Australia: The Land Where Time Began
Terra Australis Orogen
According to Peter A. Cawood:
'The record of the initiation of the Pacific and Iapetus margins of Gondwana, and the subsequent inception of convergent plate interaction, is preserved in a Neoproterozoic to late Palaeozoic orogenic belt here termed the Terra Australis Orogen (Fig. 2). The orogen forms a fundamental crustal element that extends along the margin of Gondwana.
The aim of this paper is to outline the distribution and character of the Terra Australis Orogen, concentrating on the differentiation and development of the major tectonic elements of the orogen between the late Neoproterozoic to the early Palaeozoic, synthesizing along and across strike comparison of rock units, and discussing termination of the orogen at the end of the Palaeozoic. Previous work has concentrated on individual segments within the orogen, reflecting in part the geographic convenience of the intracontinental segments of the orogen preserved in Australasia, Antarctica and South America rather than the geologic reality of the intercontinental distribution and original continuity of related tectonostratigraphic rock units'. See reference below.
Terra Australis Orogen
The Terra Australis Orogen stretches from the Australian northeast coast, past the tip of southern Africa into South America, via Tasmania, New Zealand, the Transantarctic Mountains and the Antarctic Peninsula (fig. 2, see source below). In the mid-Neoproterozoic the establishment of continental margin sequences along East Gondwana, associated with the opening of the Pacific Ocean. In the late Neoproterozoic to early Palaeozoic it began with establishment of continental margin sequences along West Gondwana associated with the opening of the Iapetus Ocean.
By the early Palaeozoic the Gondwana supercontinent had been established by the assembly of the continental blocks making up East and West Gondwana along the East African, Pinjarra, Damara and Breaziliano orogens ("Pan-African"). The result was the propagation of the Terra Australis Orogen that stretched along the entire Pacific and Iapetus margins of Gondwana, (Fig.3, see source below). The Gondwanide Orogeny, the Pan-Pacific Gondwana margin orogenic event, terminated the Terra Australis Orogen at about 300-230 Ma (du Toit, 1937; Ramos & Aleman, 2000; Veevers & Powell, 1994). This marked the initiation of the classic Gondwanide Orogen of the late Palaeozoic to Mesozoic, and a stepping out of the plate boundary position.
The along-strike, pre-dispersal length of the Terra Australis Orogen was about 18,000 km and a width across strike of up to 1,600 km (Fig. 2). The craton-ward extent of the deformation is taken to be the inboard orogen margin, best preserved in eastern Australia, corresponding with the Torrens Hinge Line, which marks the limit of the Delamerian Orogeny of the Cambrian-Ordovician. The boundary is masked in other places by younger deposits, including ice in Antarctica, and orogenic events that occurred later than the Terra Australis Orogen. This orogen has not been traced in northeastern Australia for Eastern Gondwana or the northwestern tip of South America. Some reasons for this are that the outboard margin of the orogen is not exposed, being beyond of the coastline of the continental fragments where it is preserved, or it may be overprinted by younger orogens of the age of the Gondwanide or younger orogenic belts, such as the Andes. It has been inferred that the segment between the northern segment of South America, extending into northwest Africa, may have consisted of a number of terranes that rifted from Gondwana in the early Palaeozoic(Avalonia-Carolina-Cadomia) incorporating into Laurentia during the late Palaeozoic (Keppie et al., 2003).
Zircon cores of Ordovician to Carboniferous age have been found in magmatic rocks from the Triassic, in New Guinea, directly along strike from northeast Australia (Crowhurst et al., 2004). This suggests that Palaeozoic material of similar age to the age of the Terra Australis Orogen may extend into this region (cf. Van Wyck & Williams, 2002).
Based on the timing and nature of orogenic activity, and the geographic disposition of units, it is traditionally divided into a series of separate structural units. The Adelaide/Delamerian Fold Belt (orogen), and the Ross Fold Belt, its along strike equivalent, and the Lachlan, Thompson, Tuhua and New England Fold Belts of eastern Australia and New Zealand, and underlying the South American Cordillera, the Neoproterozoic to Palaeozoic rock units, are all included in the orogen. It runs south of the Saldania Belt of southern Africa, of Neoproterozoic to early Palaeozoic age, part of the Pan-African orogenic system (Rozendaal et.al., 1999).
