The lachlan belt of eastern Australia and Circum-Pacific tectonic evolution

Tectonophysics,214 (1992) 1-Z Else&r Science Published B.V., Amsterdam

The Lachlan belt of eastern Australia and Circum-Pacific tectonic evolution Peter J. Coney ihpartnmt of Geosciences, 7?te Uniuersityof Arizona, Tucson,AZ 85721, USA (Received February 21991; revised version accepted July 25,1991)

ABSTRACT Coney, P.J., 1992. The Lachlan belt of eastern Australia and Ciicum-Pacific tectonic evolution. In: C.L. Fergusson and R.A. Glen (Editors), The Palaeozoic Eastern Margin of Gondwanaland: Tectonics of the Lachlan Fold Belt, Southeastern Australia and Related Grogens. Tectotwphysics,214: l-25. There is considerable evidence that the Pacific Ocean basin has had a remarkable permanency at least throughout the Phanerozoic. The erogenic systems that have evolved around its margins are aceretionary continental margin orogens and show little or no evidence of continental collisions in their evolutionary history. This is in dramatic contrast to the Circum-Atlantic and Tethyan realms which have experienced repeated openings and closures of, or successive transfer of continental lament across, ocean areas that were relatively never large. In other words, the Wilson cycle has dominated tectonic evolution of AtIantic and Tetbyan realms, but has not been important in the Circum-Pacific. The northeastern margin of the Pacific C&art is the North American Cordillera which is a “classic” continental margin-accretionary system dominated by a well-developed canplex miogeochnal terrace, significant fringing or “exotic” arc-trench systems and other “oceanic” accretions progressiveiy consolidated into North America from mid-Paleozoic times, but mainly from mid-Mesozoic times to the present. The northwestern margin of the Pacific Ocean is the collage of Asia which was produced by Tethyan tectonics, not Pacific tectonics - i.e., the progressive transfer of Gondwanaland fragments across Tethys to Baltica-Siberia. Only since the early Mesoxoic have minor Pacific accretions, such as Japan, produced the present margin. The southeastern, southern, and southwestern margins of the Pacific Ocean are South America, Antarctica, and Australia, respectively. Through Paleozoic-early Mesozoic times they were joined and a very enigmatic Pacific margin erogenic system extended for 20,000 km from northwestern South America to northeastern Australia. The Lachlan Fold Belt in particular, and the Tasman belt in general, are important windows into that enigma. Lack of a well-developed through-going miogeoctine is notable, and late Precambrian but mostly extensive lower Paleozoic, fairly deep-marine turbiditic and occasionally submarine volcanic facies are common along the margin, often directly juxtaposed against the cratonic interior. The tectonic evolution is dominated by prolonged histories of first late Precamb~an to Late ~rnb~an then Early Silurian-Early Mesozoic convergent to transpressive and accretionary tectonics, often ac~mpanied by extraordinary magmatism, which progressively consolidated a considerable “oceanic” to “quasi-continental” real estate into the Gondwanaland craton. Since the fragmentation of Gondwanaland in the mid-Mesozoic only the Andean margin has continued convergent consolidation. Large-scale tectogenesis and consolidation in the Circum-Pacific seem to correspond to periods when the “absolute motions” of the main continental blocks caused their margins to advance over the adjacent Pacific Ocean floor crust.

Introduction We have become accustomed to thinking of erogenic belts in the context of plate tectonics and although there are certain systems, such as

Correspondence to: P.J. Coney, Department of Geosciences, The University of Arizona, Tucson, AZ 85721, USA. tkR8-19.%/92/$05.00

Archean greenstone belts, several Proterozoic belts, and even a Phanerozoic system or two, for which the exact plate tectonic context is still unclear, the general consensus is hopeful that erogenic systems either have been, or will be eventually, clearly perceived within the plate tectonic model. This has led, for example, to a perception that most of the history of the earth is one of opening and closing of oceans as frag-

Q 1992 - Elsevier Science Publishers B.V. All rights reserved

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ments of either a growing or static volume of continental crust rift apart and redistribute themselves producing mountain chains as they interact convergently with oceanic crust and, finally, suture against one another during intercontinental collisions. These seemingly endless series of opening and closure of ocean basins has been

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granitic rocks

Wonominta block

Lachlan Upper Dev.-Lower Carb. sedimentary rocks

Glen&g metamorphic rocks

Lachlan Sil.-Dev. sedimentary and volcanic rocks

Kanmantoo Group

Lachlan Omeo metamorphic rocks

Adelaidean sedimentary rocks

Lachlan Ord. vokxnic

fault

ma aeromagnetic trPnds

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termed “The Wilson Cycle” (see for example Turcotte and Schubert, 1982, p. 37) and it is often viewed as the principal cause of mountain building. The point of this paper is not to discredit the Wilson Cycle. It has been, is, and will continue to be, a very useful conceptual framework within

New England belt

Precambrian basement

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rocks

Lachtan Ord.-Sil. greywacke

Lachlan greenstone belts

Fig. 1. The Lachlan belt, its regional tectonic setting and main lithotectonic assemblages.

THE LACHLAN BELT OF EASTERN AUSTRALIA AND CIRCUM-PACIFIC TECTONIC EVOLUTION

which to view much of the history of the earth. What I will try to emphasize is that the Pacific Ocean basin, which today is half the surface of the earth, has had a remarkable permanency throughout most of the Phanerozoic while the Circum-Atlantic and Tethyan realms have indeed been characterized by repeated openings and closures of, or successive transfer of continental fragments across, ocean areas that were never very large. In other words, the Wilson Cycle has dominated Phanerozoic tectonic evolution of Atlantic and Tethyan realms, but has not been important in the Circum-Pacific. This then suggests it might be useful to review the history of the Pacific Ocean basin and try to determine the character of evolution of the orogenie systems that have evolved around its rim during the Phanerozoic, and then to briefly compare them with those that have evolved in the Atlantic-Tethyan realms. In this regard the Lachlan belt of southeastern Australia (Gray, 1988; Fergusson et al., 1986) in particular, and the Tasman erogenic system of eastern Australia in general (Scheibner, 1978a,b, 1986; Coney et al., 19901, are important windows into Paleozoic Circum-Pacific tectonics. This is because the rest of the Circe-Paci~c has either been severely modified by Mesozoic-Cenozoic mountain building, such as in the Andes and western North America, or is covered with ice as is Antarctica. In other words, although modified and somewhat fragmented by Mesozoic-Cenozoic rifting, the LachIan belt of eastern Australia provides a near pristine view of Paleozoic Circum-Pacific tectonic evolution. The picture is not particularly comforting. The view that the Lachlan “window” gives us of Paleozoic Circum-Paci~c tectonics is troublesome and it is s~ptomatic that there is a remarkable lack of consensus as to what the exact plate tectonic setting of eastern Australia was during late Precambrian to mid-Paleozoic times. This difficulty is compounded by the fact that, in the context of most well-known Phanerozoic orogenie systems, the Lachlan is a very peculiar mountain belt. It is peculiar in the distribution of lithotectonic elements found within it and in its tectonic evolution. In fact, I would go as far as to

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say that I know of nothing quite like it that has been described in the literature of Phanerozoic regional tectonics anywhere in the world, I will first try to summarize what to me are the distinctive characteristics of the Lachlan belt and its history and attempt to explain why they are important to our thinking of erogenic processes. I will not try to review the details of Lachlan belt for that has been done elsewhere (Coney et al., 1990) and many recent developments can be found in this volume. I will, however, try to put the Lachlan belt into the context of CircumPacific tectonic evolution and make some comparisons with the Circum-Atlantic and Tethyan realms and erogenic system evolution in general. The La&Ian Belt of Eastern Australia General lithotectoniccharacter The Lachlan belt (Gas, 1983, Scheibner, 1985, 1987; Degeling et al., 1986; Ramsay and VandenBerg, 1986; Gray, 1988; Coney et al., 19901, or Lachlan composite terrane, of eastern Australia (Fig. 1) spreads for about 700 km across Victoria and extends some 500 km or more northwards into New South Wales. It certainly underlies at least the eastern part of the Murray Basin and a significant part of the south-central part of the Great Australian Basin as well. This means it has an across-strike width similar to the width of the Canadian Cordillera, and it is about two-thirds the width (the distance from San Francisco to Salt Lake City) of the Cordillera in western United States. The striking thing about the LachIan belt is that the same lithotectonic assemblages and general structural style and level are found across the entire length and breadth of the orogen, a min~um extent of over a million square miies (Coney et al., 1990). This is in direct contrast to, for example, the North American Cordillera (see Fig. 21, and most other Phanerozoic erogenic systems for that matter, which normally expose juxtaposed variabIe coeval lithotectonic assemblages ranging from continental margin to oceanic setting facies accompanied by variable structural styles and exposure of different structural levels. Thus, no matter what one might

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think about the Lachlan from afar, it cannot be dismissed as a “microcosm” or “red herring” of insignificant size. The Lachlan terrane Fig. 1) consists of three major lithotectonic assemblages which range from Cambrian to Upper Devonian-Lower Carboniferous in age (Coney et al., 1990). The oldest assemblage exposed are the L.ower to Middle Cambrian “greenstone belts” (Gas, 1983; Crawford et al., 1984; Crawford, 1988) actually confined to the western half of the belt where they occur in three Noah-trending elongate faultbounded bands. Composition includes “calc-alkaline andesite,” boninite, subalkaline tholeiitic basalt, dolerite, and gabbro, with serpentinized or silicified peridotites commonly found in shear

Glyelg

zones and along faults. The basement to the greenstones, whatever it might have been, has never been seen. The second major lithotectonic assemblage are the very well-dated mostly Ordovician, but locally Silurian, widespread quartz-rich turbiditic fan deposits of greywacke and lesser pelagic pelites and cherts so characteristic of the Lachlan (Gas, 1983; Powell, 1983; Ramsay and VandenBerg, 1988; Cas and VandenBerg, 19881. The assemblage dominates the Lachlan belt and extends in an e~raordina~ commonali~ across the entire belt. In the New South Wales, or northeastern, part of the Lachlan several narrow bands of submarine andesitic, tholeiitic, and shoshonitic volcanic rocks and volcanogenic sedimentary rocks seem to be

stawe”Castlemaine

La&Ian Bett

Canadian Cordiilera La&Ian

Canadian Cordiltara distal continental margin terranes - Old.-$11 greywackeshale

accratsd terranss of oceanic affinity

metamorphic rocks

Lachtan lower crust

tectonic accrettons

0)

peas&to tectonic accretton

diatai c6nttnentat mnrgin tsrranss mbgeocttne

Fig. 2. Diagrammatic structure-sections of the Lachlan Fold Belt (after Fergusson et al., 1986, from Coney et al., 1990) and the Canadian Cordillera (from Coney, 1989a) at the same scale for comparison.

