r n a l of Earth Sciences, 20, 1019-1029. Bally, A.W., and Snelson, S . , 1980. Realms of Subsidence: In: Facts and Principles of World O i l Occurrence. Canadian Society of Petroleum Geologists, Memoire, 6 , 9-94. Berg, H.C., 1979. Significance of geotectonics in the metallogenesis and resource appraisal of southeastern Alaska? a progress report (abstract). Alaska Geological Society Symposium "Alaska's mineral and energy resources, economics, and land status". Anchorage, Alaska, April 1979, Program and abstracts 42-43, m a p . Berg, H . C . , Decker, J.E., and Abramson, B.S., 1981. Metallic m i n e r a l deposits of southeastern Alaska. U . S . Geological Survey Open-File Report 81-122, 136p. Campa, M.F., and Coney, P.J., 1983. Tectonostratigraphic terranes and mineral resource distribution in Mexico. Canadian J o u r n a l of Earth Sciences, 20, 1040-1051. Collins, P.L.F., and Williams, E . , 1984. Metallogeny and tectonic development of the Tasman Fold Belt System in Tasmania. Geological Society of Australia, Abstracts, 12, 103-104.

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Collins, P.L.F., and Williams, E . , in p r e s s . Metallogeny and tectonic development of the Tasman Fold Belt System in Tasmania. Proceedings of the Symposium Metallogeny and Tectonic Development of Eastern Australia. Cox, S.F-, Ceplecha, J . , W a l l , V . J . , Etheridge, M.A., C a s , R.A.F., Hammond, R., and Willman, C . , 1983. Lower Ordovician Bendigo Trough sequence, Castlemaine area, Victoria - Deformational style and implications for the tectonic evolution of the Lachlan Fold B e l t . Geological Society of Australia, Abstracts, 9 , 41-42. Damon, P.E., Shafiqullah, M . , and Clark, K.F., 1983. Geochronology of the porphyry copper deposits and related mineralization of Mexico. Canadian Journal of Earth Sciences, 20, 1052-1071. Dawson, K . M . , 1984. Mineral deposits and principal mineral occurrences of the Canadian Cordillera and adjacent parts of the United States of America. Geological Survey of Canada, Ottawa. Degeling, P.R., Gilligan, L.B., Scheibner, E . , and Suppel, D.W., 1984. Metallogeny and tectonic development of the Tasman Fold Belt System in New South W a l e s . Geological Society of Australia, Abstracts, 12, 134-136. Lacy, W . C . , 1980. Mineralisation along the extension of the New England and Lachlan-Thomson Fold Belts into North Queensland. The Geology and Geophysics of Northwestern Australia edited by R . A . Henderson and P . J . Stephenson, Geological Society of Australia, Queensland Division, 169-277. Tooker, E.W., 1983. Correlation of m e t a l occurrences and terrane attributes in the northwest conterminous United States Canadian Journal of Earth Sciences, 20, 1030-1039.

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CONTINENTAL AND OCEANIC TERRANES OF THE INTERNAL HELLENIDES, Elizabeth Mass.

Institute of Technology,

GREECE

Schermer

Cambridge,

U.S.A,

and U . S .

Geological

Survey

For many years geologists have recognized that the Hellenides are composed of several oceanic and continental fragments accreted during c o l l i s i o n of the Apulia and European plates in the Alpine orogeny. O r i g i n a l l y interpreted in terms of geosynclinal theory, the " i s o p i c zones'® of Aubouin (1959) have since been put into a plate tectonic fraunework. Later work has shovm that many of the fragments, or t e r r a n e s , are far-travelled (>100 km) and that the d e t a i l s of the timing and nature of deformation and metamorphism related to accretion events are poorly understood. Recent summary a r t i c l e s can be found in Dixon and Robertson ( 1 9 8 4 ) . The location of terranes and their stratigraphy are shown in figure 1. Eastern Greece and southern Bulgaria consist of the Rhodope (RP) and Serbo-Macedonian (SM) massifs (figure 1 ) , composed of Precambrian to Paleozoic g n e i s s e s , s c h i s t s , aunphibolites, migmatite, and marbles which have been subjected to deformation and metamorphism during the Caledonian (early P a l e o z o i c ) , Hercynian ( l a t e P a l e o z o i c ) , and Alpine (Late Jurassic-late Tertiary) orogenies. Burchfiel ( 1 9 8 0 ) interpreted the two massifs as parts of a single Rhodopian fragment whrch c o l l i d e d with the European platform in pre-Cenomanian (Albian?) time. Other workers consider the Rhodope massif to be a d i s t i n c t , Alpine-age fragment, separate from Serbo-Macedonia, which is a fragment r e l a t i v e l y unaffected by Alpine events (Jacobshagen et a l . , 1 9 7 8 ) . The boundary between these two terranes has been disrupted by Pliocene thrusting. The circum-Rhodope (CR) terrane contains T r i a s s i c platform carbonates overlain by Lower to Middle Jurassic flysch and thick intermediate to acid metaigneous rocks, and o p h i o l i t e s . A l l were deformed and metamorphosed to greenschist grade during the Upper Jurassic-Lower Cretaceous. West of CR, the Vardar zone (VZ) is a composite terrane of remnants of Paleotethyan (Permo-Triassic) and Neotethyan ( J u r a s s i c ) ocean f l o o r . Ujpper Jurassic o p h i o l i t e s , Kimmeridgiam island arc volcanic rocks, T r i a s s i c to Jurassic greenschist grade metasediments and upper Maestrichtian flysch apparently represent a suture zone which closed in the latest Cretaceous (Mercier et a l . , 1975; B u r c h f i e l , 1 9 8 0 ) . Overlapping Lower Cretaceous limestone and Tithonian age conglomerate with ophiolite debris date the amalgamation of VZ terranes; however, the oldest unequivocal overlap sequences with the Pelagonian zone terranes to the west and with CR and SM to the e a s t are no older than Miocene. Disrupted Paleocene-Eocene volcanic rocks may have overlapped the eastern VZ, CR, and SM terranes, and Eocene-Oligocene flysch is present in VZ, CR, amd Pelagonian zone terranes.

Ophiolites presumed to have originated in the Vardar ocean basin (or basins) are now present >150 km to the west in the Othrys terrane (OT), a composite of several o p h i o l i t e massifs which were deducted westward onto the external Hellenides as early as latest Jurassic (Tithonian) or Early Cretaceous (pre-Albian). Evidence for the timing of obduction and subduction includes: a ) the presence of Upper Jurassic

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arc volcanics in the Vardar zone, b) flysch with ophiolitic debris of Tithonian age in the Vardar zone, Barremian-Turonian age in the Pindos zone, and Albian age in Beotia (central Greece), and c) Cenomanian Limestone which overlaps the ophiolite allochthons. These data indicate amalgamation of ophiolitic terranes and underlying Triassic-Jurassic marbles by mid-Cretaceous; however, f i n a l accretion was not complete until Bocene-Oligocene as locally ophiolites are thrust over rocks of this age. Large-scale strike slip movement may have been important in the early stages of accretion and amalgamation (Smith and Spray, 1984; Burchfiel, 1 9 8 0 ) . The principal tectonostratigraphic terranes of the Pelagoniaui zone in mainland Greece consist of the following, from structural base to top: The Olympos terrane ( O L ) , contains Triassic to Eocene neritic carbonates overlain by Eocene (Lutetian) flysch. A recently discovered Late Triassic to late Early Cretaceous hiatus (Schmitt, 1 9 8 3 ) , suggests that previous correlation of Olympos with the External Hellenides (EH) or Parnassos zone (PS) may not be v a l i d . Structurally overlying the Olympos platform is the Ambelakia terrane (AM) (Papanikolaou, 1984; Schmitt, 1 9 8 3 ) , composed of marble, metaclastic rocks, gneisses, metabasalts, and serpentinite of unknown ages; this terrane is apparently a composite of oceanic and continental lithologies metamorphosed to blueschist grade during the early Tertiary. Possibly correlative rocks on the Cyclades islands (Northern Cyclades unit of Papanikolaou, 1984) have been dated as mid-Late Eocene, but the metamorphic event could be as old as late Cretaceous, or several metaunorphic terranes could be tectonically juxtaposed (Blake et a l . , 1981; 1 9 8 4 ) . The Flambouron terrane (FK) is a composite of the Kastoria and Flambouron units of Papanikolaou, ( 1 9 8 4 ) , with a highly varied tectonic stratigraphy consisting of Carboniferous granitic rocks and feldspathic orthogneisses at the base intercalated with amphibolites, and tectonically and unconformably overlying paragneisses and schists, pelagic marbles, metavolcanics, and ultramafic rocks. The age of the ultramafic series is unknown, while the metasediments are in pari: upper Paleozoic. Upper greenschist to lower amphibolite facies metamorphism in FK occurred in the lower Cretaceous. The Almopia terrane (AP) contains a thick sequence of Triassic-Jurassic marbles which have been considered to be the cover of the Flambouron basement terrane; however the contact i s a f a u l t . Northeast trending mineral lineations and isoclinal fold axes which are common in most Pelagonian zone lithologies are interpreted as a result of either northeastward or southwestward directed thrusting during amalgamation and accretion of the nappes of the Pelagonian zone. Mylonitic rocks dated at 40 m.y. (Barton, 1976). are interpreted to date the thrusting of metamorphic rocks of AM and FK over OL; amalgamation must be younger than mid-Eocene, the age of the youngest rocks in OL. West of the Pelagonian zone, Triassic-Jurassic marbles deformed during the Late Jurassic-Early Cretaceous and tectonically overlain by OT are often termed the "sub-Pelagonian" zone ( S P ) . These may be equivalent to the Pelagonian zone marbles of AP, although their presumed crystalline basement i s not exposed. A Triassic to Maestrichtian neritic carbonate platform, the Parnassus terrane (PS) crops out in south central Greece. The carbonates are overlain by Eocene flysch. Some workers have correlated these rocks with those of the Olympos tectonic window, however, as discussed above, this correlation is no longer v a l i d . The Pindos terrane (PN) consists of Triassic to Maestrichtian pelagic limestone and radiolarian chert, and Eocene flysch. The age and nature of the basement (oceanic or continental) is unknown. TSiese rocks are folded and thrust in far-travelled (>100 km) nappe sheets probably derived from east of the Olympos window (Jacobshagen et a l . , 1978; Burchfiel, 1980; Papanikolaou, 1 9 8 4 ) . Deformation began in the eastern Pindos zone in late Eocene to Oligocene time and progressed westward from Oligocene to Pliocene. Oligocene-Miocene molasse strata overlap deformed rocks in the Pelagonian and Pindos terranes, indicating accretion of these terranes was complete by this time.

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Terranes of the internal Hellenides are thrust onto Triassic to Eocene platform carbonates of the external Hellenides (EH); imbrication of EH began in the early Miocene in the east, and migrated westward through Miocene time, as terranes to the east were consolidated between the European and Apulian plates. EH has long been considered the margin of the Apulian plate as Triassic-Eocene carbonates overlap a l l regions west of PN. However, pre-Mesozoic rocks are not exposed in EH; whether this region experienced an earlier history distinct from the Apulia plate is unknown. I f several hundreds of kilometers of Alpine shortening and Triassic and Jurassic extension in continental terranes of Greece are taken into account, reconstrtiction of western Tethys prior to the Alpine orogeny indicates this region was dominantly continental crusto While many terranes may be far-travelled, accretion was apparently accomodated by much intra-continental shortening and probably involved subduetion of lower continental crust in addition to oceanic subduction. This is consistent with the geochemical evidence that Hellenic ophiolites were derived from small, arc-related basins, and with the lack of large accretionary wedge terranes such as occur in the western U . S . cordillera. REFERENCES CITED Aubouin, J . , 1959. Contribution a 1'etude geologique de la Grece septentrionale: les confins de I ' E p i r e et de la Thessalie. Ann. Geol. Pays. H e l l . , 10: 483 pp. Barton, C . M . , 1976. The tectonic vector and emplacement age of an allochthonous basement slice in the Olympos area, N . E . Greece. Bull. Soc. France, ser. 1% 253-258.

Geol.

Blake, M . C . , J r . , Bonneau, M . , Geyssant, J . , Kienast, J . R . , Lepvrier, C . , Maluski, H . , and Papanikolaou, D . J . , 1981. A geologic reconaissance of the Cycladic blueschist b e l t , Greece: Geol. Soc. Amer. Bull. 92s 247-254.

Greece:

•1984. A geologic reconaissance of the Cycladic blueschist b e l t . Reply: Geol. Soc. Amer. Bull. 95: 119-121.

Burx^fiel, B . C . , 1980. Eastern European Alpine System and the Carpathian orocline as an example of collision tectonics. Tectonophysics 63: 31-61. Dixon, J . E . , and Robertson, A . H . F . , e d s . , 1984. The Geological Evolution of the Eastern Mediterranean, Geol. Soc. Lond. Spec. Publ. 17. Jacabshagen, V . , Durr, S t . , Kockel, F . , Kopp, K . O . , and Kowalczyck, G . , 1978. Structure and geodynamic evolution of the Aegean region, in Alps, Appenines, Hellenides: Stuttgart, E. Schweizerbart'sche V e r l . 537-564. Jtercier, J . , Vergely, P . , and Bebien, J . , 1975. Les ophiolites helleniques sont-elles les vestiges d'un ocean tethysien ou d'une mer marginale peri-europeene? Comptes Rendu Sommaire des Seances de la Soc. Geol. France, (17) 4: 108-112. Papanikolaou D . J . , 1984. An introduction to the geology of Greece: the pre-Alpine u n i t s . I . G . C . P . Project #5, Field Guide September, 1984. Athens, Dept. Geology, 64 p.

