Abstracts No.11: State-of-the-art symposia: Fractures and veins & accretionary prisms, 1983, Sydney

Geological Society of Australia

ABSTRACTS Number 11

FRACTURES AND VEINS AND

ACCRETIONARY PRISMS

State-of-the-art symposia Sydney, 1983

GEOLOGICAL SOCIETY OF AUSTRALIA ABSTRACT SERIES NUMBER 11

FRACTURES AND VEINS:

THEIR FORMATION

AND ECONOMIC IMPORTANCE and ACCRETIONARY PRISMS:

CHARACTERISTICS

AND PROCESSES

STATE-OF-THE-ART SYMPOSIA UNIVERSITY

OF

SYDNEY

1984

SPONSORED BY THE SPECIALIST GROUP IN TECTONICS AND STRUCTURAL GEOLOGY AND THE NEW SOUTH WALES DIVISION OF THE GEOLOGICAL SOCIETY OF AUSTRALIA

ACKNOWLEDGEMENTS The symposia "Fractures and veins: their formation and economic importance", and "Accretionary prisms: characteristics and processes", are supported financially by the following companies: Aberfoyle Exploration Pty. Ltd. Anaconda Australia Inc. The Broken Hill Proprietary C o . Limited Esso Australia Ltd. Consolidated Gold Fields Australia Ltd. Western Mining Corporation Limited. Financial support was also given by: Australian-American Educational Foundation Australian Academy of Science - 25th IGC Fund Geological Society of Australia Inc. Logistical support was provided by: Geological Survey of New South Wales Macquarie University University of Sydney The Earth Resources Foundation. The convenors thank all these companies and organizations for their generous assistance.

2.

3.30 p.m.

4.00 p.m.

A. J. Mauger, J. W. Creasey, J. F. Huntington (CSIRO Mineral Physics). Fracture patterns of the Sydney Basin and their economic significance. Afternoon Tea

Session 4s

Chairman - N. L. Markham

4.30 p.m.

R. A. Glen^ J. Pogson, E. Scheibner (New South Wales Geological Survey) . Landsat fracture systems aroiind Cobar and economic implications.

5.00 p.m.

B. Marshall (New South Wales Institute of Technology). Lineament - ore relations.

5.30 p.m.

E.S.T. O'Driscoll (Western Mining Corporation). Structural signatures to mineralization.

6.00 p.m.

Refreshments

3.

Joints: Their Initiation^ Propagation, and Role of Abnormal Fluid Pressure: Examples from the Appalaohian Foreland, Terry Engelder, Lamont-Doherty Geological Observatory, Palisades, New York 10964 U.S.A. Planar discontinuities within the crust of the earth may form in one of two common manners: by shear and by extension (Price, 1966; Stearns, 1968). Those formed by the former mechanism are known as a shear fractures and are the same as small faults. Discontinuities formed by the latter mechanism are known as joints and are recognized primarily by lack of shear offset parallel to the planar discontinuity. In the upper crust joints are far more common than shear fractures except in the vicinity of fault zones. Hence, properties of the upper crust such as permeability and stiffness are controlled by joints, their orientation and density. Joints propagate in the plane of the greatest principal tensile stress when that stress exceeds the tensile strength of the rock. For this reason joints may be used to map paleostress fields (Engelder and Geiser, 1980) and the contemporary tectonic stress field (Engelder, 1982). During the history of burial, lithification, deformation, and denudation of clastic rocks of the Appalachian foreland four types of joints may develop; here named: Tectonic, Hydraulic, Unloading, and Release. Tectonic and hydraulic joints form at depth prior to uplift in response to abnormal fluid pressures, whereas unloading and release joints form near the surface in response to thermal and mechanical stresses accompanying erosion and uplift. Tectonic joints are distinguished from hydraulic joints in that tectonic compaction is a mechanism for achieving abnormal pore pressures leading to the propagation of the former whereas compaction by overburden loading leads to the abnormal pore pressures in the latter case. The orientation of tectonic joints (cross-fold joints) indicates that they formed during the Alleghanian Orogeny. Evidence for abnormal pore pressure is indirect and comes from the sequence of joint formation in sandstone-shale interlayers. Propagation of joints in sandstone layers occurs first and stopped by the shale layers whereas joints initiate in the shales later and propagate across the sand-shale interface into the sandstones. Plumose markings are common on these joints. Release joints (strike joints) post-date the Alleghanian Orogeny as they butt the tectonic joints within the deeper parts of the Devonian clastic section. These joints have a less regular appearance and their surfaces show no plumose markings. Unloading joints form during the final stages of erosion when contraction is extreme. The orientation of the unloading joints is controlled by the contemporary stress field, which differs from that accompanying the Alleghanian orogeny. REFERENCES Engelder, T., 1982, Is there a genetic relationship between selected regional joints and contemporary stress within the lithosphere of North America?, Tectonics, 161-177 Engelder, T. and Geiser, P., 1980, On the use of regional joint sets as trajectories of paleostress fields during the development of the Appalachian Plateau, New York: Jour. Geophys. Res., 85, 6319-6341. Price, N.J., 1966, Fault and joint development in brittle and semi-brittle rock: London, Pergamon Press, 176 p. Steams, D.W. , 1968, Certain aspects of fracture in naturally deformed rocks: in NSF Advanced Science Seminar in Rock Mechanics, Riecker, R.E., ed., Special Report, Air Force Cambridge Research Laboratories, Bedford, Mass., p. 97-116.

4.

S O m APPLICATIONS OF FRACTURE MECHANICS TO THE STRUCTURAL CONTROL OF MINERALIZED VEINS Michael A. Etheridge Bureau of Mineral Resources, G.P.O. Box 378, Canberra 2601

Mineralized veins are natural rock fractures filled with minerals precipitated from hydrothermal solutions that pass through the fractures. In order to understand vein formation it is therefore necessary to have a sound knowledge of (i) rock fracture processes and mechanics, (ii) flow of fluid to and through the fracture, and (iii) the chemistry of hydrothermal fluids and precipitation reactions from them.

This talk will concentrate

on the first of these points to provide a basis for understanding vein geometry, and will deal more briefly with fluid access, permeability, fluid volumes and the precipitation process.

1.) The Fracture Process - Macroscopic fracture (failure) takes place by the propagation of microscopic flaws (microcracks).

The

energetics and mechanics of microcrack propagation under an applied stress were first developed by Griffiths (1924), and subsequently enlarged upon by a number of other workers (see Jaeger and Cook, 1976; Paterson, 1978 for comprehensive reviews).

Means, 1976;

Failure results in two broad

categories of fracture - extension fracture and shear fracture, and there are simple relationships between fracture geometry, the magnitude and orientation of the principle stresses and displacement across the fracture (Fig. 1).

The fracture process, particularly as it applies to the formation

of veins at significant depth within the earth, is best illustrated by the Mohr Diagram (see Means, 1976 for a full explanation).

The application

of the Mohr diagram to hydrothermal vein formation will be outlined (Fig. 2), and it will be shown that very high pore fluid pressures are essential for the majority of veins (Secor, 1975, 1969;

Phillips, 1972).

Evidence

for such high fluid pressures will also be briefly summarized (Fig. 3).

2.)

Fracture and vein geometry - Fracture orientations are

controlled by the orientations and magnitudes of the principal stresses. However, the stress field is difficult to determine in rocks and we rely on the less direct but similar relationships between fracture geometry and the strain or displacement fields.

Displacement fields are directly

i

5.

reflected by the geometry of rock structures such as folds, faults, foliations, lineations, shear zones, etc.

Therefore, predictively useful

analysis of vein geometries requires determination of the relationship between veins and synchronous rock structures.

Basically, fracture will

take place in a deforming rock body where o^ is least and when a^ drops below a critical value.

A number of examples of the relationship between

the orientation/distribution of veins, the geometry of other rock structures and the displacement field are illustrated in Fig. 4, and will be discussed in this and other talks today.

3.)

Fluid Migration and Fluid Volumes - This is a complete topic

in itself, and consequently only a few key aspects will be touched on here. (i)

At all but the shallowest depths, open fractures that are

potential veins can only form at extreme pore fluid pressures ( P ^ ^total^' Such high fluid pressures open pores and microcracks to significantly enhance permeability, enabling easy fluid migration from the rock porosity to the fracture and vice versa (Etheridge et al., 1983). (ii)

The volume of fluid required to precipitate a vein fill is

generally several orders of magnitude larger than the volume of the vein fill.

Extensive fluid throughput is therefore required to form all but

the narrowest veins. (iii)

At depth it is difficult to hold a fracture open and

pass substantial fluid volumes through it.

It is therefore likely that

most veins form, at least in part, by successive fracturing and healing by precipitation.

Evidence for this so-called crack-seal process will be

outlined, and the resultant vein textures discussed (Ramsay, 1980; Cox and Etheridge, 1983). (iv)

Because of the above, vein formation is intimately related

to fluid migration paths, fluid pressure variations and long-term access to large fluid volumes.

Both large and small scale convective motion of

hydrothermal fluid will be locally controlled by lithologically and structurally determined variations in permeability (Fig. 5). The understanding of mineralized veins therefore requires a

6.

coordinated Gtudy of vein geometry and textures, the orientation and distribution of other rock structures and the nature and flow paths of the hydrothermal fluid (esp, from the character and distribution of wall rock alteration). Published with the permission of the Director, Bureau of Mineral Resources, Geology and Geophysics.

i

1.

REFERENCES

\.)

FRACTURE MECHANICS

GRIFFITH,

(1924)

The theory of rupture.

Proc. 1st Intnl. Congr.

Appl. Mech., 55-63. HOBBS, B.E., IIEANS, W.D. and PATERSON, M.S. (1976) structural geology.

An outline of

New York, John Wiley and Sons, 571pp.

JAEGER, J.C. and COOK, N.G.W. (1976) Fundamentals of rock mechanics (2nd edition).

London, Chapman and Hall, 585pp.

iiEA^TS, W.D. ( 1976)

Stress and strain.

PATERSON, M.S. (1978)

New York, Springer-Verlag, 339pp.

Experimental rock deformation - The brittle field.

Berlin, Springer-Verlag, 254pp. PRICE, N.J. (1966) rock.

Fault and joint development in brittle and semi-brittle

Oxford, Pergamon Press, 176pp.

SECOR, D.T. (1965)

Role of fluid pressure in jointing.

Am. Jour. Sci.,

263, 633-646. SECOR, D.T. (1969)

Mechanics of natural extension fracturing at depth

in the Earth's crust. 2.)

Geol. Survey Canada Paper 68-52, 3-48.

VEIN FORI>IATION AND GEOMETRY

BEACH, A.

(1975)

The geometry of en-echelon vein arrays.

Tectonophysics,

245-261. > BEACH, A. (1977)

Vein arrays, hydraulic fractures and pressure solution

structures in a deformed flysch sequence.

Tectonophysics, 40,

201-225. KERRICH, R. and ALLISON, I. (1978) Yellowknife mineralization. NEWHOUSE, W.H. (1942) In:

Vein geometry and hydrostatics during

Canad. J. Earth Sci., J^, 1653-1660.

Structural features associated with ore deposits.

Ore deposits as related to structural features, ed. W.H.

Newhouse, Princeton, Princeton Univ. Press, 9-53. PHILLIPS, W.J. (1972)

Hydraulic fracturing and mineralization.

J. Geol.

Soc. London, J ^ , 337-359 3.)

METAIIORPHIC FLUIDS AI^ THEIR MIGRATION

BEACH, A. (1976)

The interrelations of fluid transport, deformation,

geochemistry and heat flow in early Proterozoic shear zones in the Lewisian Complex.

Phil. Trans. Roy. Soc. Lond., A280, 529-604.

8.

BRACE, W.F. (1980)

Permeability of crystalline and argillaceous rocks.

Int. J. Rock. Mech. Min. Sci.,

241-251.

ETHERIDGE, M.A., WALL, V.J. and VERNON, R.H. (1983).

The role of the

fluid phase during regional metamorphism and deformation.

J. Metamorphic

Geol., U 205-226. FERRY, J.M. (1980)

A case study of the amount and distribution of heat

and fluid during metamorphism.

Contrib. Mineral. Petrol., 7J_, 373-385.

FYPE, W.S., PRICE, M.J. and THOMPSON, A.B. (1978) crust.

Fluids in the earth's

Amsterdam, Elsevier, 383pp.

KERRICH, R., FYFE, W.S. and ALLISON, I. (1977)

Iron reduction around

gold-quartz veins, Yellowknife district. Northwest Territories, Canada.

Econ. Geol., 21, 657-663.

KNAPP, R. and KNIGHT, J.E. (1977) fluids:

Differential thermal expansion of pore

fracture propagation and microearthquake production in hot

pluton environments.

J. Geophys. Res.,

NORRIS, R.J. and HENLEY, R.W. (1976) Geology

2525-2522.

Dewatering of a metamorphic pile.

333-336.

NORTON, D. (1978)

Sourcelines, sourceregions and pathlines for fluids

in hydrothermal systems related to cooling plutons. NORTON, D. and KNAPP, R. (1977) systems:

the nature of porosity.

NORTON, D. and CATHLES, L.M. (1979) In:

Econ. Geol., T^, 21-28.

Transport phenomena in hydrothermal Am. J. Sci., 277, 913-936

Thermal aspects of ore deposition.

Geochemistry of hydrothermal ore deposits, ed. H.L. Barnes.

New York, Wiley, 2nds Edn., 611-631. PRICE, N.J. ( 1975) Fluids in the crust.

Sci. Progress,

59-87.

RUMBLE, D., FERRY, J.M., HOERING, T.C. and BOUCOT, A.J. ( 1982) Fluid flow during metamorphism at the Beaver Brook fossil locality. New Hampshire.

Am. J. Sci., 2 ^ , 886-919.

RUMBLE, D. and SPEAR, R.S. (1983)

Oxygen-isotope equilibration and

permeability enhancement during regional metamorphism.

J. Geol. Soc.

London, 140 (in press). SIBSON, R.H., MOORE, M.M. and RANKIN, A.H. (1975) hydrothermal fluid transport mechanism.

Seismic pumping - a

J. Geol. Soc. London, 131,

653-659. 4.)

