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Geological Society of Australia

ABSTRACTS Number 4

International Conference on Deformation Processes in Tectonics ALICE SPRINGS -

AUSTRALIA

9-15 AUGUST 1981

INTERNATIONAL CONFERENCE ON DEFORMATION PROCESSES IN TECTONICS

ALICE SPRINGS - GLEN HELEN NORTHERN TERRITORY - AUSTRALIA 9-15 AUGUST, 1981

Organised by :

Specialist Group in Tectonics and Structural Geology of the Geological Society of Australia

Conference Committee:

C.J.L. Wilson M.A. Etheridge G.P. Price P. Fleming S. Cox

This International Conference has been supported financially and/or logistically by a number of Mining Companies, Universities and Research Institutes. For their generous assistance the Conference Organising Committee would like to thank the following : Mount Isa Mines Limited The Broken Hill Proprietary Company Limited CRA Limited ESSO Australia Ltd. Australian Academy of Science - 25th I.G.C. Fund Australian Government - Department of Education Bureau of Mineral Resources, Geology and Geophysics Northern Territory - Department of Mines and Energy Monash University - Department of Earth Sciences University of Melbourne - School of Earth Sciences James Cook University of North Queensland CSIRO - Division of Applied Geomechanics

INTERNATIONAL CONFERENCE ON DEFORMATION PROCESS IN TECTONICS GLEN HELEN - ALICE SPRINGS AUSTRALIA 9-15 AUGUST, 1981 ABSTRACT OF PAPERS PRESENTED AT THE CONFERENCE Page STEWART, A.J. , SHAW, R.D. & BLACK, L.P: Geological Evolution of the Arunta Block Part Is Stratigraphy

SHAW, R.D., STEWART, A.J. & BLACK, L.P: Geological Evolution of the Arunta Block Part 2: Tectonics

WARREN, R.G: Retrogressive Metamorphism Related to Major Fault Zones at the Northern Margin of the Strangways Range

SANDIFORD, M: Deformation and Metamorphism during the Exhumation of the High Grade Gneisses of the Napier Complex, Enderby Land, East Antarctica

TURNER, N.J: Palaeozoic Deformation and Granitoid Intrusion in North Eastern Tasmania

NXSBET, B.W., ETHERIDGE, M.A., HOBBS, B.E. & WALL, V.J: Early Bedding-Parallel Thrusting in the East Pine Creek Geosyncline

WILLIAMS, F. PAUL: Large Scale Transposition in Holandsfjord Norway

WEBER, KLAUS: Unusual Mechanisms of Nappe Emplacement at the Southern Margin of the Damara Orogen (Namibia)

DALGARNO, C.R: Structural Concepts for the Breccias of the Adelaide Fold Belt, Gravity Slides, Thrusts, Diapirs, or Fault Block Regolith?

WILSON, C.J.L: Ice-Mica Models and their Relationship to Deformation and Texture Development in Rocks

SHELLEY, DAVID: Quartz and Sheet-Silicate Preferred Orientations of Low Symmetry, Pikikiruna Schist, New Zealand

LISTER/ G.S. & WILLIAMS, P.F: The Partitioning of Deformation in Flowing Pock Masses POWELL, C.McA: Conjugate Cleavage in Weakly Deformed Quartzose Sandstone COX, S.F. & ETHERIDGE, M.A: The Development of Deformation Microfabrics in Low Grade Metamorphosed Silicic Volcanics & Associated Rocks, Mt. Lyell Area, Tasmania BOSWORTH, WILLIAM: Microstructural Observations on Phacoidal and Slaty Cleavage, and the Significance of True Time in Measures of Noncoaxiality AHMAD R. & WILSON, C.J.L: Activity of a Fluid Phase in the Development of Cleavage MULLENAX, ARTHUR C. & GRAY, DAVID R: Kinematics of Fold Development in Mudrock Layers of a Multilayered Limestone and Mudrock Sequence COX, S.F. & ETHERIDGE, M.A: Crack-Seal Deformation and the Development of Layer Silicate Preferred Orientation WINSOR, C.N: Syntectonic Vein and Fibre Growth Associated with Multiple Slaty Cleavage Development in the Lake Moondarra Area, Mount Isa, Australia VERNON, R.H., FLOOD, R.H. & D'ARCY, W.F: Development of Foliations in Deformed Granitiods HOLCOMBE, R.J: The Origin of Foliation in a Deformed Granite COHEN, P.H: Application of Interactive Colour Graphics to the Display and Interpretation of Structural Data NICOLAS, A: High and Low Temperature Plastic Deformation in Peridotites from Ophiolites : Implications for Flow in Oceanic Mantle CHOPAR, P.N: The Deformation of Dunite GREEN, H.W: Deformation of Peridotite in the Mantle and Transport of Xenoliths to the Surface HOBBS, B.E: Mechanisms of Deformation of Olivine

CASEY, M. , DIETRICH, D. & RAMSAY, J.G: Methods for the Determination of Deformation History for Chocolate Tablet Boundinage with Fibrous Crystal Intergrowths DURNEY, D.W: Dilatancy and the Angle of Obliquity of En Echelon Fractures RUBENACH, M.J. & BELL, T.H: Sequential Prophyroblast Growth and Crenulation Cleavage Development During Progressive Deformation CASEY, M., DIETRICH, D. & RAMSAY, J.G: The Use of Pressure Shadow Fibres to Determine Finite Strain and Deformation History in Rocks DURNEY, D.W. & KHAIAMI, R: Log-Polar R f /0 f Analysis of Strain and Depositional Fabric, witn Applications to some Oolitic and Peletal Limestones WHITE, S.H., WHITE, J.C. & JOHNSTON, D.C: Changes in Deformation Mechanisms and Softening Processes with Crustal Depth MAINPRICE, D.H. & PATERSON, M.S: The Experimental Deformation of Flint FITZGERALD, J.D: The Science of Analytical Electron Microscopy Applied to Deformed Geological Materials

GEOLOGICAL EVOLUTION OF THE ARUNTA BLOCK PART 1: STRATIGRAPHY A . J . Stewart, R.D. S h a w , and L . P . Black Bureau of Mineral Resources P.O. Box 3 7 8 , Canberra C i t y , A . C . T . , 2601, Australia

The Arunta Block is the region of Proterozoic ensialic crystalline rocks that extends for about 1000 km by 400 km across the southern p a r t of the Northern Territory of A u s t r a l i a . It is surrounded on m o s t sides b y younger sedimentary cover, b u t in the northwest it passes transitionally into similar schist and gneiss of The Granites-Tranami B l o c k , and to the north into slate, schist, and quartzite of the Tennant Creek B l o c k . The rocks of the Arunta Block have been grouped into three Divisions. Rocks of Division 1 occupy the lowest stratigraphic level, and include m a f i c , felsic, and pelitic granulite and gneiss, and lesser amounts of marble and calc-silicate r o c k . Metamorphism is dated at 1800-1650 M a . Division 2 is unconformable on Divison 1, and in the northern Arunta Block consists m o s t l y of metamorphosed micaceous sandstone, shale, calc-silicate r o c k , and a few flows or sills of mafic rock. Metamorphism is generally greenschist, b u t rises to granulite near A i l e r o n . In the southern Arunta B l o c k , Division 2 consists of pelitic gneisses, amphibolite, and extensive quartzofeldspathic gneisses; metamorphism probably occurred at 1800-1600 M a . Division 3 unconformably overlies Division 2, and crops o u t as synclines and fault slices of orthoquartzite above a basal pebbly a r k o s e , followed by shale and carbonate. In the Reynolds R a n g e , these rocks are intruded by sills and a lopolith of microgranite. Metamorphism w a s coeval with or slightly younger than that in Division 2, and shows a similar range in facies from greenschist to granulite. Granites intruded all three Divisions, and include foliated augen orthogneisses, and massive or flow-textured porphyritic and even-grained granites. The granitic rocks are dated at 1800-900 Ma; some, particularly the orthogneisses, m a y have been intruded and metamorphosed before Division 3 was deposited. Metamorphism and migmatisation occurred again at about 1000 Ma in the southern Arunta B l o c k , and caused Rb-Sr isotopic homogenisation in the north. The Arunta Block lacks the rock types of an ensimatic environment, such as ophiolite, turbidite, andesite, and paired metamporphic b e l t s . Instead it is characterised by mafic and felsic meta-igneous rocks, abundant quartzose and arenitic or finer-grained metasediments and c a r b o n a t e , and low-pressure metamorphism. H e n c e , the Arunta Block is interpreted as a Proterozoic ensialic geosyncline floored by continental crust of Archaean or Early Proterozoic a g e . The Arunta Geosyncline then proceeded through a rather long-lived evolution, involving sedimentation of increasing m a t u r i t y , vulcanism, folding, metamorphism, and granite intrusion, lasting for about 900 M a .

GEOLOGICAL EVOLUTION OF THE ARUNTA BLOCK PART 2: TECTONICS R.D. Shaw, A.J. Stewart and L.P. Black Bureau of Mineral Resources P.O. Box 378, Canberra City, A.C.T., 2601, Australia

The Arunta Block is divided into three latitudinal, fault-bounded tectonic zones, which differ in terms of rock type, metamorphic grade, times of metamorphism, abundance of granite, and metal content. The Central Zone is largely composed of the oldest Arunta rocks (Division 1), viz., mafic and felsic meta-igneous rocks. The earliest metamorphism formed granulites and occurred around 1800 Ma. The eastern part of the Zone consists mostly of pelitic gneiss of Division 2. Granites are few, small, and anatectic. The characteristic metalliferous occurrence is the stratabound Pb-Zn-Cu Oonagalabi type, comprising sulphides in marble and anthophyllite rock in a sequence of cordierite gneiss, magnetite quartzite, and in some cases, amphibolite. Mica pegmatites are abundant in the pelitic gneiss of Division 2. The Southern Zone consists mostly of quartzofeldspathic gneisses of Division 2, and resembles the Musgrave Block in northern South Australia although generally at lower metamorphic grade. The Southern Zone is overlain by quartzite and pelitic schist of Division 3. Metamorphic facies ranges from greenschist to amphibolite, and is dated at about 1600 Ma. Intrusive granites, commonly deformed to orthogneiss, are abundant. The Zone underwent further amphibolite facies metamorphism at about 100 Ma, and again at about 350 Ma, when thrust-faulting and greenschist retrogression also involved • the lower part of the Amadeus Basin sequence south of the Arunta Block, and formed the nappe complexes in the Ormiston, Alice Springs, and Arltunga areas. Gold was concentrated in the root zone of the Arltunga Nappe Complex at this time. The Northern Zone cosists largely of complexly folded pelitic schist, meta-sandstone, and calc-silicate rock of Divison 2, unconformably overlain by synforms of quartzite, shale, and carbonate of Divison 3. Metamorphic facies is generally greenschist, but rises to granulite near Aileron. Division 1 granulites are dated at about 1650 Ma. Granites constitute 75 percent of parts of the zone, and contain tungsten, tin, tantalum, molybdenum, copper, and uranium. Stratabound Cu-Pb-Zn sulphides occur in pelitic and iron-rich schist of Division 2, and late Proterozoic hydrothermal barite and fluorite veins occur at the margins of the Ngalia and Georgina Basins. Thrust faulting took place at about 350 Ma, but associated greenschist retrogression was more limited than in the Southern Zone. Large Bouguer anomaly ridges and troughs correspond to the three tectonic zones, and their magnitude indicates continuance of the zones to depths of at least 20 km, and possibly to the mantle (35 km) . Hence, the major faults between the zones, especially between the Central and Southern Zones, may mark sites of cratonic collision or suturing. Tholeiitic magma, now preserved as mafic granulite of the Central Zone, then erupted in an