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interbedded in the more generally non-volcanogenie quartz-rich greywackes (Wyborn and Chappell, 1983). Thickness estimates of the original protolith turbiditic fan deposits average around 2.5 to 5 km (Gas, 1983). The greywackes are normally broken out into several fault-bounded sub-assemblages the original paleogeography of which is much debated (Fergusson et al., 19861, but the association in total seems to constitute a broad “overlap” of much if not all of the early Paleozoic Lachlan belt (Powell, 1983). The source of the greywackes is certainly “continental”-“recycled orogen,” and based on northerly to northwesterly flow patterns, the source is usually assumed to have been Gondwanaland. In one small but important exposure in the Heathcote belt of central Victoria the greywackes are seen to be depositional on the older greenstones. Elsewhere, the basement is never seen or contacts with the older greenstone belts are faulted. The third lithotectonic assemblage is made up of a very complex sedimentary, volcanic and plutonic association of about Lower-Middle Silurian to Upper Devonian-Lower Carboniferous age (Gas, 1983; Powell, 1984; Chappell et al., 1988; Marsden, 1988; VandenBerg, 1988). These rocks mark a major transition in the history of the orogen and record an end to the deep-marine-oceanic aspect of the early Paleozoic Lachlan and a progressive consolidation of the belt into a mature continental margin orogen. The patterns of sedimentation, volcanism, and plutonism are very complex. Most, but not all, of the volcanism is concentrated in the eastern Lachlan where a number of elongate fairly deep marine troughs, or basins, either formed or persisted. Adjacent “highs” expose shallow marine to terrestrial rocks. The volcanic rocks are generally mixed bimodal, but mainly felsic in composition. Extensive detrital-fluvial depositional sheets characterize the Late Devonian-Early Carboniferous (shown separately on Fig. 1) and are often considered a separate assemblage of “molasse-like” character. The granitic plutons are very widespread and account for almost one-third of the outcrop in the Lachlan belt. These now classic suites (Chappell and White, 1974; Chappell et al., 1988), for the most part mid-Silurian to Upper Devonian in

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age, are spread across the entire 700 km width of the Lachlan. The “I-S” line (Fig. 21, which separates only I-types in the east from both I- and S-types to the west, runs north-south down the easternmost Lachlan. The plutons are mostly bulbous to elongate masses in part syn-, but mostly generally late-tectonic cross-cutting bodies. Age trends are not obvious and some of the youngest intrusions, in fact, are in the central part of the Lachlan north of Melbourne. Patterns and timing of deformation in the Lachlan are extremely complex (Gas, 1983; Fergusson and Coney, 1992-this volume), but the style of deformation is remarkably uniform (Gray, 1988). The Ordovician greywackes are universally upright isoclinally folded (Fig. 2) with well-developed cleavage over the entire expanse of the Lachlan. Poly-deformed fabrics are frequently described. The individual “sub-terranes,” or structural belts, are usually bound by steep fault zones that have been interpreted to be thrusts and/or transpressional faults, and some belts are characterized by narrow “melange-like” bands of broken formation (Fergusson et al., 1986; Fergusson and VandenBerg, 1990). Vergence is variable, but the generally upright geometry and style is usually portrayed as slightly east-vergent. The Howqua belt southwest of the Omeo metamorphics, however, verges southwest. The fold style alone suggests up to 50-60% shortening across the entire Lachlan (Coney et al., 1990; Fergusson and Coney, 1992-this volume). If thrust faults, particularly low-angle, are added the shortening could be much more. The only departure from this uniformity of lithologies and structural style in the Lachlan orogen is the Omeo Metamorphic Belt, which extends over 500 km in a north-northwesterly trend through the central Lachlan and is up to about 150 km wide (Gray, 1988). Completely fault bounded it is characterized by amphibolite and greenschist-grade, fairly steeply dipping, foliated pelitic schists with local development of gneiss, and a great deal of granite. The protolith, for the most part at least, is the Ordovician greywacke. Structurally overlying members of the SilurianDevonian sedimentary-volcanic assemblage are less deformed and unmetamorphosed. In spite of

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the metamorphism, which is high temperature-low pressure in aspect and seems to be more a regional contact aureole more than anything else, the belt exposes the same structural level as the adjacent greywacke belts. The mid-Silurian to Upper Devonian-Lower Carboniferous sedimentary-volcanic assemblage is usually somewhat less deformed than the Ordovician-Lower Silurian greywacke. For the most part an angular unconformity, itself usually deformed, separates the two assemblages. In contrast to structure in the underlying greywackes, which is tightly compressional in style, complex geometries and patterns suggestive of episodes of compression, extension, transtension and transpression, are often reported as associated with the younger sedimentary and volcanic rocks. Younger on older faults are not uncommon. In the Melbourne trough, however, in the central Lachlan, the entire Ordovician to Middle Devonian sequence is conformable, but isoclinally folded (Coney et al., 1990). The timing of the deformation in the Lachlan extends from the Early Silurian to the Early Carboniferous in numerous time-honored “orogenies” (Gas, 19831, some of which are quite locally defined while others seemed to have affected the entire belt. The total time-spread of deformation represents almost 100 Ma (Fergusson and Coney, 1992-this volume). Discussion The principal issue here is the question as to what the plate tectonic settings of the Lachlan belt were throughout its evolutionary history. This question has proved a difficult one (see an excellent discussion in Cas, 1983). A number of scenarios have been proposed down the years (for example see Griffiths, 1971; Powell, 1984; Packham, 1987) ranging through almost every possible plate tectonic setting imaginable to sentiments often expressed informally implying that the belt has nothing to do with plate tectonics. Part of the problem is that no unequivocal convergent, transform, or divergent plate margin signatures have been universally recognized in the rock record of any part of the exposed Lachlan belt. To be sure,

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many of the “actors” are recognized, such as interpreted submarine arc rocks, belts of “melange-like” broken formation, plutonic and volcanic magmatic belts of compositions normally associated with convergent margins, with or without extensional “back-arc” aspects, etc., but the script is usually incomplete and the exact paleogeographic plot has remained obscure. Many of the proposed plate tectonic settings are actually quite reasonable, and quite clever - it is hard to think of anything new - but they have been very difficult to prove. The first question relates to the Lachlan belt’s origins. In other words, what was its earliest, or “pre-erogenic,” plate tectonic setting? To address this problem we have to place the belt in its regional tectonic context with respect to the Australian, i.e. Gondwanaland, craton. To get to the point, did the Lachlan begin as the result of rifting and opening of an ocean, or did it somehow develop along, or off the edge of, an already “exposed” Gondwanaland continental margin that faced a proto-Pacific basin? The Lachlan belt lies east of the Archean and lower to middle Proterozoic cratonic crystalline basement of Australia. This basement is exposed in the Adelaide region and around Broken Hill where it is covered by the thick trough-like deposits of the upper Proterozoic Adelaidean sequences and a thin lower Paleozoic platform cover (Preiss, 1987, 1988; Parker, 1986). The rift-related structures which seem to have controlled the late Proterozoic deposition of the Adelaidean are at least in part oblique to the apparent edge of the Australian craton (the so-called Tasman line) and apparently extend northwestward into the Australian craton and into the east-west-trending Amadeus basin of central Australia. Thrust up against and upon the cratonic edge of Australia from Kangaroo Island to northeast of Adelaide is a 500~km-long narrow arcuate strip of quite intensely deformed deep-marine turbidites of Cambrian age known as the Kanmantoo Group (Parker, 1986). Most of the contacts between the Kanmantoo Group and the Adelaidean-Lower Cambrian platform are faulted, but facies relationships and several much discussed exposures have been interpreted to suggest the Kanmantoo

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is a distal deep-water equivalent of the Cambrian platform. The Kanmantoo rocks were thrust upon the Australian cratonic shelf during Late Cambrian-Early Ordovician times, perhaps in a westfacing “roll-back’ tectonic setting, and quickly intruded by granites in what is called the Delamerian orogeny - long correlated with the Ross orogeny of Antarctica. The Kanmantoo greywackes and the scattered granites most likely continue southeastward in the subsurface for 300 km and actually reappear quite metamorphosed juxtaposed directly against the westernmost exposures of the Lachlan belt in the Glenelg region of western Victoria (Gibson, 1988). The Glenelg rocks are in fault contact with typical, but undated, muds and greywackes of the Lachlan to the east around the Stavely greenstone belt. These rocks in turn are juxtaposed against well-dated Ordovician Lachlan greywackes to the east along the Avoca Fault. It is of interest to take note that if the depositional contact between the Ordovician greywackes and the greenstone belt rocks in the Heathcote region is correct the Lachlan belt did not suffer, Delamerian Orogeny. It has been suggested, however, that the Ordovician Lachlan greywackes themselves might have been shed from a rising and eroding Ross-Delamerian erogenic edifice to the west and southwest (Gas, 1983). In the Wonominta terrane (Leitch et al., 19871, just east of the cratonic promontory of Broken Hill, and also in western Tasmania (Collins and Williams, 1986; Berry and Crawford, 1988), older metamorphic rocks are exposed at least structurally below sedimentary and volcanic rocks somewhat similar in aspect to those found in the Lachlan. The metamorphic rocks are probably upper Proterozoic to possibly lowest Paleozoic in age and yield isotopic ages suggestive of “PanAfrican” erogenic events. To date, nothing older than Cambrian has been identified anywhere in the Lachlan belt, and what is more, most of the Cambrian and Ordovician rocks exposed have a fairly deep-marine “oceanic aspect.” Otherwise, no unequivocal exposures of, or geophysical-isotopic geochemical evidence of, an ancient Archean-mid-Proterozoic crystalline basement has been reported anywhere