Univ.

Schmitt, Ac, 1983. Novelles contributions a I'etude geologique des P i e r i a , de L'Olympe, et de I ' O s s a (Grece du Nord). These, Faculte Polytechnique de itans (Belgium), 215 p.

216

CARBONATE/NERITIC FACIES PELAGIC FACIES FLYSCH I T ^

MOLASSE, NON-MARINE FAQIES INTERMEDIATE-SILICIC VOLCANICS MAFIC VOLCANICS

I •

•

OPHIOLITE

Ry^

GRANITE

I X ^

GNEISS/SCHIST

n n

UNCONFORMITY THRUST FAULTING, FOLDING EVENT

—-

SEDIMENTARY OVERLAP

t ^

PROVENANCE LINK

METAMORPHISM: a

LOW-GRADE

•

BLUESCHIST

GREENSCHIST-AMPHIBOLITE

EH T3 T2

TTT

T1 UK

; •

T ' r

•

UJ - V MJ 1

LJ Tr Pz

.1 • r'

1 1

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Figure 1.

r r

Tectonostratigraphic terranes of the internal Hellenides, Greece. Terrane abbreviations in text. Tl= Paleocene, Eocene; T2= Oligocene, Miocene; T3= Pliocene^ Pleistocene.

217

A REVIEW OF AUSTRALIAN PALAEOZOIC PALAEOMAGNETIC POLES AND SUSPECT TERRANES

P.W, Schmidt and B«J.J. Embleton

CSIRO Division of Mineral Physics, North Ryde, Australia In 1960 Munk and MacDonald speculated derisively on the interpretation which would be placed on the hypothetical situation arising from scattered palaeomagnetic poles of the same age from the same continent. Their remarks were both uninformed and ironic because they were unaware of the work of Cox (1957) on the Eocene Siletz River volcanics, Oregon, which indeed yielded poles unlike any others previously determined from North American Tertiary rocks and paved the way for the widespread recognition given to displaced terranes in the 1970s and 1980s, lliere is now little doubt of the reality of displaced terranes and reviews of their current status from a palaeomagnetic point of view are given by McWilliams (1983) and Irving (1979). The reliability of Cox's results is enhanced by the presence of both normal and reversed magnetisations and a positive fold test (Graham, 1949) of which the latter is proving to be a most important tool for distinguishing between suspect terranes and displaced terranes in Palaeozoic fold belts. The Lachlan Fold Belt (LFB) developed during the Palaeozoic orogenies which have imparted a strong north-south structural grain to eastern Australia. As a Palaeozoic analogue of the North American Cordillera, the temptation to interpret the geological history of the LFB in terms of displaced terranes is very strong, but quantitative evidence of the magnitudes of displacements can only come from palaeomagnetic studies, and then only azimuthal rotations and latitudinal displacements may be discernable due to the inherent longitude indeterminacy of the palaeomagnetic method. Results of palaeomagnetic studies of Australian Palaeozoic rock units are summarised in Table 1. From the results available at the time, Embleton et al. (1974) and McElhinny and Embleton (1974) argued for an anti-clockwise rotation through 90® of south-eastern Australia with respect to the more geologically mature central, northern and western Australia since the mid-late Silurian, with final docking of south-eastern Australia taking place in the late Devonian. Since then Schmidt and Morris (1977) pointed out that such a mobilistic view was not necessary if the polarities of the early Palaeozoic results were actually reversed from what is conventionally assumed and if the magnetisation age of the Mereenie Sandstone was late Devonian rather than Siluro-Devonian. This enabled a single path to be constructed, thus requiring no large relative rotation or displacement. Both these interpretations are consistent with the data and known geological constraints. That many sedimentary sequences such as the Mereenie Sandstone are magnetised long after deposition is quite possible (Roy and Park, 1974 and Larson et al., 1982) and only a fold test will enable better resolution of the timing. Other efforts to interpret the Australian Palaeozoic palaeomagnetic data have been made by Morel and Irving (1978) and Goleby (1980), but

218

without the addition of definitive results the choice between any of the models is controvertible. Although Goleby gives results from many rock units, his sampling density was insufficient to f u l f i l the requirements of a conventional fold test. Fold tests are not given for individual rock units, rather they are given for many different units of differing rock types and a spread of ages. Whilst representing a useful reconnaissance, these results require confirmation through follow-up work on selected formations. Secondary magnetisation and overprinting mechanisms are undoubtedly responsible for the multi-component character of the magnetisations of rock units in the LFB which have undergone such complex deformational and thermal histories. It is becoming increasingly apparent that to establish the palaeo-relationships of terranes in south eastern Australia with the main Australian block requires intensive work on individual rock units whose structural attitudes are well understood. Palaeomagnetic studies (Table 1) of the early Devonian Snowy River Volcanics (SR) and Buchan Caves Limestone (both from Victoria) and the late Devonian Comerong Volcanics (CV - from N . S . W . ) have been reported by Embleton et al (1985) and Schmidt et al ( 1 9 8 5 ) . Both the volcanic units yield positive fold tests which confirm that their magnetisations (after cleaning) are pre-folding and most probably related to i n i t i a l formation. Pronounced overprint magnetisations were also identified possibly dating from the Carboniferous. The limestone was completely overprinted, or remagnetised. The two primary poles are i) for the early Devonian, located at 74.3®S and 222.7°E (dp = 11®, dm = 15®) and i i ) for the late Devonian, located at 76.9®S and 330.7°E (dp = 5 ® , dm = 9 ° ) . These poles are plotted in Fig. 1 where they are compared to other midlate Palaeozoic poles from Australia (Fig. 1a) and Africa (Fig. l b ) . The dashed curves represent paths that have been proposed previously. Uie mid-late Palaeozoic trend for south eastern Australia appears to be confirmed. Therefore if the age of magnetisation of the Mereenie Sandstone (MS from N . T . ) is indeed Silurian (?)-Devonian i . e . comparable in age to that of the Snowy River Volcanics, the only logical conclusion would be that the earlier allocthonous model of Embleton et al (1974) and McElhinny and Embleton (1974) is correct. However, without a field test available for MS, the age of magnetisation cannot be firmly established. This problem is the subject of a present investigation. In short i t remains possible to draw a single path through all existing pole positions, taking due cognizance of the real nature of the geological constraints present. Ihe age of magnetisation of the Mereenie Sandstone can be regarded as Carboniferous. Similar conclusions can be drawn from African Poles. The pole for the Msissi Norite (MN), Morocco, (Hailwood, 1974) often used as evidence in support for a Devonian magnetisation for the Siluro-Devonian MS actually intrudes late Devonian sediments, and must therefore be either latest Devonian or younger. A Carboniferous magnetisation age for both MS and MN fits all the facts presently known. While the LFB is a suspect terrane(s) i t must be stressed that in our present state of knowledge, the palaeomagnetic data do not support the notion of displaced terranes.

219

References Briden, J . C . , 1966. Estimates of directions and intensity of the palaeomagnetic field from the Mugga Mugga Porphyry, Australia. Geophys. J . R . astr. Soc., 11, 267-278. Briden, J . C . , 1967. Secondary magnetization of some Palaeozoic rocks from Tasmania. Pap. Proc. R. Soc. Tasm., 101, 43-48. Chamalaun, F . H . , 1968. The magnetization of the Dotswood Red Beds (Queensland). Earth Planet. Sci. L e t t . , 3, 439-443. Cox, A . , 1957. Remanent magnetization of Lower to Middle Eocene basalt flows from Oregon. Nature, 179, 685-686. Embieton, B . J . J . , 1972a. The palaeomagnetism of some Proterozoic-Cambrian sediments from the Amadeus Basin, central Australia. Earth Planet. Sci. L e t t . , 17, 217-226. Embieton, B . J . J . , 1972b. The palaeomagnetism of some Palaeozoic sediments from central Australia. J . Proc. R. Soc. N . S o W . , 105, 86-93. Embieton, B . J . J . , 1977. A late Devonian palaeomagnetic pole for the Mulga Downs Group, Western New South Wales. J . Proc. R. Soc. N . S . W . , 110, 2527. Embieton, B . J . J . , 1981. A review of the palaeomagnetism of Australia and Antarctica. In: M.W. McElhinny and D . A . Valencio (Editors), Palaeoreconstruction of the Continents, Geodynamic Ser. 2, AGU, pp. 77-92. Embieton, B . J . J , and Giddings, J . W . , 1974. Late Precambrian and Lower Palaeozoic palaeomagnetic results from South Australia and Western Australia. Earth Planet. Sci. L e t t . , 22, 355-265. Embieton, B c J . J . , McElhinny, M.W., Crawford, A . R . and Luck, G . R . , 1974. Palaeomagnetism and the tectonic evolution of the Tasman Orogenic Zone, J . Geol. Soc. Aust., 21, 187-194. Embieton, B . J . J . , Palmer, H.C. and Schmidt, P . W . , 1985. Pre- and postfolding magnetizations from the Devonian Snowy River Volcanics and Buchan Caves Limestone, Victoria. To be submitted. Facer, R . A . , 1977. Palaeomagnetism, radiometric age and geochemistry of an adamellite at Yetholme, N . S . W . , - Reply. J . Geol. Soc. Aust., 24, 122123. Fisher, R . A . , 295-305.

1953.

Dispersion on a sphere.

Proco R. S o c . , Ser. A . ,

Goleby, B . R . , 1980. Early Palaeozoic Palaeomagnetism in South East Australia. J . Geomag. Geoelectr., 32, Supp. I l l , 11-21. Graham, J . W . , 1949. The stability and significance of magnetism in sedimentary rocks. J . Geophys. Res., 54, 131-167.

220

217,

Haiiwood, E . A . , 1974. Palaeomagnetism of the Msissi Norite (Morocco) and the Palaeozoic reconstruction of Gondwanaland. Earth Planet. Sci. L e t t . , 23, 376-386. Irving, E . , 1960, 1965. Palaeomagnetic directions and pole positions, and V I I . Geophys. J . R . astr. Soc., 3, 444-449 and 9, 185-194.

Irving, E . , 1979. Paleopoles and paleolatitudes of North America and speculations about displaced terrains. Can. J . Earth S c i . , 16, 669-694. Irving, E. and Parry, L . G . , 1963. Hie magnetism of some Permian rocks from New South Wales. Geophys. J . R . astr. Soc., 7, 395-411. Irving, E. and Stott, P . M . , 1963. Palaeomagnetic directions and pole positions, V I . Geophys. J . R . astr. Soc., 8, 249-257. Kirschvink, J . L . , 1978. The Precambrian-Cambrian boundary problem: Palaeomagnetic directions from the Amadeus Basin, central Australia. Earth Planet. Sci. L e t t . , 40: 91-100. Larson, E . E . , Walker, T . R . , Patterson, P . E . , Hoblitt, R.P. and Rosenbaum, J . G . , 1982. Paleomagnetism of the Moenkopi Formation, Colorado Plateau: Basis for long-term model of acquisition of chemical remanent magnetism in red beds. J . Geophys. Res., 87, 1081-1106. Luck, G . R . , 1972. Palaeomagnetic results from Palaeozoic sediments of northern Australia. Geophys. J . R . astr. Soc., 28, 475-487. Luck, G . R . , 1973. Palaeomagnetic results from Palaeozoic rocks of southeast Australia. Geophys. J . R . astr. Soc., 32, 35-52. McElhinny, M.W., 1968a, b, 1969, 1972. Palaeomagnetic directions and pole positions, V I I I , IX, X and X I I . Geophys. J . R . astr. Soc., 15, 409-430, 16, 207-224, 19, 305-327 and 27, 237-257. McElhinny, M.W. and Cowley, J . A . , 1978. Palaeomagnetic directions and pole positions, XIV and XV. Geophys. J . R . astr. Soc., 49, 313-356 and 52, 259-276. McElhinny, M.W. and Embleton, B . J . J . , 1974. Australian palaeomagnetism and the Phanerozoic plate tectonics of eastern Gondwanaland. Tectonophysics, 22, 1-29. McElhinny, M.W. and Luck, G . R . , 1970. The palaeomagnetism of the Antrim Plateau Volcanics of northern Australia. Geophys. J . R . astr. Soc., 20, 191-205. McWilliams, M . O . , 1983. Paleomagnetism and the motion of large and small plates. Rev. Geophys. Space Phys., 21, 644-651. Morel, P. and Irving, E . , 1978. Tentative palaeocontinent maps for the Early Phanerozoic and Proterozoic. J . Geol., 86, 535-561. Munk, W.H. and MacDonald, G . J . F . , 1960. The Rotation of the Earth, a geophysical discussion. Cambridge University Press, London, 323 pp. Roy, J . L . and Park, J . K . , 1974. The magnetization process of certain red beds: vector analysis of chemical and thermal results. Can. J . Earth S c i . , 11, 437-471.

221

Schmidt, P.W. and Morris, W . A . , 1977. An alternative view of the Gondwana Can. J . Earth S c i . , 14, 2674-2678. Palaeozoic apparent polar wander path. Schmidt, P . W . , Cudahy, T . J . , Powell, C.McA. and Embleton, B . J . J . , 1985. Late Devonian magnetisation of the Comerong Volcanics, N . S . W . , Australia: Implications for the Gondwanaland pole path and midCarboniferous megakinking in the Lachlan Fold Belt. Tectonics, under review. Schmidt, P.W. and Embleton, B . J . J . , 1981. Magnetic overprinting in southeastern Australia and the thermal history of its rifted margin. Geophys. Res., 86, 3998-4008.

the paleomaqnetic data for Australia for the Paleozoic

enonic

Age (Myr)

Pole Lat.