3. ensialic rift or short-lived, narrow ocean basin between the cratonic plates. The Northern and Southern Zones formed by geosynclinal sedimentation on the Central Zone; flysch deposition was followed by multiple folding, regional metamorphsim, granite emplacement, and faulting. In addition to the latitudinal zones, the distribution of isotopic dates, and gravity and magnetic lineaments delineate significant cross-cutting northwesterly tectonic trends. Tectonic activity along these trends is most pronounced in a zone extending from Arltunga, through the Reynolds Range to Mount Solitaire. Metamorphism, deformation, and intrusion of granite and porphyry took place along this zone between 1650 and 1400 Ma. Subsequent deformation and metamorphism in the late Proterozoic and late Palaezoic took place along the old sutures and faults. Deformation became more brittle as the crust thickened and deyhdrated during each tectonic episode. Renewed tectonic movements in the Tertiary account for the positive relief along the Arltunga-Reynolds Range zone, and for the shape and distribution of non-marine Tertiary basins. The long history of tectonism in the Arunta Block may result from its position at the intersection of several linear zones of intracontinental mobility in Australia.

RETROGRESSIVE METAMORPHISM RELATED TO MAJOR FAULT ZONES AT THE NORTHERN MARGIN OF THE STRANGWAYS RANGE* R.G. Warren Bureau of Mineral Resources, Canberra, A.C.T. Australia

A b o u t 80 km north of Alice S p r i n g s , at the northern m a r g i n of the Strangways Range adjacent to the Wallaby Knob Schist Z o n e , h i g h grade anhydrous m i n e r a l assemblages, including orthopyroxene-orthoclase i n felsic rocks are preserved in g r a n u l i t e s . These granulites have b e e n affected b y s e v e r a l episodes of h y d r a t i o n , in and adjacent to the S c h i s t Z o n e , each occurring along a continuum of successively lower temperatures and pressures. T h e first episode w a s pervasive and left only rare enclaves of completely unaltered g r a n u l i t e . The characteristic minerals formed during this episode w e r e biotite in felsic rocks and paragasitic hornblende in mafic granulites. Succeeding hydrations are m o s t clearly shown in rocks that originally contained cordierite-quartz o r cordierite-spinel a s s e m b l a g e s . In these rocks the second stage of hydration occurred w i t h i n the gedrite f i e l d , and locally, within gedrite-kyanite f i e l d . The third stage occurred w i t h i n the cordierite-sillimanite-staurolite stability f i e l d , and the last w i t h i n the clorite-talc f i e l d . The retrograde rocks are confined to zones w h i c h become narrower w i t h each successive lowering of metamorphic g r a d e . This suggests the W a l l a b y K n o b schist Zone b e g a n as a b r o a d , ductile feature deep w i t h i n the c r u s t , b u t w i t h continuing uplift and erosion the zone became n a r r o w e r , and the hydration and retrogression m o r e localised.

* Presented w i t h the permission of the D i r e c t o r , Bureau of M i n e r a l R e s o u r c e s

5. DEFORMATION AND METAMORPHISM DURING THE EXHUMATION OF THE HIGH GRADE GNEISSES OF THE NAPIER COMPLEX, ENDERBY LAND, EAST ANTARCTICA M. Sandiford, School of Earth Sciences, University of Melbourne, Parkville, 3053, Vic. Australia

The deformation processes operating during the exhumation of the high grade gneisses of the Napier Complex, Enderby Land, East Antarctica, from depths of 25-30 km are intimately related to the prevailing metamorphic conditions and are reflected in the macroscopic structure. Exhumation was initiated during granulite facies conditions and structures produced overprint earlier F^ and F^ folds in an essentially anhydrous gneissic pile. Prior to the introduction of water a series of large scale assymetric F^ folds were produced. Steep dipping southern limbs were loci for high strain and sites for the subsequent introduction of water and amphibolite facies metamorphism. Following the onset of amphibolite facies conditions strain was largely accommodated in sub-vertical ductile shear zones with the intervening granulitic blocks acting as rigid bodies.

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PALAEOZOIC DEFORMATION AND GRANITOID INTRUSION IN NORTH EASTERN TASMANIA N.J. Turner Geological Survey of Tasmania, Department of Mines, Hobart, Tasmania, Australia

Structural relationships within and between Ordovician to Early Devonian sedimentary rocks (Mathinna Beds) and Devonian to Carboniferous granitoids in North Eastern Tasmania demonstrate an involved history of deformation and intrusion. Folds and cleavage developed in the Mathinna Beds initially. Granodiorite was intruded later and followed in one area by equigranular adamellite. Several fabric elements defined by statistically preferred orientations of grains of most mineral species (grain foliations) were imposed on these granitoids. Shearing (conjugate and probably conjugate) followed development of grain foliations in several places and mylonite is well developed in one locality. Major intrusion of porphyritic adamellite and other granitoids occurred after the shearing event. Several fabric elements defined by statistically preferred orientations of feldspar phenocrysts (phenocryst foliations) were imposed on the porphyritic adamellite. In the Mathinna Beds the strain probably corresponding to one, or an aggregate of both, of the periods of post-granitoid deformation is represented by post-thermal metamorphism crenulation cleavage. Late igneous activity takes the form of dykes mainly of quartzfeldspar porphyry and dolerite. These show strong structural control by a regional fracture system.

EARLY BEDDING-PARALLEL THRUSTING IN THE EAST PINE CREEK GEOSYNCLINE B.W. Nisbet 1 , M.A. Etheridge2, B.E. Hobbs 2 , and V.J. Wall 2 iGeoscience Field Surveys, Pty. Ltd., Cammeray, N.S.W. 2062, Australia 2Dept. of Earth Sciences, Monash University, Clayton, Victoria, 3168, Australia

The Pine Creek Geosyncline of the Northern Territory can be divided into two structurally and metamorphically distinct regimes. I. A central area of generally low grade metamorphic rocks in which previous workers have described the structural history as simple. This picture may be complicated in the vicinity of granitoid plutons. II. High grade metamorphic rocks in the eastern and western parts, in which the structure is known to be complex. The work described here is part of a continuing project to investigate the structure, metamorphism and mineralization in the geosyncline as a whole. However, the work to date has concentrated on the South Alligator Valley - Barramundie Creek area which contains the transition between the regimes of contrasting metamorphic and structural style. Metamorphic grade in this area is low, but the structural history is quite complex, with four significant deformation events, as follows: 1. An early deformation (B\) in which strain is concentrated in zones up to a few hundred metres wide parallel or sub-parallel to bedding. Structures associated with this deformation include a foliation that increases in intensity towards the zones and which is sub-parallel to layering, widespread transposition of layering on the mesoscopic scale with rare isoclinal folds, and zones of brecciation. 2. B2 is manifest as close to tight, upright folds on all scales from microscopic to megascopic, with a moderate to well-developed axial surface foliation. These folds generally plunge gently to moderately to the NNW or SSE. 3. B3 folds occur on microscopic to macroscopic scales and have variable (but commonly steep) plunges towards the WNW or ESE. They are open to close, with steeply dipping axial surfaces, and axial surface foliations that are weak or absent. The Bi deformation is interpreted to have been dominantly by lowangle thrusting within an essentially flat-lying sequence. Throughout the area mapped, the contact between the Koolpin Formation and the Mt. Partridge Group is such a thrust zone, and others have been preliminarily identified in the Frances Creek and Coirwong Creek areas. The presence of extensive thrusts of this type has a number of stratigraphic and structural implications for the geosyncline as a whole. First, the current stratigraphic interpretation includes cyclic repetition of sequences, partial unconformities and rapid changes in formation thickness all of which are consistent with extensive sub-horizontal thrusting. Second, the rapid increase in metamorphic grade betwen this area and the East Alligator uranium province may be explained by the rapid thickening of a thrust pile. Finally, the relationship between structural/metamorphic history and the mineralization will be discussed.

LARGE SCALE TRANSPOSITION IN HOLANDSFJORD NORWAY Paul F. Williams University of New Brunswick, Department of Geology Fredericton, New Brunswick, Canada E3B 5A3

An area of continuous outcrop recently emerged from under the Engabre glacier has been mapped in detail. The layering in a sequence of amphibolite facies metamorphic rocks has been transposed several times and all sizes of isoclinal folds with amplitudes up to 1.5 kilometres can be observed. Sheath folds and at least two generations of folds are involved in the transposition and the outcrop offers an unusual opportunity to look at the strain involved. The large strains and the regional setting are discussed and it is concluded that the transposition foliation developed in a thrusting environment for which the deformation path is best approximated by a simple shearing. It is further concluded that the foliation is probably a "steady state" structure in which more than one generation of fold developed during a single protracted deformation.