CRACK-SEAL VEINS AND THEIR TEXTURES

COX, S.F. and ETHERIDGE, M.A. (1983)

Crack-seal fibre growth mechanisms

and their significance in the development of oriented layer silicate microstructures.

Tectonophysics, 92, 147-170.

9,

DURNEY, D.W. and RAIISAY, J.G, ( 1972) syntectonic crystal grwoths.

In:

Incremental strains measured by Gravity and tectonics, K.A. De Jong

and R. Scholten, eds., New York, John Wiley and Sons, 67-96. RAMSAY, J.G. (1980) ^84, 135-139.

The crack-seal mechanism of rock deformation.

Nature

10.

f

t uu

11.

fl^^UKE 2

5

10

(a)

Mohr envelope derived from laboratory tescs of a particular rock-type. Each circle represents one test to failure at a particular confining pressure. (After Brace, 1964, Brittle fracture of rocks. In: State of stress in the earth's crust. Ed. W,R. Judd, New York, Elsevier).

(b)

Mohr diagram illustrating effect of pore pressure on failure. Circle I represents a stable stress state at low pore pressure. Circle II represents failure at the same differential stress (same size circle), but at higher pore pressure. The difference in pore pressure between I and II is given by the length of the arrows joining the circles.

SHEAR STRESS

-T

(c)

O

T

2T 3T NORMAL

4T 5T STRESS

Mohr diagram illustrating stress states for various styles of fracture. A-tensile fracture; B - maximum differential stress for tensile fracture; C - extensional shear failure; D - compressional shear failure.

12.

i) _l/^tn/M

SVK.CC

'l^s^le fac-ZxAr-^^

roc ^

^^-zLck^

rc^sju^

:

Pf =

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niujAA

kxHa^

fyc^ssi^E

/ / ^ o •'i^Jt^^^JudJiU^^

(ii

P ^ ^ )

J.

S^-HT

Pr^

OJ^^cn^

-is T H e

f ..r

jorn,

^ ^

JLuAs

aCMMON TYPES OF VEIN GEXHETRY.

13.

Fracture (and therefore vein) orientations are generally related in specific ways to tiia principal stress orientations. Since rock structures sudi as folds, foliation/lineaton ard faults are also systenatically related to principal stress directions, there axe comonly sijnply associations between the geaietri' of veins and that of other rock structures.

4- f-

iW

A3 A.

Fault-related. A1

-

in fault plane, especially where fault plane changes orientation (e.g., Victoria "leatherjackets"),

A2

-

tension gashes adjoining fault, usually at 20^ to 40® to fault plane.

A3

-

tension gashes within fault zone or ductile shear (schist) zone, form at -45® to zone, but oomnonly signxDidal - can also be strongly folded/boudinaged into parallelism with zone during continuing deformation.

A4

-

intersection of faaltwith another plane (e.g. fault, bedding plane), prtxiucing rDd-li3<e geometry.

14.

B1

£>2

{^'I'SMU^

V^HKS

Jv-

^

/ineJtc

F i L r © ^ iJUArfe.

B.

C,

Fold/Foliatim - related, B1

-

Saddle reefs, with or without bedded legs.

31

-

tension gashes (ladders) at high angle to oonpetent beds in fold lij±>s.

33

-

perpendicular to foliation and lineation especially in lower grade inetanorphic rocks,

34

-

bedding - parallel veins in fold liirbs, especially in well layered rocks.

Other Types. CI

-

stockworks, randan or non-randcra

C2

-

regular arrays in unfolded rocks, usually horizontal i steep sets - related to stress field and rocJc-type variations.

15.

LARGE SCALE

r/(^u/LE S

MYLONITE OR BRECCIA SCHIST-^ -^DILATION GRAINSCALE POROSITY SOLUTION?-

IRREGULAR FAULT PLANE ± TENSION FRACTURES -^DILATION

SMALL SCALE 1) LITHOLOGICAL VARIATION

2) STRUCTURES- AS ABOVE ON SMALLER SCALE

16.

Interpretation of mesofraature systems: mesofractures in forelands^ fault zones and fold belts P. L. HMCOCK Department of Geology, University of Bristol, Queen's Building, University Walk, Bristol BS8 1TR, U.K. PRINCIPLES The investigation of brittle and semi-brittle mesofractures yields important information about the evolution of major tectonic units, the location of buried structures, and the orientations of past and presentday stress trajectories. Moreover, mesofractures may be the only widespread structures capable of analysis in forelands, especially the platform covers of 'stable' cratons. Their arrangement in platforms generally reflects a variety of responses to intraplate and plate boundary processes: for example, compression sind extension related to deformation in neighbouring orogens; the subsidence, uplift and inversion of sedimentary basins; deformation over a buried palaeorelief; the rejuvenation of basement structures; and the orientation of the contemporary stress field (Price 197^, Choukroune 1976, Hancock & Kadhi 1978, Engelder h Geiser 1980, Letouzey & Tr^moli^res 1980, Engelder 1982a,b, Hancock et al. 198U). Thus in tectonically quiet settings, where there have not been substantial intraterrane horizontal or vertical rotations, mesofractures are likely to reflect regionally significant stress trajectories. In contrast, many mesofractures in macrofault zones or fold belts are accommodation structures, only some of which are related to the regional strain pattern. An ideal family of mesostructures comprises kinematic indicators such as faults, shear zones, en Echelon cracks, kink bands, stylolites and fibrous veins [see Choukroune (1976), Angelier (1979), Letouzey & Tr^molidres (198O) and Eyal & Reches (1983) for examples of analyses]. Depending upon the combination of elements comprising a coeval assemblage the resulting strain is uniaxial, biaxial or triaxial (Nelson 198I, Reches 1983) (Fig.l). Interpreting mesofractures in rock masses which have experienced noteworthy horizontal or vertical rotations poses special problems. As Jackson et al. (1982) have pointed out, after about of rotation in the hangingwall of a listric normal fault a former antithetic normal fault becomes a reverse fault. The extensional character of the fault should, however, remain clear provided that sedimentary layering is regarded as the plane of reference. Where kinematic indicators are absent it is necessary to analyse systematic joints. The name joint is regarded here as a serviceable field term to describe a barren closed fracture on which there is no evidence of displacement or dilation at the scale of observation. Although the organisation of a joint pattern commonly mirrors the bulk strain history of an area, even a swarm of closely spaced joints is unlikely to achieve more than 2% elongation. Deciding whether a joint set defines a single set of normal extension fractures, or is only one set in a conjugate system of either shears or hybrid surfaces, can be attempted using a variety of criteria. These include (1) surface markings (Bahat 1979), (2) micro-offsets visible in thin section.

17.

UNIAXIAL STRAIN

(J2,r(£'0) BIAXIAL STRAIN

TRIAXIAL STRAIN Cz

CYCLOGRAPHIC

TRACES

POLES

extensional v«in, fissure

(7,5 a^^aj

or joint

principal stress axes

i styJolite or other pressure I solution surface

principoi strain axes

^

£ « natural or true

fault with slip direction •given by errors

strain

Fig.1. Block diagrams and schematic stereograms illustrating uniaxial strain (a&lD), biaxial strain (c&d) and triaxial strain (e) resulting from the development of brittle and semi-brittle mesostructures, (e) is after Heches (1983, Fig.lb).

(3) symmetry with respect to nearby kinematic indicators and (U) fracturesystem architecture. The architecture of a joint system normal to a plane of view can be visualized from the patterns made by the fracture traces, and can be characterised by reference to the stylised shapes of letters of the alphabet (Fig.2). Extension joints initiated in a weak differential stress field display a mud-crack geometry (Engelder 1982b) and thus a K-shaped pattern of traces resiats; successively younger joints abutting older ones. Unidirectional extension jointing gives rise to an I-shaped pattern, whereas two orthogonal episddes of systematic extension jointing yield a T-shaped pattern, again the younger trace abutting the older one. If the later phase of orthogonal extension jointing involved the development of nonsystematic crossfractures they will be short and hence an H-shaped pattern results. Conjugate joints generally make V-, Y- or X-shaped patterns. Care should be exercised when interpreting X-shaped patterns that the 'X' is not an artefact of crossing, but genetically unrelated joints. Cross-joints superimposed on older conjugate joints commonly lead to an A-shaped pattern. Fig.2. Joint-trace architectural styles characterized The acute dihedral angle (29) between by reference to the stylised conjugate sets permits their genetic shapes of letters. classification assuming a generalized

H

V y X A

18.

composite failure envelope and initially isotropic rock. Conjugate sets enclosing a 29 value of less than may be interpreted as oblique extension (hybrid) fractures, while those enclosing 26 values from h3 to 59^, while also hybrids, are not extension fractures. Where 29 is or greater the joints can be interpreted as shears. Field evidence favouring a joint following a shear direction might includes its parallelism or continuity with an attendant fault, shear zone or lineated surface. Some oblique extension joints pass laterally into arrays of barren en Echelon cracks. Most vertical and high-angle joints are normal or oblique extension fractures but shear fractures are better represented among joints that are gently inclined or at a small angle to bedding. Some joints are initiated before folding whereas others form during or after folding. Joints which cut cleavage surfaces and abut, but do not displace, bedding places were probably initiated after the close of folding. Where a joint system was established before or during folding, many of the fractures become the sites of later slip, dilation or pressure solution (e.g. Marshak et al. 1982),. CASE STUDIES Three joint domains each containing sets of normal and oblique extension joints symmetrically orientated about different macrostructural trends have been recognized in the eastern Saudi Arabian platform (Hancock et al. 198U). The joints are interpreted as being related to three mechanisms: (1) strike-parallel stretching during formation of the central Arabian arch; (2) stretching normal to the arcuate central Arabian graben system; and (3) longitudinal extension of the Arabian foreland parallel to the length of the Zagros deformation belt. Amplification of the arch sind displacement of the East Arabian block with the consequential development of the central Arabian graben system occurred in the late Cretaceous -earliest Palaeogene, when a peripheral bulge formed in response to the emplacement of ophiolite nappes on the northeastern Arabian subplate margin. Peripheral expansion of the Arabian foreland and post-collisional shortening in the Zagros ranges were late Neogene events.

Fig.3o Possible en Echelon structures in a right-lateral fault zone, R & Ri, Riedel shears; P, P shears; X, X shears; Y, principal displacement shear; t, thrust; st, stylolitic pressure solution seam; Si, cleavage or schistosity; f, fold or pressure ridge; e, extension fracture (vein or fissure); n, normal fault

Transcurrent fault zones in the upper crust are characterized by arrays of en Echelon structures whose geometry and kinematics permit the sense of displacement along the maj.or zone to be inferred (Fig.3)« Bartlett et al. 1981) describe the controlled experimental replication of such structures in limestones and discuss their significance for the interpretation of natural fault zones. The right-lateral North Anatolian fault zone contains a complex association

19.

Fig.U. Block diagrams illustrating mesofracture sets and systems symmetrically arranged with respect to the plane of sedimentary layering and fold axes, which throughout the fold define the orientation of the fabric axial cross. of mesostructures most of which are compatible with the present-day sense of shear. However, some sets of joints and mesofaults in the zone are anomalously orientated unless explanations other than right-lateral shear are considered (Hancock & Barka 198I, 1983). The pinnate joints that accompany some faults are particularly valuable kinematic indicators because they intersect the fault perpendicular to the slip vector anci subtend an acute angle with it that closes in the direction of motion of the block• containing the joints. Furthermore, the distribution of the pinnate joints may be related to the arrangement of compressional and dilational quadrants around a fault. Mesofractures in folds are commonly symmetrical about the structure containing them. Figure h illustrates sets which might develop when competent layers act as stress guides. The relative abundance of particular sets can be interpreted as reflecting the bulk strain regimes within which a fold was evolving. Examples of mesofracture sets orientated symmetrically about layering and fold axes (Fig.U) occur in the southern part of the Variscan fold belt in southwest Wales (Hancock et at. 1983), and in the Palaeogene molasse of the Jaca basin in the southern Pyrenees. Although the control of mesofracture geometry is identical in both areas each setting is characterized by a different assemblage of mesofractures. The sets shown in Figs.U (f), (g), (j), (k) and (m) are especially abundant in Wales, because during folding there was layerparallel shortening perpendicular to hinge lines and complementary elongation parallel to them. In contrast, the Pyrenean folds are cut by a prevailing set of normal extension joints (Fig.Ue), locally accompanied by the conjugate systems shown in Figs.U (l) and (n). The Pyrenean assemblage indicates that layer-parallel elongation normal to hinge lines was the dominant process during folding: a conclusion in accord with the observation that the folds possess many of the characteristics of large-scale synsedimentary growth structures.

20.