Position Long.

Reference (dp,dm)

12/148 14/413 14/395 9/1 21

Antrim Plateau volcanics Hudson Formation JinducJcin Formation Ootswood Red Beds

JF OR

01 Du

09-S 18*N 13*S 46»S

340*E 019*E 025*E 136»E

McElhinny & Luck 17* Luck (1972) 13Luck (1972) 11* ( 1 4 * . 1 5 » ) Chamalaun (1968)

Lower Arumbera Sdst. Upper Arumbera Sdst. Todd River Dolomite Huqh River Shale Stairway S d s t . Mereenie S d s t . Ross River Overprint

ASL ASU TRD HRS SS MS RRO

latest Pe Latest Pe £l ei-m Om S-0 0

44*S 47»S 43'S 11'N 02*N 41*S 60"S

342-S 337-E 340»E 037-E 0S0*E 040*E 068*E

(8*,14*) (3*,5-) (5».3*) 8" 8* 10* (5*.7*)

<irschvink (1978) Kirachvink (1978) Kirschvink (1978) Embleton (1972a) Embleton (1972b) Embleton ( I 9 7 2 b ) Kirschvink (1978)

Silurian volcanics Huqqa Muqqa Porphyry Ainslie Volcanics Bowninq Group Snowy River Volcanics Mulqa Downs Comeronq Volcanics Visean Volcanics Yetholme Adamellite Main Glacial Staqe Rocky CreeJc Conqlomerate Currabubulla Formation Snowy-Buchan Overprint Permo-Carb. Volcanics Upper Marine Latites Milton Monzonite

SV MP AV BG SR MD CV W YA MG RC CF SB PC UM MM

Sm-u Su(423) 01 01 D1 Ou Ou CI Cu(320) CU Cu QJ C P-C Pm-u(248) Pu(240)

54-S 80»S 71-S 64«S 74"S 54*S 76.9'S 73*S ' a4»s 53»S 52»S 43'S 64»S 44-S 46-S 29*S

271*E 340»E 353»E 045"E 222-E 096»E 330.7-E 214-E 321*E 148*S 138»E 135*E 127*E 132*E 136*E 166*E

7* 7* 10* 9(IT,15*) 11* 7.2* 21* (8*.13*) 11*, 17* 24(4-,5*) 26* 15* 7-

Luck (1973) Briden (1966) Luck (1973) Luck (1973) Embleton et a l . (1985) ESnbleton (1977) Schmidt et a l . (1985) Luck (1973) Facer (1977) Irvinq (1966) Irvinq (1966) Irvinq (1966) Embleton et a l . (1985) McElhinny 6 Einbleton (1974) Irvino 6 Parry (1963) Schmidt & Embleton (1981)

14/348 8/127 14/368 14/369

Oundas Group Housetop Granite

OG HG

Dm(375)

South Australia

Pound Quartzite Lake Prome Group Arooma Dan Seds.

PQ LFG AO

latest Pe em-u

western Australia

TUmblaqooda Sandstone

Northern Auscralia

Central Australia

Southeastern Australia

APV

(1970)

14/415 14/393 14/376

14/352 15/118 8/103 8/104 8/105 -

15/108 7/39

23"S 67-S

013*E 094* E

12* 27*

Giddinqs 6 Embleton Briden (1967)

(1974)

15/226 9/123

006*E 02S*E 033'E

24* 13* 17*

Embleton & Giddinqs Qableton 6 Giddinqs Embleton 6 Giddinqs

(1974) (1974) (1974)

60*S 16*S 36'S

0?(£»-S1 )

30*S

031

Embleton & Giddinqs

(1974)

14/405

14/409 14/416

Myr - million years; Lat. - latitude? Lonq. - lonqitude: A^g » semi anqle of the cone of 95% confidence (Fisher 1953) around the pole: dp,da - the semi-axes of the e l l i p t i c a l error around the pole at a probability of 95%, dp in the coiatitude direction and dm perpendicular to i t . GJ - cataloque ceterence to pole lists compiled by Irvinq (i960, 1965); Irvinq £ Stott ( 1 9 6 3 ) ; McElhinny ( 1 9 6 8 a , b , McElhinny 6 Cowley (1977, 1978) and published in the Geophysical Journal of the Royal Astronomical Society. Geoloqical periods are marked as follows: e « Cambrian; O « Ordovician; TR « Triassic; J - Jurassic; ic - Cretaceous. Period subdivisions are denoted by:

1 « lower; m » middle;

u • upper.

222

S » Silurian;

1969„

D - Devonian; C - Carboniferous;

1972)

and

P - Permian;

Fig. la)

Mid-late Palaeozoic pole paths for Australia based on various interpretations discussed in detail by Goleby (1980), Schmidt et (1985) and Embleton et al (1985). The pole mnemonics are those used by these authors and Embleton (1981). rtie preferred Australian pole path compared to African poles after continental reconstruction. While this single path does not rule out allochthonous models it represents the simplest interpretation of presently available results.

223

LATE PALEOZOIC AND MESOZOIC ACCRETIONARY GROWTH OF JAPAN AND EASTERN ASIA Asahiko Taira Ocean Research Institute, University of Tokyo 1-15-1, Minamidai, Nakano-ku Tokyo, Japan The Pre-Neogene geotectonic framework of Japan can be divided into four basic terranes: older ( mostly Paleozoic) "exotic" rock mass, Permian-Early Triassic subduction complex, Jurassic subduction complex and CretaceousTertiary subduction complex (Figure 1 , 2; Taira, 1985). SW Japan is characterized by the subparallel arrangement of these terranes along its strike. The continuation of this arrangement can be traced to the northeast up to the Tanakura Tectonic Line. The pre-Neogene systems of NE Japan are poorly exposed because of the widespread Neogene and Quaternary sediments and volcanics except for the Kitcikami massif which contains exposures of Paleozoic-Mesozoic "cratonic" rock masses exotic to the surrounding trench accretionary prisms. The Hokkaido area shows a complex framework of terranes in which the eastern side is a continuation of NE Japan. The Cretaceous to Paleogene collision in central Hokkaido is the major event responsible for the creation of the basic tectonic frameworkc Permian to Early Triassic was the most important time in the development of eastern Asian continental juxtaposition. Many continental masses including Yangtze, Sino-Korea, Tarim, Breya and other microcontinental blocks collided with each other and against the Siberian platform which is the nucleus of the Asian continent (Fig. 3). The Hida massif, one of the older terranes, may be a part of such a continental block, possibly the SinoKorean block. During this time, a subduction complex was formed in which Carboniferous to Permian seamounts were emplaced into the trench fill clastics. The Jurassic period saw the development of the fundamental framework of SW and NE Japan. The Jurassic siabduction complex includes Carboniferous to Triassic limestone^chert, and metabasalt together with Jurassic trench-fill clastics creating either melange facies or thrust sheet piles (Fig. 4). There are two tectonostratigraphic complexes in the Jurassic subduction complex: one is the middle Jurassic complex and the other is the upper Jurassic complex. The first one was emplaced on top of the latter by decollment, probably in late Jurassic time. This emplacement was triggered by the collision of the Kurosegawa, Abukuma and South Kitakami allochtonous terranes (microcontinents and/or remnant arc). The collision was oblique and the succeeding strike-slip motion which probably continued to the late Cretaceous reorganized the Jurassic subduction complex and the collided masses. The consequence was the development of major strike-slip mobile zones such as the Median Tectonic Line, Kurosegawa Tectonic Zone and Hayachine Tectonic Zone. The Jurassic subduction complex was thus duplicated laterally at least once along the Median Tectonic Line by strike-slip motion. In Hokkaido, an important event occurred after the accretion of the Jurassic subduction complex. This was the emplacement of latest Jurassic to earliest Cretaceous ophiolite together with Valanginian deep-water clastic sediments which overlie the ophiolite. The emplacement of this sheet on top of the Jurassic siabduction complex (Kamuikotan Belt) took place during late Neocomian time judging from the deposition of the Yezo Group on the top of it. At present the mechanism of this ocean floor or oceanic island arc obduction is not certain, but it should be noted that this event is more or less contemporaneous with the collision and strike-slip events recorded in SW and NE Japan.

224

The late Cretaceous subduction complex was developed continuously from SW Japan to Hokkaido. However, the nature of the ocean floor which were consumed seems quite different. The Shimanto belt of SW Japan includes Tithonian ocean fllor as the oldest accreted material (Taira, 1981). On the other h a n d , the Hidaka belt of Hokkaido contains Permo-Triassic limestones and cherts in Cretaceous clastic matrix. The age of the ocean floor subducted in Hokkaido during Cretaceous time was probably quite old compared with the case for the Shimanto b e l t . The uplift of the Hidaka mountain ranges which was accompanied by thrusting of the Hidaka metamorphic belt occurred during Miocene time associated with the opening of the Okhotsk Sea and the collision of the K u r i l forearc (Kimura, 1983). The opening of the Japan Sea which took place during 15-14 Ma BP was accomplished by the clockwise rotation of SW Japan accompanied by anticlockwise rotation of NE Japan. This more o r less created the configuration of the present-day island arc system. B

Fig. 3

Accretion of eastern Asia A: Permian B: Triassic-Jurrassic C: Late Jurassic to Early Cretaceous D: Late Cretaceous to Paleogene Cold front represents subduction zone.

225

1: Siberia Block 2: Sino-Korea Block 3: South China Block 4: Indochina Block 5: Chukotka Block 6: Tarim Block . 7: Okhotsk Block 8: India Block

Dotted area is accretionary belts.

Hida Metamorphic Belt (a part of continental mass). Sangun Metamorphic Belt (metamorphosed Permian subduction complex). Joetsu Metamorphic Rocks (same as above?). Yamaguchi Belt (Permian subduction complex). Maizuru Belt (Permo-Triassic shelf facies and Yakuno ophiolite). Pre-Cretaceous shelf facies in Joetsu Belt. Tamba-Mino Belt (Jurassic subduction complex). Ashio Belt (Jurassic subduction complex). Northern Kitakami Belt (Jurassic subduction complex). 6-1 and 6-II; Zones I and 11 by Minoura (1985). Jurassic subduction complex; (largely covered by Tertiary). Nappes of Jurassic subduction complex. Ryoke Metamorphic Belt (metamorphosed Jurassic subduction complex). Sambagawa Metamorphic Belt (metamorphosed Jurassic subduction complex). Nishisonogi Beit (same as above?).

9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21.

Chichibu Belt (Jurassic Subduction complcx). Sambosan Belt (Jurassic subduction complex). Shimanto Belt (Cretaceous to Tertiary subduetion complex). Abukuma Belt (Prc-Cretaceous shelf facies). Southern Kitakami Belt (Pre-Crctaceous shelf facies). Kamuikotan Metamorphic Belt (Metamorphosed Jurassic subduction complex). Horokanai Ophiolite (Late Juras.sic ocean floor or island arc). Yezo Group (Cretaceous clastics). Hidaka Bell (Cretaccous subduction complex). Yubetsu Belt (Cretaccous subduction complcx). Hidaka Metamorphic Belt (island arc crust being thrust over). Tokoro Belt (Jurassic subduction complcx). Ncmuro Belt (Late Cretaccous lo early Tertiary forearc basin and intra-urc volcuniclastics).

Strike-slip Mobile Zones A: Circum-Hida Tectonic Zone. B; Nagato Tectonic Zone. C; Median Tectonic Line, D; Ruroscgawa Tectonic Zone. E; Fossa Magna (Itoigawa-Sliizuoka Tectonic Line), F; Tanakiira Tectonic Line. G; Futaba Tectonic Line, II; Hayachine Tectonic Line.

Fig. 1

Geologic Terranes

in Japan (after Taira, 1985)

22 11

JAPAN

Kurosegawa Tectonic Zone Median Tectonic Line Nakatsuyama

/ sanbosan

Hida Belt

Circum-Hida Belt S a n g u n Metamorphics

Punw® Miumorphics

Sanbagawa Metamorphics Chichibu Belt

Shimanto Belt

HOKKAIDO

Toshima Belt

Horokanai

Yubetsu Belt^ Hidaka Metamorphics

Kamuikotan Metamorphics

Fig. 2

Tokoro Belt

Ophiolite ^Oceanic Plate

Highly schematic cross sections of Hokkaido and SW Japan (after Taira, 1985)

226

/. LATE JURASSIC

Accretionary Prism(Tamba-Mino-Chichi 8 « i t , « t c tbuki seamt.

Kuruma Group

Circum-Hida T«ctonic Zone

Kurosegawa Block

Mikabu Aseismic Ridge

t Oicollement Sangun-Yamaguchi-Maizuru Belt

2 LATEST

JURA.

Tamba Nappes Chichibu Nappes

Ohga Nappes ^Collided Kurosegawa Block Sanbagawa Nappes

J. LATEST JURA. - EARLY CRETACEOUS ' Buko Seamt. Sambosan Belt

Kurosegawa Tectonic Zone Median Tectonic Line

4. LATE CRETACEOUS ,

„

^ - I z u m i Stnke-Slip Basin • Chichibu-Nakatsuyama Nappes Shimanto Belt

Ryoke Metamorphics

Fig. 4

Late Mesozoic development of SW Japan (after Taira, 1985) References

Kimura, G., 1985, Mode of subduction and plate motion in Hokkaido. 24-31 (in Japanese).

Kagaku, 55,

Taira, A., 1981, The Shimanto belt of southwest Japan and arc-trench sedimentary tectonics. Recent Progress in Natural Sciences Japan, 6, 147-162. Taira, A., 1985, Pre-Neogene Accretion Tectonics in Japan: A Synthesis. Progress of Natural Sciences in Japan, 10, 51-63.