UNUSUAL MECHANISMS OF NAPPE EMPLACEMENT AT THE SOUTHERN MARGIN OF THE DAMARA OROGEN (NAMIBIA) Klaus Weber Geologisch-Palaontologisches Institut, Goldschmidtstr. 3, 3400 Gottingen West-Germany

Approximately 50 km south of the present southern margin of the late Precambrian to early Palaeozoic Damara Orogen the Naukluft Nappe complex is exposed. This nappe complex overlies the autochthonous Nama beds which belong to a platform area adjacent to the Damara mobile belt. The total displacement from the NW to the SE amounts to 50-80 km. The base of the Naukluft Nappes is formed by a dolomite horizon changing in thickness between zero and about 30 metres. Because of its unconformable relation with the over- and underlying rocks this strata-like dolomite layer was named "Unconformity Dolomite". Recent investigations of the mineral content, of fluid inclusions grain fabric and deformation lead to the conclusion that continental playa-lake evaporites have to be assumed as source rocks of this dolomite, called in the following "Sole Dolomite". 34 different minerals have been found in the Sole Dolomite. The main components are sparitic dolomite, albite, quartz, tourmaline, Mg-riebeckite, talc, and sericite. The Sole Dolomite contains numerous rock fragments of granites, granodiorites, gneisses and micaschists with tourmaline and Mg-riebeckite as metasomatic minerals. Very frequent are fragments of quartz-albitolites containing more than 50 Vol. -8% of albite and various amounts of tourmaline and dolomite. The Sole Dolomite is extremely rich in fluid inclusions of different, but mostly high salinity. The trapping temperatures are about 100 C higher than the metamorphic temperatures of the overlying Naukluft Nappes and about 200 C higher than in the underlying autochthonous Nama beds. The mineral assemblage of the Sole Dolomite resulted from intense interaction of highly concentrated, hot hydrothermal waters with the rock fragments and the dolomite matrix, but not from metamorphic reactions. The Sole Dolomite may be developed as a compact, sometimes well laminated sparitic rock rarely displaying SE-facing internal folds. Its base forms a sharp boundary against the limestones of the underlying autochthonous Nama Group. The minerals of the Sole Dolomite display no primary recrystallisation and no crystal plastic deformation. No preferred lattice orientation is developed. All deformation found in the Sole Dolomite is of the brittle type. Cataclastic mylonitisation without any preferred lattice orientation has been encountered only in the lowermost few centimetres of the Sole Dolomite. This cataclastic mylonite displays grain sizes less than 1 micron. In contrast to the Sole Dolomite the underlying Nama limestones have been transformed over a vertical distance of several metres into fine-grained mylonites which reveal a well developed preferred lattice orientation. The formation of these two types of mylonites will be discussed in more detail.

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From the present facts the formation of the Sole Dolomite may be interpreted as a discordant intrusion under high pore-fluid pressure into the base of the nappe complex. The development may be characterised by the following steps: 1. During increasing burial the hypersaline playa-lake sediments have been transformed into a tourmaline-bearing albite-dolomite rock rich in interstitial brines. Dewatering of hydrous carbonates and silicates, crystallisation and compactional disequilibrium may have produced abnormally high pore-fluid pressure. 2. During Damaran deformation a nappe sequence was formed and thrust to the SE. At the same time, the evaporitic sequence progressively subsided together with the Damara foreland due to a southward migration of the Damara front. During this stage the pore-fluid pressure has been further increased by aquathermal-pressuring and deformation. 3. The now highly mobile material intruded into higher levels along the structural discontinuity at the base of the earlier formed nappe complex. Here it acted as a lubricant and led to further SE movement of the nappe complex. Fluid inclusion development points to osmotic inflow of water from surrounding rocks during this stage of the Sole Dolomite development. 4. The final displacement of the nappes with the Sole Dolomite at its base must have taken place after lithification of the intrusion. Otherwise, the intense low temperature mylonitisation of the underlying autochthonous Nama limestones cannot be adequately explained. Due to the high stiffness of dolomite under confining pressure only the lowermost few centimetres of the Sole Dolomite plate were cataclastically mylonitized without any preferred lattice orientation.

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STRUCTURAL CONCEPTS FOR THE BRECCIAS OF THE ADELAIDE FOLD BELT, GRAVITY SLIDES, THRUSTS, DIAPIRS, OR FAULT BLOCK REGOLITH? C.R. Dalgarno Seltrust Mining Corporation Pty. Ltd., Adelaide S.A.

Conflicting concepts exist concerning the origin of breccias exposed in the northern part of the Adelaide Fold Belt. Genesis of breccias such as in the Witchelina Structure in the Willouran Ranges Lat. 30°05'S, Long. 138°00'E is interpreted by Sprigg (1949) as a crush zone in the sole of a low angle thrust fault, by Coats (1964) as the exposed core of a diapiric anticline and by Murrell (1977) as a Torrensian (Early Adelaidean) sedimentary overlap on to a pre-consolidated megabreccia of sedimentary origin. Other hypotheses have been invoked for the bodies mapped by the Geological Survey as diapiric breccias. Thus the Blinman, Beltana and Wirrealpa bodies have been interpreted as fault blocks overlapped by Sturtian to Cambrian sediments over a regolith of brecciated Early Adelaidean rocks. Late Precambrian and Early Cambrian deposits of the Adelaide Geosyncline are of relatively shallow water origin and consist of an initial graben-fill basic volcanic-evaporite association, followed by cyclic deltaic sandstones, siltstones and dolomites which pass up to regressive playa lake deposits. Murrell (1977) and others have postulated contemporaneous gravity sliding within parts of these sequences to account for megabreccias in the Willouran Ranges. No convincing evidence of pre-Sturtian breccias has yet been presented in this area. Early tectonism is evidenced by unconformable truncation by Sturtian glacial deposits of isoclinal folds developed in Torrensian rocks near Chintapanna in the central part of the Willouran Ranges. Local subsiding troughs were infilled with glaciomarine diamictites which were followed by interglacial laminated siltstones and dololaminites. Widespread conglomerate and slump features developed in this sequence and local angular unconformities developed adjacent to fault-localized diapiric breccia bodies in the Late Sturtian. Regional instability may have resulted from movement on major structures such as the Northwest Fault as the "Amberoona Slump" of Coats (pers. comm.) extends over portions of four 1:250,000 sheet areas, extending 125 km eastwards from the Torrens Hinge Zone near Stuart Creek and from Marree, some 40 km south to Witchelina. Red-brown siltstones, sandstones and carbonates typical of a relatively stable shelf environment dominate after the Marinoan glaciation and these extend eastward into the geosyncline. Diapiric domes were intermittently exposed resulting in bald-cap structures and draped unconformities with local conglomerates derived from the breccia cores e.g. Puttapa. The feature of sedimentation is the interplay of vertical tectonics and sedimentation illustrated by numerous contemporaneous or growth faults. In the locus of thickest sedimentation canyons and turbidite facies developed in the latest Precambrian and local megabreccias formed on hinges in the Early Cambrian. The Mid Cambrian is a molasse-type

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deltaic sequence of red shale and arenite. Major faults such as the Norwest structure were active during Cambrian deposition. This structure is a high angle reverse fault with north-block-east strike slip. The Cambrian syncline on the south side has an attenuated overturned limb, with conglomerate developments and injections of "diapiric" breccia along the fault adjacent to the Early Adelaidean quartzite (Copley Quartzite). A comparison may be drawn between the Puttapa area and The Illamurta Structure of Central Australia, both in development and relationship to a major structural zone. In the case of breccias in the core of the contiguous Beltana Diapir, blocks of pre-Adelaidean basement occur adjacent to rafts of Cambrian sediments thousands of metres out of stratigraphic context. A diapiric origin is the only satisfactory explanation for this body and its extension at Puttapa. REFERENCES Coats, R.P., 1964. Geology and mineralization of the Blinman Dome Diapir. Rep. Invest., geol. Surv. S. Aust., 26. Murrell, B., 1977. Stratigraphy and tectonics across the Torrens Hinge Zone between Andamooka and Marree, South Australia. University of Adelaide Ph.D. thesis (unpublished). Sprigg, R.C., 1949. Thrust structures of the Witchelina Area, South Australia. Trans. R. Soc. S. Aust., 73, (1):40-47.

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ICE-MICA MODELS AND THEIR RELATIONSHIP TO DEFORMATION AND TEXTURE DEVELOPMENT IN ROCKS

C.J.L. Wilson

School of Earth Sciences, Melbourne University, Parkville, Vic. 3142, Australia

Polycrystalline specimens of ice and ice-mica have been deformed in plane strain under conditions of pure and simple shear in the temperature range -10 C to -1 C . Processes involving intracrystalline slip and recrystallization have been observed in the hexagonal ice. Under coaxial deformation conditions a strong c-axis "end orientation" consisting of a double maximum lying in the 25 to 40 small-circle girdles about the shortening axis is always developed with less than 10% shortening. This pattern is modified under non-coaxial conditions and its strength is reduced where there are finely dispersed mica flakes in the ice. Dispersed tabular micas in the ice aggregate undergo rigid rotation in the plastically deformed ice aggregate. The degree of rotation has been correlated with finite strain, determined from elliptical markers. The degree of mica rotation of an initially random fabric is not appreciable until there has been a shortening strain greater than 15%. The initial strain is taken up by the ice matrix. These observations are compared to the deformation of a non-random mica distribution. Annealing the ice-mica aggregates does not change the mica patterns of preferred orientation. However, pronounced differences arise from annealing ice deformed at -1 C and -10 C . In the colder finer grained samples (-10°C) the grain size increase was not as rapid as the coarser hot worked (-1°C) samples. Little textural change was observed in the hot worked samples, however the strong preferred orientation of the colder samples was destroyed with annealing. The change in texture corresponded to a decrease in the grain growth rate and the formation of a stable grain structure. Increasing strain and decreasing the deformation temperature both reduce annealing time and the annealed grain size.

14.

QUARTZ AND SHEET-SILICATE PREFERRED ORIENTATIONS OF LOW SYMMETRY, PIKIKIRUNA SCHIST, NEW ZEALAND David Shelley Geology Department, University of Canterbury, Christchurch, New Zealand

Slightly micaceous quartzites in the Palaeozoic, almandine-bearing Pikikiruna Schist of Nelson, New Zealand contain remarkably asymmetric fabrics. Patterns of quartz c-axis preferred orientations, the poles to planes of inequidimensional quartz grains, and the statistical maxima of poles to sheet-silicate cleavages are oblique to each other. Quartz c-axes form type-1, and occasionally type-2, crossed girdle patterns. The asymmetric fabric can be explained in terms of one complex rotational deformation of an essentially plane-strain nature in a ductile shear zone, in which bedding was rotated approximately 90 about the intermediate strain-axis with subsidiary late rotations about the extension axis. Quartz c-axis preferred orientations can be related to the kinematic framework whereas the quartz dimensional preferred orientation probably indicates the orientation of the finite strain—axes.

15.