REFERENCES Angelier, J. 1 9 T 9 . Determination of the mean principal directions of stresses for a g5.Yen fault population. Teotonophysics 56, T17-T26. Bahat, D. 19T9. Theoretical considerations on mechanical parameters of joint surfaces based on studies on ceramics, Geot. Mag, II6, 81-92. Bartlett, W. L., Friedman, M. & Logan, J. M. 198I. Experimental folding and faulting of rocks under confining pressure: Part IX: wrench faults in limestone layers. Tectonophysics 79, 255-277. Choukroune, P. 1976. Strain patterns in the Pyrenean chain. Phil. Trans. R. Soa. A283, 271-280. Engelder, T. 1982a. Is there a genetic relationship between selected regional joints and contemporary stress within the lithosphere of North America? Tectonics 1, 161-177. Engelder, T. 1982b. Reply to a comment by A.E.Scheidegger on 'Is there a genetic relationship between selected regional joints and contemporary stress within the lithosphere of North America? by T. Engelder. Tectonics 1, U65-t70. Engelder, T. & Geiser, P. 198O. On the use of regional joint sets as trajectories of paleostress fields during the development of the Appalachian Plateau, New York. J. geophys. Res. 85, 6319-63^1. Eyal, Y. & Reches, Z. 1983. Tectonic analysis of the Dead Sea Rift region since the late-Cretaceous based on mesostructures. Tectonics 2, 167-185. Hancock, P. L. & Barka, A. A. 198I. Opposed shear senses inferred from neotectonic mesofracture systems in the North Anatolian fault zone. J. Struct. Geol. 3, 383-392. Hancock, P. L. & Barka, A. A. 1983. Tectonic interpretations of enigmatic structures in the North Anatolian fault zone. J. Struct. GeoZ. 5, 217-220. Hancock, P. L. & Kadhi, A. 1978. Analysis of mesoscopic fractures in the Dhruma-Nisah segment of the central Arabian graben system. J. geol. Soc. Lond. 135, 339-3^7. Hancock, P. L., A1 Kadhi, A. & Sha'at, N. A. 198U. Regional joint sets in the Arabian platform as indicators of intraplate processes. Tectonics 3. (in press) Hancock, P. L., Dunne, W. M. & Tringham, M. E. 1983. Variscan deformation in southwest Wales. In: The Variscan Fold Belt in the British Isles (edited by Hancock, P. L.). Adam Hilger, Bristol, U7-73. Jackson, J. A., King, G. & Vita-Finzi, C. I982. The neotectonics of the Aegean: an alternative view. Earth Vianet. Sci. Lett. 61, 303-318. Letouzey, J. & Tr^moli&res, P. 198O. Paleo-stress fields around the Mediterranean since the Mesozoic derived from microtectonics: comparisons with plate tectonic data. Mem. Bur. Rech. G&ol. Min. 115, 261-273. Marshak, S., Geiser, P. A., Alvarez, W. & Engelder, T. 1982. Mesoscopic fault €Lrray of the northern Umbrian Apennine fold belt, Italy: geometry of conjugate shear by pressure-solution slip. Bull. geol. Soc. Am. 93, 1013-1022. Nelson, R. A. I98I. Significance of fracture sets associated with stylolite zones. Bull. Am. Ass. Petrol. Geol. 65, 2U13-2U25. Price, N. J. 197^. The development of stress systems and fracture patterns in undeformed sediments. Proc. 3rd Congr. Int. Soc. Rock Mech. I, U87-U96. Reches, Z. 1983. Faulting of rocks in three-dimensional strain fields. II. Theoretical analysis. Tectonophysics 95, 133-156.

21.

Major

Brecclation

Styles

At

The

Ardlethan

Tin

Mine,

N.S.W,

G.W. Clarke?" R.G. Patterson? R.G. Taylor?' 1 D^jartrnent of Geology James Cook tJhiversity Tbwnsville QLD, 4811.

2 Aberfoyle Ejqjloration PA' 144 Ccin±)erwell Road Hawthorn East VIC, 3123.

«

The mineralization at Ardlethan is localized ty a series of major breccia pipe systens, which have subsequently been overprinted by the development of minor fracture and fracture intersection-contrblled veins and pipes. The breccia systems are centred vapon. each of the major mineralized zones (Wild Cherry ( Ardwest-South Cherry), Carpathia, Stackpool and White Crystal). They range considerably in size and shape and at least two ages of formation are revealed ty cross cuttii^ relationships at the Wild Cherry, Carpathia, and StadqxxDl centres. The system is analogous to the breccia-pipe, porphyrytin style of mineralization in southern Bolivia. Three styles of major brecciation have been recognized: 1) Collapse Style Brecciation

(White Crystal).

The White Crystal breccia zone extoids sane 80 m frcm sxorface over an area approximately 20 x 80 m .The econonic mineralization terminates sharply in depth, althoiogh small pror^s are known to extend boieath the main opei cut. The alteration zone continues beneath the econonic zone to at least 100 m. The ore zone consists of quartz-tourmaline-topaz rock containing numerous mineralized quartz-lined vughs. Towards the margins of the ore zone, the alteration is less intense and it is apparent that the vughs r^resent zones of infill between numerous fragments of granite. The fragments are essentially rectangular, slab-like, and range up to 2 m in length. In places they can be seen preserved in the process of "peeling off" fron the roof and side v/alls. The elongate fragments are frequently aligned and their plunge indicates a dip towards a cannon epicentre, i.e. a crude funnel shape. The infill consists of quartz-cassiterite tourmaline and sulphides. The entire ore zone is surrounded by a shell of argillisation , which is succeeded by sericitisation , with an inner core of intense tourmalinisation. The origin of the large scale tensional brecciation is uncertain. 2) Intrusive Style Brecciation . Type 1. Hydrothermal Intrusive Style (Wild Cherry Stackpool, Carpathia).

(Ardwest-South Cherry),

This style of brecciation forms the bulk of the mineralization within the system, and hosts the two main ore bodies within the main Wild Cherry Pit. The Ardwest ore zone is essentially carrot-shaped, tapering downwards (50 m diameter x 150 m deep), whilst the South Cherry is tabular (200 x 50 x 150 m deep). The irain breccia zones are surrounded ty an intense shattering (crackle breccia) which is superiirposed on close-spaced jointing. The resulting permeability has allowed widespread fluid access and consequent widespread sericiticchloritic, and minor argillic alteration . The intense alteration obscures many of the brecciation c±aracteristics fron casual observation. Etching reveals the main breccia zones to consist of rounded to siobrounded fr^ents, ranging fran cobble size to microscopic , set within a matrix of rock flour. Both the matrix and rock flour often display local alignment/foliation and void space is limited to small elongate infilled vughs. The fragments are predoninantly

22-

altered granite, vath altered quartz-feldspar porphyry beconing locally comoi. Within the Ardwest zone the porphyry fragments form a declining dispersion trail extending upwards from the point where the breccia zone cuts through a porphyry c^ke systan. The vughs are infilled with variox;is conbinations of q-Jartz, tourmaline, sxiLphides, cassiterite, fluorite and siderite. The above characteristics are strongly s\:iggestive of intrusive breccia via a flmdised systan,.i.e. hydroth^nnal intrusive breccia. Type 2. Gaseous-hydrothermal Intrusive Breccia? (C&rpathia, Stac3qxx>l) Brecdation of this style has only been observed at two points. Both exartples cross cut the Type 1 style, are smaller, and contain relatively low econanic tin values. The shape of both exartples is less well established. The larger Carpathia zone forms a vertical pipe (approx. 50 m diameter and j 250 m de^), whilst the Stackpool zone is also pipelike (15 m diameter?). The breccia is characterised by an extensive, loosely cemented matrix of rock flour, with mixed cobble sized fragments of metasedunaits, altered granite, and occasionally altered porphyry. The granite and porphyry are subrounded to rounded and extensively altered, whilst the metasediments are angular with minor rounding and are essentially unaltered. The matrix is tightly packed, occasionally foliated, and often extends outwards frati the generally sharp contacts as small prongs/veins along fractures and joints. Vughs are virtually absQit and no major associated alteration is observed. Sane fragments are clearly of Type 1 breccia, and the lack of alteration within the metasediments may indicate that the granitic fragments are all derived fran the Type 1 breccia enclosii^ wall rocks. The origin of the brecdation poses a few queations. The main features strorgly support an intrusive style, while the prolific rock flour, loose canentirg, limited alteration, tightly filled joints, and local foliation suggest a relatively dry gaseous driving force. The angularity of the mstasediment fragments is initially puzzling, but could relate to their general resistance to alteration and their propensity to splinter into elongate-oblong shapes. This model also iitplies that areas of metasediment exist below the outcropping granite. The systen does not appear to be a mineralizing systan and it is possible that the erratic-minor tin values relate to incorporated fragments of the surrounding Type 1 breccia. A small pebble dyke noted within the Ardwest systan may also relate to this phase of brecdation. Several localised zones of brecdation and mineralisation have been noted overprinting both the Type 1 and T^pe 2 major brecda zones. These are primarily fracture fault controlled resiiLting in tourmaline, and chloritetourmaline pipes and veins. Late stage faulting also brecdates and overprints the earlier major brecdas and it is dear that the systan represents a long history of evolving fluids and tectonic adjustn-^ents.

23.

STRUCTURAL VEIN

CONTROLS

DEPOSITS

IN

ON THE

THE

DEVELOPMENT

BENDIGO-BALLARAT

OF SYNTECTONIC

GOLD-QUARTZ

TROUGH,

VICTORIA.

CENTRAL

S.F. Cox Dept. of Earth Sciences, Monash University, Clayton, Victoria

3168.

Auriferous quartz vein systems in low grade regionally metamorphosed rocks have constituted a major source of gold on a world-wide basis. Ordovician slate belts of the Tasman Fold Belt have yielded particularly productive examples. This paper examines the regional setting of such deposits in the Bendigo-Ballarat Trough of central Victoria, then considers the structural geometry of auriferous vein systems in this region, their relationships to the deformation history, and the structural controls on their development. Throughout, particular emphasis will be placed on recently studied examples of quartz vein systems and their setting in the Castlemaine-Chewton region, however well-documented examples from other areas within the Bendigo-Ballarat gold province will also be discussed. The Bendigo-Ballarat Trough consists of a sequence of Ordovician quartzwackes, siltstones, and slates having a total thickness of about 5000 metres. Regional deformation has folded the sequence about dominantly upright northsouth trending axial surfaces, and has produced open to tight and usually moderately to gently plunging folds having wavelengths between about 150m and 600m. The development of a penetrative solution cleavage has been associated with fold growth, and is most pronounced in pelitic and semi-pelitic units, though arenites typically have a strongly developed divergent fanning solution cleavage in hinge zones. Folds are cut by abundant steeply to moderately west-dipping and east-dipping reverse faults which strike approximately parallel to fold axial traces. Faults having displacements less than several tens of metres are common. Major fault zones having displacements of the order of 100-300 metres appear to have a spacing of one kilometre or so. Larger regionally significant faults such as the Whitelaw, Sebastian, Muckleford, Cambelltown, and Djerriwah Faults are high angle reverse faults having displacements in excess of several hundred metres. Displacement on such faults has been a fundamental shortening mechanism during regional deformation. Varying fold styles throughout the region reflect strain variations in part due to changes in the partitioning of strain between folding, cleavage development, and shortening on reverse faults. The auriferous quartz vein deposits in the Bendigo-Ballarat gold province have developed in fault-related and fold-related dilatant fracture arrays which have formed late during the regional folding history in a high fluid pressure, low grade metamorphic regime. Regional stratigraphic and structural relationships suggest that regional deformation, associated low grade metamorphism, and gold mineralisation occurred during the middle Devonian Tabberabberan Orogeny. Auriferous quartz vein deposits occur dominantly in the lower Ordovician (Lancefieldian to Castlemainian) part of the sequence, and form a gold province up to sixty kilometres wide and at least one hundred and fifty kilometres long in the central and western parts of the Bendigo-Ballarat

24.

Trough. Auriferous vein systems are typically abundant only within narrow north-south trending belts which are parallel to major fold axial traces« For example, in the Chewton gold belt, gold deposits occur dominantly within a zone approximately eight kilometres long, but less than one kilometre wide. Within this belt, the major vein deposits are restricted to the eastern limb zones of two major anticlines. The known major gold belts within the Bendigo-Ballarat gold province are separated by intervals of approximately 20 to 30 kilometres• Gold production from vein systems and alluvial deposits derived from them within the Bendigo-Ballarat gold province has been in excess of 2 x 10® kg. Major producing areas include the Bendigo field (> 660,000 kg total Au production, with 240,000 kg Au from vein depsoits), the Ballarat gold fields C> 500,000 kg total Au, with 58,000 kg Au from vein deposits), and the Castlemaine-Chewton gold fields (> 120,000 kg Au, with 24,000 kg Au from vein deposits)• Several types of auriferous quartz vein systems may be recognized. Dominant are within-fault veins which have developed by dilation within faults during their displacement history. The geometry and internal structure of such veins and associated fracture arrays is dependent upon whether the fault zone is discordant or concordant with bedding. Very large vein systems can develop where faults are oblique to bedding and where refraction of faults across folded structures has caused the local fault orientation to be inclined to the bulk displacement direction^ This type of vein geometry is particularly important in the Ballarat East, Chewton, and Daylesford gold fields. The development of vein systems in bedding-parallel faults, which may in part be related to flexural slip during folding, has provided an important source of gold in most goldfields, but particularly Ballarat West and Bendigo. Flat-lying vein systems are typically spatially related to fault zones, and have been generated in extension fracture arrays which have developed in response to the modification of the local stress field during fault movement. Fault intersections have been major dilation sites. Saddle reefs are an important type of fault intersection structure whose development is also related to fold growth and the generation of abnormally high fluid pressures below low permeability rock units capping anticlinal hinge zones. Such structures are best developed in the Bendigo gold field, but are also found in many other areas in the Bendigo-Ballarat gold province. Many veins exhibit microstructures indicative of vein growth during an extended history involving repeated increments of cracking by hydraulic fracture and sealing by deposition of quartz and other phases. Crack-seal inclusion bands and related features (Ramsay, 1980; Cox and Etheridge, 1983) particularly in fibrous veins, indicate that vein growth may involve up to 5 X 10^ crack-seal growth increments per centimetre of vein width. Both sub-critical and unstable crack growth mechanisms have been involved in the development of fracture arrays. The development of auriferous quartz vein systems during regional deformation but predominantly late in the shortening history, the lack of extensive wall rock alteration, and the isotopic and chemical compositions of the fluids, involved, all attest to the formation of the mineralisation from regional metamorphic fluids. The we11-documented spatial association between locally high gold grades in vein systems and the presence of carbonaceous and pyritic slate wall rocks suggests that gold deposition may be controlled by the mixing

25.

of locally-derived methane-bearing'reducing fluids with more oxidized metamorphic fluids traversing faults and associated fracture arrays (Ceplecha and Wall, 1975; Wall and Ceplecha, 1976; Cox et al; 1983). The important structural control of high angle reverse faults is that they have played a fundamental role in controlling metamorphic fluid transport, and in providing access for the auriferous fluids to locally-derived reducing fluids. Fault zones and associated fracture systems have thus acted as fluid mixing sites. Extended histories of cyclic hydraulic fracturing and vein sealing associated with faulting in these regimes of fluid flow have generated dilation sites in which large vein systems have formed. The development of auriferous vein systems comprising individual gold fields must involve through-put of cubic kilometres of fluid within very narrow belts of crust, and indicates significant structural control and focussing of fluid transport. Consideration of total gold production and the gold content of the Ordovician sequence and probable underlying sequences CGlasson and Keays, 1978) indicates that source region volumes for individual gold fields must have been of the order of at least 100 km^. The large volumes of both fluids and source regions imply deep circulation of metamorphic fluids. The geometry and spacing of gold belts suggests that their development is related to that of major deep level fault zones which have tapped auriferous metamorphic fluids and focussed fluid flow into restricted upper crustal^ regions. The reasons for the restriction of gold mineralisation to particular fault zones, and not others, are unclear. However they may be explained in terms of the temporal and spatial sequence of reverse fault growth during the regional deformation history, the ability of developing faults to tap auriferous metamorphic fluids, and the development of regimes suitable for gold deposition in and adjacent to faults or at zones of fault termination. The restriction of major gold deposits in the Bendigo-Ballarat Trough to the lower Ordovician sequence indicates a large scale stratigraphic control on gold deposition. This may reflect in part the availability of suitable reducing fluids in this level of the sedimentary sequence, as well as depletion of gold from metamorphic fluids reaching higher crustal levels. Within individual gold fields the controls on the generation of significant gold deposits are largely structural. The geometry of fault zones and folded rock units are controlled by the regional stress field and strain history during formation of these structures. Fault and fold geometry in turn control fluid migration paths, the development of high fluid pressure regimes, and the geometry of hydraulic fracture arrays in which auriferous vein deposits develop. The spatial association between the generation, in high fluid pressure regimes, of large hydraulic fracture arrays, and sites in which auriferous metamorphic fluids traversing these arrays may mix with locally-derived more reduced fluids would seem to be the ultimate requirement for the development of gold deposits. The association of well developed vein systems, high gold grades, and domal or culmination zones in anticlines found in many parts of the BendipBallarat gold province suggests that low permeability horizons capping anticlinal hinges, particularly in culmination zones, may play a major role in controlling fluid migration and mixing. A consideration of fluid dynamics indicates that such sites may partially trap locally-derived CHt^-bearing fluids below them, thus facilitating gold deposition from

26.

auriferous metamorphic fluids traversing faults in these zones. Such locally capped structural sites are also expected to develop abnormally high fluid pressures, thus enhancing the development cf hydraulic fracture arrays suitable for fluid mixing and gold deposition.