227

Recent

DEVONIAN ZOOGEOGRAPHY IN RELATION TO TECTONOSTRATIGRAPHIC TERRANES IN EASTERN AUSTRALIA John A . Talent School of Earth Sciences, Macquarie University, North Ryde 2113, Australia

Eastern Australia lacks an array of Palaeozoic ophiolite suites comparable to those of China, central Asia and SW Siberia where it is a matter of relative ease to discriminate micro-continents and other crustal terranes. Nonetheless, despite the apparently divergent collages of structural zones and definitions presented by various workers, a general pattern can be recognized. This has been interpreted primarily in terms of the orthodoxies of crustal dilation, accretion and cratonization, often without significant recourse to strike-slip tectonics. There is in fact no a priori reason why the contemporary configuration of structural zones need bear much relationship to past palaeogeographies and basin geometry. It will be contended here that large scale translocations of the order of hundreds of kilometres may have grossly altered relative positions of 'terranes' and that past configurations therefore can be deciphered only in a very general w a y . The evidence may not be compelling, but neither is the evidence for former contiguity and relative immunity to major translation. Instance the Melbourne 'Trough' o r , better and non-committally, Melbourne Terrane [nomenclature used h e r e , for convenience, is that of Scheibner (1984)]. There is still no data that necessarily constrains one to the popular belief that the 'Trough' was closed to the E or SE. Though palaeocurrent patterns may have changed through Silurian and Early Devonian time, the directions range only from S to W and NW (predominantly from the SW to NW quadrant). There is no documented sedimentary input from the N E , E or SE nor other data suggesting closure in those directions. The pattern so far documented is more consistent with some sort of prograding wedge than with a substantially closed trough or basin. It has been postulated (Talent et a l . , 1975, p . 16; Jell & Talent, in press) that the present geometry of the 'Trough' and specifically its E 'margin' may be an artefact of predominantly post-Silurian, pre-Late Devonian translocation of the order of hundreds of kilometres along the 'Dolodrook-Howqua-Benalla-Dookie tectonic complex'. Further candidates for major translocations in central and E Victoria include other boundaries of the Melbourne Terrane: the tectonically complex Heathcote 'Axis' (its W boundary) and the Walkerville Fault (Talent, 1965), its 'boundary' to the SE. Add to these the Indi Fault in E Victoria, with its extension, the Long Plain Fault of southern N.S.W. and the Coolac-Goobarragandra serpentine belt (cfo Crook & Felton, 1975, for review of E Australian 'ophiolites') and there results a notional division of SE Australia (Fig. 1) that forms the basis for subsequent discussion. Palaeozoogeographic principles. There are obvious dangers in the traditional approach of drawing biogeographic conclusions from one or two taxa, proceeding by hunch rather than through principles of quantitative biogeography (Yolkin et al., 1985). Before attempting to draw conclusions about the significance of similarities or dissimilarities of faunas relative to various crustal models, it is essential that: le Comparison be made between faunas of precisely the same age; this usually requires re-evaluation of accepted correlations.

228

2. Taxonomies of the faunas being compared should be consistent, preferably the work of the same worker or team. Only thus can one avoid the biogeographic phantoms that arise from uncritical acceptance of the work of a medley of workers with differing taxonomic styles. 3. Zoogeographic data be clearly differentiated from palaeoecologic data. There is little point in comparing faunas from contrasting sedimentary environments. An adequate sample of the faunas of an autochthonous or suspect terrane should include a goodly sampling of most, preferably the entire suite of its 'palaeocommunities*. Stratigraphic alignments. We are far from developing trustworthy stratigraphic alignments for much of the Devonian of E Australia, especially for the Late Devonian (Mawson et al., 1985). In central Victoria there are large areas of monotonous, sparingly fossiliferous and unfossiliferous Silurian-Early Devonian mudstone-sandstone sequences for which biochronologic control is deficient or patchy. This has made it difficult to rigorously avoid the old conceptual problem of commingling lithostratigraphic, chronostratigraphic and now palaeoecologic approaches (Garratt, 1983a, 1983b). Though dependent on chronostratigraphic inferences for historic analysis, a palaeoecologic (community/benthic assemblage) system should be geographically (not stratigraphically) sequential for any given time interval. The suite of depth-controlled benthic communities recently proposed (Garratt, 1983b, Fig. 3), however, groups associations of various ages: Early Ludlow to Late Pragian. Though outcropping over large areas, the Early Devonian rocks of central Victoria do not have elegant successions displaying several graptolite zones or have enough carbonate developments to supply conodonts for formulating tight correlations. For the most part, the record of fossils with high correlative potential is rather spotty but persistent efforts (N.W. Schleiger, unpub.; Garratt, 1983a, 1983b) are gradually yielding harder data. The Silurian-Devonian boundary is still placed somewhere among the poorly preserved and nondescript shallow water faunas of the thick (c. 1600 m) Mclvor Formation (Jell & Talent, in press), but there are continuing difficulties in aligning the units of it and the overlying Mt Ida Formation (2500+ m) with units lying E of the Moormbool Fault (Talent, 1964), a possible c. 25 km dextral wrench. Publication of a vast amount of work by N.W. Schleiger (pers. comm.) in the BailliestonPuckapunyal-Seymour-Kilmore area should help reduce this tenuousness. There is, nevertheless, still no firm basis for correlation of the post-Dargile (?Late Ludlow, Pridoli to Pragian) units across the Melbourne Terrane from Heathcote through Seymour and Yea to the well-mapped Eildon area. Similarity/dissimilarity of faunas The present account is concerned solely with rugose corals (see Hill, 1978, for literature) and brachiopods. Contrasting ostracode faunas have been described from the Lilydale Limestone, the Receptaculites Limestone at Taemas, and the Buchan Caves Limestone at Buchan. Similar contrasts between the mollusk faunas of the same units and, as well, those of the Kilgower Member of the Tabberabbera Formation, the Coopers Creek Formation and the Bell Point and Burdekin Limestones may be attributed to the sporadic nature of data to date as well as to age and environmental differences. Melbourne Terrane vs other terranes in SE Australia. It is difficult to draw meaningful comparisons between the incredibly diverse brachiopod faunas (c. 170 spp.) from the limestones of the Garra Formation at Wellington (Lenz & Johnson, in press) and The Gap (J.R. Farrell, in prep.) [both in the

229

Molong-Monaro Terrane] and the low diversity faunas from equivalent, approximately Early Pragian (pesavis and sulcatus Zones) horizons elsewhere, namely the Waratah Limestone and the limestones at Lilydale, Loyola, Coopers Creek and Tyers [in the Melbourne Terrane] and the Point Hibbs Limestone in western Tasmania. In all cases the faunas are small (c« 8-15 spp.) and mostly undescribed. The great contrast between any or all of these and the Garra faunas could lead one to argue, if one were uncritical, for provincial contrast» The impact of silicified faunas in boosting species-counts by 5 to 10 times and effects of this on 'palaeocommunity discrimination' has been emphasized elsewhere (Talent, 1983b). What of rugose corals from the same units? Only a single species of rugose coral from the carefully-studied Garra Faunas (38+ spp.) has been identified confidently from the muchcollected Lilydale and Loyola faunas (9 spp.); 7 of 9 species known from the latter two faunas occur among 22 specifically identified Rugosa at Tyers, also in central Victoria. Around 50% of the specifically identified brachiopods and rugose corals (only 6 and 7 spp. respectively) from Point Hibbs have been recorded elsewhere; the sample may be too small for zoogeographic significance. Present data is therefore consistent with great geographic separation or profound zoogeographic barriers between the coeval Garra and central Victorian faunas but caution is needed: The Garra faunas come from a wider spectrum of carbonate facies than occurs at Lilydale, Loyola or Tyers. • Silverwood

• Cobar

BROKEN RIVER EMBAYMENT

Loyola Seymour Heothcore Yea \ '.MtEtna Creek \ r ^ .o^rkh^mntrtn Craigilw • ^ L^ckhampton • Springsure

100

200

^^^^^^^^ _

Eildon Kinglake

300 km BRISBANE

Fig. I

Tectonostrotiqraohic ferrones of «asttrn Australia and location of principol Devonian coral and brachiopod faunas

Non-carbonate brachiopod faunas of Early Pragian age (approx. pesavis and sulcatus Zones) include those of Loyola, the Coopers Creek Formation at Tyers and Marble Creek and the upper Humevale Formation about Mooroolbark, Lilydale and Kinglake West in the Melbourne Terrane, the Kilgower Member (lower Wentworth Group) of the Tabberabbera Terrane and may include undescribed faunas S of Mudgee, N.S.W. All of these have much in common lithologically and may be assumed to have broadly similar spectra of biofacies.

230

Brachiopods of the Maradana Shale at Manildra, N.S.W, (Savage, 1974) are demonstrably older (?Early Lochkovian), underlying (by c. 280 in) a ?delta Zone conodont fauna and there is uncertainty as to precise correlation of the Florence Quartzite and Bell Shale faunas from the vicinity of Zeehan in NW Tasmania. Brachiopod faunas from the mudstones and conglomerates of the Coopers Creek Formation (18 spp.) have 617o in common with LilydaleMooroolbark and Loyola faunas [all on the Melbourne Terrane] and a mere ITk in common with the Kilgower Member of the Tabberabbera Terrane; the latter (35 spp.)» however, has a slightly higher similarity (317o) with the combined Lilydale and Loyola faunas. In short, the brachiopod faunas of the Kilgower Member are less similar to those of central Victoria than one might have imagined from a recent comparison (Garrett, 1983, p. 89) focussed, however, on correlation rather than palaeozoogeography. If one applies criteria for discriminating contemporary marine provinces (507o species-contrast), the faunas from the two areas would be assigned to different provinces.

It can

be argued that the contrasts may be no more than artefacts of incomplete sampling. Incidentally, the contemporary marine circulation and climatic patterns along longitudinally aligned continental coasts produce provincial change on a rough average every 15-18 degrees of latitude (1600-2000 km). Moreover a 750-1000 km longitudinal separation of shelf faunas by deep ocean without shallow-water staging points will ultimately lead to generation of discrete provinces (Talent, 1983a, 1985). In view of sampling problems with the Devonian faunas, however, it would be perilous to attempt explicit inferences using such oversimplified generalizations. The data to hand, nevertheless, though not necessarily compelling, would be consistent with a model suggesting juxtaposition of terrains that may once have been hundreds of kilometres apart latitudinally. The Early Devonian brachiopod and m o H u s k faunas about Cobar have long been assumed to be closely comparable with the Mclvor and Mt Ida faunas of the Heathcote-Redcastle area. Unpublished descriptions (Landrum, 1975) suggest c. 2 5 7 o similarity between the brachiopod faunas of the Amphitheatre Formation and those of Unit 3 (Pleurodictyum beds) of the Mt Ida Formation. The significance of this is uncertain because there may be significant age differences between the faunas compared. Parts of the Amphitheatre Formation are indubitably Lochkovian and Zlichovian (Pickett, 1 9 8 0 ) ; the presence of a Spinella assemblage indicates portion of it to be probably Pragian (kindlei and/or dehiscens zones). Much of it thus may be younger than the youngest Mt Ida faunas. There is virtually no similarity to faunas of the Mclvor Formation nor to the Wentworth Group of the Tabberabbera Terrane. Only one useful comparison seems possible between coeval brachiopod faunas from within the Molong-Monaro Terrane. Of 40 species described from the Receptaculites and basal Warroo Limestones (broadly perbonus Zone) at Taemas (Chatterton, 1973), at least 24 are known to occur in the perbonus Zone portion of the Buchan Group at Buchan and Bindi (Talent, unpub. data). Curiously, this similarity is not matched by the rugose corals. There are too few species of Rugosa firmly identified specifically (3 spp.) from the dehiscens Zone at Buchan and Bindi for meaningful comparison with the coeval faunas from Taemas and Wee Jasper (8 spp.), though the abundance of Chalcidophyllum recessum in both areas is striking. Of 7 spp. of Rugosa from the perbonus Zone portion of the Buchan Group only one, Xystriphyllum mitchelli, is known from coeval horizons at Taemas and Wee Jasper; some of this bias may be due to 4 of the Buchan species being from a more pelagic context, associated with ammonoids and dacryoconarids. Similarities are more obvious with the Tabulata but records are in need of revision.