THE PARTITIONING OF DEFORMATION IN FLOWING ROCK MASSES G.S. Lister1 and P.F. Williams2 department of Structural Geology, Institute for Earth Sciences, University of Utrecht, The Uithof, Utrecht, The Netherlands department of Geology, University of New Brunswick, Fredericton, New Brunswick, Canada

The partioning of deformation in a flowing rock mass is discussed, with examples from rock bodies deformed under crustal conditions. Flow partitioning refers to division of the instantaneous velocity field into components related to translation, strain and rotation. Heterogeneity leads to the development of flow domains with different characteristics, notably the degree of non-coaxiality of deformation, and the partitioning of the rotational component of flow into shear-induced vorticity and spin. Flow in the crust and mantle of the earth leads to the development of geological structures on all scales, and involves many different types of rheological response. Most geological structures owe their development to heterogeneous and/or non-steady flow, and spin is usually an important element in such flow fields. Flow partitioning during the development of a geological structure usually changes with time, and different histories of flow are involved at different points in the developing structure. For example bulk shear-induced vorticity can be locally converted into spin, and with time there can be repartitioning of vorticity between the spin and shear-induced components. The factors that affect flow partitioning (e.g. spin versus non-coaxial deformation) are therefore of interest. Non-coaxial flow is favoured by a suitably oriented material anisotropy and/or the presence of well-defined boundary constraints involving relative translation. Deformation of isotropic materials in contrast tends to involve coaxial flow in the regions near weakly constrained boundaries. A developing anisotropy (e.g. an axial plane cleavage) can influence a change from coaxial to non-coaxial flow. We are led to re-examine the rheological significance of competency contrast, and the nature of the factors that determine whether a foliation is active or passive.

16.

CONJUGATE CLEAVAGE IN WEAKLY DEFORMED QUARTZOSE SANDSTONE

C. McA. Powell School of Earth Sciences, Macquarie University, North Ryde, N.S.W., 2113

Conjugate cleavage is sporadically developed in weakly deformed quartzose sandstone of the Lambie facies in the Lachlan Fold Belt. In one example from the Late Devonian Mandagery Sandstone, central-west New South Wales, the conjugate cleavage occurs as planar zones, 0.01 m m to 0.05 mm wide and marked by phyllosilicates f in the lithons between the principal cleavage: a widely spaced (2 to 5 cm apart) anastomosing, disjunctive cleavage parallel to the axial surface of the open regional folds. Angles between the conjugate directions range from 4 0 ° to 8 0 ° (mean 64 ), with the acute angle bisected by the principal cleavage. The principal cleavage zones are domains of concentrated quartz dissolution, accompanied by recrystallization and growth of phyllosilicates. Microfabric indications are that phyllosilicates in both the conjugate cleavage and the principal cleavage have grown synchronously. A speculative model of formation for the conjugate cleavage involves its initiation as dilatant shear zones in a deforming granular body where the greatest principal stress has been locally reoriented normal to bedding during elastic buckling. Formation of the conjugate shear zones relieved the elastic stresses, and thereafter the rock responded to the regional sub-horizontal principal compressive stress with development of the anastomosing principal cleavage zones, accompanied by quartz dissolution and phyllosilicate growth on these zones, and the already-formed conjugate zones.

17.

THE DEVELOPMENT OF DEFORMATION MICROFABRICS IN LOW GRADE METAMORPHOSED SILICIC VOLCANICS AND ASSOCIATED ROCKS, MT. LYELL AREA, TASMANIA S.F. Cox and M.A. Etheridge Department of Earth Sciences Monash University, Clayton, Victoria, 3168, Australia

Deformation microfabrics in low grade metamorphosed silicic volcanics and associated rocks in the Mt. Lyell area of western Tasmania have developed largely by dissolution/solution transfer/redeposition processes. Dissolution has occurred preferentially along discrete subplanar zones subperpendicular to the shortening direction. The oriented growth of layer silicates in these sites has generated layer silicate films having an (001) preferred orientation subparallel to the dissolution plane. Oriented growth in such sites has probably been controlled by the interaction of anisotropic growth kinetics, the shape anisotropy of the growth sites, and non-hydrostatic thermodynamics constraints. Redeposition of material removed from dissolution sites has occurred dominantly in opening intergranular, transgranular, and intragranular microfracture sites. Phases deposited in these sites typically have fibrous to platy habits which have developed by a crack-seal mechanism. Layer silicates have grown with (001) subparallel to the extension direction. Quartz fibres in some cases have [1012] subparallel to the extension direction across microfracture sites. Bulk rock shortening during microfabric development has been accomplished largely by volume loss from dissolution sites, though in some cases shortening has also occurred by displacement along oblique extension and shear microfractures. Opening across pure extension and oblique extension microfractures has resulted in bulk extension subperpendicular to the shortening direction. The dissolution/solution transfer/redeposition process has been controlled at least partly by thermodynamically predicted stress-controlled solubility differences. However, for a realistic model in which the opening of microcracks has been a significant deformation mechanism, transient differences in fluid pressure are likely to develop between differently oriented microcracks and intergranular fluid films. Under such conditions fluid migration will be an important mass transfer mechanism, and the dissolution/redeposition process is expected to be driven partly by solubility differences due to fluid pressure variations between different microstructural sites.

18. MICROSTRUCTURAL OBSERVATIONS OF PHACOIDAL AND SLATY CLEAVAGE, AND THE SIGNIFICANCE OF TRUE TIME IN MEASURES OF NONCOAXIALITY William Bosworth Department of Geology, Colgate University, Hamilton, NY 13346 U.S.A.

Exposures in the Taconic Mtns. of eastern New York, U.S.A., provide an opportunity to study a sequence of Cambrian and Ordovician pelites, sandstones and graywackes spanning from undeformed units in the west through a folded and thrust faulted central zone to far-travelled allochthonous units in the east. The deformation of these units resulted in the initiation and progressive strengthening of an east-dipping tectonic foliation. The character of the foliation is variable and relationships between cleavage morphology and larger-scale structural styles may be useful in understanding the origin of the foliation itself. CLEAVAGE MORPHOLOGIES Slaty Cleavage In the western least deformed units, cleavage first appears in shaly horizons as a weak alignment of platy minerals approximately parallel to axial planes of mesoscopic and megascopic folds. The foliation is anastomosing in thin section, but would be referred to as "slaty" in hand sample. The strength of the slaty cleavage as measured by parting characteristics at the outcrop steadily increases to the east. In thin section this appears as stronger alignment of platy minerals, decreasing significance of anastomosing or deviant folia, and the truncation of quartz and calcite grains by cleavage lamellae. In the allochthonous (most highly deformed) units, fine-scale differentiated layering (not recognizable in hand sample) is also important in defining the slaty cleavage. Although generally referred to as an axial planar cleavage, more precisely large-scale folds are transected, with maximum dihedral angles between cleavage a,nd axial planes in the fold profile of 34° and between cleavage and fold axes of 7° (d and A of Borradaile, 1978) . XY planes of finite strain ellipsoids, as measured from reduction spots, are not, in general, exactly parallel to slaty cleavage (also noted by Williams, 1976). Phacoidal Cleavage Overthrusting in the central zone has produced a large-scale imbricate structure with an east-over-west sense of displacement. Most of the faulting occurs along shear zones defined by disrupted folds and blocks lying in a strongly foliated shaly matrix. In hand sample the east-dipping foliation in the shear zones appears scaly and not to be a single distinct cleavage, but rather a strongly anastomosing fabric of shaly lamellae enclosing lens- or "phacoid" - shaped chips of mudstone, siltstone, graywacke and chert. The phacoids are typically a few millimeters to a centimeter in length, lying in the "average" cleavage plane

19. with long axes plunging down-dip. Striations on cleavage surfaces roughly parallel long axes of the phacoids. Where larger clasts and blocks are present, the shear zone units are referred to as melange. In thin section the phacoidal cleavage lamellae appear as unresolvable dark seams. Some of the phacoids consist of sheared-off, asymmetric micro-folds or multiply crenulated shale clasts. Small offsets along phacoidal cleavage lamellae are easily demonstrated, but provide no upper limits to cleavage parallel shearing, and demonstration of consistent sense of shear on a micro-scale is difficult. Scanning electron microscopy reveals that individual striations on cleavage lamellae are actually packets of micro-grooves and ridges associated with micro-steps and cross-fractures. DISCUSSION The slaty cleavage observed in the western Taconics developed synchronously with the major, outcrop-pattern producing folds, and is not a late feature nor was it superimposed during a brief period of the deformation history. Analysis of larger-scale structures suggests that the deformation history of these units was noncoaxial not only at specific positions within folds but on a much larger scale as well. Slaty cleavage is interpreted to have tracked the stress ellipsoid through the deformation, producing transected folds. The rate of rotation of the principal directions of stress with respect to material lines, however, was slow enough to allow cleavage-producing deformation mechanisms to keep pace and to continually modify the foliation. The end result here was distinct slaty cleavage, with little deviation from planarity at the outcrop scale. In the shear zones (melange terranes), the rate of rotation of the stress ellipsoid appears to have been much faster, and early cleavage lamellae were rotated and subsequently crenulated and sheared-through. New cleavage orientations would continuously develop throughout the deformation. The end result is the "sheared-looking" phacoidal cleavage. Measures of coaxiality in deformation paths have in the past been proposed which relate the orientation of infinitesimal strain and strain rate ellipsoids to that of the finite strain ellipsoid, and the history of these relationships through "natural" time (Elliott, 1972). If the above interpretations are valid, then it would suggest that the degree of noncoaxiality of a deformation with respect to real or "true" time should be considered in the analysis of cleavage and other microstructures. This arises due to the time and stress dependence of the physicochemical process of diffusion, which probably plays an important role in most cleavage-producing deformational histories. REFERENCES Borradaile, G.J., 1978. Transected Folds: A study illustrated with examples from Canada and Scotland. GSA Bull. 89: 481-493. Elliott, D., 1972. Defamation paths in structural geology. a3: 2621-2638.

GSA Bull.,

Williams, P.F., 1976. Relationships between axial - plane foliations and strain. Tectonophys, 30/ 1 8 1 - 1 9 6 •

20.