The area covered by the Terra Australis Orogen overlaps that of the Samfrau Geosyncline of du Toit (1937). According to Du Toit, the Samfrau Geosyncline included similar rock units and events from Silurian to Early Cretaceous age that extended from New Guinea to South America. Rocks from the early Neoproterozoic to early Palaeozoic of the East Gondwana margin including the Adelaide-Ross fold belts were not included and include late Palaeozoic to Mesozoic units that are now part of separate, temporarily discrete Gondwanide Orogen.
Based on their character and affinities the lithotectonic units, reflecting the initial tectonic settings of the blocks, the Terra Australis Orogen is divided into a number of sequences and assemblages (Fig. 4). Cawood, (See source below). Structural overprints related to late orogenic events have traditionally been the basis for division of specific segments of the orogen, such as individual orogens/fold belts of eastern Australian, New Zealand and Antarctica (Leitch, 1974; Scheibner, 1996; Stump, 1995). The importance of differentiating depositional and orogenic belts with the depositional basin of the Adelaidean succession was recognised by Preiss (see also Drexel et al., 1993, 2000), referring to it as the Adelaide Geosyncline, separated from depositional boundaries of the early Palaeozoic, the Delamerian Fold Belt (cf. Adelaide Fold Belt).
Along the east and west margins of Gondwana segments of the orogen are continental margin sequences, with a series of continental and oceanic assemblages of Gondwanan, Laurentian and intra-cratonic character, lying outboard of the orogen segments. The breakup of Rodinia is recorded by the continental margin sequences, the accretionary history of the Gondwanan margin being recorded by the outboard continental and oceanic sequences. The ocean-ward limit of the autochthonous Gondwana basement is marked by the outboard limit of continental margin sequences. Cawood suggests the boundary probably corresponds with the original boundary between the continent and ocean as Rodinia broke up, though invariably modified by events occurring later, including those that occurred after the Terra Australis Orogen. The boundary corresponds approximately with the Tasman Line in eastern Australia (Hill, 1951; Mills, 1992; Scheibner, 1996; also Crawford et al., 2003; Direen & Crawford, 2003b). In Victoria Land it equates to the eastern boundary of the Wilson terrane (Lanterman Fault) (see Goodge, 2002; Roland, 1991; Borg et al., 1987). They provide a more complete discussion of the boundary location in the region. The Boundary is close to the coast in the central Transantarctic Mountains (Goodge, 2002).
According to Cawood 'in the Antarctic Peninsula it must lie inboard of the Eastern Domain, which is correlated with the peri-Gondwanan oceanic basement terranes of eastern Australia, New Zealand and Marie Byrd Land, Antarctica (Vaughn & Storey, 2000)'. In the south of South America, the limit of autochthonous Gondwanan basement corresponds to the western margin of the Sierras Pampeanas, and further to the north, with the edge of the platform succession (Ramos and Aleman, 2000). The Sierras Pampeanas is part of a peri-Gondwanan assemblage rifted from the Rio de La Plata craton, the Pampean terrane, that re-accreted at the time of the Pampean Orogeny of the early Palaeozoic (Rapela et al., 1998a).
Sequences of continental margins
Within Rodinia, the Terra Australis Orogen continental margin sequences developed on stabilised continental lithosphere. They can be divided into sequences reflecting their location in either East Gondwana or West Gondwana, as well as with the breakup of Rodinia timing.
Margin of East Gondwana
The Adelaide Fold Belt (Orogen) of eastern South Australia, as well as its continuation in western New South Wales and western Tasmania, the Ross Fold Belt of the Transantarctic Mountains, and the Anakie Inlier in central Queensland (Fig.4), have continental margin sequences of East Gondwanan origin. These preserved sequences are of mixed siliclastic and carbonate successions, of Neoproterozoic to Palaeozoic age, intercalated locally with mafic and felsic volcanic rocks. The low-grade rocks of the Adelaide Fold Belt preserve the most complete record of margin evolution (Powell et al.,1994; Preiss, 1987). Between about 830 and 500 Ma a thick succession of marine and terrestrial sediments accumulated in a series of rift and basinal successions, the succession being preserved in the Adelaide Fold belt. During the Cambro-Ordovician Delamerian Orogeny, about 500 Ma, sedimentation ended and deformation of the sequence began (Drexel & Preiss, 1995; Drexel et al., 1993; Powell et al., 1994; Preiss, 1987).