231

New England Fold Belt and north Queensland, There are no zoogeographically useful brachiopod faunas described from the Devonian of the Taraworth Terrane nor from other parts of the New England Orogen. Comparison of rugose coral faunas of the Taraworth Terrane with other regions is not particularly fruitful. The faunas of the Nemingha Limestone (Lochkovian) are too meagre. Of 16 rugosans specifically identified from the Sulcor and Loomberah Limestones, only 2 are recorded from the somewhat older Garra Formation and, surprisingly, none from the Taemas and Wee Jasper limestones [see above]o There may be a significant age-differences none of the latter being postserotinus in age, the Sulcor Limestone possibly not including pre-Eifelian horizons (Mawson et al., 1985); this needs verification. All rugose corals so far recorded from the Late Eifelian giganteum fauna (5 spp.) at Timor occur farther north in the Moore Creek Limestone (12+ spp.); this fauna has yet to be discriminated outside the Tamworth Terrane. The Grypophyllum cf. denckmanni [Givetian varcus Zone] rugosans (6 spp^) also seem to be without analogues elsewhere in Australia. Documentation of coral, conodont and other faunas of the tectonic blocks in the Queensland portion of the New England Orogen may prove interesting. A faunule from Mt Etna, perhaps significantly, has 6 of its 9 rugose coral species occurring also in the Garra Formation and, besides, has much in common with the fauna of the Martins Well Limestone Member of the Shield Creek Formation in the Broken River area (J.S. Jell, pers. comm.). This apart, the faunas of the entire New England Orogen, the Taraworth Terrane included, cannot be said, at this juncture, to place constraints on tectonic models. Rugose coral faunas from the Nogoa Anticline (near Springsure; serotinus to partitus zones), Douglas Creek (near Clermont; includes perbonus Zone), and Ukalunda are too small for meaningful analysis; closer sampling is needed to augment the coral faunas and improve conodont zonation. Rich rugose coral faunas (20+ spp.) of broadly Givetian age occur in the Burdekin Limestone at the Fanning and Burdekin Rivers but, unexpectedly, these contrast with coeval coral faunas from Givetian horizons near the top of the Broken River Formation (J.S. Jell, pers. comm.). Though located on different tectonostratigraphic terranes, an ecologic explanation is favoured because of difficulty visualizing a regional tectonic history that would have generated the isolation necessary to produce contrasting faunas. Late Devonian faunas of eastern Australia, in contrast with those of Western Australia, are too poorly known for establishing correlations (Mawson et al., 1985) much less analysis of zoogeographic significance. Summary. No eastern Australian terranes are known to be 'fingerprinted' by manifestly exotic Devonian faunas whose composition compels us to infer long-range (thousands of kilometres) displacement. There are, nevertheless, some zoogeographic anomalies (approximately equivalent to the level of species-contrast between adjacent present-day provinces) that could be consistent with less dramatic movements (hundreds of kilometres) from former greater distances apart (latitudinal or longitudinal) than at present, eog. between the the Melbourne and Tabberabbera terranes and between the Melbourne and Molong-Monaro terranes; divergence in ages of well-documented faunas precludes inferences regarding relative juxtaposition of the MolongMonaro and Taraworth terranes. For many areas, no explanation can be hazarded because of paucity of data, imprecisions in stratigraphic alignments and uncertainties in discriminating palaeoecologic from zoogeographic factors.

232

References Chatterton, B.D.E., 1973. Brachiopods of the Murrumbidgee Group, Taemas, New South Wales. Bull. Bur. Min. Res. Geol. Geophys. Aust. 137, 146 pp. Crook, K.A.W., & Felton, E.A., 1975. Tasman geosyncline greenstones and ophiolites. J. geol. Soc. Aust. 22, 117-131. Garratt, M.J., 1983a. Silurian and Devonian biostratigraphy of the Melbourne Trough, Victoria. Proc. roy. Soc. Vict. 95, 77-98. Garratt, M.J., 1983b. Silurian to Early Devonian facies and biofacies patterns for the Melbourne Trough, central Victoria. J. geol. Soc. Aust. 30, 121-147. Hill, D., 1978. Bibliography and index of Australian Palaeozoic corals. Pap. Dep. Geol. Univ. Qd 8 (4), 1-38. Jell, J.S., & Talent, J.A., in press. The Silurian of Australia: the most instructive sections. ^ C.H. Holland (ed.), A global standard for the Silurian System, Nat. Mus. Wales. Landrum, R.S., 1975. Biostratigraphy, palaeontology and palaeoecology of some Silurian and Devonian deposits in the Cobar Basin, New South Wales. Aust. Nat. Univ., Ph.D. thesis (unpub.) Lenz, A.C., & Johnson, B.D., 1985. Early Devonian brachiopods from the Garra Formation. Palaeontographica A (in press). Mawson, R., Jell, J.S., & Talent, J.A., 1985. The Early Devonian-Middle Devonian boundary in Australia: key sequences and conodont succession. Senckenberg Courier (in press). Pickett, J.W., 1980. Conodont assemblages from the Cobar Supergroup (Early Devonian), New South Wales. Alcheringa 4, 67-88. Savage, N.M., 1974. The brachiopods of the Lower Devonian Maradana Shale, New South Wales. Palaeontographica A 146, 1-51. Scheibner, E., 1984. Suspect terranes in the Tasman Fold Belt System (eastern Australia). Pub. geol. Sci. Stanford Univ. 18, 170-174. Talent, J.A., 1964. The Silurian and Early Devonian faunas of the Heathcote district, Victoria. Mem. geol. Surv. Vict. 26, 1-55. Talent, J.A., 1965. Stratigraphic and diastrophic evolution of central and E Victoria in Middle Palaeozoic times. Proc. r. Soc. Vic. 79, 179-195. Talent, J.A., 1983a. Middle Palaeozoic faunal provinciality in relation to crustal blocks in Asia. Abs. geol. Soc. Aust. 9, 223-4. Talent, J.A., 1983b. K voprosy ob "evolyutsii soobshchestv". Trudy Inst. Geol. Geofiz., Sib. Otd., Akad. Nauk SSSR 569, 46-53. Talent, J.A., 1984. Australian biogeography past and present: determinants and implications. J.J. Veevers (ed.), Phanerozoic earth history of Australia, Oxford Univ. Press, 57-93. Talent, J.A., 1985. Provinciality as a means for qualified resolution of separation of continental blocks in the past: preliminary exemplification from western Pacific borderlands. XIV Pacific Sci. Cong. Talent, J.A., Berry, W.B.N., & Boucot, A.J., 1975. Correlation of the Silurian rocks of Australia, New Zealand and New Guinea. Sp. Pap. Geol. Soc. Am. 150, 1-105. Yolkin, E.A., Talent, J.A., & Gratsianova, R.T., 1985. Osnovnye problemy taksonomii i biostratigrafii v svyazi s paleobiogeograficheskim rayonirovaniem. Geol. Geofiz. 1985 (11) (in press).

233

PALEOMAGNETISM OF FRANCISCAN LIMESTONES OF CALIFORNIA AND OREGON : CONSTRAINTS ON TERRANE AMALGAMATION AND TRANSPORT

John Anthony Tarduno Michael McWilliams Department of Geophysics, Stanford University, Stanford CA, U.S A . Introduction Pelagic limestones which originally capped oceanic plateaux, seamoimts or ridges are a minor component of the Franciscan Complex of California and southwestern Oregon, yet they contain vaJuable information constraining the tectonic evolution of these areas. Detailed paleomagnetic, paleontologic and stratigraphic analysis of limestones from the Franciscan Central belt melange (CE, Figure 1), and the coeval Permanente (P) and Sixes River (SR) terranes, constrain the interaction of oceanic plates which emplaced these limestones within the North American Cordillera. Our techniques follow those of Alvarez ct al. (1980). All the limestones studied were deposited during the Cretaceous normal polarity superchron (118-83 Ma), so that the geomagnetic polarity during deposition is known. Facing directions of the blocks are determined by foraminiferal biostratigraphy and sedimentological control. As the limestone blocks have likely rotated with respect to each other during incorporation into the melange, paleomagnetic inclinations from each block are evaluated independently. Since the magnetic polarity and stratigraphic facing are known, the paleomagnetic inclination relative to the paleohorizontal defines uniquely the paleolatitude of magnetization. Blocks of varying attitude are sampled to provide "mega-conglomerate'' tests which show that the age of magnetization predates accretion. Assuming magnetization shortly after deposition, as demonstrated in paleomagnetic studies of similar pelagic limestone sequences (Lowrie and Alvarez 1977), the paleolatitude during a paleontologically-dated time interval is known. Sampling individual sequences yields paleolatitude versus time trends which reinforce the interpretation of primary magnetization. These trends can be used to compute the poleward component of plate velocity and constrain models for the motions of oceanic plates during the Cretaceous normal polarity superchron, an interval where the lack of marine magnetic anomalies causes nonunique solutions to oceanic plate reconstructions. Calera Limestone of Permanente terrane The Aptian to Coniacian (115-88 Ma) Calera Limestone occurs as a series of blocks in the Permanente terrane (Blake ct aL, 1984, Sliter tt ai, 1984, Figure 1) on the San Francisco Peninsula. The Permanente terrane also contains blocks of greenstone and Valanginian chert (Murchey tt al. 1984) as well as graywacke and shale of continental origin. Sampling sections exposed at Permanente Quarry (Figure 1), Courtillot ct ai (1985) demonstrated a positive "megaconglomerate'* test and, together with paleontologic dating, concluded that the Calera Limestone originated at 24^ north. More detailed study of Calera Limestone sections from Pacifica Quarry (Figure 1) and Permanente Quarry confirms this interpretation. Paleomagnetic analysis of thirty paleomagnetically dated samples from Permanente Quarry reveals a paleolatitude v«. time trend suggesting a poleward component of plate velocity of approximately 5 cm/yr (Tarduno tt aL, 1985). Laytonville Limestone We have found 5 blocks of red and white pelagic limestone in the Laytonville area (Figure 1) which are fragments of a once-larger terrane now incorporated into the Franciscan Central melange belt (Tarduno tt a/., 1985b). Both right-side-up and overturned blocks occur, with

234

individual blocks containing as much as 25 m of continuous section. A composite section ranges in age from Albian to Coniacian (10188 Ma). Foraminiferal study of these sections provides unambiguous facing directions. Paleomagnetic study of 42 paleomagnetic samples from these blocks reveab a mean paleolatitude of 14^ ±5^ south, over the depositional interval of 97-89 Ma (Tarduno ct al. 1985b) confirming original work by Alvarez et ai (1980). In addition, a megaconglomerate test significant at the 99% confidence level demonstrates magnetization prior to accretion. Assuming magnetization shortly after deposition, these data imply northward components of plate velocity of 30 to 14 cm/yr for accretion times of Late Cretaceous to Eocene, respectively. A systematic change in inclination is visible within the outcrop, similar to that predicted by the mean southerly paleolatitude and geologically reasonable accretion ages (Figure 2). A linear regression of the paleolatitudes obtained from 65 paleomagnetic samples (7 sections) reveab a northward component of approximately 26 cm/yr. This rate is two to three times faster than the fastest absolute velocity of any present plate. Whitsctt Limestone of Sixes River terrane The Whitsett Limestone of southwestern Oregon occurs as a linear belt of blocks in melange called the Sixes River terrane (Blake ct al., 1985). Based on identical lithology, structural style, age and nature of the tectonic blocks, the Sixes River terrane is thought to be an equivalent of the Franciscan Central Belt terrane (Blake et ai, 1985). Limestone deposition begins with a shallowwater bioclastic limestone, grading upward into a calcareous black shale facies and a pelagic limestone facies. Algae species from the shallow water facies have low-latitude tethyan affinities (A. Niem, personal commun., 1985), as do foraminifera from the pelagic facies (Sliter, 1984). In contrast to the Califomian localities, the Sixes River terrane has not been disrupted by Neogene faulting. A clear depositional contact between the Sixes River terrane and overlapping Eocene {" 50 Ma) sediments (Baldwin, 1974) is preserved. This overlap assemblage provides a minimum

235

Figure 1. Tectononstratigraphic Terrane Map of California and Oregon after Blake et ah, 1084; Blake et aL 1985, showing the Permancntc terrane (P), Franciscan Central belt (CE), and the Sixes River terrane (SR)-

DCPLANATION OREGON SR- Sixes River terrane S a Sao* Camp terrane OP. Otter Pornt terrane El- Elk .ubterrane & EaMem Franciacan belt T- Tyee Formauon

EDCPLANATION

CALIFORNIA QT- Cenotoic deposits GV- Great VaUey Sequence S-SaUnia Fraaciacan Complex £-Eaat«ra belt CE- Central belt P- Permaaente terrane CO- Coanal belt

•

SAMPLE LOCALITIES

accretion age for the Whitsett Limestone to North America. Paleomagnetic study of 30 samples of late Aptian to early Albian (115-105 Ma) Whitsett Limestone, together with sedimentary features denoting facing direction, suggests deposition at approximately 30' south paleolatitude. Exhibiting over 70' of poleward motion, the Whitsett Limestone may be the farthest-traveled Cretaceous rocks in the North American Cordillera. Scarcity of suitable outcrop, however, makes fold or megarconglomerate tests unlikely. The age of magnetization is constrained only by a consistency test with the magnetizations far from the present-day earth's field and any post-accretionary North American field direction. If the magnetization of the Whitsett Limestone was acquired shortly after deposition, these new results suggest an extrapolated poleward velocity of approximately 14 cm/yr for the plate that carried the Whitsett Limestone from the southern hemisphere to North America. Such an interpretation is in accordance with the southern hemisphere origin and fast minimum plate velocities suggested by the paleomagnetic data from the Laytonville Limestone.