REFERENCES

Ceplecha, J.P., ajid Wall, V.J., 1975. The Chewton Goldfield and Wattle Gully Mine : A model for Gold-Quartz Mineralization in Slate Belts. (Abstr.) S.G.I.G.O.D. Conf. on Geol. and Min. Lachlan Fold Belt, Macquarie Univ. Cox, S.F., Wall, V.J., Etheridge, M.A., Sun, S.S., and Potter, T.F., 1983, Gold-quartz mineralization in slate belts : The ChewtonCastlemaine example. Geol. Soc. Aust. Ahatv.j 9, 260-261. Cox, S.F. and Etheridge, M.A., 1983. Crack-seal fibre growth mechanisms and their significance in the development of oriented layer silicate microstructures. Tectonophysicsj 92, 147-170. Glasson, M.J., and Keays, R.R., 1978. Gold mobilization during cleavage development in sedimentary rocks from the auriferous slate belt of central Victoria, Australia. Eoon. Geol.j 73, 496-511. Ramsay, J.G., 1980. The »'crack-seal" mechanism of rock deformation. Nature, 284, 135-139. Wall, V.J. and Ceplecha, J.P., 1976. Deformation and metamorphism in the development of gold-quartz mineralization in slate belts (abstr.). XXI st. Int. Geol. Congr., 1, 142-143.

27.

RoZz

oi

pxz-Qxl^tlng

yjdiru

diViinQ

V^ coppdA

irUntAjOitizatiori

at

Mi.

Cees Swager, Department of Geology, James Cook University (gj), 4811, Australia. The Mount Isa bxse metal deposits are characterized by the spatial separation of stratiform Ag-Pb-Zn orebodies in a doloniitic shale sequence and discordant Cu orebodies in a i-ecrystallized and/or brecciated host rock, the silica-dolomite. A "syngenetic** origin for the galena and sphalerite layers within the ^ l e s is widely accepted (Gustafson and Williams, 1981), but several models have recently been proposed for the chalcopyrite mineralization. Tliese include models of essentially contenporaneous deposition of Cu and Pb-Zn (Finlow-Bates and Stunpfl, 1979), and a model of separate, consideably later Cu deposition (Perkins, 1983; Swager, 1983). Detailed (micro) structural studies of tlie chalcopyrite mineralization and its hostrock, and a comparison with the deformation history of the footwall dolomitic shales, produced evidence for extensive carbonate replacement processes after the regional D2, and probably during the D^ deformation phase. The first of the three deformation phases recognized in the dolcanitic shale sequence was found cmly locally. The N-S D^ event produced a weak Sg cleavage and widespread Dg veins, and the Ntfr/^-SSE Dg liiase was responsible for development of the macroscopic fold zones in the mine area and Sg cleavage. The D^ extension veins commonly developed subperpendicular to bedding, ana were deformed dxiring D^; however, they were also in a favourable orientation for reopening during D^. In particular, these D2 veins played a significant role as fluid pathways during the post-D2 and/or syn-Dg silica-dolomite fonration and copper mineralization. The complex history of such veins is inferred from a variety of (micro) structures suggesting several processes, such as preferential grain growth of the original vein dolomite, replacement of the vein dolomite by quartz or chalcopyrite, deposition of new material during Dg reopening of the Dg veins, etc. BEFBRENCES Finlow-Bates, T. and Stunpfl, E.F., 1979. The copper and lead-zincsilver orebodies of Mt. Isa Mine, Queensland: products of one hyrothermal system. Annales Soc. Geol. Belgique, 102, 497-517. Gustafson, L.B. and Williams, N., 1981. Sediment-hosted stratiform deposits of copper, lead and zinc Seventy-fifth Anniv. Vol., Econ Geol., 139-178 Perkins, W.G., 1983. Mount Isa 'Silica-dolomite' and copper orebodies: The result of a syntectonic hydrothermal alteration system. Econ. Geol. in press. Swager, C.P., 1983. Microstructural development of the silica-dolomite and copper mineralisation at Mount Isa, M Queensland, with special enphasis on the timing and mechanism of mineralization. Ph.D. thesis, James Cook University.

28.

FRACTURE PATTERNS OF THE SYDNEY BASIN AND THEIR ECONOIIC SIGNIFICANCE Mauger, A.J., Creasey, J.W. and Huntington, J,F. CSIRO, Division of Mineral Physics, North Ryde, NSW The Sydney Basin is a Permo-Triassic structural basin developed between the Palaeozoic Lachlan and New England Fold Belts, To the west and south the sediments lie unconformably on the Lachlan Fold Belt and to the northeast the boundary is defined by the Hunter-Mooki thrust system, which marks the western edge of the New England Block. The Sydney Basin has been subdivided into several structural units (Bembrick et al, 1973), Fig. la, that are based on minor basins, structural plateaux and folds. Previous lineament interpretations within the basin had been carried out on ERTS-1 images (Scheibner, 1976), elements of which have been included in Fig. lb, and Landsat imagery at CSIRO. A detailed fracture analysis of the Sydney Basin was recently conducted by CSIRO as part of a NERDDP funded project to evaluate structural patterns within present and future coal mining areas of the basin. The fundamental strategy of the project incorporated a multiscale, multi-element approach that included remotely sensed data, surface and subsurface geological, geophysical and mining data. The fracture analysis involved interpretation of 1:80,000 scale air photos, Landsat imagery enhanced on a colour TV monitor, and regional sampling of bedrock fracturing. Data was compiled onto eleven 1:100,000 scale base maps comprising the study area. Fig. la. Digital techniques were then employed to assist the geologist/ interpreter in a data reduction process that drew together the corroborative evidence for the existence of fracture patterns and discrete lineaments. The resulting structural synthesis for each map sheet has been further summarised to produce a regionally coherent lineament pattern. Fig. lb, that contains four clearly identifiable trends. Fig. Ic. The fracture pattern is better expressed in areas of outcropping Triassic Narrabeen and Hawkesbury Group sediments and poorly expressed within the Permian outcrop of the Hunter Valley Dome Belt and Upper Triassic Wianamatta Shales in the Cumberland Basin. The lineaments are related spatially and geometrically to older basement structural trends. The WNW and ENE lineament trends (I and IV, Fig. Ic) parallel basement transform and transcurrent faults, and megakink planes within the Lachlan Fold Belt (Sheibner, 1976 and Powell, 1983) . The ENE trending lineaments are widely distributed across the basin. They parallel the Pyramul Creek Lineament (Scheibner, 1976) that is located on the Bocoble Orocline within the Lachlan Fold Belt. The Gospers Mountain Lineament in the central Blue Mountains Plateau is coincident with a dextral ENE trending transcurrent fault in the Devonian basement of the Capertee Valley. The WNW trending lineaments parallel the Lachlan River Lineament (Scheibner, 1976) along which the Carboniferous Bathurst Granite is emplaced. These lineament trends are bimodal, with the more northerly trending group distributed north of the Lachlan River Lineament. Both the WNW and ENE trending lineaments are associated with intrusive and extrusive volcanism in the basin. These lineaments are also well expressed in the Upper Devonian Bindook Porphyry off the south western edge of the basin. NNE trending lineaments (III, Fig. Ic) are spatially concentrated on the Blue Mountains Plateau, west of the Lapstone Monocline. Detailed

29.

multiscale structural analysis on the Gaidiners Gap Lineament Zone, Deans Creek Lineament and Clarence Lineament on the western margin of the plateau define sinistral wrench faulting in the Permian and Triassic strata that are associated with basement discontinuities. The Blackheath Lineament extends into basement south of the plateau and parallels basement fold trends. The East Coast Lineament or Coastal Lineament (Scheibner, 1976) is a poorly expressed but extensive lineament that trends more easterly than the other group 111 lineaments. The NW trending lineaments (group II, Fig. Ic) are widely distributed. To the northwest they parallel Lachlan Fold Belt trends north of the Bocoble Orocline. Throughout the basin this trend is associated with normal faults and dykes especially along the eastern edge of the basin (Branagan et al, 1979). On the south western edge of the Hunter Valley Dome Belt a northwesterly trend basement reverse fault has been interpreted that coincides with a lineament expressed in the Triassic cover, (Scheibner, 1976). A model for the origin of these lineaments is proposed based on the direct inheritance of basement structural trends under the influence of a regionally active shear couple. Evans and Roberts (1979) proposed a NNWSSE dextral rotational shear couple for central eastern Australia up to Middle Triassic and a reversal of the shear couple since Late Triassic. The Permian deformation in the Hunter Valley Dome Belt producing en-echelon N-S folds has been attributed to the dextral shear couple (Evans and Roberts, op cit). However, the essentially post-Triassic lineament pattern and structural deformation in the plateau areas of the basin has resulted from the later sinistrally acting shear couple. The ENE, WNW and NNE trending lineaments have resulted from simple shear reactivation of basement discontinuities. The NW trending lineaments conform to a tensional direction within the sinistral shear couple. Such a model provides a general framework to account for the interpreted lineament pattern within the Sydney Basin. Other specific tectonic events need to be assessed in order to form a complete structural history of the basin (Mayne et al, 1974, Branagan et al, 1979). The most significant economic effect of geological structures in the basin has been in underground coal mining. Seam displacing faults have restricted mine development in most coal mining areas, structures have influenced roof stability and provided the loci for outbursts of coal and gas in the Southern Coalfield. Fracture analysis has been used to delineate structural zones in developing and new collieries in the Western Coalfield (Shepherd et al, 1981). The 1:100,000 scale fracture analysis that has been conducted forms the framework for structural assessment in areas under consideration for future mine leases. Other applications of fracture analysis in the basin are fracture permeability studies for gas extraction, structural control of deep seated intrusives especially in the west and north west region of the basin, and in a variety of surface and subsurface engineering projects.

30.

References Bembrick, C.S., Herbert C., Scheibner, E. and Stuntz, J., 1973, Structural subdivision of the New South Wales portion of the Sydney-Bowen Basin. Q. Nbtes Gaol. Surv. NS^f, 11, 1-13• Branagan, D., Herbert, C. and langford-Smith, T. 1979, An Outline of the Geology and Geomorphology of the Sydney Basin. Science Press, University of Sydney. Evans, P.R., and Roberts, J. 1979. Evolution of central eastern Australia during the late Palaeozoic and early Mesozoic. J. Geol. Soc. Aust., 26, 325-340. Mayne, S.J., Nicholas, E., Bigg-Wither, A.L., Rasidi, J.S., and Raine, H.J. 1974. Geology of the Sydney Basin - A Review. Bureau of Mineral Resources, Geology and Geophysics, Bull. 149• Powell, C.MC.A., 1983. Geology of the NSW South Coast. Geol. Soc. of Aust., S.G.T.S.G. Field Guide §1. Scheibner, E., 1976. Explanatory Notes on the Tectonic Map of New South Wales. Geol. Surv. N.S.N^ Shepherd, J., Huntington, J.F. and Creasey, J.W. 1981. Surface and underground geological prediction of bad roof conditions in collieries of the Western Coalfield, New South Wales, Australia. Trans. Instn. Min. Metall. (Sect. B: Applied earth sci.), 90, B1-B14.

Fig. la) b) c)

Structural subdivision of the Sydney Basin (after Bembrick et al, 1973) Summarised lineament pattern within the study area, Sydney Basin Frequency azimuth diagram of lineament trends in (b)

REFERENCE —

- v

.

Unconfoimity

,

Monocline

showing

d i i e c l i o n of

dif

Foull

showing

d u t c i i t n of d i p ^

Th»usl showing d n « c l i o n oI dip B o u n d 0 1 y of b a s i n Position unceHoin B o u n d o i y ol O Sl'ocluiol Subdivision H i n g e line d o t i n g mount Permian H i n g e line d u t i n g c o o l m e a s u r e lime S y n c line A n t i c line

S t u d y

A r • •

L l n « « m « n l

Ltn««m«nl

• •• •• •

W O L L O N G O N G J

KILOMETRES

FIG.1

1

i i — L i

1

i

i

i

(Schttibner, 1 9 7 0 )

B s l h u r t l

O r « n l l •

Fold

-

B«lt

-

B « » « m « n i

Q t u m a n l

32.