The western margin of the succession is situated at the Torrens Hinge Zone (Thompson, 1970), passing west into the platform strata, of a similar age, of the Stuart Shelf. As well as marking the limit of orogen-related deformation the Torrens Hinge Line marks the eastern boundary of the Gawler Craton, and the change from thick sedimentary sequences to the east to thin platform sedimentation to the west. At up to 25 km wide, it is a major crustal feature that has been interpreted as a number of different structures, such as a half-graben fault system, a monoclinal flexure, and a thrust front active during the Neoproterozoic, Palaeozoic and Cainozoic (Drexel et al., 1993, and references therein).
In southeast Australia, the western margin of the successions of the continental margin is usually believed to be situated along the Moyston Fault (Cayley & Taylor, 1991, 1997; Korsch et al., 2002, and references therein). The continental margin sequences are juxtaposed against the ocean margin assemblages of Gondwana, that were deformed by the orogensis of the mid-Palaeozoic between 450 and 340 Ma, by this long-lived structure.
The along-strike extension of the continental margin sequences of the Adelaidean are inferred to lie within the Wilson terrane in Antarctica (Flottmann et al., 1993; Goodge et al., 2002; Stump, 1995, and references therein). They range from lithologies of low grade siliclastic, limestone and calc-silicate to the high grade metamorphic equivalents of them, migmatite gneiss and anatectic granite. The East Antarctic ice sheet covers the western margin of the terrane. In northern Victoria Land the faulted eastern margin abuts the Bowers terrane along the Lantermann Fault (Gibson, 1987, Stump, 1995).
Multiply deformed greenschist amphibolite facies, pelitic and psamitic schist, marble calc-silicate schist, mafic schist and serpentinite comprise the Anakie Inlier of central Queensland (Fig. 4) (Fergusson et al. 2001; Withnall et al, 1996). Included in the succession are strata with an age of at least the Cambrian, based on detrital zircons dating to 510 Ma (Fergusson et al. 2001). These sequences situated east of the Tasman line are believed to be an extension of those of the Adelaidean succession.
Crystalline basement of Mezoproterozoic of older age, the age and character varying along strike and includes the Gawler Craton and Curnamona Province of southeast Australia, is overlain by the sequence of the East Gondwana continental margin (Drexel et al., 1993; Preiss, 2000), as is the Nimrod Group of the East Antarctic Shield in Antarctica (Goodge et al., 2001).
About 830 Ma rifting associated with the initiation of breakup of Rodinia began in east Australia (Fig. 5 in Cawood), based on the ages of volcanic rocks and feeder dykes from near the base of the Adelaidean succession (Wooltana volcanics, Gairdner dykes, Wingate et al., 1998). About 780-750 Ma magmatic activity related to rifting began (e.g. Holm, 2003). Within this sequence the estimated timing of the transition from rifting to drifting, reflecting the final continental breakup, Pacific Ocean generation and, along the margin of East Gondwana, the establishment of a passive margin sequence, range from at least 755 Ma, based on palaeomagnetic constraints, as well as the assumption that Australia was joined to Laurentia; (Wingate & Giddings, 2000), to about 700-680 Ma, the time of the influx of the first marine sediments (Powell et al., 1994; Preiss, 2000), and based on extension-related igneous activity in southeastern Australia, to about 600-560 Ma (Crawford et Al., 2003; Drieen & Crawford, 2003a; Veevers et al., 2007).