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in the Lachlan belt, or for that matter, anywhere in the entire Tasman Orogenic System (Coney et al., 1990). Present crustal thicknesses range from about 30 to over 50 km so the problem is to guess what makes up the lower Lachlan crust (Fig. 2). Assuming most of this crust is late Precambrian to Paleozoic in age, which some would question, crustal budget calculations and isotopic data from the granites suggest that lower crust is not primarily made of anything we see today at the surface, i.e. specifically the Ordovician greywackes. It could be some attenuated upper Proterozoic “Pan-African” basement, or perhaps a vast upper Proterozoic distal submarine fan deposit possibly equivalent to the Adelaidean deposits on the Australian shelf. The remarkable thing here is the apparent abrupt juxtaposition of lower Paleozoic rocks which display fairly deep-marine aspects directly against the platform-like margin of cratonic Australia. Unlike Phanerozoic mountain belts in North America, for example, which preserve welldeveloped miogeoclinal “transitional” facies, there is no obvious through-going miogeoclinal terrace between the Lachlan and the craton, or for that matter, anywhere in eastern Australia. This raises the important issue as to what the transition was from the craton to the Lachlan, say in late Proterozoic to earliest Paleozoic times. It could have been a rifted margin as has been often suggested (Scheibner, 1985). In this light, the Adelaidean Amadeus basin sequences might have formed in an aulocogen. But this then raises the question as to what was rifted away. About the only possible candidate is North America, an idea already suggested by Jefferson (1978; see also Bell and Jefferson, 1987) and most recently again by Hoffman (1991), Moores (1991) and Dalziel (1991). This would suggest a “supercontinent” including Australia, East Antarctica and India rotated away from Laurentia in the late Proterozoic to open the Pacific Ocean. The fact that a through-going, long enduring late Proterozoicearly Paleozoic miogeoclinal terrace is not obvious along the entire Australian margin would then suggest this rifted margin was at times at least convergent in some sense, or at least different (i.e., an “upper plate” rifted margin as sug-

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gested by Scheibner during the Lachlan Fold Belt 1991 Conference) from western North America, during “Pan African” late Proterozoic-early Paleozoic times. The next problem to address is the plate tectonic setting of the Lachlan during CambrianEarly Carboniferous times. There are several issues here. First, the Cambrian greenstones and the Ordovician greywackes, and second, the complex deformational patterns from the Early Silurian to the Early Carboniferous. There could have been a convergent plate boundary somewhere between the Cambrian greenstones and the convergent Australian cratonic margin during Delamerian-Ross orogeny in Late CambrianEarly Ordovician times. Obduction of the greenstones onto the late Proterozoic “basement” in Tasmania has been proposed (Berry and Crawford, 1988), perhaps in a “roll-back” tectonic setting, and a similar setting in the Adelaide region could explain the Kanmantoo greywacke prism and its thrusting onto the craton. There is, however, no proof of this as yet in Victoria and South Australia. In any event, the Ordovician turbidite fans of the Lachlan seem to have spread over everything across the Lachlan from sources in the Ross-Delamerian orogen to the south and west, and this was a mighty turbidite fan (Powell, 1983). Recall that structural analysis of the Lachlan suggests a minimum of 50% shortening in a belt which today is over 700 km wide. This means the original, no doubt complex, fan was almost 2000 km across and from 2 to 5 km thick. The only thing that even approaches this in the present world is the Bengal fan spreading onto oceanic crust of the eastern Indian Ocean from the Himalayas. It has been argued that the estimates of thickness of the protolith Lachlan fan, when combined with isotopic data from the Lachlan granites and present crustal structure and thickness in the Lachlan, suggests the fan must have been deposited on some sort of attenuated “quasi-continental” substrate that could not have been much thicker than 10 km. The thickness estimate is based on isostatic arguments. You cannot deposit a vast S-km-thick deep-marine turbidite fan on “normal” 35 km continental crust. The reasoning also

f’.1 (‘ONEY

concludes the substrate could not have been young 5 km thick oceanic crust unless the shortening in the Lachlan is dramatically greater than the 50-60% calculated from structural analysis (Coney et al., 1990). It is possible, of course, to argue that the bounding fault zones of the several “structural belts” of the Lachlan represent major strike-slip or even transform boundaries that brought in separate segments of a long narrow fan spread out along the Gondwanaland margin. Numerous models have been proposed with both right and left lateral kinematic schemes and some are quite plausible (see Fergusson et al., 1986; Packham, 1987). They have, however, been difficult to prove. I have not been impressed by the through-going character of any of these fault zones, particularly into the sub-surface outside the exposed Lachlan. Furthermore, it is not obvious just how much lateral movement they represent, and the endless debates about timing and sense of movement are not comforting. I can, however, feel quite comfortable envisioning the fault zones as belts of major intraplate failure just as well, a position which, of course, is also difficult to prove. The several narrow bands of Ordovician submarine volcanic rocks in the eastern Lachlan have been interpreted as arcs. They also have been interpreted as “hot spot” related volcanic rocks (Wyborn, 1992-this volume). Preliminary age dates on these volcanics span most of the Ordovician (Perkins et al., 1990). In any event, complementary trench assemblages of appropriate age and position have not been recognized to everyone’s satisfaction. It would seem that if a major Ordovician plate boundary existed between the protoPacific and the Australian craton it was either at, or east of, the present most easterly exposures of the Lachlan Fold Belt, for example in the present position of the New England Fold Belt. This suggests the Ordovician Lachlan was a vast quasi-oceanic realm, swamped in mud and sand, up to 2000 km wide which lay between Gondwanaland and the Proto-Pacific Ocean basin. A plate boundary between the Lachlan and the Australian craton may have briefly existed in Late Cambrian-Early Ordovician Delamerian orogeny of South Australia, but as has already been men-

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tioned, no unequivocal plate boundary has been generally accepted anywhere within the present Lachlan itself. The numerous small ocean basins of the present-day western Pacific Ocean are probably not inappropriate analogues, but the substrate of the possible “quasi-continental” Lachlan would make it somewhat, and perhaps importantly, different. Whatever it was, this vast terrane was progressively telescoped and thickened by folding, thrusting, and strike-slip faulting, intermittently thinned by extensional faulting, partially disrupted by strike-slip faulting, engulfed in massive outbursts of volcanism and plutonism, and finally consolidated into the Australian craton during Early Silurian-Early Carboniferous times, a period of almost 100 Ma. The patterns of collapse and consolidation are quite complex and different parts of the Lachlan seem to have failed at different times (Fergusson and Coney, 1992-this volume). One of the youngest to fail, the Melbourne trough, persisted as a marine basin until the Middle Devonian right in the-middle of the present-day Lachlan. Much of the magmatism seems to follow the major telescoping seen in the greywackes of individual structural belts with a time lag up to lo-20 Ma. No age trends, such as younging to the east or west, are obvious and some of the youngest plutons in fact are in the center of the Lachlan just postdating the collapse of the Melbourne trough in the Middle Devonian. This suggests to me the magmatism is more due to some combination of crustal and lithosphere thickening and melting, and possibly delaminations, than completely due to complex patterns of “flapping” Benioff zones from some distant trench. As the various belts failed, previously deformed adjacent belts were also affected and some of the younger deformational pulses, such as the Middle Devonian Tabberaberan, and Early Carboniferous Kanimblan “orogenies,” may have affected the entire Lachlan Orogen in one form or another as the vast edifice progressively tightened, wrenched, and consolidated. I prefer to view all this as a progressive massive intraplate failure of a vast somewhat unusual inherited quasi-oceanic-continental lithosphere. Aborted

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efforts at “A’‘-type intraplate “subduction,” extensional “roll-back” effects of sinking overthickened lithosphere, crustal and lithospheric melting, all may have played important roles in this very peculiar and unusual evolution. On the other hand, no deep-seated crustal rocks are brought-up on large-scale ramping thrusts in the Lachlan, as is so commonly seen in many erogenic systems. This suggests to me that the Lachlan belt and its deeper crustal and lithospheric substrate must have sequentially telescoped by isoclinal folding and thrusting at upper crustal levels, but more or less homogeneously by accordian-like shortening and thickening at deeper levels. This style of consolidation over a present-day width of nearly 1000 km I find singularly and curiously unique in Phanerozoic erogenic systems. A seemingly necessary major convergent plate boundary between the proto-Pacific plates and the Lachlan must have existed somewhere to the east of present Lachlan exposures. The Lachlan belt in the context of Circum-Pacific evolution The Lachlan belt today is part of the very complex “Pacific Rim” of continental margin mountain belts such as the American Cordilleras, festoons of fringing intraoceanic arc-trench systems and back-arc basins more typical of the western Pacific, and lesser segments of recently evolved rifted and passive margins such as West Antarctica. The Pacific Ocean basin itself, almost half the surface of the earth, has no ocean crust older than Jurassic. The floor is swept from East Pacific rise spreading centers which lie asymmetrically along the southern to southeastern sector such that the central and western to northern floor is the largest of plates, the Pacific plate, the oldest parts of which (as old as Jurassic in age) are what is being subducted beneath most of the western Pacific intraoceanic arcs. The asymmetry seen on the Pacific Ocean floor is matched by a marked asymmetry seen in its margins most dramatically displayed by the complex continental margin systems of the Americas opposed to the fringing arc-trench-back-arc basins of the Asian-Australasian margins. Our principal con-

IO

tern here, however, is to try to reconstruct the Phanerozoic history of the Circum-Pacific and try to place the Lachlan Fold Belt in that context. The principal objects we need to identify and track the movements of in trying to reconstruct the evolution of the Circum-Pacific are the several major Precambrian cratons. We also need to take account of the generally Phanerozoic accretionary terranes which either surround, or are caught between, the several cratons and the orogenie systems their amalgamation into the cratons represents. Thirdly, there is the Pacific Ocean floor itself. This has certainly been recycled several times at least during the Phanerozoic, but passengers on it have perhaps been incorporated into the present Pacific rim and certainly relative and “absolute” motions of the Pacific floor and adjacent cratons have consolidated numerous oceanic to distal continental margin terranes into those cratons through erogenic processes. The principal Precambrian cratons are Baltica, Siberia, North America, South America, Africa, East Antarctica, Australia, and India. During most of the Phanerozoic, the last five were one, the continent of Gondwanaland. Sy mid-Paleozoic times, Baltica was joined to North America, and briefly Gondwanaland may have joined with North America also, but then moved away some unknown and much discussed distance. By the end of the Paleozoic, Siberia joined Baltica and Gondwanaland either returned to North America-Eurasia or nudged ever more tightly against it to produce the supercontinent Pangaea. The Mesozoic-Cenozoic has seen the separation of North America from Gondwanaland and its near separation from Eurasia, and the progressive fragmentation of Gondwanaland. The point of this brief description of the cratons is to emphasize that we actually have a fair control on where they were, particularIy since middle Paieozoic times, based on known interactions between them, paleomagnetic data, and paleoclimatic-biological arguments. There are probably over 300 “suspect terranes” that have been recognized around the Pacific Rim (Howell et al., 1985). In spite of all this complexity, for our purposes the major terrane types can be grouped quite simply into 4 major