LANVSAT

FRACTURE PATTERNS AROUNV COBAR M.S.W., AMV ECOmiC

mPLICATIONS

R.A. Glen, D.J. Pogson, E. Scheibner. Geological Survey of NSW, Box 5288 GPO Sydney, N.S.W. 2001. Much of the Paleozoic geology of the Cobar region in Central Western New South Wales is obscured by deep weathering and by surficial cover. Landsat imagery has proved to be of little use in the differentiation of mapped lithologies under areas of no outcrop, but has been of considerable assistance in establishing a structural framework for the region. The current level of understanding of the tectonic development of the Cobar region owes a great deal to Landsat lineament studies. Conversely, the significance of linear patterns on Landsat imagery could not have been clearly established without the geological knowledge obtained fran detailed mapping in areas of outcrop or sub-outcrop. Landsat imagery has contributed to this understanding of Cobar geology in three ways. Firstly, recent detailed mapping in the Cobar region has identified a number of major faults which played a key role in ^ e geological development of the region. Recognition of co-incident linears in Landsat imagery has, in many cases, enabled these faults to be extended into areas of non outcrop. Secondly, this imagery has enabled suspected faults separating areas of different structural style to be more confidently inferred. Thirdly, the presence of lineaments/inferred faults/established faults with inferred wrench movements during regional east-^est ccmpression in the Carboniferous provides a consistent explanation for fold orientations in part of the Early Devonian - PEarly Carboniferous cover sequences in the area. Ultimately, analysis of faults and lineaments bounding basanent crustal blocks with the Cobar region will enable us to propose models for the formation of extensional stratotectonic features in the region. Our work on lineaments in the Cobar region follows early pioneering work by Hills (1956) on the Darling River Lineament, and more recent work by Scheibner (1973, 1974) based on ERTS-1 imagery. Results of the Cobar mapping project are found in Pogson & Felton (1978), Barron et al (1982) and Glen (1982a, 1982b). StAuctu/ial

FJLammoA,k

o^

tkn

RaqloYi

The structural framework of the Cobar region corprises several fault sets shown in figure 1. In nearly all cases lineaments are expressed by drainage patterns, and in very few cases has it been possible to identify mesoscopic structures/ (mainly closely spaced fractures and joints) on the ground which correspond to lineaments. J. WNW tA^zndlYiQ ^Zt. Included here are the Crcwl Creek Lineament (CCL) and Nymagee Lineaments (NL) (the latter is in part a fault) both of which were identified by Scheibner as the western extension of his Lachlan River LineaiT^t. (Scheibner 1973, 1974, Scheibner & Stevens 1974) . The less welldefined Buckambool Lineament (BL)^the Elliston Lineament (EL) (in part a mapped fault) and other unnamed lineaments also belong to this set. 2. NE^ncUng Included in this set, which lies parallel to the Darling River Lineament, is the Cobar-Inglewood Lineament (C-IL) the Mount Hope Lineament (MHL) (Scheibner, 1973, 1974) and other unnamed lineaments.

3. urn iAzndlng This set includes the Jackermaroo Lineament (JL) (Scheibner, 1973), the NNW trending part of the Rookery Fault (RF) v ^ c h separates deep-water Early Devonian rocks on the west frcm shallow-water Devonian ones, and fran basement on the east. The Yanda Lineament (YL) and

33.

Airphitheatre Fault (AF) are also part of tiiis set. 4. W ^encUng This set incluc3es the N trending part of the Rookery Fault, the Thule Fault (TF) along the eastern sic3e of the Thule Granite, and the Myrt Fault (MF) . 5. A NWE ^eX consists, to date, of only the Buc3cwaroon Fault (BF) northwest of Cobar, and a parallel? extension of it to south west of Cobar. Together these faults and lineaments divide rocks of the Cobar region into a number of discrete blocks. Mov^ient on sane of these faults can be shown to have occurred at several times during the evolution of the region. Pxz-CoAboyiyLieAou^ Fault lAovmzYvU

Our view that faults have been repeatedly activated, is siminarised below. We think that many of the major faults were initiated during latest Ordovician to earliest Silurian deformation of the Girilambone and Tallebung Groups. 1. OKdoyjloAJxn to S^iluAlan. Euplacement of the Nymagee Igneous Corplex (foliated phase c. 440Ma) along part of the Nymagee Lineament. Emplacsnent of the Erimeran Giranite (c.420Ma) was controlled by NNW and NE faults. Silicic dykes within the Erimeran Granite point to an episode of extension along the Growl Creek Lineament. NNW faults also controlled large-scale juxtaposition of slices of Girilambone Group rocks of differing metamorphic grade. 2. EaJiZy VzvorUan. The Rookery Fault, parts of the Thule Fault, and possibly part of the Buckambool Lineament^acted as hinge zones, controlling areas of subsidence and thus the margins of deepwater troughs and basins. Silicic volcanics were associated with NNW faults. During the extensional phase of basin/trough formation, some faults of the WNW set may have acted as transforms. 3.

LateJit Ea/tly Vtvonian

- EoAly CaAb0yii^2A0(jb&. The change from marine dep-

osition of the Cobar Supergroup to fluviatile deposition of the Mulga Downs , Group in the latest Early Devonian, accctnpanied by paraconformable and disconformable relations between the two around Cobar, suggests block rearrangement in the Cobar region at this time. Local angular unconformities between these two imits SW of Cobar suggest south-block up movement on a fault of the WNW set (Glen, 1982a) . Thickness variations in the basal formation of the Mulga Downs Group, and a SSE palaeoflow direction in the lower part of this formation (C.Powell pers. conm) suggest that faults of the WNW set had topographic expression and controlled deposition around the later Early - Middle Devonian. CoAboyU^eJiotu RzactLvatLon

o^ MajoA FacLttd

During the Carboniferous deformation, major faults underwent a further period of re-activation. Sane propagated through the Cobar Supeirgroup and Mulga Downs Group, undergoing either high-angle reverse movement, strike slip movement or both. These are represented by faults in areas of cover or exposed basenent. In other cases, fault movement was restricted to basement lying below cover rock. Re-activation of these faults is inferred from patterns in folding and cleavage developed in cover sediments, with visible Landsat lineaments probably representing narrow zones of jointing (and fracturing) in cover rocks, leading to greater weathering and thus to the localization of drainage channels.

34.

Around Cobar itself, we suggest that (i) the N and NNW faults underwit high angle reverse rnovetnent, sane with later left-lateral i ^ ^ t , (ii) the WNW trending faults underwent left lateral moveinent, and ( m ) ^ r e was locally right lateral mDveitent on satie NE trending faults (Glen, 1983). Most of the faults in this set lie parallel to F, folds in the cover rocks, and sane may have undergone sane high angle revere movement. Economic Imptication^ The relationship between lineaments, faults and major ore-bodies of the Cobar type is indirect and two-fold. In the first case, C o ^ - ^ ore bodies a ^ restricted to muddy and silty turbidites (Glen, 1982b),and to to areas of dsepwater sedimentation. These are areas which underwent tiie greatest subsidence and extension in the Early Devonian - extension which was controlled by movCTient on pre-existing faults. In the second case, the Cobar-type orebodies are structurally c ^ t r o l l ^ and have been emplaced/reitobilized into structural locations in a stress field: controlled by inovenent on pre-existing faults and lineaments.

Figure 1.

Siirplified geology of the Cobar r e g ^ . B a s ^ on w r k ^ Baker, Brown, Felton, Glen, M a c r a e , f ^ o n , Scheitner, Trigg.

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

35.

Barron, L.M., Scheibner, E. and Suppel , 1932. The Mount Hope Group and its ccmagnatic granites on the Mount Allen 1:100,000 sheet New South Wales. Geological Survey of N.S.W., Quarterly Note 47, 1-17. Glen, R.A., 1982a. Nature of late-Early to Middle Devonian tectonics in the Buckainbool area, Cobar, New South Wales. J . Geol. Soc Aust., 29, 127-138. Glen, R.A. 1982b. The Ainphitheatre Group, Ccbar, New South Wales. Preliminary results of new mapping and inplications for ore search. Geological Survey of N.S.W., Quarterly Note 49, 1-4. Glen ,R.A 1983. Basement control on cover deformation. An example fron the Cobar area in the Lachlan Fold Belt of N.S.W., Australia. Abs. Int Cnfce on Multiple Deformation and Foliation Developnent (SGTSG Newsletter 9, 122) Hills, E.S. 1956. A contribution to the morphotectonics of Australia. J . geol. Soc. Aust, 3, 1-15. Pogson, D.J. and Felton, E.A., 1978. Rea^raisal of geology, CobarCanbelego - Mineral Hill region. Central Western New South Wales. Geological Survey of N.S.W., Quarterly Note 33,1-14. Scheibner, E. 1973. ERTS-1 geological investigations of New South Wales. Geological Survey of N.S.W. Report GS1973/382 (unpubl) . Scheil^ner, E. 1974. Fossil fracture zones (t^^osform faults), segmentation, and correlation problems in the Tasman Fold Belt System. In: The Tasman Geosyncline - A syir^xDsim in Honour of Professor Dorothy Hill, Qld. Div., Geological Society of Australia, Brisbane, pp 65-96. Scheibner, E. & Stevens, B.P.J. 1974. The Lachlan River Lineament and its relationship to metallic deposits. Geological Survey of N.S.W. Quarterly Notes 14, 8-18.

36.

Oil

FIG.1

Mulga Downs Group

Silurian Granitoids

Cobar Supergroup shelf; old, young trough volcanics

Tallebung Group

Devonian Granitoids

Q

Girilambone Group Fault or Lineament

37.

LINEAMENT - ORE RELATIONS Brian Marshall, Department of Applied Geology, NSWIT, Broadway, 2007

Relationships between rectilinear and circular (cyclolith) lineaments and ore deposits.continue to be recorded despite adverse comment on the methodology of correlation and doubt as to their exploration application• Finding the "controlling lineament" after the ore discovery presents little problem, but reversing the process necessitates an appreciation of the complex controlling relations (Marshall, 1979) between lineaments and ores. This will be examined with respect to rectilinear lineaments in the present paper. Controlling relations are direct, indirect, marginal or absent (Marshall, 1979). They arise through the interaction of factors specific to the ore deposit, and factors specific to the lineament. The principal factors pertaining to the deposit are its type, in terms of mineralogy and the major geological processes comprising its genesis; its age, in that certain deposits and settings for those deposits became extinct or evolved through geological time; and its scale, in that considering a plethora of minor mineralized showings diverts attention from the regional relations of exploration value to the local factors that usually become apparent once the discovery has been made. The principal factors pertaining to the lineament are its dimensions (length, width) and internal complexity, since small, simple lineaments are less likely to exercise indirect and marginal control relations than more complex types; its time-persistence, since longstanding, reactivated lineaments are more likely to influence ore-forming processes than short-lived types; and its parallelism or discordance to the regional tectonic grain, particularly since the advent of plate tectonic processes. Based on the above, deposits will be classified according to whether they have direct, indirect, marginal or absent controlling relations. An attempt will then be made to assess the control relations of differing types of lineament and thereby develop lineament significance criteria such that the exploration potential of lineaments is substantially upgraded. Reference Marshall, B., 1979. The Lineament-Ore Association, Economic Geology, 7£, 942-945.

38.

S T R U C T U R A L SIGNATURES TO M I N E R A L I S A T I O N E.S.T. O'Driscoll, Western Mining Corporation, Adelaide M i n e r a l deposits are c o m m o n l y found where t h e i r host rocks are i n t e r s e c t e d by t r a n s c u r r e n t regional l i n e a m e n t s (> 80 km long) w h i c h belong t o one or more of several lecognised systems. Examples have been described f o r N i , A u , Cu and U deposits ( R e f s . 4, 5, 6 & 7). One of the most c o m m o n , and o f t e n the most conspicuous, of ore l i n e a m e n t s is a W N W - t r e n d i n g line of s t r u c t u r a l disturbance r e p r e s e n t i n g the T e t h y a n s y s t e m , and giving the appearance of imposing a l e f t - l a t e r a l ( n o r t h - b l o c k - w e s t ) dislocation or d e f l e c t i o n of the host rocks where i t passes through t h e m , and through the position of the ore body. This c h a r a c t e r i s t i c s i g n a t u r e has been c o l l o q u i a l l y c a l l e d " t h e T e t h y a n t w i s t " . I t has the essence of an S-bend, and is f r e q u e n t l y seen both at a broad regional scale and in m i n i a t u r e at more l o c a l prospect scales. I t is e x e m p l i f i e d by m i n e r a l deposits and ore fields f e a t u r e d in the eight numbered figures accompanying this a b s t r a c t , to w h i c h the f o l l o w i n g captions correspond:F i g . 1. Ni gossans at Selcast L o c a t i o n - 1 , S p a r g o v i l l e , Western A u s t r a l i a occur on the u l t r a m a f i c / b a s a l t c o n t a c t , but only where t h a t c o n t a c t is crossed and d e f l e c t e d by a WNW l i n e a m e n t giving the c h a r a c t e r i s t i c l e f t - h a n d T e t h y a n t w i s t signature. S i g n i f i c a n t Ni-sulphides in depth are also c o n f i n e d to this d e f l e c t e d segment of the c o n t a c t . ( R e f . Auselex records). F i g . 2. The old Kapunda Cu mines. South A u s t r a l i a , are seen to be c o n t a i n e d in a WNW c o r r i d o r of s t r u c t u r a l disturbance independently recognisable in a p l o t of t o t a l geological ingredient o u t l i n e s . In the i l l u s t r a t e d d o u b l e - i n g r e d i e n t plan, the t w o ingredients are Tapley H i l l F o r m a t i o n ( c l e a r - l i n e areas) and Ulupa S i l t s t o n e (black areas). The d e f l e c t i v e e f f e c t imposed on these P r o t e r o z o i c ingredients by the l e f t hand T e t h y a n t w i s t can be seen along the path of the c o r r i d o r (see R e f . 10). F i g . 3. The C u - Z n deposit at Woodlawn, N.S.W. occurs in Silurian host rocks (S) at a position where the host rocks are crossed and d e f l e c t e d by the T e t h y a n WNW l i n e a m e n t A - B . The gossans and ore body occupy the elbow of the d e f l e c t i o n , w h i c h shows the t y p i c a l l e f t - h a n d c h a r a c t e r o f the T e t h y a n t w i s t signature (see R e f . 3). F i g . 4. The C u - A u - U ore deposit at O l y m p i c Dam, South A u s t r a l i a , is l o c a t e d on a long g r a v i t y ridge at the position where the ridge is crossed by a WNW photol i n e a m e n t c o r r i d o r w h i c h d e f l e c t s i t w i t h a t y p i c a l l e f t - h a n d T e t h y a n t w i s t . I t is where the c o r r i d o r and the g r a v i t y ridge i n t e r s e c t t h a t the ore occurs. For this deposit, the T e t h y a n t w i s t signature has h i s t o r i c a l s i g n i f i c a n c e in t h a t i t was used to define the t e c t o n i c t a r g e t in w h i c h the ore body was subsequently discovered (see R e f .