Cambrian magmatism has been reported from the Pensacola Mountains in Antarctica that has been related to margin rifting (Curtis et al., 1999). The suggestion that the Pacific Ocean was already established by the end of the Neoproterozoic is indicated by the overlapping of margin rifting and associated with convergent margin igneous activity along the East Gondwana margin (Googe, 2002; Googe et al., 2002). It also suggests igneous activity associated with extension occurred in an upper plate setting (cf. Miller & Storey, 1995), that is thought to possibly reflect the calving off of a ribbon of microcontinent, rather than the separation of the main Australian-Antarctic craton from Rodinia.
The Anakie Inlier, being to the east of the Tasman Line, could then be part of a detached lithospheric ribbon of the mainland of East Gondwana, if the Tasman Line does in fact reflect the original boundary between the continent and the ocean. A review of rifting timing of the Australian and Antarctic segments found that by the end of the Proterozoic the true passive margin must have been established, concurring with Preiss (2000) that it was established by 700-680 Ma (Fig. 5, Cawood) (Googe, et al, 2002).
During the Ross/Delamerian Orogeny, a protracted phase of tectonism occurred near the end of the Neoproterozoic to the Palaeozoic, during which continental margin sequences underwent deformation, as indicated by stratigraphic, structural and geochronological data. Sedimentation within the continental margin sequences was terminated by the Ross/Delamerian Orogeny. It also terminated metamorphism and regional deformation, as well as widespread emplacement of granite. The age of the main Delamerian orogenic phase is constrained from about 515-490 Ma by U-Pb zircon dating in the Adelaide Fold Belt of syn- to post-tectonic granitoids (Drexel & Preiss, 1995), the main deformation and metamorphism occurring between 515 and 500 Ma. The emplacement of Granite Harbour Intrusives in the Transantarctic Mountains, drowning of archaeocythian reefs and associated development of clastic sedimentary wedge, and unconformities in the Pensacola Mountains in the Late Cambrian to Early Ordovician, about 510-490 Ma, are all related to the Ross Orogeny (Encarnacion & Grunow, 1996; Myrow et al., 2002; Storey et al., 1996; Stump, 1995).
West Gondwana margin
Along the Andean margin of West Gondwana the continental margin sequences are mostly obscured by later tectonic events associated with the Andean margin that is convergent. Examples are the extensive sequence of foreland basins that developed inboard of the Cordillera during the Phanerozoic (Milani & Filho, 2000) that mostly cover any sedimentary sequences that were deposited along the western margin of the Amazonian Craton and the Rio de La Plata Craton at the time of the lithospheric extension and separation of West Gondwana from its inferred conjugate margin in Rodinia. It has been suggested that the passive margin sequences are probably preserved only beneath overthrusts in the Andes, possibly being situated west of the Arequipa-Antofalla terrane, based on data from the central Andes in Chile and Argentina (Ramos, 2000).
From northern Argentina to Venezuela autochthonous basement lies beneath shallow water platformal cover of Cambrian and Ordovician age (Fig. 4.6 see source below). In the northern Andes to the west of the platform succession, pre-Gondwanan terranes preserve deep-water deposits that are inferred to represent the original platform facies of the platform sequences (Ramos & Aleman, 2000; and references therein). The basement is indicated to be largely Grenvillian, 1300-1000 Ma, or older, based on isotopic studies, though locally it contains a record of Braziliano events (700-550 Ma; (Aleman & Ramos, 2000; Kroonenberg, 1982; Priem et al., 1989; Restrepo-Pace et al., 1997; Ruiz et al., 1999).
Along the West Gondwanan margin the rifting initiation, and the associated sediment deposition, as well as the transition from rifting to drifting is poorly constrained. Most of the continental margin strata were deposited after the transition from rifting to drifting that occurred as the Iapetus Ocean opened in the Early Cambrian (Cawood et al., 2001), as indicated by data from the inferred conjugate margin of East Laurentia, that is well-preserved.