1’ I (‘ONLY

classes: (1) continental fragments; (2) continental margin terranes; (3) disrupted continental margin terranes; and (4) oceanic magmatic-sedimental terranes (Coney, 1990a). Continental fragments are smaller pieces of the larger cratons which have uncertain paleogeographic settings in the erogenic systems within which they are found. Some may have crossed oceans but around the Pacific Rim most seem to be laterally displaced pieces of the continental margin along which they are found. Examples would be the Caborca and Cassiar terranes of the North American Cordillera (Coney, 1989a). Fragments of continents are abundant in the Asian “collage” (Sengor, 1987), but they are not particularly prominent around the Circum-Pacific. Continental margin terranes are of considerable variety but usually display thick deeperwater, quartz-rich distal detritaf accumuIations shed from adjacent continental source areas. Many are interpreted to be initially rift-related distal toes, or continental slope-rise deposits, of adjacent miogeoclines or continental platforms, like the Yukon-Tanana terrane of western Canada (Coney, 1989al, while others seem to be vast off-shore turbiditic fan deposits perhaps in some cases shed from degrading continental margin erogenic belts like the Lachlan Ordovician greywackes of eastern Australia (Scheibner, 1987; Coney, 1990). There is usually uncertainty as to the basement below in these terranes, but if continental crust is known or assumed, thickness, water depths and subsidence estimates of the typical muds, grits, and greywackes usually suggest the basement could only be thinned or attenuated continental crust. These terranes are very prominent around the Circum-Pacific and most are late Precambrian to Paleozoic in age. The large turbiditic fan types are particularly characteristic of the Pacific margin of GondwanaIand. Disrupted continental margin terranes are characterized by large-scale voluminous disrupted sequences of greywacke and shale, usually accompanied by varying but lesser amounts of banded chert, mafic volcanic rocks, occasional ultramafic slices and other “oceanic” assemblages. MeIange and broken formation fabrics are usually common. Examples would be the Franciscan terrane

THE

LACHLAN

BELT

OF EASTERN

AUSTRALIA

AND

CIRCUM-PACIFIC

of California (Blake and Jones, 19811, the Chugach-Prince William terranes of southern Alaska (Page et al., 1986), much of Japan (Taira et al., 1989), the Wandilla-Gwydir terranes of the New England orogen in eastern Australia (Murray et al., 1987), and at least parts of the Torlesse terrane of New Zealand (Howell, 1980; Bishop et al., 1985). The usual interpretation is that these terranes form inboard of subduction zones against continental margins and represent accretions of trench-fill sediments, off-scrapings of layer 1 from oceanic crust, and occasional slices of layer 2. These terranes are often voluminous and imply accretion of off-shore submarine fan deposits and/or trench fill which were originally shed from nearby continental source areas and are now telescoped against, or tectonically underplated beneath, a convergent margin. The fact that they are not ubiquitous around the presentday Pacific suggests that continental marginparallel transport in oblique convergence to transform settings, and tectonic accumulation against continental margin salients or “elbows,” might favor their development. Intraoceanic magmatic-sedimentary terranes represent several settings, but can be summarized as made-up of mostly submarine volcanic and plutonic constructions with associated sedimentary aprons, platforms, and basins usually with a strong “oceanic” affinity. Some seem to have been intraoceanic magmatic arcs, like the Stikine terrane of western Canada (Monger and Berg, 19871, others seem to have been large “hot spot” magmatic-sedimentary oceanic plateaus and seamount chains such as the Triassic part of Wrangellia terrane, while others may be mixed assemblages of oceanic plateau, oceanic crust, intra-arc and back-arc basins, etc., like the Slide Mountain-Angayucham terranes of Alaska and western Canada (Coney and Jones, 1985; Coney, 1989b). They often record long and complex histories with little or no evidence of continental input (Samson et al., 1990). There is usually no evidence of continental basement in these terranes. When accreted to a continental margin these terranes are for the most part the only significant net additions to continental crust and thus have played an important role in either

TECTONIC

11

EVOLUTION

maintaining, or adding to, the total global budget of continental crust. These terrane types are particularly prominent in the Circum-Pacific in a belt extending from northeastern Siberia through the North American Cordillera and the CaribbeanMiddle America region into the northern Andes of Columbia and Ecuador (see Fig. 11). Late Paleozoic to Mesozoic protolith ages are particularly common here. Phanerozoic

tectonic

evolution of

the Circum-

Pacific

The discussion which follows will try to briefly summarize the salient features of Phanerozoic Circum-Pacific tectonic evolution utilizing several reconstructions of the positions of the major Precambrian cratons and their evolving margins. For a more detailed discussion see Coney (1990). In order to perceive better the evolution of the Circum-Pacific we need to focus our attention on a perspective centered on about 14O”W to 17o”W, i.e. the center of the present-day Pacific Ocean, rather than the more ethnocentric “old world” perspective of the Greenwich Meridian which is what is usually done. We also need to focus attention on Gondwanaland, for, as I have argued elsewhere, the Lachlan belt in particular and the Tasman Orogenic System in general share a commonality with the rest of the Paleozoic-Early Mesozoic tectonic evolution of the Pacific margin of Gondwanaland. The principal vehicle for the reconstructions shown on Figures 3-9 is the program Terra Mobilis (trademark) of Scotese and Denham (1988) with selected modifications based on Van der Voo (19881, May et al. (19891, and Kent and Van der Voo (1990). Early Paleozoic

During the early Paleozoic (Fig. 3) Gondwanaland extended from the South Polar regions into equatorial latitudes with North Africa near the South Pole and Australia on the equator. North America lay east of Gondwanaland mostly on or south of the equator. Most of the cratons were in the southern hemisphere and the paleo-Pacific Ocean extended northward from the Pacific margins of Gondwanaland and North America across

12

North

Early Paleozoic

Fig. 3. Early Paleozoic. 10”. The light shading terranes. occasional

On this and subsequent represents

rocks

reconstructions

the main cratons,

Heavy dark lines are generalized volcanic

Pnle

typical Andes;

supposed

of eastern

fold pattern

represents

orogeny,

zones of plate convergence.

Australia;

4 = continental

(Figs. 4-9) the projection

margin

2 = Ross-Delamerian terranes

and beyond the North Polar regions. The Pacific margin of North America was a passive margin. The Pacific margin of Gondwanaland from Australia to South America, however, seems to have been much different from the Pacific margin of North America. No through-going long evolving miogeoclinal terrace like that found in western North America has been recognized. Instead, during late Precambrian to lower Paleozoic times we have variable settings recorded in generally thick deeper-water continental margin elastic rocks, submarine mafic to intermediate volcanic rocks, and deformation. There was deformation in northwestern Argentina during the Middle Cambrian (Ramos, 1988) and there are Cambrian turbidites in Northern Victoria Land (Bradshaw et al., 1985) and South Australia (Parker, 1986). There are variable mafic to intermediate Cambrian submarine volcanic associations such as the greenstone belts of the Lachlan Orogen and in Tasmania (Cas, 1983). Late Cambrian-Early Ordovician deformation and plutonism is associated with the Ross and Delamerian Orogenies from South Australia through the Transantarctic Mountains (Craddock, 1982; Parker, 1986). During the Ordovician, the Pacific margin of Gondwanaland is much typified by thick, continentally derived

darker

is orthographic shading

I = largely Ordovician orogeny;

of the North American

full-globe

are various

3 = continental

and the grid is

continental

greywacke, margin

margin

mudstone, terranes

and of the

Cordillera.

deep-water turbiditic quartz-rich sands and muds which must have shed over vast deeper marine areas. These fan deposits are occasionally associated with submarine mafic to intermediate volcanic rocks. The widespread Ordovician greywackes of the Lachlan (Scheibner, 1987; Powell, 1983; Cas, 19831, the Excelsior Group in Peru (Aubouin et al., 1973; Megard, 1978; Laubacher and Megard, 19851, the so-called “Faja Eruptiva” (Ramos, 1988), or Puna terrane, of the south-central Andes, and the Central Andean, or Zamora, terrane of Columbia-Ecuador (Restrep0 and Toussaint, 1988) are all examples. All this suggests that much of the Pacific margin of Gondwanaland was in some sense convergent with possible subduction zones some distance off-shore from its continental edge. About the only actualistic plate tectonic setting that comes to mind is something like the present-day southwestern and western Pacific east of Australia and Asia. Middle Paleozoic

During the Silurian-Devonian and into the Early Carboniferous (Fig. 4a and b) there was considerable orogenesis along the Pacific margin of Gondwanaland which brought to an end the deep marine settings typical of the early Paleozoic. There is orogeny and plutonism reported

THE

LACHLAN

BELT

OF EASTERN

AUSTRALIA

AND

CiRCUM-PACIFIC

TECTONIC

EVOLUTION

13

North Pole

Middle Pakozoic NorthPole

170w Fig. 4. Middle Paleozoic. (A) Late Silurian-Early Devonian. This figure shows possible effect of the excursion of Gondwanaland’s APW path front North Africa to southern South America rotating Australia sharply into the Pacific (heavy black arrow). I= Early Silurian deformation in the Lachlan belt; 2 = continued development of the miogeocline in western North America. (B) Devonian-Early Carboniferous. Heavy dark arrows show “advance” of Gondwanaland over the Pacific based on Gondwanaland’s APW path excursion back into central Africa discussed in text. I = orogeny in eastern Australia, New Zealand, and West Antarctica; 2 = orogeny in the central Andes; 3 = orogeny in western North America and the Arctic. NC = North China; SC = South China.

from eastern Australia in the Lachlan, Tasmanian, Thompson, and Broken River-Hodgkinson sectors of the Tasman belt (Coney et al., 1990), in the Nelson region of South Island New Zealand Victoria Land (Sporli, 19871, in Northern (Bradshaw, 1987), Marie Byrd Land (Grikurov et al., 19821, and in the central Andes of Peru (Laubacher and Megard, 1985; Herve et al., 1987).