8). F i g . 5. A plot of regional f a u l t s and f r a c t u r e s in the Tennant Creek Block ( d o t t e d outline) in the N o r t h e r n T e r r i t o r y shows t w o WNW d i s t r i b u t i o n a l i g n m e n t s , A A* and B - B ' , w h i c h impose l e f t - h a n d Tethyan t w i s t s on the Block o u t l i n e . The main C u - A u ore deposits of the Tennant Creek (TC) f i e l d are found to be segregated along the A - A ' t r e n d (see R e f . 2). F i g . 6. A WNW s t r u c t u r a l c o r r i d o r passes through the Devonian core of the Melbourne trough, d e f l e c t i n g the whole assembly w i t h a l e f t - h a n d T e t h y a n t w i s t . The n o r t h e r n edge of this T e t h y a n c o r r i d o r also c o n t r o l s the upper reaches of the Goulburn R i v e r (GR) w i t h i n t h e trough, and Victorians g r e a t e s t g o l d f i e l d , Bendigo, is also l o c a t e d on the w e s t e r l y extension of this edge (see R e f . 11).

39.

R G U R E S 1, 2, 3 & 4. SEE TEXT FOR CAPTIONS, Fig. 7. Queensland's Etheridge gold deposits (black dots and clusters) near Georgetown (G) are located on the edges of a WNW Tethyan echelon c o r r i d o r independently defined from t o t a l geological ingredient outlines. The corridor has generated a t y p i c a l l e f t - h a n d Tethyan t w i s t in the margins of the post-Palaeozoic b a t h o l i t h (heavy outlines) surrounding the "Georgetown Window" (see R e f . 1). Fig. 8. T o t a l geological ingredient outline map of Queensland showing how the Tethyan WNW Toowoomba-Charleville lineament (T-C L) has imposed a l e f t - h a n d Tethyan t w i s t on the southern p a r t of the Adavale Basin ( c e n t r a l black area w i t h w h i t e margins). This example carries the Tethyan control to basin structures associated w i t h oil search (see Refs. 7 & 9).

40.

17'S

V

:

MCLBOURNe THOUGH DEVONIAN 100 Km

.

500

KM

8

R G U R E S 5, 6, 7 & 8. SEE T E X T FOR CAPTIONS. I t is emphasised that WNW Tethyan trends represent only one of several systems t h a t appear in concert and conjunction in the s t r u c t u r a l signature to mineral deposits. For instance, the Tethyan t w i s t has a conjugate partner which is its m i r r o r image i.e., an ENE trend w i t h a r i g h t - h a n d d e f l e c t i v e e f f e c t . This partner is known colloquially as "the Laurasian t w i s t " . Each partner in turn has its own orthogonal companion. They are never all equal c o n t r i b u t o r s to the f u l l s t r u c t u r a l signature. The WNW Tethyan t r e n d , however, appears to be the most common, and t h e r e f o r e to possess a fundamental p r i o r i t y in s t r u c t u r a l status. Together w i t h the a t t a c h e d eight figures, the additional slides displayed in the presentation and the listed references give

41.

examples of the Tethyan twist signature at a variety of scales and for a variety of Australian mineral occurrences of different kinds and ages. Among the many major deposits in other countries which exhibit the Tethyan trend prominently are the Zambia Cu-belt; the Rand (Au) deposits; Bingham Canyon (Cu) in Utah; and Homestake (Au) at Lead, South Dakota (see Refs. 6 & 7). Regional Tethyan trends and their associates are interpreted as representing major crustal disturbances which control the origin, distribution and structural setting of both syngenetic and epigenetic deposits. REFERENCES (1)

Bain, J.H.C., and Withnall, I.W., 1980. Mineral deposits of the Georgetown region. Northeast Queensland. In Henderson, R . A . and Stephenson, P.J. (Eds.), Geology and Geophysics of Northeastern Australia, p.129.148. Geol. Soc. Austral. Queensland Division, Brisbane.

(2)

D'Addario, G.W., 1976. Notes to accompany the Geological Map of the Northern Territory. Bureau of Mineral Resources, Geology and Geophysics, Australia. Bull. 180., Map scale 1:2,500,000.

(3)

Malone, E.J., et al., 1975. The Woodlawn copper-lead-zinc ore body, in Knight, C . L . (Ed), Economic Geology of Australia and Papua New Guinea, Vol.1, Metals, p.701-710. Austral. Inst. Min. Metall., Melbourne.

(4)

O'Driscoll, E.S., 1980. V.63, p.397-417.

(5)

O'Driscoll, E.S., 1981a. A broad-scale structural characteristic of major nickel sulfide deposits of Western Australia. Econ. Geol. v.76., p.1364-1372.

(6)

O'Driscoll, E.S., 1981b. Structural corridors interpretation. Mineralium Deposita, v.l6, p.85-101.

(7)

O'Driscoll, E.S., 1982. Patterns of discovery - the challenge for innovative thinking. 1981 P E S A Australian Distinguished Lecture, P E S A Jour., No. 1, p . l l 31. Petroleum Exploration Society of Australia.

(8)

O'Driscoll, E.S., 1983. Broken Hill at the cross-roads, in Broken Hill Conference - 1983; Conference Series No.l2, p.29-47. Australas. Inst. Min. Metall., Melbourne.

(9)

O'Driscoll, E.S., and Keenihan, S.L., 1980. The Toowoomba-Charleville lineament in Southern Queensland. Austral. Petrol. Explor. Assoc. Jour., v.20(l), p.16-24.

(10)

Thomson, B.P., 1969. S.A. Geol. Atlas Series Sheet SI 54-9 (Adelaide), Scale 1:250,000. Geol. Surv. South Australia, Adelaide.

(11)

VandenBerg, A.H.M., et al., 1976. Silurian-Middle Devonian, in Douglas, J.G., and Ferguson, J.A. (Eds), Geology of Victoria, p.45-76. Geol. Soc. Australia, Spec. Pub. No 5.

The double Helix in global tectonics, Tectonophysics,

in

Landsat

lineament

42.

N O T E S

43.

SYMPOSIUM 2:

ACCRETIONARY PRISMS: AND PROCESSES

(Convenor:

E. C. Leitch,

CHARACTERISTICS

University of Sydney)

PROGRAMME Session 1:

Chairman - E. Scheibner

8.55 a.m.

Convenor's welcome and introduction

9.00 a.m.

J. C. Moore (University of California, Santa Cruz). Depositional systems and structural styles of m o d e m forearcs.

10.00 a.m.

J. B. Keene (University of Sydney. Tectonic setting of the New Britain Trench and the Trobriand Trench, western Solomon Sea.

10.30 a.m.

Morning Tea

Session 2:

Chairman - R. Offler

11.00 a.m.

K. A. W. Crook (Australian National University). Tectonic models and con^arative studies of ancient fore-arc terrains.

12.00 noon

C. L. Ferguson (University of New England). The New England Subduction Complex, eastern Australia.

12.30 p.m.

Lunch

Session 3:

Chairman - V. A. Gostin

1.45 p.m.

Magmatism in foreR. H. Flood (Macquarie University) . arc complexes: the timing, the character, and the causes.

2.15 p.m.

J. C. Moore (University of California, Santa Cruz). Structural evolution of the Kodiak accretionary complex: consequences of ridge-trench interaction in a more southerly latitude.

2.45 p.m.

B. L. Wood (University of New South Wales). Cenozoic accretion in Taiwan and stmctural trends around the South China Sea.

3.15 p.m.

Afternoon Tea

44.

Session 4:

Chairman - H. J. Harrington

3.45 p.m.

R. J. Norris (University of Otago). The Rangitata Orogen of New Zealand - accretionary wedge^ accreted wedges or exotic terranes?

4.15 p.m.

C. McA. Powell, P. J. Conaghan (Macquarie University). Assessment of the accretionary prism model for the eastern Lachlan Fold Belt in the Late Ordovician.

4.45 p.m.

General Discussion

5.15 p.m.

Refreshments

45.

Depositional Systems and Structural Styles of Modern Forearcs. J. Casey Moore, Earth Sciences and Marine Studies> University of California, Santa Cruz, CA 95064 Depositional systems of modern forearcs subdivide into three main groups: continental margin forearcs; oceanic forearcs with lateral sediment supply, and sediment-starved oceanic forearcs. Continental margin forearcs (e.g. S. Mexico & E. Aleutians) are dominated by terrigenous sediment with turbidite filled trenches, slope basins and forearc basins, and have slope aprons of terrigenous mud. Oceanic forearcs with lateral sediment supply (e.g. Lesser Antilles and Central Aleutians) have turbidite trench-fill, but slopes covered principally by pelagic to hemipelagic sediment. Relatively sediment-starved oceanic forearcs (e.g. Marianas) have little trench fill and slopes mantled by pelagic to hemipelagic deposits. Continental margin forearcs or oceanic forearcs with lateral sediment supply construct macroscopically "ductile" prisms of accreted sediment that show distributed deformation (e.g. Barbados, E. Aleutians, S. Mexico). Fold-and-thrust belt structural style characterizes the initial deformation of overpressured sediments in ductile forearc regions. Seismic reflection profiles reveal decollements with intervening thrust ramps. Thickness of initial thrust packages is directly proportional to the thickness of sediment on the incoming oceanic plate. Because of their weak consolidation, continuing deformation of ductile forearcs ultimately develops a complicated, stratally disrupted structural style characteristic of onland accretionary complexes. Ductile forearcs reveal high rates of tilting or kneading of the slope apron deposits, with an exponentially decreasing rate landward of at least one trench (S. Mexico). Available estimates suggest less than 50% of total convergence is released near the seaward deformation front of "ductile" forearcs. For example, only about 5% of the total convergence is taken up near the seaward edge of the northern Barbados Ridge with decreasing strain arcward. Here deformation is partitioned into distributed deformation (small-scale faulting and fabric development) and discrete faults that separate large-scale structural units within the accretionary prism. The discrete faults constitute potential terrane boundaries where substantial strike-slip may occur. Convergent plate boundaries with low sediment supply develop forearcs underlain by both oceanic and island arc ophiolitic basement complexes with deformation localized near the base of the trench slope (e.g. Marianas, Tonga, Guatemala). DSDP penetrations of these relatively "rigid" forearcs reveal low rates of tilting of slope apron deposits and locally conspicuous faulting. Analysis of faults from cores in the Mariana forearc reveals conspicuous strike-slip faulting supporting transform scenarios for tectonic erosion and terrane removal. Small-scale structures of DSDP cores from forearcs include stratal disruption, scaly and spaced foliation, kink folds, vein structure, and microfaults. Stratal disruption is associated with cataclasis of sands, perhaps with overprinting solution affects. Intensity of cataclasis correlates with magnitude of bedding dip and is developed in sediments 200 to 300m below the sea floor. Cataclastic fabrics of the DSDP cores are precursors to web structure or cataclastic shear networks common in accretionary complexes onland. Scaly foliation is preferentially associated with reverse faults and shows anastomosing ultramicroscopic shear surfaces. Mesoscopic to microscopic kink folds are formed by

46.

mechanical crenulation of sedimentary layers during initial layer-parallel shortening in the proto-thrust zone of the Nankai Trough. Axial surfaces of the kink folds dip at about 60 degrees and correlate with seismically resolved thrust faults. Spaced foliation involving the development of new phyllosilicate phases occurs in late Miocene sediments that have never been buried more than 300m. Micro- and ultramicroscopic studies of pseudoveins show no evidence of hydrofracturing but rather suggest slow fluid expulsion following disaggregation by grain-boundary sliding and displacement in an extensional upper slope environment.

47.

TECTONIC SETTING OF THE NEW BRITAIN TRENCH AND THE TROBRIAND TROUGH, WESTERN SOLOMON SEA J. B, Keene and Shipboard Scientists

During December 1983 and early January 1984 a geological investigation of the Solomon Sea was carried out using the Japanese research vessel M/S Natsushlma. Data from single- and multi-channel seismic reflection, magnetics, coring and dredging have enabled new interpretations to be made regarding the origin and tectonic significance of the New Britain Trench and Trobriand Trough. The ship-track and principal morphologic features of the Solomon Sea are shown on the bathymetric map in Figure 1. Along the northern margin the New Britain Trench ranges in depth from 6 km in the west to 8.5 km in the east. South of the trench the sea floor rises to a series of east-west ridges and valleys (water depth 3 to 5 km) occupying the central part of the western Solomon Sea. Along the southern margin is the sediment-filled Trobriand Trough (5 to 5.5 km deep). Several mid-slope sedimentary basins characterise the southern side of this trough. The shallow waters of the Trobriand Platform are cut by several large sxabmarine canyons and further to the west the axis of the Huon Gulf is occupied by the Markham Canyon. Bathymetric trends (N105®E) on the oceanic plate are controlled by normal faulting of the sedimentary cover as well as the underlying basaltic basement. Sheared lithified mudstone and pillow basalts are exposed on these scarps. To the west the distinct arching of the plate (with many faults) is masked by recent turbidite sedimentation. Our data confirm that the Trobriand Trough is a subduction system and deformation in the near-surface sediment suggests recent tectonic activity. An abrupt deformation front occurs in the trench fill at the base of the slope. The basement in the axis of the Trobriand Trough is at a depth comparable to that in the New Britain Trench. Accretionary ridges of deformed sediment compose structural highs on the inner slope of the trench and form perched basins filled with slump deposits, pebbly mudstones and turbidites. The deeper strata show distinct tectonic tilt. In contrast the New Britain Trench is relatively free of sediment. Pelagic sediments are overlain by a relatively thin turbidite sequence restricted to the axis of the trench. In the west the sediment fill thickens and the trench curves south-west and continues along the southern margin of the Huon Peninsula and its submarine extension. The inner slope of the trench appears to lack a large accretionary prism and the slope surface is very irregular.