FRANCISCAN LIMESTONES

SO 50

5 cm/yr

O C3

10 0

-J

-10

-30

•

30 20

-20

-40

-50

26 cm/yr

-SO 90

100

105

115

120

Figure 2. Paleomagnetic data from the Calera Limestone (*), Laytonville Limestone (o) and the Whitsett Limestone (+)• Data points from the Laytonville and Calera represent individual field-drilled samples. The Whitsett Limestone represents a mean of 30 samples. Discussion Complicated oceanic plate interaction and post-accretion translation is required to reconcile the southern hemisphere origin of the Laytonville and Whitsett limestones^ now located north of the coeval Calera Limestone (Figure 1). Plate velocity constraints obtained from the paleomagnetic data and plate motion models of the paleo-Pacific basin (Engebretson ct ai 1984), correspond well with an origin of Calera Limestone on the Farallon plate (Tarduno et al 1985). During the same time interval, however, the Laytonville Limestone was riding on a plate moving at significantly greater poleward velocitieSc Several modifications of extant plate models (Engebretson tt aL, 1984) can account for the Laytonville Limestone motion. One involves initiation of the Kula-Farallon spreading during the Cretaceous normal superchron. This conflicts with the assertion that Kula-Farallon spreading could not have begun prior to 85 Ma (Woods and Davies, 1982). Another possibility involves the presence of another plate that ha^ subsequently subducted; we call this the Escondido Plate (Tarduno et aL 1985b). Li any model, the Franciscan Central belt melange and equivalent terranes must form by the interaction of two or more oceanic plates. The plate that carried the Laytonville and possibly

236

the Whitsett Limestone had very high poleward components of velocity, suggesting highly oblique subduction. This oblique subduction may have triggered strike-slip faulting, which in turn may have transported previously-accreted Franciscan terranes to their present positions within the Central belt melange (Tarduno et ai 1985b). Defining the plates which interacted with North America during the Late Cretaceous to Eocene is an area of current research. We propose two elements of a possible scenario to produce the fast minimum plate velocities suggested by the Laytonville Limestone paleomagnetic data. First, it is likely that the plate that carried the Laytonville Limestone was attached to an old, dense subducting slab. Second, it is possible that the 'plate', rather than being a smgle plate, was actually a series of smaller plates separated by fast-spreading ridges. The latter element is consistent with a model of the Mesozoic California borderland resembling the present-day region between Australia and Indonesia (Blake tt ai, 1984; Silver and Smith, 1983). Acknowledgements We thank William V. Sliter for paleontological analysis and M. Clarke Blake for help with structural interpretations, without which much of this work would not have been possible. References

Alvarez, W., Kent, D.V., Premoii-Silva, L, Schweickert, RA., and Larson, RA., 1980, Franciscan Complex limestone deposited at 17^ South paleolatitude. Geological Society of America Bulletin, 91, 476-484. Baldwin, E.M., 1974, Eocene stratigraphy of southwestern Oregon, Oregon Dept. of Geology and Mineral Industries, 83, 40 pp. Blake, M.C. Jr., Howell, D.G., and Jayko, AS., 1984, Tectonostratigraphic terranes of the San Francisco Bay region, in Blake, M.C. Jr., ed., Franciscan Geology of Northern California, Pac. Section, SEPM, 43, 522.

Blake, M.C. Jr., Howell, D.G., and Jones, D.L., 1982, Preliminary tectonostratigraphic terrane map of California, U.S. Geol. Survey Open-File Report 82-593. Blake, M.C. Jr., Engebretson, D.C, Jayko, AS., and Jones, D.L., 1985, Tectonostratigraphic terranes in southwestern Oregon, in Howell, D.G., ed., Tectonostratigraphic terranes of the Circumpacific region: Earth Sci. Series, Circumpacific Council for Energy and Mineral Resources, in press. Courtillot, v., Feinberg, H., Ragaru, J.P., Kerguelen, R., McWilliams, M., Cox, AV., 1985, Franciscan Complex limestone deposited at 24^ N, Geology, 13, 107-110. Engebretson, D.C., Cox A, and Gordon, R.G., 1984, Relative motions between oceanic plates of the Pacific basin. Journal of Geophysics Research, 89, 10,291-10,310. Lowrie, W., and Alvarez, W., 1977, Upper Cretaceous-Paleocene magnetic stratigraphy at Gubbio, Italy, Geological Society of Am. Bulletin, 88, 367-389. Murchey, B.L. and Jones, D.L., 1984, Age and significance of chert in the Franciscan Complex in the San Francisco Bay region, Pacific Section, SEPM, 43, 23-30. Silver, EA., and Smith, R.B., 1983, Comparison of terrane accretion in modern Southeast Asia and the Mesozoic North American Cordillera, Geology, 11, 198-202. Sliter, W.V., 1984, Foraminifers from Cretaceous Limestone of the Franciscan Complex, Northern California, Pacific Section, SEPM, 43, 149-162. Tarduno, J.A, McWilliams, M., Debiche, M.G., Sliter, W.V. and Blake, M.C. Jr., 1985, Franciscan Complex Calera Limestones- Accreted Remnants of Farallon Plate Oceanic Plateaux, in press. Tarduno, J.A., McWilliams, M., Sliter, W.V., Cook, H.E., Blake, M.C. Jr., and Premoli-Silva, I., 1985b, Southern hemisphere origin of the Laytonville Limestone of California, in prep. Woods, M.T., and Davies, G.F., 1982, Late Cretaceous genesis of the Kula plate. Earth and Planetary Science Letters, 58, 161-166.

237

THERMAL CONSEQUENCES OF TERRANE ACCRETION, FRANCISCAN COMPLEX OF NORTHERN CALIFORNIA *

Michael B. Underwood ^^ M. Clark Blake, Jr. and David G. Howell •k

^^University of Missouri, Columbia, MO., USA U.S. Geological Survey, Menlo Park, CA., USA Introduction The Franciscan Complex of northern California includes several tectonostratigraphic terranes; each is distinguished on the basis of structural style, lithologic content, metamorphic grade, and biostratigraphy (e.g., Jones et al., 1978; Bachman, 1982; McLaughlin et al., 1982). The terranes were progressively accreted to the western margin of North America during late Mesozoic to midTertiary interactions with subducting oceanic plates. The thermal consequences of terrane accretion are complicated, and several types of thermal events can be recognized. The Central Belt of the Franciscan Complex is a classic pol3naict melange; fossil ages range from Late Jurassic to Late Cretaceous. The melange matrix consists of intensely-deformed sedimentary rock (mostly scaly argillite), and common exotic blocks include blueschist, greenstone, diabase, serpentinite, chert, limestone, and graywacke. Large blocks of internally-coherent turbidites are also present, and some of these blocks reach kilometers in dimension. The Yager complex is a relatively undeformed sequence of Paleogene turbidites, mudrocks, and conglomerate. Yager strata are juxtaposed with the Central Belt along a prominent east-dipping thrust, termed the Eel River fault (Fig. 1) (Bachman et al., 1984). The structural style of the Yager terrane consists of polyphase folding, but bedding-parallel shear fabrics are generally absent. Turbidite lithofacies and depositional cycles are typical of slope, feederchannel, and intrabasinal environments (Underwood, 1985). The Franciscan Coastal Belt is broadly coeval with the Yager complex (uppermost Cretaceous to Oligocene), but the structural style is dramatically different, Turbidite sequences display chaotic mesoscopic fabrics such as scaly argillite, web structure, pinch—and—swell, boudinage, isoclinal folding, and bedding transposition. Rare exotic elements include mostly large greenstone blocks and limestone. The contact with Yager strata is a structural transition rather than a clearly-delineated fault (Fig. 1). The King Range terrane includes rocks as young as middle Miocene, but there is probably some overlap in age with the adjacent Coastal Belt. Lithologies are identical to those of the Coastal Belt; however, most of the King Range exhibits polyphase folding with little bedding-parallel stratal disruption. The terrane boundaries are difficult to outline with precision because the structural style grades into the pervasive shear zones of the Coastal Belt. According to one recent designation (McLaughlin et al., 1982), strata exposed at Pt. Delgada comprise a sub-terrane of the King Range (Fig. 1). These rocks include thick amalgamated sandstones, thin sandstone turbidites, limestone, pillow basalt, volcanic breccia, tuff, chert, and rare blueschist. The structural style is typical of a melange. Fossil control is limited; pelagic intercalations within pillow breccias are Campanian to Coniacian in age, but the melange matrix remains undatedc

238

Figure 1. Map showing the principal geologic elements of coastal northern California, Terrane boundaries are modified from Bachman and others (1984). Strike-slip fault zones mapped by Herd (1978) and McLaughlin and Nilsen (1982). Eel River fault places the Yager complex beneath the Franciscan Central Belt. Coastal Belt transition juxtaposes Yager strata with more highly-deformed rocks of the Franciscan Coastal Belt. Boundaries of the King Range terrane are from McLaughlin and others (1982).

Methods Estimates of paleotemperature for the Franciscan terranes of northern California were obtained using the technique of vitrinite reflectance (Ro) and the empirical Temp-Ro correlation of Price (1983).. Vitrinite reflectance is a powerful tool because it is sensitive only to the highest temperature event in the history of a stratal section. Moreover, it covers the entire temperature range from early diagenesis into metamorphism (i.e., 25®C to at least 400°C). All shale samples used in the study were collected from surface exposures, and vitrinite was concentrated using acid maceration methods. Problems encountered during our analyses include: (1) weathering and oxidation, which reduce Ro values, (2) shales lean in total organic carbon and vitrinite, (3) increasing reflectance anisotropy at elevated levels of thermal maturity, and (4) possible inclusion of recycled vitrinite. In most cases, recycled particles can be routinely identified using morphologic criteria, but reflectance data were also plotted on histograms for statistical evaluation. Results The results of vitrinite-reflectance measurements and paleotemperature estimates are summarized in Table 1. In general, values for the Yager complex and Franciscan Coastal Belt are similar, averaging about 0.70% Ro. This corresponds to a paleotemperature of roughly 140®C (Price, 1983). The Central

239

TABLE

SUMMARY OF FMNCISCAN TERRANE CHARACTERISTICS AND PARAMETERS OF THERMAL MATURITY STRUCTURAL STYLE

No.

Low

PALEOTEMP. (®C) High Ave. Low

0.55

232

180

108

0.71

0.40

280

143

1.50

0.70

Oo^

Zl^i

140

2.21

1.39

0.77

290

229

153

2.50

2.27

2.15

305

293

282

MEAN Ro {%) High Ave.

TERRANE

AGE

LITHOLOGIES

Central Belt

JurassicCretaceous

scaly argillitet exotic blocks1 turbidites

melange

Coastal Belt

CretaceousOllgocene

turbidites, mudrocks; minor greenstone

broken formation

2.03

Yager complex

Paleogene

turbidites, mudrockSt conglomerate

polyphase folding

King Range

EoceneMiocene

turbidites, mudrocks, rare exotic blocks

polyphase folding 4 broken fm.

Point Delgada

Cretaceous

scaly argillite, exotic blocks, turbidites

melange

Most mean values based upon at least 50 randomly oriented particles of vltxlnite. Temperature estimates based upon temp-Ro correlation of Price (I983).

Belt is significantly more mature, with most values ranging between 0.85% and 1,20%. Interestingly, the youngest Franciscan terrane displays the highest level of thermal maturity; strata in the King Range produce mean reflectance values which are as high as 2«21% and average 1.39%. The Point Delgada subterrane is similar, with all values in excess of 2.10% Ro and paleotemperatures greater than 280®C. The striking thermal anomaly centered at Point Delgada dissipates in all directions, but abnormally high levels of thermal maturity extend well into both the King Range and Coastal Belt. This regional-scale thermal anomaly is attributed to a hydrothermal event which occurred during middle Miocene time. Adularia veins associated with the hydrothermal activity have been dated radiometrically at 13.8 mybp (K-Ar method), and fluid-inclusion studies of early quartz and calcite veins support temperatures of 287-243®C (McLaughlin et al. , 1985). Thus, the homogenization temperatures of the fluids are slightly lower than the maximum rock temperatures recorded by vitrinite particles. There are several types of thermal anomalies within the study area which have other origins. For example, three isolated highs occur within the Coastal Belt southeast of Fort Bragg (Fig. 1). The most prominent of these anomalies is defined by a change in mean reflectance from 0.53% to 1.07% over a distance of roughly 1 km; this corresponds to a temperature change of approximately 90®C. The higher reflectance values are perhaps related to shear heating along unrecognized faults; alternatively, they simply could be due to differential uplift. If differential uplift has occurred, the anomaly pattern seemingly requires either diapiric movement or vertical displacement of perhaps 4 km along closelyspaced faults. Throughout most of the study area, the Ro gradient within the Yager complex decreases systematically from east to west. This trend is opposite to the expected stratigraphic gradient, however, because homoclinal sections between Garberville and Willits are upright and dip to the northeast. The structural pattern becomes more complicated north of Garberville (Fig. 1), and no obvious Ro gradient is evident in that area. The original Ro gradient associated with depositional burial has almost certainly been overprinted by one or more phases of structural burial. This contention is further supported by the fact that Ro values^for most Yager shales exceed 0.55%; assuming a geothermal gradient of less than 3®C/100m, this level of thermal maturity requires burial depths that exceed the cumulative thickness of overlying Neogene-Holocene formations (Underwood, 1985) .