Most of the these belts were consolidated into Gondwanaland at this time. During this period Gondwanaland may have moved quite rapidly with respect to the South Pole. This possibility is based on an excursion in Gondwanaland’s APW path from a well-established and somewhat stable South Polar position near North Africa in the Ordovician to a position somewhere near south-

14

ern Chile by the Late Silurian-Early Devonian (see Van der Voo, 1988). Some data then suggests the South Pole shifted back into central Africa by the Late Devonian-Early Carboniferous (Van der Voo, 1988; Li et al., 1990; Kent and Van der Voo, 1989), but this is not accepted by all. Alternatively, the pole may have simply moved into southern Africa. In any event, these rather abrupt movements through Silurian-Devonian times seem to encompass most of the variable but widespread orogeny typical of the 20,000~km-long Pacific margin of the supercontinent (Coney et al., 1990). The Late Ordovician-Late Silurian movement of Gondwanaland has the effect of causing the South American Pacific margin to retreat rapidly out of the Proto-Pacific as Africa-northern South America moves into a position just south of or adjacent to North America-Baltica. This suggests a “proto Pangaea” formed as Gondwanaland collided with Laurentia to produce the Acadian orogeny typical of the northern Appalachians (Van der Voo, 1988). This same movement may have been conducive to significant left lateral transpressive shear along the Australian margin as the Australian end of Gondwanaiand rotates sharply into the proto-Pacific. This might explain complex Silurian deformation typical of the Lach-

Late Pakozoic

lan. If the pole then shifted into central Africa by the Late Devonian this would cause the entire Pacific margin of Gondwanaland to push northward into the proto-Pacific and presumably over Pacific Ocean floor perhaps explaining the variable Devonian orogeny so typical of almost all of Gondwanaland’s Pacific margin (Coney et al., 1990). Late Paleozoic

During the late Paleozoic (Fig. 5) significant changes begin and the Pacific Ocean basin starts to have a more familiar aspect. The amalgam North America-Baltica-Siberia begins to move into a position which eventually forms a more confined Pacific Ocean basin with continents on its northern borders. This period also marks the formation of Pangaea as Gondwanaland tightened against North America-Baltica to produce the ~leghanian-Variscan orogeny and established Tethys as an embayment of the greater Pacific Ocean. The Pacific margin of North America was affected by Antler and related orogeny and several “off-shore” fringing arcs may have typified the paleogeography. Gondwanaiand moved across the South Pole as it collided with Laurentia and this seems to have placed the southern Andes-

North Pole

Fig. 5. Late Paleozoic. Gondwanaland moves into collision with North America-Baltica (B) and Siberia (S) to form Pangaea. 1 = fold pattern is accretion in the New England belt; 2 = accretion in southern Chile; 3 = collision of Gondwanaland with North America. Checkered areas in south-central United States and central Australia are Wichita-Ancestral Rockies and Ahce Springs Orogeny respectively (see Coney et al., 1990). Spreading centers (heavy stippled lines) are speculative.

THE

LACHLAN

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Early Meson

AND

CIRCUM-PACIFIC

TECTONIC

EVOLUTION

15

North Pole

Fig. 6. Early Mesozoic. 1 and 2 = speculative and much discussed possible positions of the several oceanic magmatic-sedimentary terranes of the North American Cordillera. Pat = Pacific plate; Fur = Farallon plate; Ph = Phoenix plate; I. = Izangai plate.

Antarctic Peninsula and eastern Australia in favorable positions for significant accretion of oceanic materials and development of continental margin magmatic belts. The Middle Carboniferous to early Mesozoic accretionary and magmatic terranes of the New England belt of eastern Australia (Murray, 1986) and the Chiloe terrane of the coastal regions of southern Chile (Dalziel and Forsythe, 1985) were the result.

Mesozoic-Cenozoic

During the Mesozoic (Figs. 6-9) the Pacific Ocean basin takes a form more familiar to modem perceptions in that it is more confined with Eurasia bounding the northwestern sector. Gondwanaland bounds the southwest, southern, and southeastern sector while North America is moving toward its northeastern position as it breaks away from Africa-South America.

Late Mesozoic North Pole

Fig. 7. Late Mesozoic. I =“accretion” of Guerrero, and western Columbia oceanic-magmatic-sedimentary terranes in the American Cordilleras; 2 = accretion of Greater Wrangellia terrane. Ku = Kula plate; In = Indian plate; LH-C = Lord Howe rise and Campbell plateau.

Early Cenozoic

North Pole

Fig. 8. Early Cenozoic. Heavy dark arrows show advance of North and South America into the Pacific Ocean basin and associated orogeny in the leading edge Cordilleras.

The history of the Tethys Ocean and its evolving borderlands is not a principal concern here, but its evolution led to the construction of the Tethyside super erogenic complex (Sengor, 1987; SengSr et al., 1988) the eastern part of which today forms the northwestern margin of the Pacific rim. The Tethyside erogenic complex is composed of oceanic to quasi-continental margin accretions, micro-continent fragments, and fringing arcs, which progressively accumulated against the “back-stop” of Laurasia. This process seems to have begun in the late Paleozoic and continues to

Present

the present-day. Much of this process involved the progressive transfer of fragments from the northern or Tethyan margin of Gondwanaland across the several Tethys oceans to the margins of accreting Asia. In other words, the northwestern margin of the Pacific was constructed from Tethyan tectonics rather than Pacific tectonics. The tectonic evolution is collisional in origin and most of the major tectonic trends lie perpendicular to the Pacific margin. All Pacific tectonics has done is wrap a narrow band of oceanic accretions and associated magmatic belts around the “col-

North Pole

Fig. 9. Present. 1 = collision of Australia with New Guinea arc; 2 = accretion and deformation in southern Alaska; 3 = Basin and Range extension; 4 = Caribbean plate moves into the Atlantic realm. Ph = Philippine plate; I-Au = India-Australia plate; C = Cocos plate; An = Antarctic plate.

THE

LACHLAN

BELT

OF EASTERN

AUSTRALIA

AND

CIRCUM-PACIFIC

lage” since latest Paleozoic-early Mesozoic times, best known in the geology of Japan (Taira et al., 1989). Convergence seems to have characterized the Pacific margin of Gondwanaland and also North America during the early Mesozoic. Magmatic belts either stood off-shore or were draped along the margins, but their tectonic signatures seemed to have been “neutral” if not extensional in most cases. No widespread consolidation or massive orogenesis seems to have been typical. As Gondwanaland began to break-up and fragment, however, this commonality is lost. The principal development is the opening of the Atlantic Ocean which influenced the tectonic evolution of the

TECTONIC

EVOLUTION

17

Pacific Rim. As first North America in the Late Jurassic, then South America in the mid- to Late Cretaceous finally break free, they begin a fairly rapid advance over the Pacific Ocean in an “absolute motion” sense (see Fig. 10). In each case this marked advance seems to correlate with massive orogenesis in the American Cordilleras which continues to the present-day (Coney, 1973, 1987; Dalziel, 1988). The Antarctic-Australian sector, however did not experience this rapid advance. Continued fragmentation of Gondwanaland and collision of southern Pacific spreading centers seems to have disrupted the Antarctic margin and most of the so-called West Antarctic “terranes” are probably fragments of this aborted break-up

Middle Cretaceous

Fig. 10. The American Cordilleras. The three figures show the seeming correlation between “absolute” motions of the North American and South American plates over the Pacific Ocean basin and major orogenesis on their Pacific margins. “Neutral” or extensional arc magmatism along the margins precedes the “advance” (see text for discussion). The 10 degree grid remains fiied in all reconstructions. The reconstructions are manipulated from Scotese and Denham (1987).

18

(Dalziel and Grunow, 1985). The Lord HoweCampbell fragment rifts away and moves northward. Major changes take place during early Cenozoic times around the Pacific Rim. Besides India’s initial contact with Asia, West Antarctica-New Zealand-Australia move into a passive margin setting as the Tasman Sea rifts open and Australia leaves Antarctica. Major changes take place during the Eocene as the Pacific plate veers to a more westerly movement as recorded in the Hawaii-Emperor seamount chain. The change in motion coincides with, or eventually influences, initiation of numerous off-shore intraoceanic arcs in the western Pacific from Tonga Kermadec through the Philippines, the Marianas, JapanKamchatka and into the Aleutians. This sets up the dramatic “asymmetry” of the Pacific Ocean basin noted earlier with major continental margin mountain systems evolving on the edge of the “advancing” Americas and fringing “off shore” arcs and back-arc basins evolving outboard of the nearly stationary, or Pacific margin-parallel moving (Australia), continental back-stops of the western Pacific.

I’ , (‘ONI~,t

Australia. There are important disrupted continental margin terranes of late Paleozoic-early Mesozoic age in the southern Andes and adjacent Antarctic Peninsula, then again in the New England orogen of eastern Australia. Later Mesozoic-Cenozoic examples would be the Franciscan terrane of California and the younger ChugachPrince William terranes of southern Alaska. Somewhat lesser developed examples are also known along the Asian margin such as is seen in Japan. Younger Mesozoic-Cenozoic examples are significantly lacking in the central and southern Andes in spite of the fact that we know convergence (i.e. subduction), and ample nearby source areas, both have and do exist. In fact, most of the so-called “suspect terranes” of the central and southern Andes are Paleozoic in both protolith and “accretionary” age. Probably the same is true of West Antarctica, and it is certainly true as well in Australia. The most important distribution of large “oceanic” to “quasi-oceanic” magmatic-sedimentary terranes is the 13,000~km-long belt from northeastern Siberia through the North American Cordillera into the northwestern Andes (Fig. 11). Most are upper Paleozoic to Mesozoic in age of

Discussion The first thing to be discussed is the tectonic evolution of the Pacific Ocean basin and its rim and the role the “suspect” terranes have played in its evolution (Coney, 199Oa,b). It appears their role has been quite minimal, at least as a driving force of erogenic system evolution. The principal terrane types found around the Pacific Rim are largely continental margin terranes or disrupted continental margin accretionary terranes, while intraoceanic magmatic-sedimentary terranes are also identified as particularly important in some sectors. As was noted earlier, in terms of rock volume most of the continental margin terranes have late Precambrian to lower and mid-Paleozoic protoliths and the thick, fairly deep marine, turbiditic early Paleozoic examples, such as the Lachlan, are particularly characteristic of the Pacific margin of Gondwanaland from the central Andes through to Queensland in northeastern

Fig. 11. Northeast Pacific Margin. Figure shows in fine shading the major areas of oceanic magmatic-sedimentary terranes in northeast Siberia (I), Alaska and western Canada (2), western Mexico (3), the Caribbean-Greater Antilles(4), and Ecuador-Columbia (5).