Igneous assemblages of the Gondwana margin
Associated with the continental margin successions, as well as outboard, but proximal to the margin (Fig.4, see source), are igneous rocks of a convergent margin type. They mostly date to the Cambro-Ordovician, and generally associated with them are shallow marine or terrestrial siliclastic strata. Included among these are the Mount Windsor province, northeast Queensland (Henderson, 1986; Stolz, 1995), the Wright Volcanics, western New South Wales (Crawford et al., 1997), the Mount Stavely Belt, western Victoria (Crawford et al., 1996; Crawford, 1988), western Tasmania sequences (Crawford & Berry, 1992), the Bowers terrane, North Victoria Land, Antarctica (Cooper et. al.,1996; Weaver et al., 1984), the Takaka terrane, New Zealand (Cooper & Tulloch, 1992; Munker, 2000; Munker & Cooper, 1995), the Delamerian Granites, southeast Australia (Foden et al., 2002a; Foden et al., 1999; Foden et al., 2002b), the Granite Harbour Intrusives and related bodies of East Antarctica (Encarnacion & Grunow, 1996; Vogel et al., 2002), and the Western Sierra Pampeanas and Famatina belts of Argentina (Ramos, 2000; Rapela et al., 1998a).
The majority of the igneous bodies are associated with continental lithosphere of Gondwana, according to the geological relationships and geochemical data, and are of supra-subduction zone character, of type-A, extension related bodies, associated temporally and spatially with convergent plate magmas in Antarctica (Encarnacion & Grunow, 1996; Munker & Crawford, 2000; read et al., 2002). The mafic igneous rocks of the Takaka and Bowers terranes the geochemical signature shows an oceanic signature (Munker, 2000; Munker & Cooper, 1995; weaver et al., 1984). Within these 2 terranes the igneous sequences are interstratified with, or underlie, siliclastic strata of Gondwana derivation (Cooper, 1997; Cooper et al., 1996), their formation and development being constrained close to the Gondwana margin. Along the western margin of the Pampean Craton, a peri-Gondwanan terrane that rifted off the Rio de La Plata Craton, is situated the Sierra Pampeanas and Famatina belts, both from the Late Cambrian to Middle Ordovician, 510-460 Ma (Ramos, 2000; Ramos & Aleman, 2000). In East Antarctica are the oldest dated igneous bodies in this assemblage, dating to the late Neoproterozoic, about 550 Ma, though in this region most of the ages are about 540-480 Ma ((Encarnacion & Grunow, 1996; Rowell et al., 1993; Vogel et al., 2002).
A major phase of initiation of subduction at the continental margin of Gondwana at the start of the Palaeozoic, that terminated passive margin sedimentation, is indicated by the convergent margin character of the igneous bodies (Encarnacion & Grunow, 1996; Foden et al., 2002a; Munker & Cooper, 1995; Rapela et al., 1998b) and their association with passive margin sequences.
Parautocthonous and Allochthonous assemblages
Outboard from the continental sequences of the margin of Gondwana, is a series of parautochthonous and allochthonous assemblages comprising, continental lithosphere derived from Gondwana and Laurentia, oceanic lithosphere that formed at or near the margin of Gondwana, as well as oceanic lithosphere in an intra-oceanic setting, removed from the margins of Gondwana and Laurentia.
Peri-Gondwanan continental basement assemblages
A series of crustal fragments made up of Neoproterozoic to Palaeozoic cover succession, that accumulated on continental crust of Cambrian age, (Fig.6) (see source below) are present along the Andean segment of Gondwana. Among these are the Merida terrane, Venezuela, the Arequipa-Antofalla and Pampean terranes of Chile and Peru, the Famatina terrane of Argentina and Chile (Aleman & Ramos, 2000; Ramos, 2000; Rapela et al., 1998a). Along the Andean segment of Gondwana, basement outcrops of Precambrian age (Fig. 4, source 1) display evidence of protolith of Palaeoproterozoic and Mesoproterozoic age that has been overprinted by deformation and metamorphism during the late Mesoproterozoic, and in places, Neoproterozoic, based on geochemical and isotope data (Aleman 7 Ramos, 2000; Jairlard et al., 2000; Ramos, 2000).