240

Structural burial is particularly noteworthy along the Eel River fault. Shear heating has also occurred along this thrust, as evidenced by unusually high Ro values at Bridgeville (1.08%, 1.02%) and within the Long Valley window south of Laytonville (1.16%). Shear heating is also inferred near Leggett (Ro « 1.50%), but this anomaly may be related to younger strike-slip faults which have been mapped in the vicinity (Fig. 1). Ro-equilibration temperatures within the fault zones were elevated as much as 100®C above background values, although short-term temperatures were probably raised even higher. Thermal inversion is also evident along many portions of the Eel River fault; typically, the older Central Belt strata are more mature than footwall beds of the Yager complex. This relationship requires maximum heating of the Central Belt prior to activation of the thrust« Temperature estimates range from 108®C to 232®C for the Central Belt, and thermal alteration increases from west to east, suggesting greater uplift and unroofing in that direction. There are no systematic variations in thermal maturity across the Coastal Belt transition. This terrane boundary has been regarded as a tectonized unconformity separating trench-slope sediments (Yager) from underlying accretionary basement (Coastal Belt). If this interpretation is correct, the Coastal Belt should be more mature, and such is the case along some transects (e.g., west of Garberville). However, Yager shales from other localities yield Ro values that either equal or exceed those of adjacent Coastal Belt strata, so the terrane boundary remains somewhat enigmatic. Conclusions The thermal history of the Franciscan Complex is complicated because of several superimposed tectonic and/or hydrothermal' events. There are thermal effects associated with terrane suturing along the Eel River fault, but the nature of the Coastal Belt transition remains obscure. Potential anomalies caused by the emplacement of the King Range terrane have apparently been overprinted by a subsequent hydrothermal event centered at Point Delgada. The interpretation of paleotemperature data is complicated further by Plio-Pleistocene strike-slip faulting related to the San Andreas fault system. References Bachman, S.B., 1982, The coastal belt of the Franciscan: youngest phase of northern California subduction, Trench-forearc geology. Geological Society London Special Publication, 10, 401-417. Bachman, S.B., Underwood, M.B., and Menack, J.S., 1984, Cenozoic evolution of coastal northern California, Tectonics and sedimentation along the California margin. Society Economic Paleontologists Mineralogists, Pacific Section, 38, 55-66. Herd, D.G., 1978, Intracontinental plate boundary east of Cape Mendocino, California, Geology, 6, 721-725. Jones, D.L., Blake, M.C., Jr., Bailey, E.H., and McLaughlin, R.J., 1978, Distribution and character of upper Mesozoic subduction complexes along the west coast of North America, Tectonophysics, 47, 207-222. McLaughlin, R.J., and Nilsen, T.H., 1982, Neogene non-marine sedimentation and tectonics in small pull-apart basins of the San Andreas fault system, Sonoma County, California, Sedlmentology, 29, 865-876. McLaughlin, R.J., and others, 1982, Post-middle Miocene accretion of Franciscan rocks, northwestern California, Geological Society America Bulletin, 93, 595-605. McLaughlin, R.J., and others, 1985, Paragenesis and tectonic significance of base and precious metal occurrences along the San Andreas Fault at Point Delgada, California, Economic Geology, 80, 344-359. Underwood, M.B., 1985, Sedimentology and hydrocarbon potential of Yager structural complex - possible Paleogene source rocks in Eel River basin, northern California, American Association Petroleum Geologists Bulletin, 69, in press. Price, L.C., 1983, Geologic time as a parameter in organic metamorphism and vitrinite reflectance as an absolute paleogeothermometer, J. Petroleum Geol., 6, 5-38.

241

THE SANGUN HIGH P/T METAMORPHIC TERRANE AND SURROUNDING TERRANES IN THE INNER SIDE OF SW JAPAN: REVIEW OF RECENT PROGRESS Teruo Watanabe and Takao Tokuoka Department of Geology, Shimane University, Matsue, 690, Japan The Sangun P/T metamorphic terrane is located on the Japan Sea side of SW Japan in the Chugoku and Kyushu districts« With the Hida low P/T metamorphic terrane, it is considered to form a Permian - Jurassic paired belt (Miyashiro, 1973) as most of the radiometric ages for both belts, except for older ages in the Hida terrane, range from late Paleozoic to Jurassic. However, some part of the Sangun rocks are unconformably overlain by Upper Triassic lithologies and therefore many geologists do not consider the Sangun metamorphism continued into the Jurassic. The Sangun terrane has been interpreted as a Triassic accretion complex by Saito (1983) and a metamorphosed Permian accretion complex by Ozawa et al (1985). Bordering the Sangun belt to the southeast are rocks of the unmetamorphosed Maizuru Permo-Triassic Belt. Outcrops of a Jurassic subduction complex lie adjacent to either the Maizuru or Sangun rocks and occasionally occur within the Sangun belt as shown by Ozawa et al (1985) (Fig. 1). The occurrences within the Sangun (Fig. 1) are interpreted as tectonic windows related to the formation of nappe structures (Hara et al, 1980). Jurassic radiometric ages of some Sangun rocks are considered to date the formation of nappe structures in the Chugoku area (Hayasaka and Hara, 1982). However, Nishimura et al (1985) do not concur with this interpretation, because in some areas the Sangun nappe overlies latest Jurassic (about 145 Ma) formations. Therefore, they argue that movement related to the nappe formation should have occurred after the latest Jurassic. Radiometric ages presented by Shibata & Hishimura (1983) , Nishimura et al (1983) , and Igi (1984) amongst others reveal that the Sangun rocks fall into two groups, 250 - 310 Ma and 170 - 220 Ma (Fig. 2). Rocks of the older group, which occur spatially proximate to serpentinite masses, may be the western extension of the Hida marginal serpentinite melange zone. In contrast, rocks of the younger group occur over wide areas in the Chugoku district, rarely associated with serpentinite and do not necessarily show close relations to the nappe stinicture. Forming part of the Sangian belt (and occasionally adjacent to the older metamorphic rocks) are a thick sequence of weakly metamorphosed rocks comprising muddy sedimentary units with acidic tuff horizons and olistostromes. A Permian age is indicated by radiolarian and fusulinid fossils. Middle Permian rocks with acidic tuff are found in the adjacent Maizuru belt where they are unconformably overlain by lower Triassic shallow-water sediments. In the Gotsu area, a member of the younger group consists of pelitic schist and blueschist yielding radiometric ages of 180 - 190 Ma. However some rocks, which occur around a small antigorite serpentinite mass, show complex zoning of sodic amphibole and contain composite grains of barroisite and glaucophane, or blastoporphyritic albite crystals including glaucophane, white mica and clinozoisite. These rocks show possible duplication of glaucophanitic metamorphism. The age of the early stage of metamorphism is not yet certain, but the later metamorphic stage must coincide with the Jurassic radiometric age. It is notable that formation of the Jurassic accretion complex and metamorphism have occurred almost simultaneously, as is the case for the older members of the Sangun group.

242

The Sangun terrane may be the result of two phases of high P/T metamorphism, or it may represent a mixed metamorphic terrane composed of late Paleozoic and Middle Mesozoic complexes. Acidic tuff in the Permian rocks in the Sangun and Maizuru terranes (some of which have suffered prehnite - pumpellyite grade metamorphism) may be arc-derived.

!1 Iv^

Fig. 1 -

l:Sang\in terrane, 2:Permian accretion complex, 3:Maizuru belt and its western extension, 4:Jurassic accretion complex, 5:Hida marginal belt (based on Ozawa et al, 1985).

5 _

^ H

100

Fig. 2 -

200

UTI , ! • » , • iiOOMa 300

Radiometric ages for the Sangun metamorphic rocks. Data from Sibata & Nishimura (1983), Nishimura et al (1983), Igi (1984) and others. Solid squares are the age of hornblende in the Yakuno ophiolitic complex associated with the Maizuru belt. Squares with dot are unpublished data by Watanabe et al.

243

BIOGEOGRAPHICAL SIGNIFICANCE OF SOME EAST AUSTRALIAN ORDOVICIAN FAUNAS by

B. D. Webby

Department of Geology & Geophysics, University of Sydney, Australia The Ordovician faunas of the Tasmanides include benthonic and pelagic (planktonic and nektonic) organisms. The benthonic forms are the most useful for biogeographic analysis because such organisms must adapt to local temperature and substrate conditions as well as rely on larval stages for their dispersal. However they have limited distribution along the edge of the Gondwanan continent and around the islands of the former Tasmanide ocean. The pelagic forms are widely distributed but much less sensitive indicators of provincialism, usually only reflecting broad-scale temperature differences. The spread of isolated occurrences of Late Ordovician shallow, coral-bearing carbonates from Tasmania to north-east Queensland suggests that the entire Tasmanide belt originally lay in low latitudes, probably aligned equatorially. Ordovician graptolites are recognized as planktonic and belonging to the equatorial Pacific province, but there are also some small-scale faunal differences which may have significance in elucidating oceanic circulation patternse For example, distinctive elements of the Late Ordovician fauna in central New South Wales (Orthograptus apiculatus and Amplexograptus Inuiti) do not occur in coeval Victorian successions and may suggest the existence of a separate localized, shallower or laterally equivalent water mass in central New South Wales. Conodont faunas in the shallow carbonates of the Tasmanian Shelf (microcontinent or part of the Gondwanan continental margin) are mainly of warm North American Midcontinent type or endemic elements like Tasmanognathus ^ while the coeval, deeper 'slope' deposits, according to Burrett and others, include representatives of the cooler. North Atlantic province. Similarly in New South Wales and central Queensland, shallow coralline carbonates of Late Ordovician age have yielded conodonts of mixed North American Midcontinent and endemic aspects, while deeper Middle Ordovician graptolite shales in the Australian Capital Territoiry and carbonate blocks of an allochthonous deposit in northern New South Wales have produced North Atlantic type conodonts. This consistent depth stratification pattern suggests limits to the usefulness of conodonts for establishing palaeogeographical relationships. Apart from genera with South-East Asian (and less commonly North American) affinities, the Tasmanian Ordovician nautiloids are, according to Stait, 'largely endemic'. The moderately diverse fauna which includes nektobenthonic forms, colonized the Tasmanian Shelf early in the Ordovician and persisted through much of the period. It contrasts with the nautiloid assemblages of long ranging, near cosmopolitan, entirely nektonic stocks which invaded the shallow seas around the offshore, volcanic 'island arc' of central New South Wales in Late Ordovician times. The abundance of Tasmanian endemics may suggest that 'platformal Tasmania' was a microcontinent, long separated but not hugely displaced from the rest of the Gondwanan continental margin. The absence of nektobenthonic forms on the offshore highs of central New South Wales suggests some sort of deep-water barrier to their dispersal, but this did not stop other benthonic organisms colonizing the highs (see below). Early-Middle Ordovician trilobites of the Tasmanian Shelf include few endemic forms. Some like Tasmanocephalus also occur in western New South Wales, and others such as Asaphopsls have Asian and South American affinities. Others again, like the pelagic Carolinites, are cosmopolitan. Early

244

Ordovician trilobites of shallow, benthonic aspect also occur along t±ie Gondwanan continental margin of western New South Wales, and in a small, localized offshore 'high' (Waratah Bay Axis) in south-central Victoria. A different type of association is reported by Henderson in an Early Ordovician volcanic-sedimentary complex of northern Queensland, with graptolites and trilobites - pelagic forms (Carolinltes and Oplpeuter) and representatives of a deeper benthonic community type {Acanthopleurella and Hypermecaspis). Late Ordovician trilobite assemblages in the shallow, carbonate shelf facies of the offshore 'island arc' in central New South Wales are characterized by occurrences of Pliomerina and Amphlllchas. One likely endemic occurs in this association of nine genera. A second, outer shelf or upper slope community type is represented by occurrences of Parkesollthus and Malongullla. The assemblage includes seven genera, only one or two of which are endemic. A third association of Triarthus, Geragnostus? and Shuznardla is found in the adjoining 'basinal' graptolite facies. Overall, the New South Wales 'island arc' trilobite fauna is mainly benthonic, of relatively low diversity and low endemicity. Their biogeographical relationships are with Tasmania, South East Asia and Kazakhstan, that is, belonging to the Pliomerina province of the much larger 'band-like' equatorial Remopleuridid realm. Only four genera of the comparatively low diversity fauna were capable of diversifying to produce more than one species in the 'island arc'. This suggests that only relatively stable and conservative stocks with relatively low rates of evolutionary activity immigrated to the peri-insular setting. The 'island arc' also provided a centre for the origin and early diversification of the two lineages based on Eokosovopeltis and Encrinuraspis. The correlative Late Ordovician platform deposits of Tasmania have a similar Pllomerina-Amphilichas community but with few species in common. The deeper 'slope' environments have an assemblage including trinucleids, raphiophorids {Nanshanaspis and Bulbaspis), Shumardla and the pelagic Telephina, of mixed cosmopolitan and Chinese affinities. The fauna exhibits none of the typical elements of the equivalent New South Wales Parkesollthus - Malongullla 'slope' association. In studies of the Late Ordovician 'island arc' brachiopods of central New South Wales, Percival has shown that about one third of the total forty three genera are endemic and that nearly all the forty nine species have a restricted distribution. Percival also recognized the fauna as broadly belonging to the equatorial American realm with closest provincial affinities to the Scoto-Appalachian province and Kazakhstan. The 'island arc' was sufficiently isolated to allow a large number of endemic genera to evolve, but not so cut off as to prevent it acting as a staging post for the migration of other shallow benthonic stocks from North America and Kazakhstan. Of the pandemic stocks, some fifteen genera migrated from North America, and another eight came from Kazakhstan. Others like Rhynchotrema originated in New South Wales and subsequently immigrated to North America and Kazakhstan. Relationships with contemporaneous Tasmanian brachiopod faunas cannot yet be adequately clarified. They seem to belong to the American realm but appear to be less diverse, less endemic and with a less strong affinity to Kazakhstan. Significant provincial differences also exist between respective Late Ordovician coral and stromatoporoid faunas of the Tasmanian Shelf and New South Wales 'island arc'. At the generic level stromatoporoids Pachystylostroma and Aulacera are found in Tasmania but not in coeval New South Wales successions, whilst Cystlstroma and Cllefdenella (this latter now regarded as a sphinctozoan sponge) are typical representatives of the New South Wales

245

fauna, but not recorded from Tasmania. Among the corals, Foerstephyllum is commonly recorded from Tasmania but absent from New South Wales, and the large colonies of bank-forming Tetradium crihriforme in New South Wales successions are not apparently found in Tasmania. Late Ordovician corals in the isolated fault blocks within the Peel Fault System of northern New South Wales are similar to those of equivalent central New South Wales 'island arc' sequences. Eleven of the thirteen genera recorded by Hall have also been found in central New South Wales, but there are few species in common. A similar coral fauna is recorded from near the Anakie High in central Queensland, but not yet described^ There are also Late Ordovician corals from the Broken River Embayment, north-east Queensland, which, according to Hill, include the Asian-Alaskan genus Agetolltes. This genus is only known elsewhere in the Tasmanides from the Early Silurian of central New South Wales. Currently known patterns of faunal distribution support the generalized Ordovician palaeogeographical reconstructions of the Tasmanides originally proposed by Packham and Webby, and subsequently elaborated upon by Powell, Cass and others. As yet no examples of large-scale displacements between Tasmanide suspect terranes can be demonstrated.