THE

LACHLAN

BELT

OF EASTERN

AUSTRALIA

AND

CIRCUM-PACIFIC

protolith, These terranes, in a sense, are the type “suspect terranes” and seem to mark a concentration here of transfer of oceanic materials to continent in this sector of the Circum-Pacific during accretion, orogenesis, and consolidation from mid-Mesozoic times to the present. Lesser examples occur here and there, such as the Brook Street terrane in New Zealand (Howell, 1980) and the Gympie terrane (Murray, 1986) in eastern Australia, but the contrast to the large accumulation along the northeastern margin of the Pacific is notable. This raises the interesting question as to how “far-traveled” many intraoceanic magmatic arcs actually are. Paleomagnetic evidence is usually ambiguous in that movement across longitude is never recorded and recent re-evaluations of Cordilleran paleomagnetic data suggests to some that latitudinal shifts of the principal “exotic” arc terranes of Wrangellia, Stikine, and Quesnelia in western Canada have not been extreme (May et al., 1989). This, when coupled with the isotopic evidence from particularly Alexander, Wrangellia and Stikine which suggests these terranes spent most of their prolonged history out of reach of continental “influence” (Samson et al., 19871, is particularly puzzling. Today, three coeval late Paleozoic-early Mesozoic arc terranes are stacked side by side across two-thirds the width of the Canadian Cordillera and numerous complex traffic patterns and tectonic gymnastics have been argued about for years with little consensus to this day (Coney, 1989b). Multiple arcs are characteristic of much of the western Pacific today, one can point out, but it may be years, if ever, before anyone can definitively reconstruct the exact late Paleozoic-early Mesozoic paleogeography of the North American Cordillera. In any event, one can wonder if active oceanic arcs would, or even could, cross the Pacific Ocean. My intuition tells me it is not likely. To do so would demand they were on the leading edge of an advancing oceanic plate. With all the intra-arc and back-arc thermal softening of oceanic lithosphere it would seem the polarity of the arc-trench system would quickly flip and they would be transferred, by a reversal of the direction of subduction, to the plate which was formerly being subducted. Perhaps their fa-

TECTONIC

EVOLUTION

19

vored position is fringing continental margins, migrating off-shore to open small ocean basins behind them when relative and absolute plate motions favor it, as is seen in the western Pacific today. Margin-parallel movements of these “exotic” arcs are more probable, or as continents advance toward them they can be accreted and finally consolidated and transferred to continental crust as we see in northwestern Australia-New Guinea today. On the other hand, there are to date no known examples of large, far-traveled continental fragments the size of India or larger that have crossed the Pacific Ocean to collide and indent, and cause severe deformation, anywhere around the Circum-Pacific margin (Coney, 1990a,b). This is in direct contrast to the Circum-Atlantic and Tethyan realms where almost the entire Phanerozoic erogenic history is dominated by continental collisions or collisions of large continental fragments with larger continental margins (see Sengiir, 1987; Harris and Fettes, 1988; Sengor et al., 1988; Dalmeyer, 1989). Fu~he~ore, the oceans that opened and closed, or across which fragments were passed, never seem to have been very large, at least compared to the Pacific realm. In other words, the Wilson cycle has dominated ~ircum-Atlantic and Tethyan tectonic evolution. The North Atlantic realm opened and then closed in a period of 200 to 300 Ma in the late Proterozoic-early Paleozoic to produce the Caledonian orogeny and fuse Baltica with North America, then has opened again since the early Cenozoic. The central Atlantic realm possibly opened and closed twice during the Paleozoic to produce Acadian then ~leghanian-Variscan orogeny in central Europe-eastern and southern North America as Gondwanaland interacted with Laurentia. It has, of course, opened again since the Jurassic-Cretaceous. As was previously discussed, Tethys has seen the near constant transfer of Gondwanaland fragments across to Eurasia since mid- to late Paleozoic times down to the present (Sengijr, 1987). In contrast, the Pacific Ocean basin has experienced a remarkable permanency throughout most if not all of the Phanerozoic, a period of at least 500 Ma (Scotese, 1987; Coney, 199Ob). The

20

I I CON,

pre-Phanerozoic history of the Pacific is still obscure, but, as was discussed earlier, it could have opened in the late Proterozoic by the rotation of Australia-East Antarctica away from Laurentia. The Phanerozoic permanency of the Pacific Ocean basin is best displayed in Figure 12 which is a cumulative plot of the positions of the major continental cratons through Phanerozoic times. In the figure, the shaded area is that part of the surface of the earth where continents either are today or have been in the past 500 Ma. The central part of the projection, left blank, is that region where no continent has apparently ever entered or crossed since the early Paleozoic. This area is nearly coincident with the present Pacific Ocean basin. For some reason the Pacific Ocean basin has been a permanent feature throughout Phanerozoic times in spite of the fact that its oceanic crust has been recycled at least three times. This then suggests the Wilson cycle has not been an important geodynamic process in the Phanerozoic evolution of the Circum-Pacific. What then was the important geodynamic process in the Circum-Pacific? Accretionary process have been very significant. This seems to have been mainly the telescoping and consolidation of distal continental margin terranes into their adjacent continental margins, accretions of off-shore fringing arcs, accretions of off-scraped and/or underplated disrupted continental margin ter-

Pacific Ocean Basin

1

ranes associated with marginal subduction zones. occasional accretions of relatively small near to perhaps in some cases far-traveled oceanic terranes such as oceanic arcs and plateaus. These “exotic” accretions seem to have been rapidly detached from the oceanic floor plates upon which they rode, thus made no important and prolonged indentations, and then along with the other terranes, were subjected to long processes of telescoping and disruption as they were all progressively consolidated into the cratonic marginal erogenic systems typical of the CircumPacific. In other words, the tectonic processes were intraplate rather than interplate in character. The important geodynamic process seems to have been some combination of the “absolute” motions of the main cratonic blocks themselves and relative motions between those blocks and the Pacific Ocean floor. The motions of the continents may have been important. Almost all the major periods of orogeny and consolidation identified around the Circum-Pacific are associated with proposed significant movements of the continents, principally when the margins of these continents advance in an “absolute motion” sense over the adjacent Pacific Ocean floor. The best examples are the Mesozoic-Cenozoic evolution of the American Cordilleras. Another possible example is the Silurian-Devonian evolution of

North Pole

Fig. 12. The apparent permanency of the Pacific Ocean basin (from Coney, 1990a,b). Shaded area is region where major continental cratons are today or have been during the past 500 Ma. Area left white is region where no continent has entered or crossed during past 500 Ma. H = Hawaii hotspot. Pacific Ocean geoid high shown by 25- and 100-m contours (after Rapp, 1981).

THE

LACHLAN

BELT

OF EASTERN

AUSTRALIA

AND

CfRCUM-PACIFIC

the Tasman Orogenic System of eastern AustraIia, and perhaps the entire Devonian Pacific margin of Gondwanaland. Finally, what is the significance of all this for the Lachlan belt? The exact paleogeographic and plate tectonic setting of the early Paleozoic LachIan, mentioned earlier, has been, and no doubt will continue to be, discussed for years. Almost all the possible actualistic analogs seem to have been thought of and until new data, particularly from the deep crust, or insight materializes one is left with trying to choose between the many quite reasonable but difficult to prove scenarios that have been proposed. As mentioned earlier, some sort of vast and no doubt complex submarine back-arc realm is not unreasonable. Similar conditions, whatever they were, may have existed along much of the Pacific margin of Gondwanaland. Of more concern here is the way in which this large tract of the pre-Silurian Lachlan was consolidated into the crust of Australia during the 100 Ma or so of complex Silurian-Devonian orogenesis already summarized. The way in which it failed is quite distinctive and is worth returning to. It seems to have failed sequentially, by sectors, but in a very homogeneous way. It did not fail like oceanic crust which is usually easy to subduct, and it did not fail like most continental tracts in erogenic systems with large scale thrust ramping and juxtaposition of different structural levels. This suggests that some fortuitous inherited crustal structure and composition - perhaps an attenuated thin quasi-continental substrate favored its unique intraplate style. Presumably the compression and shortening, as well as the intermittent extension and transpressive modes, were anchored by subduction zones somewhere to the east into which convergent rates must have been - intermittently at least - quite high, perhaps interspersed with extensional “roll-backs.” If the movements of Gondwanaland during the Silurian-Devonian prove to be substantiated by further paleomagnetic work, the collapse and consolidation might have been driven in part by a relatively high intraplate stress regimen applied over an unusually large quasi-continental to quasi-oceanic realm along and “off-shore” of the

TECTONIC

21

EVOLUTION

6

1 5

4

1

(equal to BOX of Phanerozolc

gold)

Tote1 Atl.-Tsthys

Fig. 13. Gold production from Phanerozoic orogens. Production figures to 1934 from Emmons, 1937). Note contrast in Circum-Pacific as against Atlantic-Tethyan totals and preponderance of Circum-Pacific production from the northeast sector.

Gondwanaland cratonic margin as Australia “advanced” over the adjacent Pacific. In conclusion, in the context of this seemingly important contrast in Circum-Pacific as against Circum-Atlantic and Tethys tectonic evolution, it might be fruitful to ponder possible contrasts in resource distributions. Prompted by a dim and perhaps ethnocentric realization that I had never heard of a serious “classic” gold rush outside of the Circum-Pacific I tabulated total world gold production from Phanerozoic erogenic belts as reported in Emmons (1937). The results (Fig. 13) suggest something like 80% of all Phanerozoic orogen gold production up to 1934 came from Phanerozoic Circum-Pacific belts. CaledonianAppaIachian-Varis~an production in particular, and even Tethyan production in general, was remarkably low in comparison. Furthermore, it is worth pointing out that over 60% of the Pacific Rim production seems to have come from the Alaska to Columbia northeastern Pacific margin sector which, as was mentioned earlier, is dominated by suspect terranes of oceanic affinity. In other words, 60% of Circum-Pacific gold production has come from only 30% of the Pacific Rim’s circumference. Interestingly enough, the Lachlan belt of eastern Australia has produced most of the rest.

I’ 1 (‘ONI~l

22

And finally, it is worth pointing out that the Lachlan belt has a remarkable similarity to the equally enigmatic Archean greenstone belts, a point which I have heard stated informally by several knowledgeable and usually reticent Australian earth scientists. The exposed scale is actually similar. The greenstone “substrate,” thick sequences of deep-water greywacke, evolving submarine volcanic tracts, isoclinal fold styles, major transpressive to strike-slip discontinuities, a growing awareness of thrusting, all swamped in a sea of bulbous granites, no clear-cut plate boundary tectonic signatures, and an extraordinary production of gold, are all typical of both. It is not clear if calling attention to this similarity does more for our understanding of the Lachlan or of the Archean greenstone belts, but it might mean that the Lachlan, through some accident of early Paleozoic paleogeography and fortuitous crustal composition is in a sense a preserved Phanerozoic relic of whatever some of the conditions on our planet were during Archean tectonic evolution.