ACTIVITY OF A FLUID PHASE IN THE DEVELOPMENT OF CLEAVAGE R. Ahmad and C.J.L. Wilson University of Melbourne, Parkville, Vic. 3052, Australia

The formation of cleavage in some slates has been related to chemically controlled processes involving an aqueous fluid phase which may be enriched in certain mobile components. There is, however, no trace of such fluid left in the rocks after deformation and metamorphism except for the presence of fluid inclusions. However, the activity of fluids and their mineralogical and microstructural distributions can be established by locating highly mobile tracer elements such as uranium and boron using the particle track method. Though not absolute, the technique provides an insight into the activity of fluids during deformation processes. This paper describes the distribution of uranium and boron in Ordovician slates from the Brisbane Ranges, Victoria. The cleavage in these slates is domainal. Marked cleavage development occurs along the short limbs and adjacent to the hinges of Fx folds (A zones) . An axial plane differentiated cleavage occurs in the "A" zones and bedding (S ) is generally rotated parallel to the zone boundaries. "A" zones have the appearance of miniature shear zones cutting through the slates and are areas of comparatively high strain and ductile deformation. The initiation of these shear zones may be related among other processes to a high fluid flux locally built up as a consequence of indigenous fluids concentrated in the hinges of large assymetric F^ structures and/or externally introduced fluids. "B" zones are where bedding is comparatively undeformed and usually occurs at a high angle to a weakly developed cleavage. Mineralogical and microstructural changes are more pronounced in "A" zones compared to "B". The uranium distribution patterns correlate well with the domainal cleavage development. Domains of marked cleavage development ("A" zones) are outlined as areas with a high concentration of both mobile and relatively immobile uranium. In contrast "B" zones with poorly developed cleavage have lower uranium fission track concentrations. These uranium distribution patterns in turn reflect the domainal mineralogical and micro-structural characteristics. Selective removal of material appears to be one of the reasons for the enrichment of uranium in "A" zones compared to "B". Bedding, no matter how weak or diffuse, is always defined by detrital zircons (high U) and tourmalines (high B) arranged in distinct zones. The uranium distribution patterns suggest an extensive redistribution of material and a high degree of fluid activity during the development of cleavage. On the basis of uranium and boron micromapping in relation to mineralogy and microstructure, Ahmad and Wilson (1981) have shown that the metamorphic fluid phase is enriched in these elements and uranium has a high affinity for titanium bearing phases (Ruhlmann, 1980).

21.

REFERENCES Ahmad, R. and Wilson, C.J.L., 1981: Uranium and boron distributions related to metamorphic microstructure - evidence for metamorphic fluid activity. Contrib. Mineral. Petrol., 76: 35-34. Ruhlmann, F., 1980: Quelques exemples de relation uranium-titane. Bull. Min., 103: 240-244.

22.

KINEMATICS OF FOLD DEVELOPMENT IN MUDROCK LAYERS OF A MULTILAYERED LIMESTONE AND MUDROCK SEQUENCE Arthur C. Mullenax and David R. Gray Department of Geological Sciences, Virginia Polytechnic Institute and State University, Blacksburg, VA U.S.A. 24060

Fold geometry along cannot be used to specify mechanics of folding. In low grade metamorphic rocks determination of fold propagation and amplification mechanisms requires knowledge of cleavage formation, cleavage-fold relationships, grain-scale deformation mechanisms, deformation history and spatial variation in the magnitude and orientation of total strain across folds. These have been investigated for mudrock layer folds within a deformed multilayered sequence of limestone and mudrock (Ordovician Martinsburg Formation of southwest Virginia, U.S.A.). The rocks are part of the Narrows Thrust sheet which has been subjected.to temperatures of 200-300 C (anchimetamorphism). Folds have sinusoidal form and are upright to steeply inclined with subhorizontal axes. Wavelengths range from 3 to 10 metres with amplitudes between 0.2 and 2 metres. Mudrock layers most commonly have class 3 (Ramsay, 1967) geometry. Fibrous calcite laminae with overlapping step-like structure on bedding surfaces suggest movement has occurred along bedding partings during folding. A weak spaced disjunctive cleavage (Powell, 1979) defined by thin (10 - 50ym) clay selvages, is locally developed in the folded mudstone layers. Cleavage patterns include divergent fans, arcuate hinge cleavage (Roberts, 1971) and subplanar axial surface cleavage. Total strain and fabric variations around mudrock layer folds correspond to flexural-flow folds to a first order approximation. They are characterized by low strains and weak cleavage fabrics in fold hinges and higher strains and stronger cleavage fabrics along fold limbs. Curved crystal-fibers in pressure fringes on framboidal pyrite in mudrock define rotations of incremental extension directions which are compatible with flexural flow. Opposite rotation senses related to bedding-parallel shear directed toward fold hinges, occur on both limbs. Progressive total strain curves on (Ei, <j>) graphs indicate non-coaxial deformation involving progressive simple shear. Initial extension increments on fold limbs are at 5 - 35 to bedding whereas fold hinges show initial increments at 55 - 85 . Discrepancy between 1) theoretical total strains (assuming finite shearing strain along limbs was dictated by limb dip) and total strains calculated from pressure fringes, and 2) observed bedding-cleavage angle (s"si) relationships around folds with those predicted for flexural flow, suggest however that folding was not flexural flow sensu stricto. Strains are higher and cless fanning than expected for flexural-flow folding. Hinge migration during fold development has further complicated the observed relationships. This is indicated by anomalous progressive strain curves for some fold limbs and hinges, and noncoaxial deformation histories in fold hinges.

23.

Bulk flexural flow behaviour to produce a class 3 form requires movement of material from the limbs to the hinges of these folds. Microstructure and mineralogic variations across the mudrock folds suggest that fabric modification was due to pressure solution and dependent gbs concomitant with a metamorphic dehydration reaction involving transformation of illite (+ Mg, Fe) chlorite. Thickness variations around folds, involving net transfer to fold hinges, is considered largely due to grain boundary sliding facilitated by "porewater" generated by the local reactions. Cleavage microfabrics, cleavage intensity mudrock mineralogy and progressive total strain in mudrock layer folds are considered relatable to inhomogeneous bedding-parallel shear across the folds. Buckling and limb rotation of adjacent enclosing limestone layers facilitated bulk flexural flow behaviour in the mudrock. REFERENCES Powell, C. Mc.P., 1979. Timing of slaty cleavage during folding of Precambrian rocks, northwest Tasmania. Bull. Geo. Soc. Am., 85; 1043-1060. Ramsay, J.G., 1967. Folding and Fracturing of Rocks. (McGraw Hill, New York), 568pp. Roberts, J.L., 1971. Abnormal cleavage patterns in fold hinge zones from Varanger, Peninsula, northern Norway. Am. J. Sci., 271: 170-190.

24.

CRACK-SEAL DEFORMATION AND THE DEVELOPMENT OF LAYER SILICATE PREFERRED ORIENTATION

S.F. Cox and M . A . Etheridge

Department of Earth Sciences, Monash University, Clayton, Victoria, 3 1 6 8 , Australia

The microstructures of layer silicates and associated phases developed in several examples of syntectonic veins and intragranular microfracture sites indicate that layer silicate (001) and grain shape preferred orientation can develop during crack-seal deformation by solution-assisted oriented growth mechanisms. During a crack-seal increment preferred orientation may develop in response to an interaction between anisotropic growth kinetics and the displacement history, resulting in preferential rejoining, by epitaxial overgrowth, of pulled apart grains having fast growth directions parallel to the incremental extension, direction across a microcrack. Preferred orientation m a y also be enhanced by epitaxial overgrowth of previously oriented grains in the microcrack w a l l . Such mechanisms of layer silicate preferred orientation development are likely to be very significant in developing and enhancing foliation during deformation involving dissolution/solution transfer/redeposition processes.

25. SYNTECTONIC VEIN AND FIBRE GROWTH ASSOCIATED WITH MULTIPLE SLATY CLEAVAGE DEVELOPMENT IN THE LAKE MOONDARRA AREA, MOUNT ISA, AUSTRALIA C.N- Winsor Geology Department, James Cook University, Queensland, 4811, Australia

Three folding events have been recognised within the Lake Moondarra area. Each produces a slaty type cleavage and regional folds. First deformation folds have an average eastwest axial plane orientation and are refolded by second and third generation folds. Overprinting criteria are not clearly developed for D 2 and D 3 folds. However Swager (1980) has found overprinting criteria at Mt. Isa which distinguish these events enabling them to be time differentiated in the Lake Moondarra area. This evidence is supported by the timing of veins related to D^ and D^ (see below) . Syntectonic quartz veins are abundant within the metasediments having orientations geometrically related to the axial plane of D^ and D^ folds. Six vein sets can be distinguished primarily on the basis of vein orientations. Two vein sets are developed perpendicular to each other and the S^ cleavage. Cross cutting relationships indicate that extension has occurred in these two directions synchronously. Fibres have grown parallel and normal to the S^ mineral elongation direction within the respective vein sets. It is inferred that the mineral elongation is the X direction of the strain ellipsoid for D2- A further vein set is developed parallel to the Scleavage.

Offsets between veins of this

set and the other two with fibres growing in the cleavage plane show that this set developed after the other two. These veins are believed to be a result of secondary elastic stresses within the cleavage plane (cf. Edelmon, 1973) . A further three vein set with similar geometrical relations as those described above are related to the D 3 folding event. Consistent dilational offsets between veins of different sets show that the three vein sets geometrically related to D 2 developed before the vein sets related to D . Fibres in some D veins show a syntaxial curvature. 3 ^ curvature is away from the orientation of fibres in D 2 veins towards the orientation of fibres in D 3 veins. It is apparent that as S 2 and S 3 are close in orientation, D veins could undergo further dilation at an appropriate time during-D . Some fibres within veins of the set related

This

to residual stresses in S 2 (i.e. fibres normal to the cleavage plane) are observed to curve towards the orientation of fibres that lie within S 3 and are normal to the D 3 mineral elongation direction. from a D

The curvature of fibres

to a D 3 extension direction is not believed indicative of progressive

deformation (cf. Helmstaedt and Dixon, 1980), as the curvature observed is between vein sets showing different geometrical relationships to the respective cleavage planes.

26.

Edelmon, N., 1973. Tension cracks parallel with the axial plane. Bull. Geol. Soc. Finl45_; 61-65. Helmstaedt, H., and Dixon, J.M., 1980. Superposed crenulation cleavages resulting from progressive deformation. Tectonophysics, 66; 115-126. Swager, C., 1980. A preliminary report on the microstructures Urquhart shales and "Recrystallized" shales with Cu-mineralization -J-20 Crosscut, 16A level, unpublished internal report, Mount Isa Mines Limited.

27.