Gondwanan faunas are present within deposits of Late Neoproterozoic to Palaeozoic age associated with basement blocks that are interpreted as representing parautochthonous fragments of the West Gondwana craton. It has been suggested they were accreted to West Gondwana during the Grenville and Braziliano orogenic cycles (Ramos & Aleman, 2000; Westeneys, 1995). Basement of Palaeoproterozoic to Mesoproterozoic age is contained within the Arequipa-Antofalla and Pampean cratons, with Neoproterozoic to Palaeozoic cover locally. In the Ordovician this basement had been remobilised during magmatic arc development, that was succeeded in the Late Ordovician by igneous activity associated with collision (Conti et al,., 1982; Davidson et al., 1983; Ramos, 1988b; Westeneys et al., 1995). These terranes have been interpreted to represent ribbon fragments of microcontinent that rifted from Gondwana during the opening of the Iapetus Ocean in the late Neoproterozoic to the Early Palaeozoic. Remaining marginal to Gondwana, these blocks were re-accreted during the early Palaeozoic as the marginal sea closed (Bahlburg & Herve, 1997; Keppie & Ramos, 1999; Ramos & Aleman, 2000; Rapela et al., 1998b).
A major fault separates the Patagonian segment of the Terra Australis Orogen, comprised of a series of basement blocks of Precambrian age, from the rest of South America. These basement blocks have a cover, that is predominantly siliclastic, that is preserved in the Somun Cura and Deseado massifs and the Patagonian Precordillera (Ramos & Acquirre-Uretta, 2000). In the Somun Cura and Deseado massifs, basement rocks have been dated to the early Proterozoic to Palaeozoic (Pankhurst et al., 2003; Ramos & Acquirre-Uretta, 2000). A parautochthonous Gondwana origin is indicated for the basement by age signatures of the metsedimentary basement and granitic rocks inthe Deaeado Massif, that are similar to the evolution of adjacent South America and the Antarctic Peninsula (Pankhurst et al., 2003).
Conclusion (of Cawood)
'The Terra Auastralis Orogen lies along the Pacific and Iapetus margins of Gondwana forming a fundamental lithospheric element within Gondwana. Prior to the breakup of Gondwana/Pangaea, the orogen extended from the northeast coast of Australia, through the Transantarctic Mountains, and along the west coast of South America, over a distance of some 18,000 km with an across strike width of up to 1,600 km. The orogen comprises continental margin sequences recording the breakup of East and West Gondwana segments from within Rodinia, outboard of which are a series of continental and oceanic assemblages of per-Gondwanan, Laurentian and intra-oceanic character that record the accretionary history of the margin. These assemblages show significant variation between East and West Gondwana, with the former characterised mostly by oceanic assemblages of peri-Gondwanan and Laurentian character (Fig.8) Thus the accreted assemblages appear to have a memory of the contrasting history of the inboard East and West Gondwana cratonic fragments and their continental margin assemblages.
Final amalgamation of Gondwana during the Neoproterozoic and Cambrian corresponds with initiation of subduction, first along the East Gondwana margin and then its propagation along the West Gondwana margin following its amalgamation with East Gondwana. This probably reflects a global plate kinetic adjustment to Gondwana amalgamation in which termination of convergence between East and West Gondwana, along with the development of a major spreading centre between West Gondwana and Laurentia associated with opening of the Iapetus Ocean, required initiation of convergence along the Pacific/Iapetus margin of Gondwana between 570-530 Ma.
The initiation of subduction along the Terra Australis Orogen in the late Neoproterozoic and Early Cambrian marks the inception of the Pacific 'ring of fire', yet throughout the Phanerozoic the Pacific has remained a major a major ocean basin [Coney, 1992 E947]. This indicated that the longevity of the pacific and its antecedents is a result of continued production of oceanic lithosphere throughout the Phanerozoic, rather than a delayed onset of subduction. Although the Pacific has been cited as the declining stage of the Wilson cycle of ocean basins [e.g., Jacobs, 1974 #604], its protracted history of ongoing subduction, and, by inference, oceanic crust generation, contrasts with the clear evidence of the opening and closing of oceans preserved in the Iapetus/Atlantic and Tethyan realms. This contrast has important implications for models of orogenesis within orogens in the Pacific which are the result of ocean-continent collision during a continuing cycle of subduction rather than continent-continent collision following ocean closure'.
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