Bell, T.R.

and Jefferson,

Berry,

R.F.

cance

87. Australas.

and

Crawford,

of Cambrian

plexes in Tasmania. Bishop,

D.G.,

sional

Resources,

Ernst

1987. Terrane

placement

in northern

problems

and constraints.

ner (Editors), Geophys.

Ruby

of

for Enassem-

a reinter-

Volume

I

Prentice-Hall.

and

dissome

and E. Scheib-

and Orogenic

S.D. and Laird, tectonics

Belts. Am.

M.G.,

1985. Suspect

in northern

In: D.G. Howell (Editor),

Terranes

terrane

Antarctica:

Ser., 19: 199-206.

Cambrian

Council

and

Land,

In: E.C. Leitch

Geodyn.

Land, Antarctica.

boundaries

Accretion

J.D., Weaver,

graphic

California:

California.

Victoria

Terrane

Union,

terranes

Council

Sci. Ser., 1: 515-522.

(Editor), of

In:

Terranes

Cliffs, N.J., pp. 306-328.

J.D.,

Bradshaw,

Zealand.

D.L., 19X 1, The Franciscan

Development

Englewood Bradshaw,

Earth

com-

lY85. Provi-

New

Circum-Pacific

rocks in northern

In: W.G.

C.A.,

Island,

Tectonostratigraphic

ergy and Mineral

R.A.F.. ment

in the Circum-Pacific for Energy

Victoria

Tectonostrati-

Region.

and Mineral

1983. Paleogeographic

of the Lachlan

References

Circum-

Resources,

Earth

and Vandenberg,

J.G.

and

Douglas

Victoria. Chappell,

G., Charrier,

R. and Vicenti,

et structurale

Phys. Geol. Dyn.,

R., Chotin,

J.C., 1973. Esquisse

des Andes 1.5: 11-72.

Meridonales.

Tectonic

Develop-

Southeastern

B.W.

Chappell,

Australia.

and

White,

and Hine,

P.L.F.

Australia.

contrasting

R., 1988. Granite

in the Lachlan

Aust. J. Earth

fold belt,

Sci., 35: 505-521.

E., 1986. Metallogeny

of the

of

Tasman

fold

belt

and tecsystem

in

Ore Geol. Rev., 1: 153-201.

P.J., 1973. Plate tectonics

of marginal

Cordillera.

Evolution

(Editors), of the

Geodyn.

P.J.,

foreland

thrust-

1: 131-134.

P.J., 1987. Circum-Pacific

Francheteau

Coney,

terranes

and Williams,

development

Union,

1974. Two

A.J.R.

and basement

American

Coney,

A.J.R.,

In:

Geology

pp. 63-102.

B.W.. White,

fold belts. Geology, Coney,

Melbourne, 8: 173-174.

Tasmania. Coney,

(Editors),

Geol.,

southeastern tonic

1988. Ordovician.

Ferguson

types. Pacific

provinces Collins,

A.H.M.,

J.A.

Geol. Sot. Aust.,

granite

Oxford, A.V., Cecioni,

and

Belt,

Geol. Sot. Aust. Spec. Publ. 10, 104 pp.

tectonogenesis

In:

J.W.H.

Pacific

Ocean

in the North

Monger

Circum-Pacific

Orogenic Basin.

Am.

and

J.

Belts and Geophys.

Ser., 18: 59-70

1989a. The North

American

In: Z. Ben-Avraham

of the Pacific

J., Thiele,

Fold

Cas, R.A.F.

cum-Pacific.

Rev. Geogr.

J.D. and Landis, of South

Blake, M.C., Jr. and Jones,

Geotectonic

signifi-

Sci., 35: 5233533.

Region.

pretation.

1988. The tectonic

Aust. J. Earth

(Editor),

blage and related

Pacific

pp. 3Y-SO.

mafic-ultramafic

map

Howell

In: Proceedings,

Sci. Ser., 1: 467-479.

Studies by the author in the Circum-Pacific region over the past 15 years have been supported by the U.S. Geological Survey, U.S. Department of Energy, BHP Minerals International, Melbourne and San Francisco, a Williams Evans Fellowship at the University of Otago, New Zealand, and a fellowship from the University of Wollongong, Australia. Collaborative work and discussions with D.L. Jones, N.J. Silberling, J.W.H. Monger, D.G. Howell, M.F. Campa, T. Harms, D. Richards, C. Smith, Ian Dalziel, A. Edwards, R. Hine, F. Morrison, D. Windrim, A. Goode, C. Murray, S. Titley, J. Ruiz, W.R. Dickinson, C. Landis, and C. Fergusson have been most helpful. This paper was much improved by constructive and helpful reviews by C. Fergusson, E. Scheibner, and J. Veevers.

paleogeographique

A.J.,

for an

Proterozoic

Inst. Min. Metall.,

the Circum-Pacific

Cas,

J., Borrello,

in the late

allochthonous

Bradshaw.

terrane

D.G.

Acknowledgments

P., Frutosi,

1987. An hypothesis

connection

and the birth of the Pacific Ocean. Rim Congress

Pacific

Aubouin,

C.W..

Australian-Canadian

Ocean

margins.

sector

(Editor), Oxford

of the Cir-

The Evolution

University

Press,

pp. 43-52.

P.J., 1989b. Structural

accretionary

tectonics

aspects

in western

Geol., 11: 107-125. Coney. P.J.. 1990a. Terranes,

of suspect

terranes

North America.

tectonics

and

J. Struct.

and the Pacific

Rim.

THE

LACHLAN

BELT

OF EASTERN

AUSTRALIA

AND

CIRCUM-PACIFIC

In: T.J. Wiley, D.G. Howell and F.L. Wong (Editors), Terrane Analysis of China and the Pacific Rim. CircumPacific Council for Energy and Mineral Resources, 13: 49-70. Coney, P.J., 1990b. Terranes, tectonics and the Pacific Rim. In: Proceedings, Pacific Rim Congress 90. Australas. Inst. Min. Metall., pp. 19-30. Coney, P.J. and Jones, D.L., 1985. Accretion tectonics and crustal structure in Alaska. Tectonophysics, 119: 265-283. Coney, P.J., Edwards, A., Hine, R., Morrison, F. and Windrim, D., 1990. The regional tectonics of the Tasman erogenic system, eastern Australia. J. Struct. Geol., 12: 519-543. Craddock, C., 1982. Antarctica and Gondwanaland. In: C. Craddock (Editor), Antarctic Geoscience. Int. Union Geol. Sci. Ser.,B, 4: 3-13. Crawford, A.J., 1988. Cambrian. In: J.G. Douglas and J.A. Ferguson (Editors), Geology of Victoria. Geol. Sot. Aust., Melbourne, pp. 37-62. Crawford, A.J., Cameron, W.E. and Keays, R.R., 1984. The association boninite low-Ti andesite-tholeiite in the Heathcote greenstone belt, Victoria; ensimatic setting for the early Lachlan fold belt. Aust. J. Earth Sci., 31: 161-175. Dallmeyer, R.D. (Editor), 1989. Terranes in the CircumAtlantic Paleozoic orogens. Geol. Sot. Am., Spec. Pap. 230, 277 pp. Dalziel, I.W.D., 1988. Andean magmatism: the global tectonic setting. Geol. Sot. Am., Abstr. Programs, 20: A5. Dalziel, I.W.D., 1991. Pacific margins of Laurentia and East Antarctica-Australia as a conjugate rift pair: evidence and implications for an Eocambrian supercontinent. Geology, 19: 598-601. Dalziel, I.W.D. and Forsythe, R.D., 1985. Andean evolution and the terrane concept. In: D.G. Howell (Editor), Tectonostratigraphic Terranes of the Circum-Pacific Region. Circum-Pacific Council for Energy and Mineral Resources, Earth Sci. Ser., 1: 565-581. Dalziel, I.W.D. and Grunow, A.M., 1985. The Pacific margin of Antarctica: terranes within terranes within terranes. In: D.G. Howell (Editor), Tectonostratigraphic terranes of the Circum-Pacific region: Circum-Pacific Council for Energy and Mineral Resources, Earth Sci. Ser., 1: 555-564. Degeling, P.R., Gilligan, L.B., Scheibner, E. and Suppel, D.W., 1986. Metallogeny and tectonic development of the Tasman fold belt system in New South Wales. Ore Geol. Rev., 1: 259-313. Emmons, W.H., 1937. Gold Deposits of the World. McGrawHill, New York, N.Y., 562 pp. Fergusson, C.L. and Coney, P.J., 1992. Convergence and intraplate deformation in the Lachlan Fold Belt of southeastern Australia. In: C.L. Fergusson and R.A. Glen (Editors), The Palaeozoic Eastern Margin of Gondwanaland: Tectonics of the Lachlan Fold Belt, southeastern Australia and related orogens. Tectonophysics, 214: 417-439. Fergusson, CL. and VandenBerg, A.H.M., 1990. Middle Paleozoic thrusting in the eastern Lachlan fold belt, southeastern Australia. J. Struct. Geol., 12: 577-590.