DEVELOPMENT OF FOLIATIONS IN DEFORMED GRANITOIDS R.H. Vernon, R.H. Flood and W.F. D'Arcy School of Earth Sciences, Macquarie University, North Ryde, N.S.W. 2113, Australia

The deformation of several S-type and S/I transitional type granitoids in south-eastern Australia began with the formation of a gneissic foliation (Si), involving mainly deformation and recrystallizat of quartz and biotite. Locally a crenulation-style, mylonitic foliation (S2) developed, probably as part of the same deformation event. Several types of mineral aggregate are involved in the initiation of S2, and variations between different plutons have been observed. Locally the development of S2 is accentuated, with the formation of mylonite zones, in which vestiges of Sj locally can be recognized. Chemical differences between mylonites and less strongly deformed rocks may reflect changes during mylonitic deformation.

28.

THE ORIGIN OF FOLIATION IN A DEFORMED GRANITE

R.J. Holcombe Department of Geology & Mineralogy, University of Queensland, Brisbane, Qld. Australia

The Proterozoic Wonga Granite is a strongly foliated body outcropping in a long narrow belt near Mary Kathleen in northwest Queensland. The area around Mary Kathleen, including the granite, shows evidence of only a single, pervasive, ductile deformation event. Finite strain measurements in both the granite and surrounding rocks give maximum shortening values ranging from 65% to 80% in the flattening field. The dominant structural feature in the area is the regional foliation which, in the granite, is defined by a preferred dimensional orientation of both single grains and grain aggregates producing a vague lenticular layering in the rock leading to its description as a granite gneiss or a gneissic granite in the literature. In addition there is a much coarser layering on both a mesoscopic and macroscopic scale. This is defined by variation in texture and mineralogy and is subparallel to the main foliation. The foliation in the granite contains two distinct lineations. One is a biotite streaking defining an extension lineation. The other is defined by a vague compositional banding and by the long dimension of deformed xenoliths and schlieren. It is parallel to the hinges of rare folds in the broader layering in the granite. In outcrops where the granite is in contact with adjacent units this lineation is parallel to an intersection lineation in metasediments and metaporphyries. It is concluded that the second lineation in the granite foliation surface is an intersection lineation - implying that there must originally have been a pervasive layering in the rock. Furthermore, the parallalism of the intersection lineations in granite and metasediment implies that the pervasive layering was horizontal. It is suggested that the granite was originally a sill complex intruded into a sedimentary/acid porphyry pile and that the principal contribution to the observed foliation is a transposition layering derived from an original pervasive flow banding.

29.

APPLICATION OF INTERACTIVE COLOUR GRAPHICS TO THE DISPLAY AND INTERPRETATION OF STRUCTURAL DATA P.H. Cohen Geology Department, University of Adelaide, S.A., 5000, Australia

The analysis of the structural geometry of an area includes a number of time consuming processes: (1) the compilation of a structure map, (2) stereographic plotting and analysis of structural data, (3) subdivision of the map into subareas, with modifications based on the results of (2) above, and (4) presentation of diagrams. These processes have been automated to greatly speed up structural analysis. This paper describes part of the automated, process - which is the ability to rapidly define subareas in a map, analyse these subareas using both stereographic plots and a technique modified after Sander's A.V.A. process (Sander, 1970) , and finally to be able to alter at will the subarea boundaries, based on these analyses (see figure). The method of analysing areas by subdivision is conducted by the user interactively controlling the flow of the analytical process. This is achieved by using a "light pen" coupled to a colour graphics display unit. The system is run on a minicomputer. Defining a subarea boundary on a displayed map is achieved by drawing the boundary on the display with the use of the light pen. Data files for the subarea are made from the map's data files, and these can then be analysed using the stereographic plotting program and a technique, described below, termed SODA (Spatial Orientation Distribution Analysis). The subarea boundary can further be modified with the light pen and the process repeated until the user is either satisfied with or rejects the subarea. Any map or subarea can be used as a source map for subdivision. The choice of subarea boundaries is made by using the SODA technique, of which the A.V.A. process is a subset. This technique firstly polygonises the map with regular (Thiesen) polygons so that there is only one station location containing orientation data within each polygon. These polygons are then shaded in colours according to the orientations of the structural elements contained within them. The colours used can be chosen by the user. In this way regions of orientations (e.g. different fold limbs) can be distinguished by different colours. The result of the application of interactive colour graphics in the way described above is that structural analyses which may have taken days to carry out can be achieved in a few hours. REFERENCE Sander, B., 1970: An Introduction to the Study of Fabrics of Geologic Bodies. Authorised translation by F.C. Phillips and G. Windsor, (Oxford Pergammon Press) 641 pp.

30.

START

OISPLAY MAP

EXTRACT SUBAREA

NO STOP

YES

STEREONET

FINISHED v

ANALYSIS

DELETE

S.O. D. A.

SUBAREA NO YES

^SAVE ^ SUBAREA

YES

SUBAREA y O K ? > NO MODIFY SUBAREA

31.

HIGH AND LOW TEMPERATURE PLASTIC DEFORMATION IN PERIDOTITES FROM OPHIOLITES : IMPLICATIONS FOR FLOW IN OCEANIC MANTLE

A. Nicolas Laboratoire de Tectonophysique, Universite de Nantes, France

Detailed structural studies have been conducted in the peridotites and gabbros of several ophiolite massifs (Antalya, Turkey ; Cyprus ; Bay of Islands, Newfoundland ; Zambales, Philippines ; New Caledonia and Oman) and comparison made with specimens dredged and drilled in the oceans. Special attention has been paid to mapping the foliations and lineations in the tectonic peridotites underlying the mafic cumulates. The contact surface between these two units is considered as the paleo moho in the oceanic crust from which the ophiolite is derived. Once this surface is rotated to the horizontal, the structural pattern in the tectonic peridotites has been restored to its genuine orientation and a regional plastic flow pattern can be proposed. Two distinct flow conditions are easily recognized in many ophiolites. The first one, present in all massifs, concerns the main body of peridotites from the contact with cumulates downwards. It corresponds to high temperature, locally hypersolidus T conditions and therefore is ascribed to asthenospheric flow beneath an oceanic spreading center (figure, locations A and B). Depending on the investigated massifs, the geometry of flow evokes either a flat flowing asthenosphere (flow lines parallel to isotherms in the mantle) or a diapiric intrusion. In many massifs, the S-L structures associated with this flow are progressively replaced by new S'-L' structures within 1-2 km from the basal contact. This basal contact which is parallel to the peridotite cumulates surface and was therefore initially subhorizontal is also underlined by an inverse metamorphic aureole developed in the formations beneath the peridotites. At the contact with peridotites, conditions of amphibolite to granulate facies (800°C) are fulfilled. The plastic flow structures in the basal peridotites are parallel to those in the metamorphic rocks and to the contact plane. Indeed the microstructures in these peridotites are indicative of comparatively low T conditions along with extreme strain (mylonite facies). This new deformation is ascribed to an oceanic thrusting (figure location C) in a compressive environment. The facts are well explained by a model of thrusting of a very young oceanic lithosphere in a subduction zone environment.

32.

GABBROS

DUNITES

ASTHENOSPHERlC DEFORMATION (Temperature 1000-1300°C Stress: 200- AOObors)

HARZBUR6ITES

SPREADING CENTER

i © ^M£H|BOUTES

LITHOSPHERIC DEFORMATION (Temperature 700-900° Stress ~ lOOObors) OCEANIC TRUSTING

Trench

Ridge

SlEiiillllliaiSSXI H i t ' Mantle ^ ^ / / / / ^ ^^

Lithosphere Asthenosphere

Possible oceanic environment for the flow structures described in ophiolitic peridotites. Letters A, B and C refer to specific loci in the ophiolite log.

33.

THE DEFORMATION OF DUNITE P.N. Chopra Research School of Earth Sciences, Australian National University, Canberra, A.C.T. 2600, Australia

Deformation experiments have been carried out on two dunites (Anita Bay, of 100 ym grainsize, and Rheim, of 900 ym grainsize) at strain rates from 10 3 to 10~6/s and temperatures from 1000°C to 1300°C in a gas-medium deformation apparatus at 300 MPa confining pressure. For tests carried out under wet conditions, defined by the presence of small amounts of water from hydrous minerals initially present, constant strain rate and stress relaxation experiments show that there is a change in flow law in going below about 100 MPa differential stress, and that the coarser-grained rock is stronger than the finer-grained one. Power law parameters above the transition are n = 4.48 ± 0.31 and Q = 498 ± 38 kJ/mol for Rheim dunite and n = 3.35 ± 0.17 and Q = 444 ± 24 kJ/mol for Anita Bay dunite, while below the transition, relaxation tests on Anita Bay dunite give n = 2.44 ± 0.18 and Q = 386 ± 27 kJ/mol. Exgeriments with specimens of both rocks which were firstly dried at 1200 C under controlled oxygen fugacity conditions yield higher flow strengths and activation energies and n's than do their wet counterparts. In contrast to the wet experiments, an inverse dependence of flow strength on grain size has been observed for these specimens. It is concluded that there is a weakening effect of water, that this effect is mainly in the grain boundaries and that the species responsible can be characterised by the broad absorption of infrared radiation in the 3 ym region.

34.

DEFORMATION OF PERIDOTITE IN THE MANTLE AND TRANSPORT OF XENOLITHS TO THE SURFACE H.W. Green II, Department of Geology, University of California, Davis, California 95616 U.S.A.

The deformation substructure of olivine in peridotite xenoliths from the mantle indicates that extensive plastic flow at rather high stresses (up to perhaps 100 MPa) was occurring in the mantle at the time of xenolith extraction by magmas of kimberlite or alkalic basalt. The lack of appreciable recovery in many of these xenoliths requires that they reached the surface in at most a few days. The olivine crystals of these xenoliths characteristically contain bubbles of CO^. Primary bubbles are precipitates from solid solution and generally are smaller than 0.1 ]im diameter. They sometimes are accompanied by (111) platelets of chrome spinel exsolved in the (100) plane of olivine. Secondary bubbles representing healed cracks are frequently several microns diameter and yield pressure determinations up to about 1.0 GPa. Precipitation of primary bubbles takes place in the mantle during dynamic recrystallization in (spinel) peridotites from basalt, but precipitation takes place during transport to the surface in (garnet) periodotites from kimberlite. Decompression during eruption results in punching out of prismatic dislocation loops around the primary bubbles. Trains of such loops commonly are transformed into helices by interaction with screw dislocations; loops punched at the highest temperatures may show effects of dislocation climb, whereas those produced later in the eruption show no such effects. Precipitation can be a continuing process and these early loops and helices may themselves be decorated with bubbles. C09filled microcracks develop relatively late in the eruption, probably due to bursting of primary bubbles. These cracks knit themselves back together leaving arrays of bubbles whose internal pressures reflect the depth of crack healing. Further decompression results in punching of loops from secondary bubbles. In xenoliths from basalt these late loops are the last substructural feature produced, but in xenoliths from kimberlite they are followed by extensive inter-and transgranular cracking and serpentinization. No serpentine is found on primary or secondary bubble surfaces, suggesting that the H 2 0 content of the fluid phase was low. Primary bubbles in olivine can approach 0.1% by volume and represent a significant CO2 solubility at high pressure. The change in point defect populations implied by appreciable solution of a seemingly incompatible component could have a significant effect on the flow properties of olivine. We currently are pursuing this question in my laboratory.