246

SUSPECT TERRANES ALONG THE PRECAMBRIAN/PALAEOZOIC MARGIN, GREENVALE AREA, NORTH QUEENSLAND I.W. Withnall Geological Survey of Queensland, Brisbane, Australia This paper describes several terranes recognised by recent GSQ field work in the eastern part of the Georgetown Inlier, one of several cratonic inliers in northern Queensland. The inlier is bounded on the east by the Broken River Province. The rocks in the central part of the inlier include the Proterozoic Etheridge Group, which consists of fine-grained clastic sediments and minor mafic volcanics and dolerite sills (Withnall & others, 1980). The Group was strongly deformed and metamorphosed at grades from greenschist to granulite facies in two major events at 1570 and 1470 Ma. The recent mapping has shown the eastern part of the inlier to contain at least three separate NNE-trending terranes which contrast in lithology with each other and with the rest of the Georgetown Inlier (here referred to as the Georgetown terrane). Some boundaries are marked by major mylonite zones. Several major, NE to NNE-trending mylonite zones also occur within the eastern part of the Georgetown terrane, but they do not separate obviously different lithological units. Lateral displacement of at least 50 km has been postulated on one of them. The westernmost of the three terranes, the Balcooma terrane, consists of the Balcooma Metavolcanics, 4000 m of felsic metavolcanics and fine-grained metasediments, which were metamorphosed in the amphibolite facies (Withnall, 1982). At least two main folding events are evident on a small scale, but no major repetition of the sequence is apparent. The sequence is steeply dipping and overturned, younging westwards towards the contact with the Georgetown terrane which is marked by the Balcooma Mylonite Zone (BMZ). Mylonitisation affected the Georgetown terrane for up to 2 km from the contact. Affects extend a few hundred metres into the Balcooma Metavolcanics. The age of the terrane is unknown. Company geologists correlate it with a similar (although less metamorphosed) volcanic terrane, the Cambro-Ordovician Seventy Mile Range Group near Charters Towers, about 250 km southeast, (Henderson, 1982); both terranes contain similar styles of massive sulphide mineralisation. Structural geologists from JCUNQ suggest a Proterozoic age, because the folding predates the mylonitisation and no Early Palaeozoic folding is known in the rest of the Georgetown Inlier. However large-scale displacement on the BMZ might remove this restriction on an Early Palaeozoic age. To the east, a large batholith (Dido Granodiorite) separates the Balcooma and Lucky Creek terranes. The relationship between them is therefore unknown. The Lucky Creek terrane is compositionally distinct, and possibly composite, containing three main assemblages. In the west biotite gneiss, amphibolite and minor marble crop out adjacent to the batholith. A central belt consists of greenschist facies andesitic metatuff, volcanogenic metasediments, and minor marble. In the east low-grade pelitic metasediments predominate. The stratigraphic relationships are not known. A pervasive mylonitic foliation is a feature of the terrane. Stretching lineations are generally aligned ESE to ENE. The foliation is deformed by open to tight large-scale NNEtrending folds with subhorizontal axes, suggesting that the foliation was originally subhorizontal. The age of this terrane is also uncertain but could be Early Palaeozoic. A thin slice of andesitic volcanics and sediments (including Ordovician limestones) is preserved in the western part of the Broken River Province near Greenvale, and it could be related to the Lucky Creek terrane. The Seventy Mile Range Group near Charters Towers also includes andesitic to dacitic volcanics and sediments.

The contact between the Lucky Creek and Greenvale terranes is also a mylonite zone which has converted the coarse mica schist of the Halls Rewards Metamorphics to phyllonite. Prior to mylonitisation, the rocks were deformed and metamorphosed in the amphibolite facies along with mafic/ultramafic complexes within them (Arnold & Rubenach, 1976). The age of the terrane is Proterozoic, based on K/Ar hornblende ages from the mafic rocks (Black & others, 1979). The mafic/ultramafic complexes are therefore not ophiolite slices related to the Palaeozoic margin, in spite of their proximity to it. Mylonite zones also occur within the terrane and along the eastern margin with the Broken River Province. The Judea beds crop out to the east of the Greenvale terrane across a steeply dipping mylonitised contact which is part of the Burdekin Fault system. They consist of quartz-rich flysch, spilite, and keratophyre of Middle or Late Ordovician age, and are one of the oldest units in the Broken River Province. They are strongly deformed, contain abundant broken formation, and are interpreted as being thrust over the Greenvale terrane (T. Bell, JCUNQ, pers. comm). An ultramafic/mafic complex, the Gray Creek Complex, is completely surrounded by rocks of the Broken River Province, south of Greenvale. It is regarded as part of the Greenvale terrane exposed in an anticlinal core, because it also has Proterozoic K-Ar ages; It may also have a thrust contact with the Judea beds; on the east it is also bounded by the Gray Creek Fault, which is a mylonite zone (possibly a transcurrent fault) dividing the Broken River Province into the Graveyard Creek and Camel Creek Subprovinces (or terranes)« Synthesis The mylonite zones suggest either large-scale thrusting or strikeslip displacement along terrane boundaries, and the terranes can thus be regarded as suspect. Detailed studies to determine the sense and type of movement have not been completed. Because of later folding, any thrusts are now steeply dipping or vertical, and difficult to distinguish from transcurrent faults without such studies. However the Lucky Creek terrane is thought to be a thrust sheet underlying the Greenvale terrane and exposed in the core of an anticlinorium. The sense of movement is not yet certain. The BMZ may be the same roof thrust repeated on the western limb. The Balcooma and Lucky Creek terranes could thus be a single terrane thrust under the Georgetown terrane, of which the Greenvale terrane is an extension. However, if instead the BMZ represents a strike-slip fault, the Balcooma, Lucky Creek, and Greenvale terranes could have moved as a composite terrane after accreting elsewhere. The mylonite zones in the eastern Georgetown terrane may be related to such movement. The overthrusting or obduction of the Judea beds probably postdated the docking of the composite terrane, and may be related to deformation further east in the Camel Creek terrane of the Broken River Province; such deformation is characterised by imbricate stacking of slices of quartz-rich and quartzintermediate flysch, and extensive melange (Arnold, 1975). This deformation continued through the Silurian and can probably be explained in terms of a convergent margin. An upper limit on the mylonitisation and docking time is provided by the Middle or Late Silurian age for the Dido Granodiorite (which truncates the BMZ), and by Early Silurian sediments in the Graveyard Creek Subprovince which contain mylonite pebbles and unconformably overlie both the Greenvale terrane and Judea beds. This suggests that the main mylonitisation was pre-Silurian and probably Ordovician, if the postulated Cambro-Ordovician ages for the Balcooma and Lucky Creek terranes are correct. To the north the terranes are obscured by Cainozoic cover, but are probably truncated by the Palmerville Fault, which marks the Palaeozoic margin there, and which is possibly a thrust. To the south, they are truncated by the rift-like Graveyard Creek Subprovince; they have not yet been positively identified further south because of extensive Cainozoic cover and granitoid batholiths. However the Seventy Mile Range Group near

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Charters Towers may be an extension of the Balcooma and/or Lucky Creek terranes. Extensive mylonite in the Cape River Metamorphics (R. Hammond, pers. comm) could be an extension of the BMZ. The change in structural trends from NNE to ESE may be due to a large orocline, the "Big Bend Megafold" of Bell (1980). References Arnold, G.O., 1975, A structural and tectonic study of the Broken River Province, north Queensland. PhD, thesis, James Cook University of North Queensland (unpublished). Arnold, G.O., & Rubenach, M.J., 1976, Mafic-ultramafic complexes of the Greenvale area, north Queensland. Devonian intrusions or Precambrian metamorphics? Journal of the Geological Society of Australia, 23, 119-139. Bell, T.H., 1980, The deformation history of northeastern Queensland - a new framework, in Henderson R.A. & Stephenson, P.J. (eds). The Geology and Geophysics of Northeastern Australia, Geological Society of Australia Queensland Division, 307-313. Black, L.P., Bell, T.H., Rubenach, M.J., & Withnall, I.W., 1979, Geochronology of discrete structural-metamorphic events in a multiply deformed Precambrian terrain, Tectonophysics, 54, 103-107. Henderson, R.A., 1982, Notes on the stratigraphy of the Mount "Windsor Subprovince, in, Withnall, I.W. (ed), 1982 Field Conference, Charters TowersGreenvale area. Geological Society of Australia, Queensland Division, 7-11. Withnall, I.W., 1982, The geology of the Greenvale-Balcooma area, in Withnall I.W. (ed), 1982 Field Conference, Charters Towers-Greenvale area. Geological Society of Australia, Queensland Division, 31-46. Withnall, I.W., Bain, J.H.C., & Rubenach, M.J., 1980, The Precambrian geology of northeastern Queensland, in, Henderson, R.A. & Stephenson, P.J. (ed). The Geology and Geophysics of Northeastern Australia Geological Society of Australia, Queensland Division, 109-127.

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boundary ant i fan*. syntorm anticline. sync line major fault prevailing foliation mnjor mute small mine, prospect

20km

CJl o

Carboniferous-Cainozoic cover Silura-Devonian granitoids Silurian-Carboniferous post-docking flysch, shelf,& continental sediments Ordovician flysch & melange Luckv Creek terrane metapelites andesitic metavolcs gneiss, amphibolite Balcooma terrane felsic metavolcs & metasediments

Figure 1

Cairns-Townsville hinterland showing Proterozoic

Proterozoic terranes (Georgetown ft Greenvaie)

terranes

(shaded), Ordovici an -Devonian granitoids (crosses), and deformed Palaeozoic terranes (stippled)

Figure 2

Geological sketch map of the Greenvaie area showing the main terranes.

STUDY OF THE TECTONOSTRATIGRAPHIC TERRANES OF THE COASTAL REGION IN SOUTHEAST CHINA Shi Yangshen, Guo Lingzhi, Ma Ruishi and Lu Huafu Department of Geology, Nanjing University, Nanjing, China The coastal region of southeast China is a complex orogenic region, a collage of terranes consisting of folded basement of different ages including Proterozoic, Paleozoic and Me^ozoic. The Jiangnan Proterozoic terranes collaged with the Yangzi pre-Sinian continental margin and constituted the Jiangnan Proterozoic ancient arc due to the Xuefengian orogeny. Owing to the Caledonian orogeny the south China Caledonian terranes accreted to the southeast margin of the Jiangnan Proterozoic arc. Then the coastal Hercynian-Indosinian terranes in Zhejing and Fujian provinces accreted to the Caledonian orogenic belts. Consequently the continental accretion displays a spatial regularity from northwest to southeast. It can be compared with southeast Australia. The different aged terranes mentioned above have remarkable differences in paleontology, stratigraphy, sedimentation, igneous activity, metamorphism and crustal thickness. The boundaries of terranes are subduction zones, sutures or the deep fault zones. The Taiwan Island of China consists of the east Taiwan terrane and the west Taiwan terrane. The boundary between them is a longitudinal valley fault zone. The east Taiwan terrane, originally located north of the Philippines was displaced from south to north.accreted to the continental margin of southeast China during the early Miocene. The Hainan Island and Xisha, Dongsha Islands terranes were separated from the coast of Guangdong Province. The folded basement of Hainan Island discrete terrane can be compared with the Caledonian fold belt of western Guangdong completely.

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THE HIGH P/T METAMORPHIC RCX:KS OF CHINA Zh. M , Zhang,

J.G. Liou and R.G. Coleman

Department of Geology, Stanford University, Stanford, California, U.S.A. Blueschists and/or C-type eclogites have been found in almost all the accretionary fold belts and major sutures of China, which not only belong to the Circum-Pacific and Tethys-Himalayan belts but also within those areas between Precambrian cratons. The ages of these high P/T metamorphic rocks range from Early Paleozoic to late Cenozoic. The study of the high P/T metamorphic rocks has been concentrated on the easily identified exotic blocks (or knockers) within either ophiolitic melange (or serpentine melange) or olistostromic melange. Little work has been done on the possibly more widespread high P/T metamorphism in the matrix. A majority of the blueschists and C-type eclogites have experienced multiple-stage metamorphism, commonly with increasing grade at the later events. The mineral assemblages and chemical composition of sodic and sodic-calcic amphibole, garnet and clinopyroxene in some Chinese blueschists and C-type eclogites are discussed. The sodic amphiboles were commonly replaced by other amphibole, and the reverse alteration occurs in some samples. The P/T conditions of blueschist and eclogite recrystallization for a number of areas are estimated, based on clinopyroxene-gamet geothermometer, phengite geobarometer and mineral phase equilibria. The protoliths of the high P/T metamorphic rocks of China are mainly mafic volcanics and sedimentary rocks, with subordinate siliceous and carbonate rocks. The metamorphic maps and information about metamorphic facies series are available in a few areas where more studies have been done. Studies of high P/T metamorphic rocks so far are very preliminary in nature. It is hoped that this synthesis will stimulate further investigation, which in turn will provide better tectonic interpretations of China in the future.

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