TECTONIC

EVOLUTION

23

Fergusson, C.L., Gray, D.R. and Cas, R.A.F., 1986. Gverthrust terranes in the Lachlan fold belt, southeastern Australia. Geology, 14: 519-522. Gibson, G.M., 1988. Glenelg Zone. In: J.G. Douglas and J.A. Ferguson (Editors), Geology of Victoria. Geol. Sot. Aust., Melbourne, pp. 5-7. Gray, D.R., 1988. Structure and tectonics. In: J.G. Douglas and J.A. Ferguson (Editors), Geology of Victoria. Geol. Sot. Aust., Melbourne, pp. l-36. Griffiths, J.R., 1971. Continental margin tectonics and the evolution of southeast Australia. APEA J., 11: 75-79. Grikurov, G.E., Kamenev, E.N. and Kameneva, G.I., 1982. Granitoid complexes in Antarctica. In: C. Craddock (Editor), Antarctic Geoscience. Int. Union Geol. Sci., Ser. B, 4: 695-702. Harris, A.L. and Fettes, D.J. (Editors), 1988. The Caledonian-Appalachian Orogen. Geol. Sot. London, Spec. Publ. 38, 643 pp. He& F., 1988. Late Paleozoic subduction and accretion in southern Chile. Episodes, 11: 183-188. Her&, F., Godoy, E., Parada, M.A., Ramos, V., Rapela, C., Mpodozis, C. and Davidson, J., 1987. A general view on the Chilean-Argentine Andes, with emphasis on their early history. In: J.W.H. Monger and J. Francheteau (Editors), Circum-Pacific Orogenic Belts and Evolution of the Pacific Ocean Basin. Am. Geophys. Union, Geodyn. Ser., 18: 97-114. Hoffman, P.F., 1991. Did the breakout of Laurentia turn Gondwanaland inside out? Science, 252: 1409-1412. Howell, D.G., 1980. Mesozoic accretion of exotic terranes along the New Zealand segment of Gondwanaland. Geology, 8: 487-491. Howell, D.G., Jones, D.L. and Schermer, E.R., 1985. Tectonostratigraphic terranes of the Circum-Pacific region. In: D.G. Howell (Editor), Tectonostratigraphic Terranes of the Circum Pacific Region. Circum-Pacific Council for Energy and Mineral Resources, Earth Sci. Ser., 1: 3-30. Jefferson, C.W., 1978. Correlation of Middle and Upper Proterozoic strata between northwestern Canada and south and central Australia. Geol. Sot. Am., Abstr. Programs, 10: 429. Kent, D.V. and Van der Voo, R., 1990. Paleozoic paleogeography from paleomagnetism of the Atlantic-bordering continents. In: C. Scotese and S. McKerrow (Editors), Paleozoic Biogeography and Paleogeography. Geol. Sot. London Mem., 12: 49-56. Laubacher, G. and F. MCgard, 1985. The Hercynian basement: a review. In: W.S. Pitcher et al. (Editors), Magmatism at a Plate Edge: The Peruvian Andes. Leitch, E.C., Webby, B.D., Mills, K.J. and Kolbe, P., 1987. Terranes of the Wonominta block, far western New South Wales. In: E.C. Leitch and E. Scheibner (Editors), Terrane Accretion and Orogenic Belts. Am. Geophys. Union, Geodyn. Ser., 19: 31-38. Li, Z.X., Powell, CM., Thrupp, G.A. and Schmidt, P.W., 1990. Australian Paleozoic paleomagnetism and tectonics

24

-If: a revised apparent polar wander path and paieogeography. J. Struct. Geol., 12: 567-576. Marsden, M.A.H., 1988. Upper Devonian-Carboniferous. In: J.G. Douglas and J.A. Ferguson (Editors), Geology of Victoria. Geol. Sot. Aust., Mel~urne, pp. 147-194. May, S.R., Beck, M.E. and Butler, R.F., 1989. North American apparent polar wander, plate motion, and left oblique convergence: Late Jurassic-Early Cretaceous erogenic consequences. Tectonics, 8: 443-452. Migard, F., I978. Etude geologique des Andes du Peru central. ORSTOM, 86, 310 pp. Megard, F., 1989. The evolution of the Pacific Ocean margin in South America north of the Arica Elbow (IX”S). In: 2. Ben-Avraham (Editor), The Evolution of the Pacific Ocean Margins. Oxford University Press, Oxford, pp. 10X-230. Monger, J.W.H. and Berg, H.C., 1987. Lithotectonic terrane map of western Canada and southeastern Alaska. U.S. Geoi. Sure., Field Studies Map 1874-B. Moores, E.M., 1991. Southwest U.S.-East Antarctica (SWEAT) connection: a hypothesis. Geology. 19: 425-428. Murray, C.G., 1986. Metailogeny and tectonic development of the Tasman Fold Belt system in Queensland. Ore Geol. Rev., 1: 315-400. Murray, C.G., Fergusson, C.H., Flood, P.G., Whitaker, W.G. and Korsch, R.J., 1987. Plate tectonic model for the Carboniferous evolution of the New England Fold Belt. Aust. J. Earth Sci., 34: 213-236. Packham, G.H., 1987. The eastern Lachlan Fold Belt of southeast Australia: a possible late Ordovician to Early Devonian sinistral strike-slip regime. In: E.C. Leitch and E. Scheibner (Editors), Terrane Accretion and Orogenic Belts. Am. Geophys. Union, Geodyn. Ser., 19: 67-82. Page, R.A., Platker, G., Fuis, G.S., Nokieberg, W.J.. Amdos, EL.. Mooney, W.D. and Campbell. D.L., 1986. Accretion and subduction tectonics in the Chugach Mountains and Cooper River basin, Alaska: initial results of TACT. Geology. 14: 501-SOS. Parker, A.J., 1986. Tectonic development and metallogeny of the Kanmantoo trough in South Australia. Ore Geoi. Rev., I: 203-212. Perkins, C., McDougall, I. and Ciaoue-Long, J., 1990. Dating of ore deposits with high precision: examples from the Lachian fold belt, New South Wales, Australia. in: Proceedings, Pacific Rim Congress 90. Au&alas. Inst. Min. Metali., 2: 105-l 12. Powell, CMcA.. 1983. Tectonic relationship between the Late Ordovician and Late Silurian paleogeographies of southeastern Australia. J. Geoi. Sot. Aust., 30: 3533373. Powell. CMcA., 1984. Silurian to Mid-Devonian. In: J.J. Veevers (Editors), Phanerozoic Earth History of Australia. Oxford University Press, Oxford, pp. 309-328. Preiss, W.V. (Compiler), 1987. The Adelaide Geosyncline Late Proterozoic Stratigraphy, Sedimentation, Paleontology and Tectonics. Geoi. Surv. South Aust., Bull. 53. Preiss, W-V., 198X. A stratigraphic and tectonic overview of the Adelaide geosyncline, South Australia. Dep. Mines and Energy of South Australia. Rep. 8X/19, 49 pp.

?,,I ( Oh1

>

Ramos, V.A., 1988. Late Proterozoic-Early Paleozoic of South America - a collisional history. Episodes, 11: 16% 174. Ramsay, W.R.H. and Vandenberg, A.H.M., 1986. Metalogeny and tectonic development of the Tasman Fold Belt system in Victoria. Ore Gent. Rev.. 1: 213-257. Rapp, R.H.. 1981. The earth’s gravity field to degree and order 180 using SEASAT, altimeter data, terrestrial gravity data, and other data. Ohio State Univ. Dep. Geodesy, Science Survey Report No. 322. Restrepo, J.J. and Toussdint, J.F.. 1988. Terranes and continental accretion in the Columbia Andes. Episodes, 11: 189-193. Samson, SD., McClelland, W.C., Gehrels, G.E. and Patchett, P.J., 1987. Nd isotopes and the origin of the accreted Alexander and Stikine terranes in the Canadian Cordiilera. EOS, Am. Geophys. Union Trans., 68: 1458. Scheibner, E.. 1978a. Tasman fold belt system or erogenic system-Introduction. Tectonophysics, 48: 153-157. Scheibner, E. (Editor), 197%. The Phanerozoic Structure of Australia and Variations in Tectonic Style. Tectonophysics, 48: 153-427. Scheibner, E., 1985. Suspect terranes in the Tasman Fold Belt System. eastern Australia. In: D.G. Howell (Editor), Tectonostratigraphic Terranes of the Circum-Pacific Region. Circum-Pacific Council for Energy and Mineral Resources, Earth Sci. Ser., I: 493-514. Scheibner, E. (Editor). 1986. Metaliogeny and Tectonic Development of Eastern Australia. Ore Geoi. Rev., I: 147412. Scheibner, E., 1987. Paleozoic tectonic development of eastern Australia in relation to the Pacific region. In: J.W.H. Monger and J. Francheteau (Editors), Circum-Pacific Orogenie Belts and Evolution of the Pacific Ocean Basin. Am. Geophys. Union, Geodyn. Ser., 18: 133-165. Scotese, CR., 1987. Development of the Circum-Pacific Panthaiiassic Ocean during the Early Paleozoic. In: J.W.H. Monger and J. Francheteau (Editors), Circum-Pacific Orogenie Belts and Evolution of the Pacific Ocean Basin. Am. Geophys. Union, Geodyn. Ser., IX: 49-58. Scotese, CR. and Denham, C.R., 1988. Terra Mobilis. Earth in Motion Technologies, Austin, Texas. Sengor, A.M.C., 1987. Tectonics of the Tethysides: erogenic collage development in a collisional setting. Annu. Rev. Earth Planet. Sci., 15: 213-244. Sengor, A.M.C., Altimer, D., Cin, A., Ustaiimer, T. and Hsii, K.J., 1988: Origin and assembly of the Tethyside erogenic collage at the expense of Gondwana Land. In: M.G. Audiey-Charles and A. Haliam (Editors), Gondwana and Tethys. Geoi. Sot. London, Spec. Pubi., 37: 119-182. Sporii, K.B., 1987. Development of the New Zealand microcontinent. In: J.W.H. Monger and J. Francheteau (Editors), Circum-Pacific Orogenic Belts and Evolution of the Pacific Ocean Basin. Am. Geophys. Union, Geodyn. Ser., 18: 115-132. Taira, A., Tokuyama, H. and Soh, W., 1989. Accretion tectonics and evolution of Japan. In: Z. Ben-Avraham (Editor),

THE LACHLAN BELT OF EASTERN AUSTRALIA AND CIRCUM-PACIFIC TECTONIC EVOLUTION

The Evolution of the Pacific Ocean Margins. Oxford University Press, Oxford, pp. 100-123. Turcotte, D.L. and Schubert, G., 1982. Geodynamics. Wiley and Sons, New York, N.Y., 449 pp. VandenBerg, A.H.M., 1988. Silurian-Middle Devonian. In: J.G. Douglas and J.A. Ferguson (Editors), Geology of Victoria. Geol. Sot. Aust., Melbourne, pp. 103-146. Van der Voo, R., 1988. Paleozoic paleogeography of North America, Gondwana, and intervening displaced terranes: comparisons of paleomagnetism with paleoclimatology and biogeographical patterns. Geol. Sot. Am. Bull., 100: 311324.

25

Wyborn, D., 1992. The significance of Ordovician magmatism in the eastern Lachlan Fold Belt. In: C.L. Fergusson and R.A. Glen (Editors), The Palaeozoic Eastern Margin of Gondwanaland: Tectonics of the Lachlan Fold Belt, southeastern Australia and related orogens. Tectonophysics, 214: 177-192. Wyborn, L.A.I. and Chappell, B.W., 1983. Chemistry of the Ordovician and Silurian greywackes of the Snowy Mountains, southeastern Australia: an example of chemical evolution of sediments with time. Chem. Geol., 39: 81-92.