35.

MECHANISMS OF DEFORMATION OF OLIVINE

B.E. Hobbs Monash University, Department of Earth Sciences, Clayton, Victoria, 3168, Australia

T h e m a n n e r in w h i c h t h e d e f e c t c h e m i s t r y o f a m a t e r i a l i n f l u e n c e s t h e m e c h a n i c a l p r o p e r t i e s p r o v i d e s a p o w e r f u l t o o l for e s t a b l i s h i n g t h e p r e c i s e m e c h a n i s m s o f d e f o r m a t i o n in t h a t m a t e r i a l . T h i s p a p e r p r o v i d e s a n e x a m p l e for i r o n b e a r i n g o l i v i n e ; it is shown t h a t the a v a i l a b l e d a t a p o i n t t o a m e c h a n i s m i n v o l v i n g t h e f o r m a t i o n and m o t i o n of n e g a t i v e l y c h a r g e d jogs c o m b i n e d w i t h s i l i c o n d i f f u s i o n b y a v a c a n c y m e c h a n i s m as t h e rate controlling process. T w o g r o u p s o f e x p e r i m e n t s in r e c e n t y e a r s e n a b l e t h e m e c h a n i s m o f d e f o r m a t i o n o f o l i v i n e to b e s t r o n g l y c o n s t r a i n e d . O n e g r o u p o f e x p e r i m e n t s (Hornach and K o h l s t e d t , 1979; P o u m e l l e c et al., 1980) h a s demonstrated a creep rate increasing with a ^

w h e r e a is t h e t h e r m o d y n a m i c

a c t i v i t y . T h e o t h e r g r o u p (Ricoult and K o h l s t e d t , 1980) h a s shown t h a t the c r e e p r a t e i n c r e a s e s r a p i d l y a s a ^ is i n c r e a s e d o r as is d e c r e a s e d a t c o n s t a n t a ^ ; in t h e s e l a t t e r e x p e r i m e n t s t h e r e is a t e n fold c h a n g e in

s t r a i n r a t e for an o r d e r o f m a g n i t u d e c h a n g e in a ^ o r for a b o u t two o r d e r s o f m a g n i t u d e c h a n g e in ^ A n a l y s i s o f t h e d e f e c t c h e m i s t r y of o l i v i n e l e a d s to t h e f o l l o w i n g c o n c l u s i o n s (assuming t h a t e l e c t r i c a l n e u t r a l i t y in o l i v i n e is m a i n t a i n e d b y 2 [ V " = F e . ]) Mg Mg (i) The creep rate dependences on a . a . and a ^ a r e Mg Si O2 n o t c o n s i s t e n t w i t h a c r e e p m e c h a n i s m w h i c h c o n s i s t s s o l e l y of d i f f u s i v e p r o c e s s e s as r a t e c o n t r o l l i n g . ii)

T h e c r e e p r a t e d e p e n d e n c e on a ^

is c o n s i s t e n t w i t h

a c r e e p m e c h a n i s m in w h i c h p o s i t i v e l y c h a r g e d k i n k s a r e r a t e c o n t r o l l i n g . The dependence on a a n d a _ . is in t h e w r o n g sense for t h i s m e c h a n i s m . ^ Mg Si H e n c e a k i n k c o n t r o l l e d m e c h a n i c s m is r u l e d o u t . iii) O f the v a r i o u s m e c h a n i s m s i n v o l v i n g jogs p l u s d i f f u s i o n as t h e r a t e c o n t r o l l i n g p r o c e s s , o n l y t h a t i n v o l v i n g n e g a t i v e l y c h a r g e d jogs a n d s i l i c o n d i f f u s i o n b y a v a c a n c y m e c h a n i s m is c o n s i s t e n t w i t h t h e o b s e r v e d c r e e p r a t e d e p e n d e n c e s o n a M g , a g i and a Q ^ . T h i s m e c h a n i s m p r e d i c t s c r e e p r a t e s p r o p o r t i o n a l to a Q

/ e

, a ^

agi

/ l 2

36.

One conclusion from this analysis is that the activation energy for creep of olivine is made up of two parts; approximately 80-90 kcal/mol for the energy of formation, ionization and migration of a silicon vacancy and approximately 50 kcal/mol for the energy of formation, ionization and migration of a negatively charged jog. A second conclusion follows from the strong dependence of creep rates on a and a . . It is difficult with only solid phases present to Mg Si change these activities by more than one or two orders of magnitude. However, with even small melt fractions available larger variations are possible and hence even more dramatic changes in creep rates are possible once a small amount of melt phase is introduced. Thirdly, the creep rates need to be corrected for changes in a .. a , a , a before activation energies are calculated from creep Si Mg O2 Fe data. This is capable of explaining the wide variation in activation energy data obtained from experiments at the present time. REFERENCES Hornack, P. and Kohlstedt, D.L. 1979. The effect of oxygen partial pressure on the creep of olivine. EOS, Trans. Am. Geophysc. Union, 60: 369. Poumellec, M., Jaoul, 0., Froidevaux, C. and Havette, A., 1970. Silicon diffusion in forsterite: A new constraint for understanding mantle deformation. Paper presented to Symposium on Anelastic Properties and Related Processes in the Earth's Mantle, XVII General Assembly, Int. Union. Geol. Geophys. December 1979 Ricoult, D. and Kohlstedt, D.L. 1980. High temperature deformation of olivines effects of changing point defect chemistry. EOS, Trans. Am. Geophys. Union, 61: 403.

37.

METHODS FOR THE DETERMINATION OF DEFORMATION HISTORY FOR CHOCOLATE TABLET B0UDINA6E WITH FIBROUS CRYSTAL INTERGROWTHS M. Casey, D. Dietrich and J.G. Ramsay Geologisches Institut der ETH, CH 8092 Zurich, Switzerland

When chocolate tablet boudinage contains fibrous growths in the boudin necks the curvature of the crystal fibres offers the potential of tracing the sequence of break-up of the rigid layer and determining the relative displacements and rotations of rigid blocks and fibrous material. Three methods of geometric analysis of the structure are presented and their relative merits are assessed. The first method is based on the method for the determination of incremental and finite strains in pressure shadows devised by Durney and Ramsay. For the second method isogon plots of the curved fibres are first prepared and then traverses are taken across the plot to determine the total fibre length in each direction of extension. This method then continues in the same manner as the Durney and Ramsay method with the advantage that irregularities caused by uneven separation of the blocks are averaged out. The third method is dependent on the material being of high quality. The geometry of the rigid objects and the fibrous material is defined by means of a digitizing table and the relative displacements and rotations of rigid fibrous material and adjacent rigid boudins are determine as smooth curves for the complete history of separation. A deformation history for the matrix is then found which would account for all the displacements and rotations according to an assumed interaction behaviour, based on fibre loading theory, of the matrix and rigid plates. The three methods are applied to natural examples and to simulations and their relative merits are assessed.

38.

DILATANCY AND THE ANGLE OF OBLIQUITY OF EN ECHELON FRACTURES D.W. Durney Macquarie University, N.S.W. 2113, Australia From the geometrical boundary conditions for a planar zone of deformation between two blocks of undeformed material (an "ideal" shear zone) it can be shown that dilation or volumetric strain, as well as shear, is allowed in the deformation inside the zone. The deformation in such zones is thus generally a "dilational shear" and possesses a degree of freedom greater than that of simple shear. New theoretical relations are given which describe infinitesimal dilational shear deformation in terms of the components of dilation and shear strain. The infinitesimal strain relations for a single zone imply a dependence of the directions of propagation of minor en echelon structures in zones of sheared isotropic rock on the ratio of the increment of shear (6y) to the increment of dilation (6A_) operating at the time of propagation, a factor which expresses inverse dilatancy, according to the relation tan 20 = - 5y/6A. Usually only the latest directions can be deduced from field observations of these structures. Examples of en echelon extension fractures described in the literature and observed by the writer commonly appear to display late propagation directions at < 45 to the zone, which indicates positive dilation during late increments of strain.

39.

SEQUENTIAL PORPHYROBLAST GROWTH AND CRENULATION CLEAVAGE DEVELOPMENT DURING PROGRESSIVE DEFORMATION M.J. Rubenach and T.H. Bell James Cook University, Townsville, Qld. 4811, Australia

Six stages of crenulation cleavage development during D 2 can be recognised in both matrix and porphyroblast inclusion trails in the Robertson River Metamorphics, N.E. Australia. These stages progress from layer parallel shortening, crenulation and differentiated crenulation of Si through development of differentiated crenulation cleavage (S2), differentiated schistosity (S2) and homogeneous foliation (S2) . These rocks underwent prograde metamorphism during D 2 and the chloritoid garnet, staurolite, andalusite, and sillimanite isograds have been mapped. Most rocks are rich in porphyroblasts which commonly contain well defined inclusion trails. The geometry of these trails varies from one mineral type to the next, depending on the timing of porphyroblast growth relative to the stage of crenulation cleavage development in the matrix of that specimen. The deformation history involved progressive bulk inhomogeneous shortening and the strain is very heterogeneous on all scales, partly as a consequence of this. Hence the 6 stages of crenulation cleavage development were locally developed at various times during D 2 and even if the porphyroblast grew late in the deformation, it could sometimes locally overgrow very early stages of crenulation cleavage development. The temperature increased during metamorphism in any one locality and consequently some porphyroblasts overgrew others, others which were unstable in the new conditions, preserving them from destruction. Hence the precise timing of reactions (as deduced from isograds and mineral chemistry) relative to the stages in schistosity development can also be determined. For example, in the andalusite zone the sequence of porphyroblasts, in decreasing order of age, is biotite-garnet-biotitestaurolite-andalusite. Dissolution of garnet occurred during growth of staurolite and especially andalusite growth, while dissolution of staurolite accompanied andalusite growth.