48.

Figure 1:

Bathymetric map of the western Solomon Sea (500 m contour interval) showing ship-track for Leg 1 and seismic line numbers and station locations.

49.

TECTONIC MODELS AND COMPARATIVE STUDIES OF ANCIENT FORE-ARC TERRAINS KEITH A.W, CROOK Department of Geology, Australian National University Canberra Australia Conceptual models, which relate observed rock associations and relationships to formative processes, are widely used in sedimentology and tectonics. Some are specific to particular regions: no general applicability is claimed. Others usually include variants of some common theme, thereby accommodating observed differences between.regions. In tectonics, unlike sedimentology, the development and application of models is unsystematic. Procedures for their construction and criteria for their application and validation are needed. A general tectonic model must summarise the essential features of several particular cases. It must serve as a norm for comparative purposes; be a guide for future observations; have potential as a predictor for new geological situations; and provide a basis for process-interpretation of the system it represents. Tabulation of observed tectonic attributes, followed by systematic comparison of regions, provides a systematic basis for constructing a general model. Its validity or relevance to any region may be assessed by comparing tabulations of model attributes and observed regional tectonic attributes. Useful working hypotheses may result. Process aspects remain conjectural. This procedure is exemplified using a fore-arc model of crustal evolution and regions in Australia and other continents.

50.

THE NEW ENGLAND SUBDUCTION COMPLEX, EASTERN AUSTRALIA

CoL. Fergusson, Department of Geology and Geophysics, University of New England, Armidale, N.S^W., 2351e

The main tectonic elements of the New England Fold Belt in the Late Palaeozoic were: a continental margin volcanic chain in the west, a forearc basin in the centre, and a subduction complex in the east- In the southern part of the Fold Belt remnants of the volcanic chain are covered by the PermoTriassic Sydney Basin, the forearc basin is preserved in Zone A (Leitch, 1974) and the subduction complex is exposed in Zone B (Leitch, 1974). During the Early Permian the subduction complex was oroclinally folded and disrupted by strike-slip faulting. The subduction complex in the southern New England Fold Belt consists of three principal types of melange, relatively coherent units and a suspect terrain. The melange types are: (1) serpentinite-matrix melange; (2) melange formed by the tectonic disruption of coherent marine strata and associated igneous rocks; and (3) deformed olistostromes. The relatively coherent units consist of folded and faulted thick marine sequences. Serpentinite-matrix melange (type 1) occurs along the Peel Fault System and in the Nowendoc, Port Macquarie, Yarras and Baryulgil areas. This type is dominated by autoclastic melange and consists of tectonically moulded mafic, exotic and serpentinized ultramafic blocks contained in a sheared serpentinite matrix (Cross, 1983). Type 2 melange is widespread throughout Zone B. A specific example is the Gundahl Complex of the central Coffs Harbour Block. The principal lithologies in the Gundahl Complex are greywackes, greywacke-argillite, argillites, argillite-tuff, bedded cherts and greenstones. Structurally, the Gundahl Complex consists of slabs and blocks contained in a relatively deformed matrix^ The matrix is comprised of argillite with many slickensided and polished shear fractures that finely divide the rock into many small chips. Slabs and blocks within the Gundahl Complex display varying intensities of internal deformation. Greywacke and greenstone blocks are characterised by »web structure' (networks of cataclastic veins) and the blocks themselves pinch and swell. Slabs containing well-bedded sequences are disrupted by abundant cryptic bedding-parallel faults and may contain numerous folds. In some instances an axial surface cleavage is associated with these folds. On a map-scale much of the Gundahl Complex has an imbricate structure with slabs ranging up to 10 km in length effectively repeating the disrupted pre-melange marine sequence. A tectonic origin for the Gundahl Complex is evident from the abundance of tectonically formed structures and the lack of any characteristics diagnostic of sedimentary mixing. Other examples of type 2 melanges in Zone B are the Woolomin-Myra beds, the Sandon beds and parts of the Texas beds. An example of the areally restricted type 3 melange has been described by Leitch and Cawood (1980) from the Woolomin Association.

51,

Relatively coherent units are abundant throughout Zone B. An example is the Coffs Harbour beds in the central Coffs Harbour Block. Most of this unit consists of thin-bedded turbidites with interbedded thick-bedded and very thick-bedded turbidites which were probably deposited in a sedimentfilled trench. These rocks are deformed into tight to isoclinal macroscopic folds, the vergence of which is consistent with the predominant sense of younging to the northeast. Axial surfaces are either steeply dipping to the southwest or vertical, and are typically faulted. In addition to thickening related to macroscopic faulted folds, the Coffs Harbour beds have been tectonically thickened by repetition along cryptic bedding-parallel faults. Other examples of relatively coherent units include the Girrakool beds and parts of the Texas beds. A suspect terrain, the Silverwood Association, occurs in the northeastern part of Zone B. This terrain consists of a monotonous assemblage of tuffaceous mudstones and greenstones, with minor limestone breccias, conglomerates, greywackes, argillites, and cherts. The Silverwood Association formed in either an island arc or intraoceanic back-arc setting prior to its incorporation into the subduction complex of Zone B. The subduction complex of Zone B formed by the off-scraping and accretion of layer 1 (±?layer 2) oceanic crust and overlying trench-fill material at a subduction zone active throughout the Devonian and Carboniferous. Type 2 melanges were produced during episodes of rapid subduction and/or lower sedimentation rates, whereas relatively coherent units reflect slower subduction and/or higher sedimentation rates. Type 3 melanges apparently represent shoaling and erosion of parts of the subduction complex. CROSS, K.C., 1983. Unpub. Ph.D. thesis, Univ. New England. LEITCH, B.C., 1974. Geol. Soc. Aust. y J., 21, 133-56. LEITCH, E.C. § CAWOOD, P.A., 1980. Sediment. Geot., 25, 5-22.

52,

Magmatism in Fore-Arc Complexes: the Timing, the Character and the Causes R.H. Flood Macquarie University At convergent plate boundaries the fore-arc is the 100-200 km wide belt lying between the trench and a line, normally parallel to the trench, that marks the limit of volcanism closest to the trench. Although in this definition the fore-arc is non-volcanic, there are several volcanoes which are developed atypically close to the trench and not forming part of the trench-parallel volcanic arc, that are termed fore-arc volcanoes. These volcanoes, which erupt either calc-alkaline andesites, boninites or tholeiitic basalts, obviously have sub-crustal source rocks. Some but not all of these volcanoes have been explained as being formed where a spreading ridge and a convergent-plate boundary intersect. These occurrences are so rare and the volcanic products, with the exception of the boninites, so similar to the arc volcanoes that they offer little chance of being useful in reconstructing ancient convergent plate boundaries. Ancient fore-arc complexes in Alaska, New England and ?Newfoundland have been intruded by large S-type granitoid batholiths during episodes of changing plate boundary configuration. In two of these examples the change has been inferred to involve the intersection of the spreading ridge and the trench. These S-type batholiths have'been attributed to the partial melting of the deeper part of the fore-arc complex metasediments with or without a basaltic component coming from the spreading ridge. Because these S-type plutons are both distinctive and reasonably abundant they hold^ considerable promise as indicators of the position and polarity of ancient plate boundaries. Although most of these S-types are developed outboard of the earlier and ?coeval arc volcanism the S-type granites of the Lachlan Fold Belt where they were first defined are the oldest plutons exposed and lie "inboard" (in most interpretations) of the slightly younger I-types. Perhaps the older arc is hidden beneath the younger cover to the west or perhaps i5-type granites can form in different tectonic situations. Many of the larger I-type granite batholiths are emplaced into sediments of deep water origin, some (?most) of which are part of earlier fore-arc complexes. These plutons are emplaced so long after the formation of the fore-arc complex 50 Ma) that a connection between this magmatism and the initial formation of the fore-arc complex becomes tenuous. Many authors would prefer to see this"later magmatism as marking the development of a new magmatic arc "outboard" of the earlier arc. Certainly these granitoids are derived, in part at least, from mantle-derived basic rocks (or melts) that would not be exx^ected to develop simply as a consequence of fore-arc aging.

53.

Structural Evolution of the Kodiak Accretionary Complex: Consequences of Ridge—Trench Interaction in a more Southerly Latitude• J, Casey Moore, Earth Sciences and Marine Studies, U.C. Santa Cruz, Santa Cruz, CA 95064 The Kodiak accretionary complex consists from landward to seaward of (1) a lower Mesozoic blueschist, (2) an Upper Cretaceous melange, (3) an uppermost Cretaceous slate and greywacke belt, (4) a Paleocene turbidite and volcanic sequence and (5) an Eocene submarine fan complex. The accreted Eocene and Paleocene units are stratally disrupted with evidence for widespread development of cataclastic shear zones (web structure) and limited pressure solution. The uppermost Cretaceous slate and greywacke belt exhibits minor stratal disruption and cataclasis but conspicuous isoclinal folding and pressure solution. The Upper Cretaceous melange is stratally disrupted and was subjected to calaclasis now overprinted by pressure solution. At least four penetrative deformations characterize the blueschist: two phases of isoclinal folding, kink folding, and late-stage faulting. An early Paleocene magmatic event invaded the Kodiak accretionary complex with mid-ocean ridge basalt (MORB), andesite, and granodiorite. Geochemical evidence suggests both the andesite and granodiorite are mixtures of MORB and sediment. The volcanic rocks are interlayered with coarse clastic sediments and deformed with the latter; the granodioritic intrusions, with K-Ar ages as old as 62 Ma postdate emplacement of the early Paleocene sedimentary and volcanic rocks. Interaction of the subduction complex with the Kula-Farallon ridge most simply explains this early Paleocene magmatic episode. Paleomagnetic studies of the volcanic rocks indicate this interaction occurred at 40.3+6® north latitude, which is 25.3+9® south of the expected position of the Kodiak Islands relative to North America. The magmatic arc presently northwest of the Kodiak accretionary complex was active during the Late Cretaceous and early Paleocene, diminished after the ridge-trench interaction, and resurged from late Eocene into Miocene time. The magmatic resurgence is coeval with the obductive offscraping of Eocene to 01igocene(?) submarine fan deposits in the Kodiak Islands. The petrology of these fan deposits indictes they are the most likely proximal equivalent for the Zodiac fan, thereby limiting its relative motion with respect to the Kodiak Islands. Possible models accounting for the northward motion of the Kodiak accretionary complex include (1) coupling the accretionary complex and possibly related arc to the northward-moving Kula Plate and closing an ocean basin north of the arc in Paleogene time, (2) emplacing the Kodiak accretionary complex at the northward limit of the paleomagnetic data, with subsequent northward motion accomplished by closure of a small back arc basin and/or intracontinental shortening, and (3) transcurrent faulting and northward motion of the Kodiak accretionary complex with or without the presently adjacent arc.

54.

Cenozoic Accretion Structural

i n T a i w a n and

Trends around

the S o u t h China

Sea

B . L . Wood, S c h o o l o f A p p l i e d n e o l o ^ U n i v e r s i t y o f New S o u t h Wales Taiwan c o m p r i s e s accreted

two m a j o r C e n o z o i c p l a t e

a t a NNE t r e n d i n g

and i s one o f collisional

the L o n g i t u d i n a l

Valley,

the w o r l d * s y o u n g e s t ,

most c o m p l e t e l y

exposed

terrains.

From e a s t tectonised

to west,

andesitic

the C o a s t a l Range c o n t a i n s

arc

and t r e n c h

Sea P l a t e ,

the

a massive

Pliocene alistostromal

Melange)

margins

suture,

fault-aligned

as w e l l

deformed Eocene Eurasian Plate

to Miocene

The d o m i n a n t C e n o z o i c trend r e p r e s e n t e d metamorphic

terrestrial

piedmont

contain thick

(Lichi

structural

molasse,

strongly

and s l o p e d e p o s i t s Palaeozoic

by o v e r p r i n t e d

basement

includes

melange

shelf

a Late

a

Philippine

ophiolitic

Provinces

overlying

the

Longitudinal Valley

a s more r e c e n t

and t h e C e n t r a l - W e s t e r n

slope of

schist

of

the

basement.

t r e n d s a r e NNE, b u t an

folds

and l i n e a t i o n s

in

t r e n d s WNW and i s

probably Late

Permian

earlier

the

i n age • A similar relationship where f o l d e d

f o l d s and l i n e a t i o n s

c l a s t i c s h a v i n g NE t r e n d s . schist

Cretaceous

ophiolite. i n basement

separated

Oligocene- Miocene

I n s o u t h e r n Palawan,

exhibits

volcanifolds

i n Late

a l s o p l u n g e NTV b e n e a t h NE t r e n d i n g

The s i m i l a r i t y

without

a t Hong Kong

( T o l o Harbour Formation)

p l u n g i n g NW b e n e a t h J u r a s s i c

Palaeozoic

three widely

c a n be r e c o g n i s e d

Permian p h y l l i t e

structural

localities

seafloor

is

spreading

significant rotation of

trends at

attributed

these

to

i n the South C h i n a

the d i s p l a c e d

segments.

Sea

55.

The Rcingitata Orogen o f New Zealand - Accretionary wedge, accreted wedges or exotic terranes? R.J.

Norris

CUniversity

of Otagoj

New

Zealand)

The Rangitata Orogen developed as part o f the P a c i f i c margin of Gondwanaland from Permian to Cretaceous time. The only proven Pre Cambrian or Lower Paleozoic continental crust i s exposed in the early Paleozoic Tuhua Orogen west o f the Median Tectonic L i n e . East o f this l i n e , only rocks o f Late Paleozoic age and younger are found, even in the Southern Alps where u p l i f t has exposed rocks o r i g i n a l l y buried below 30 km depth. Much of the New Zealand continental block therefore, appears to have formed by accretion during late Paleozoic and Mesozoic time. The precise manner of this accretion, however, and the o r i g i n of the accreted materials, continue to be a subject o f some controversy, i n the same way that divergences of opinion have arisen over the interpretation of other similar Circum-Pacific terranes. The Rcuigitata Orogen may be subdivided into a series o f subparallel terranes. Immediately northeast of the Median Tectonic Line is the Hokonui terrane, which consists o f a number of u n i t s . The western. Brook S t r e e t , u n i t represents the flanks of an early Permian ensimatic volcanic arc. This i s overlain unconformably by T r i a s s i c and Jurassic volcanoc l a s t i c rocks o f the Murihiku Supergroup, interpreted as representing a fore-arc b a s i n . These are underlain on the northeast side by the late Permian volcanogenic deep-marine Maitai Group which rests in sedimentary contact upon the Dun Mountain O p h i o l i t e . I n many places beneath the Maitai rocks, the o p h i o l i t e i s essentially i n t a c t , but elsewhere, and particularly along the northeast margin, i t i s composed of a series o f tectonic melanges. These melange zones probably represent a major thinist complex separating the Caples and Hokonui terranes. To the northeast o f this suture the rocks are much more deformed and metamorphosed than those to the southwest. The Caples terrane i s composed largely of Permo-Triassic, volcanoclastic, redeposited sediments i n the form o f a series o f l a r g e , i s o c l i n a l l y folded coherent sequences cut by zones o f broken formation and melange, the latter containing disrupted spilite-chert sequences. I n contrast to the Hokonui terrane, Caples rocks are poorly f o s s i l i f e r o u s and contain no recognised shallow water deposits. The Caples terrane appears to pass northeastward into the Aspiring terrane, at the same time becoming progressively overprinted by regional metamorphism which obscures many o r i g i n a l structural and stratigraphic relationships. The Aspiring terrane is dominated by metabasites, cherts and p e l i t i c schists with occasional serpentinite pods, presumably representing a disrupted oceanic association. The remainder o f the exposed orogen, and by far the largest part i n voliame, is composed o f the Torlesse terrane, comprising mainly quartzofeldspathic greywackes and their schist equivalents. These may be subdivided into the older Torlesse, or Rakaia terrane, of Permo-Triassic age, and eastern younger Torlesse or Pahau terrane of Jurassic-early Cretaceous age. These two are separated by the Eskhead melange. Torlesse petrography suggests an e n s i a l i c arc provenance, and sediments are largely redeposited although important occurrences of shallow and non-marine facies are also found. The structure i s complex and stratigraphy uncertain - i n some areas pockets of coherent strata separated by imbricate faults suggest an accretionary wedge o r i g i n ; i n other areas this structure is less w e l l developed. Fragments of f u s u l i n i d warm-water

56.

limestones associated with basaltic volcanics^ cherts and quartzose sandstones suggest the incorporation of truly exotic elements. Most interpretations accept the Hokonui terrane as an arc-fore-arc complex with the ophiolite as oceanic crust beneath the outer fore-arc basin• Early models had this arc complex in its present position adjacent to the Western Province continental margin, but the absence of a large part of the volcanic arc and the scarcity of continental detritus in the Murihiku sediments suggest that importaint displacorients may have occurred on the Median Tectonic Line. This is further indicated by the presence west of and structurally beneath the Brook Street rocks of the Drumaduan terrane, comprising faulted slivers of probably Jurassic tuffs, volcanogenic sediments and silicic volcanics, in places containing metamorphic lawsonite. Major displacements on the Median Tectonic Line prior to the mid Cretaceous led to the accretion of the Brook Street arc to the Gondwana margin. Early plate tectonic interpretations combined Caples-Aspiring and Torlesse terranes as trench sediments forming an accretionary prism related to the Brook Street arc. The quartzofeldspathic provenance of the Torlesse and its age equivalence to Caples suggests this is unlikely. Also the presence of the Aspiring terrane between the two suggests open ocean at some stage outboard of the Caples. Subsequent models for the origin of Torlesse terranes have involved either strike-slip displacements of laterally equivalent accretionary prisms and/or submarine fans, or the collision and subsequent disappearance of a lost continent. Current evidence tends to favour the lateral accretion, by oblique-slip movements along the active margin, of slices of sedimentary acciimulations from further east. The Caples-Aspiring terrane is usually accepted as a trench slope accretionary prism associated with the Brook Street arc, although even here, there is little direct evidence to tie it to the rocks west of the ophiolite belt, and at least one model suggests it is an allochthonous terrane. The accretion of the older Torlesse and Caples-Aspiring terrane took place in the late Triassic, at which time they were deformed and metamorphosed and subsequently supplied detritus to the younger Torlesse. Despite the main features of the Rangitata Orogen being fairly clear, the degree of allochthonicity and significance of lateral displacements is still uncertain. However it is suggested that the lateral accretion of slices by oblique-slip parallel to an active margin is widespread around the Pacific and perhaps is as characteristic of the "Pacific-type" orogeny as the more traditional accretionary prism model.

57.

MAJOR TECTONIC SUBDIVISIONS OF THE RANGITATA OROGEN SOUTH ISLAND, NEW ZEALAND

EM RISTCHURCH 100 km

DUNEDIN

I

Western Province (Tuhua Orogen) Hokonui Terrane I Brook Street

EZH^ Murihiku/Maitai Dun Mountain Ophiolite Belt Caples Terrane Y / ' / A Aspiring Terrane F T T l Rakaia (older) ^

Torlesse M Pahau (younger) Terranes

D Drumaduan Terrane MTL Median Tectonic Line EM Eskhead Melange

58.

SELECTED REFERENCES ON THE TECTONIC INTERPRETATION OF THE RANGITATA OROGEN

(in order of publication) LANDIS, C.A. & COOMBS, D . S . 1967. Metamorphic belts and orogenesis in Southern New Zealand. Tectonophysics 4: 501-518. FLEMING, C.A.

1969. The Mesozoic of New Zealand: Chapters in the history of the Circxim-Pacific mobile belt. Q . J . Geol. Soc. Lond. 125: 125-170.

LANDIS, C.A. & BISHOP, D . G .

1972.

Plate tectonics and regional

stratigraphic-metamorphic r e l a t i o n s i n the southern p a r t o f the New Zealand g e o s y n c l i n e . B u l l . G e o l . S o c . Am. 8 3 :

2267-2284. BRADSHAW, J . D .

& ANDREWS, P . B . 1973. Geotectonics Geosyncline. Nature P h y s . S c i . 2 4 1 :

and the New Zealand 14-16.

COOMBS, D . S . , LANDIS, C . A . , NORRIS, R . J . , SINTON, J . M . , BORNS, D . J . & CRAW, D. 1976. The Dun Mountain Ophiolite Belt, New Zealand, its tectonic setting, constitution and origin, with special reference to the southern portion. Am. J . Sci. 276: 561-603. NORRIS, R . J . , LANDIS, C.A. & WARD, C.M. 1977. The relationship of the Rcingitata Orogen to possible plate configurations in the South Pacific. (Abs.) Geological Society of New Zealand, Programme of Abstracts, Queenstown Conference. CARTER, R . M . , H I C K S , M . D . ,

NORRIS, R . J .

& TURNBULL, I . M .

1978.

Sedimentation patterns in an ancient arc-trench-ocean basin complex: Carboniferous-Jurcissic Rangitata Orogen, New Zealand. ^ D . J . Stanley & G. Kelling (eds.) "Sedimentation in submarine canyons, fans and trenches", Dowden, Hutchinson & Ross, Stroudsberg, V a . : 340-361. SPCiRLI, K.B.

1978. Mesozoic tectonics. North Island, New Zealand. Geol. Soc. Am. 89: 415-25.

Bull.

BRADSHAW, J . D . , ADAMS, C . J . & ANDREWS, P.B. 1980. Carboniferous to Cretaceous on the Pacific margin of Gondwana: the Rangitata phase of New Zealand. Proc. 5th Int. Gondwana Symp.: 217-221. HOWELL, D.G.

1980. Mesozoic accretion of exotic terranes along the New Zealand segment of Gondwanaland. Geology 8: 487-491.

KAMP, P . J . J .

1980. Pacifica and New Z e a l a n d : proposed eastern elements Gondwanaland's h i s t o r y . Nature 2 8 8 : 659-664.

McKINNON, T.C.

1983. Origin of the Torlesse terrane and coeval rocks. South Island, New Zealand. Bull. Geol. Soc. Am. 94: 967-985.

in

59.

ASSESSMENT OF THE ACCRETIONARY PRISM MODEL FOR THE EASTERN LACHLAN FOLD BELT IN THE LATE ORDOVICIAN By C.McA. Powell § P.J, Conaghan School of Earth Sciences, Macquarie University, North Ryde, N.S.W. 2113 Regional stratigraphic relations for the Late Ordovician in southeastern Australia suggest that the eastern Lachlan Fold Belt was part of an east-facing island-arc marginal-sea system (Cas et at. 1980, Powell 1983a, h). In the prefeired model (Fig.l), the palaeogeography of the eastern Lachlan Fold Belt consisted of a N- to NW-trending island arc running from near Kiandra towards Molong, and thence northwestward under the cover of the Great Artesian Basin (Powell, 1983Z?). Behind the island arc (i.e. to the west) was a small marginal sea which may have been floored by ocean crust in part, and in front was an accretional prism complex in which the extensive Middle and Late Ordovician sediments of the eastern Lachlan Fold Belt were deposited. The dimensions of the system may have been similar to the m o d e m Andaman System (Fig.l).

2 ^

Shoalwafer ctasfic sedimsnfs

Shoaiwafer limestones

|v y I

Primary volcanic sequences

Land

Areas of greater rfian 200m wofer depfft (Andaman system)or flyscnoid fac:es (S £ Australia)

| o

Small volcanic seamounr (submerged greater than 200m)

Submarine disfnoufary system

A

Fig,

I '

Alcoch Seamounr

1 Comparison of Late Ordovician facies patterns of southeastern Australia with the present-day facies of the Andaman-Nicobar system. Modified from Cas et al. (1980, fig. 5), with Andaman System inverted and routed 15® counter-clockwise to improve its match with the Ordovician of southeastern Australia.

60.

In the postulated accretionary prims complex, two major fades associations can be distinguished: (a) a black shale/mafic volcanics/chert facies (the Wagonga Beds), in places overlain conformably by (b) quartzose turbidites of both distal and proximal facies. Palaeocurrent and provenance studies in the quartzose turbidites has enabled further subdivision into a feldspathic sublitharenite facies and a quartzarenite facies. The feld^pathic sublitharenite corresponds to Packham's (1969) coastal greywacke and slate, and has palaeocurrents indicating an eastward slope. The quartzarenite facies occurs further inland and generally has unimodal sole markings indicating south to north palaeoflow. Powell (1983a) suggested the junction between these facies probably lay along the line of the Budawang Synclinorium in eastern N.S.W., and that the eastern feldspathic sublitharenite was probably deposited on, or at the foot of, the trench slope. The quartzarenite turbidites were interpreted as craton detritus fed axially along the forearc basin (Fig.2). Powell (1983a) also suggested that the higher proportion of feldspathic material in the turbidites most distant from the inferred island-arc source was a result of mixing of quartz-poor arc derived material with the craton-derived quartzarenites by slumping down, and resedimentation at the foot of, fans on the outer-arc slope.

Skm

Cratonic detritus (quartz flysch) Arc detritus Mixed cratonic a arc detritus Mafic

• Fig 2

volcanics

Layer I Layer 2 a 3

Ocean floor

MIDDLE to LATE ORDOVICIAN GEOGRAPHY ( Darriwilian S Gisbornian) 460 Ma

Scaled block diagram of Middle to Late Ordovician paiaeogeography. Arrows indicate palaeoflow direction.

The accretionary prism model is consistent with most of the known sedimentological information, but we point out that the level of knowledge is still scanty, and other interpretations may be possible. Three features which need further investigation are: (1) the nature of the mafic volcanics in the Wagonga Beds, particularly since they contain relatively large vesicles suggesting eruption in relatively shallow C<500m) water; (2) the lack of any recognized chaotic bedding, sedimentary melange or imbrication in the proposed subduction complex; and (3) the possibility that much of the quartzarenite turbidite is younger than the last volcanic episode in the andesitic volcanic-island arc immediately to the west.

61.

Feature (1) may be explained if the mafic volcanics are part of exotic seamounts added to the accretionary prism during subduction. Feature (2) may indicate a rather slow rate of subduction allied with a rapid rate of sedimentation, and feature (3) may indicate that the defunct island arc continued to be buried beneath the blanket of quartzose detritus derived from the Gondwanan craton to the south. This last feature is also consistent with the general observation that in most areas the uppermost part of the Ordovician turbidite succession is a black cherty shale with very thin silty laminations suggesting deposition a long way from major feeder fans.

6 km

Cratonic detritus (quartz flysch) Other symbols as for Middle to Late Ordovician

LATEST ORDOVICIAN to EARLIEST SILURIAN GEOGRAPHY

(Eastonian, Bolindian a early Llandoverian) ^

440 Ma

Fig. 3 Scaled block diagram of the latest Ordovician to earliest SUuran geography of the N.S.W. South Coast. Note that upper surface is 2 km below sea level.

REFERENCES CITED CAS, R.A.F., POWELL, C.McA. § CROOK, K.A.W., 1980: Ordovician palaeogeography of the Lachlan Fold Belt: A m o d e m analogue and tectonic constraints. Geol. Soa. Aust. y J., 27, 19-31. PACKHAM, G.H. (ed.), 1969: Aust.j J.y 16.

The geology of New South Wales.

Geol. Soa.

POWELL, C.McA., 1983CI: Geology of the N.S.W. South Coast and adjacent Victoria with emphasis on the Pre-Permian structural history. Geol. Soa. Aust., SGTSG Field Guide^ 1. POWELL, C.McA., 1983Z?: Tectonic relationship between the Late Ordovician and Late Silurian palaeogeographies of southeastern Australia. Geol. Soa. Aust., J., 30, 353-373.

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