40. THE USE OF PRESSURE SHADOW FIBRES TO DETERMINE FINITE STRAIN AND DEFORMATION HISTORY IN ROCKS M. Casey, D. Dietrich and J.G. Ramsay Geologisches Institut der ETH, CH 8092 Zurich, Switzerland

Pressure shadows around rigid objects, which in the deformed rocks of the Helvetic Nappes consist of quartz, calcite and chlorite fibres around pyrite crystals or aggregates, are very important features because they record the develpment of the finite strain during deformation episodes. Methods of geometric analysis of pressure shadows rely on assumptions about fibre growth, behaviour of the matrix, and interrelationships between the growing fibres and the deforming matrix. To explore the consequences of these assumptions two extremes of deformation behaviour are considered for the fibres. In the first the fibres are assumed to deform homogeneously with the matrix and in the second they are assumed to be perfectly rigid. Both models assume that the fibres grow at the surface of the rigid object. Simulated fibre geometries are presented for each model of behaviour and various deformation histories. The simulation models are used to devise corresponding methods for determing strain histories from natural examples of pressure shadows and results of the methods applied to material from the Western Helvetic Nappes and to the simulated geometries are presented. The finite strains and deformation histories obtained from the Western Helvetic Nappes are related to the finite geometry of the nappes and to proposed models of their kinematic development.

41.

LOG-POLAR R f /0 f ANALYSIS OF STRAIN AND DEPOSITIONAL FABRIC, WITH APPLICATIONS TO SOME OOLITIC AND PELETAL LIMESTONES D.W. Durney and R. Khaiami Macquarie University, NSW 2113, Australia

Procedures are discussed for analysing total (compactional and tectonic) finite strain (R , 0 ) and initial shape fabric (R., 0.) s s —i —i from sections through ovoid objects, using combined R /0 (Dunnet, 1969; —T — Dunnet and Siddans 1971) and Polar Graph (Elliot, 1970) methods. Plotting and interpretation of the measured R f (final axial ratio) and 0 f (final orientation) data are made simpler by a logarithmic - polar R f /0 f graph and standard Ri curves. Interpretation of log-polar initial fabric data is assisted by a simplified 3-dimensional depositional fabric model which uses initially spheroidal particles of uniaxial ratio U , distributed with frequency to and aligned (with respect to uniform) according to the piano-linear transformation I?. A common feature of the resulting log-polar density diagrams is the occurrence of a central (FL = 1) maximum and a restricted peripheral (R. 1 conditions of + J

max u ) limit; features which persist for various R and section orientation and which arise even in ur uniform orientation distributions of homogeneous prolate and oblate particle populations. Similar fabric features are observed in undeformed specimens of oolitic and peletal limestone where FL max ranges from < 1.3 to > 10. This suggests a combination of density mode, obtained with a moderately large counting circle or grid, and FL envelope as the most promising

estimators of R. = 1 (and hence of strain in deformed specimens) , while —l quartiles and vector means are useful for defining initial fabric symmetry and intensity. However, R = 1 is estimated to within A R = 0.05 by quartile division for particles of low initial ellipticity (ooliths), and this requires a sample size of only 50-100. Particles with higher initial ellipticity (superficial ooliths, pellets, abraded fossil fragments) display broader scatters with increasing tendency towards bedding preferred orientation, which requires a larger sample size, but bedding symmetry is more distinct. Some deformed fabrics of oolitic and peletal limestone, including particles with differing relative competency, and two samples of deformed fossils with restricted, sub-uniaxial initial shapes (belemnites and echinoids) are discussed in the light of the natural and model initial fabrics.

42.

REFERENCES Dunnet, D., 1969. A technique of finite strain analysis using elliptical particles. Tectonophysics, 1\ 117-136. Dunnet, D., and Siddans, A.W.B., 1971. and their modification by strain.

Non-random sedimentary fabrics Tectonophysics, 12^: 307-325.

Elliott, D., 1970. Determination of finite strain and initial shape from deformed elliptical objects. Geol. Soc. Am. Bull., 81: 2221-2236.

43.

CHANGES IN DEFORMATION MECHANISMS AND SOFTENING PROCESSES WITH CRUSTAL DEPTH S.H. White, J.C. White and D.C. Johnston Geology Department, Imperial College, London S.W.7, U.K.

Attempts at quantifying deformation within major fault zones have mainly been limited to a consideration of monominerallic rocks. They have also concentrated on either the totally brittle or totally ductile regimes. The intermediate zone, viz. the area of the ductile-brittle transition, has been largely avoided or if avoidance is neither practical nor politic, simply referred to as a 'grey area of complexity'. However, in seismically active fault zones, it is in this area that most seismicity is located. In this contribution deformation in the 'grey area' w i l l be considered using the Alpine Fault Zone in New Zealand as a model. Additional data from the Moine Thrust Zone and from experiments will be used. It will be argued that the fault rock distribution across the Alpine Fault Zone is a depth profile and can be used to deduce changes in deformation mechanism and softening process with depth. Detailed studies of the Alpine Fault Zone mylonites indicate that cataclastic deformation may extend as deep as amphibolite facies conditions. Pressure solution and dislocation processes are imprinted on the cataclasites; pressure solution is particularly important in the region between 10 15 km depth with dislocation processes becoming dominant at between depths of greater than 20 km. The zone between 10 to 20 km is one in which microstructural evidence suggests a cyclicity in processes: cataclasis-pressure solution and dislocation mechanisms. This is in agreement with seismic data, namely a high stress phase, relaxing to a low stress and increasing again to rupture. The cataclasis produces a 'cataclastic' microstructure which is healed and converted into a mylonitic one by pressure solution. Dislocation processes then impart a crystallographic fabric on the mylonite grains. Softening processes must occur to locate the deformation in a limited zone. No single process predominates. The dominant processes recognized are reaction enhanced ductility and fabric softening in the 10 - 20 km zone and these plus shear heating at greater depths.

44. THE EXPERIMENTAL DEFORMATION OF FLINT D.H. Mainprice and M.S. Paterson

Research School of Earth Sciences, Australian National University, A.C.T. 2600, Australia

The rheology of flint has been studied in the temperature range 500 to 1000°C at a confining pressure of 300 MPa using a gas confining medium deformation apparatus. In constant strainrate deformation/ at strain rates from 10 3 to 10 5 s stress-strain curves characteristic of hot working have been observed. An initial yield drop followed by a constant flow stress at faster strainrates gives way to cyclic stressstrain behaviour at slower strainrates. The hot working regime above the critical weakening temperature of approximately 500°C can be described by an empirical exponential law of the form, &

K exp (-Q/RT)exp(a/aQ)

where e is the strainrate (s

, K is a constant (s •), Q is the apparent

activation energy (kJ mole 1), R is the gas constant, T the absolute temperature, a is a constant (MPa) and a is the differential stress (MPa). Least squares analysis of the constant strainrate data yields the values = 69.0 ± 6.6 for of log K •= -1.61 ± 0.32, Q = 108.5 ± 15.2 and a the constants where the errors are quoted at 95? cofidence level. Infra-red spectroscopy shows that flint contains about 100,000 H/106Si ("1-2 wt%) in the "gel1 form. However it is inferred from the value of the critical weakening temperature that only 1% of the available hydroxyl is effective in the hydrolytic weakening of flint. The activation volume, V*

2.303 KT

3 log £ 3a

where K is Boltzmann's constant, determined from constant strainrate and stress relaxation tests suggests that dislocation motion is limited by Peierls-Nabarro lattice friction near the critical weakening temperature and by a point defect interaction mechanism in the hydrolytically weakened state. At low stresses, stress relaxation testing has revealed a change in flow law from a hot working type to a strainrate sensitive regime which can be described by an empirical power law of the form e

A exp (-Q/RT)on

where A is a constant (s 1 MPa n ) and n is the stress exponent (dimensionless) Least squares analysis yields the values of the constants as log A = -4.87 ± 0 Q - 64.2 ± 7.3 and n = 1.2 ± 0.1 at a specimen strain of "22%. The strainrate sensitive regime exhibits a strain dependent behaviour which is shown to be a function of the volume fraction of recrystallized grains.

45.

Ductile shears of recrystallized grains are present in many of the specimens deformed at the faster strainrates, even if no significant strain softening is evident in the stress-strain curve. The fabric in homogeneously deformed axially symmetric specimens, as revealed by X-ray analysis, has a maximum parallel to and a subsidiary maximum normal to the compression direction, whereas optical flat stage analysis indicates that the maximum is rotated in the ductile shears. Using the image matching technique the pressence of 1/3 < a >(c) and 1/3 < a >(r) slip systems has been established by T.E.M. The presence of 'growth accidents' in the form of Brazil twins and high local dislocation densities near grain boundaries confirm the importance of grain boundary migration during hot working. The present study has implications for the development of quartz mylonites.

46.

THE SCIENCE OF ANALYTICAL ELECTRON MICROSCOPY APPLIED TO DEFORMED GEOLOGICAL MATERIALS

J.D. Fitzgerald Research School of Earth Sciences, Australian National University, Canberra, A.C.T. 2600, Australia

Conventional EM methods have consistently proven to be powerful tools for investigating deformed materials. To date, most geological studies have required largely qualitative conventional TEM observations of micro-textures. However, with the new generation of techniques and instrumentation developed for materials science applications the potential for collection of quantitative microfabric data has been realized. Many avenues to aid the identification and understanding of fabrics have been thrown open. 1. Characterization of vectors of stacking faults and microtwins. Results from feldspars to be presented. 2. Characterization of dislocations, either unbound or present in low density arrays 1) Geological studies of dislocation densities - to date consistent results are sparse; b) Confirmation of dissociation important implications for dislocation - controlled deformation; c) 1 Positive identification of dislocation Burgers vectors - comprehensive image matching has not been spectacularly successful in silicate minerals. Considerations of the symmetry and stability of dislocation images may allow more rapid determinations. Results from olivine deformed at T>1000°C in a gas-medium apparatus to be presented. 3. Intergranular relationships - Grain boundary misorientation, thickness and dislocation content currently fascinate materials scientists. Data gathered using TEM are now used to interpret the role of grain boundaries in deformation. Some results are available from important mineral groups. 4. Fabric development - Sample preparation and analytical techniques developed for EM as applied to experimentally deformed thick samples facilitate quantification of some aspects of deformed fabrics. Selected area electron channelling patterns in calcite and grain boundary sliding measurements in marble and dunite are to be presented. 5. Accurate chemical analyses of minute volumes using x-rays produced in the EM. New but expensive developments signal that this technique is now ready for application to geological problems.