Terra Australis Orogen: Rodinia breakup and development of the Pacific and Iapetus margins of Gondwana during the Neoproterozoic and Paleozoic
Introduction
The breakup of the end-Mesoproterozoic supercontinent Rodinia and its transformation into the end-Neoproterozoic to Paleozoic supercontinent Gondwana is recorded in the life cycle of four main ocean basins and their margins: the Mirovoi, Mozambique, Pacific and Iapetus oceans (Fig. 1). At the end of the Mesoproterozoic, Rodinia is envisaged to have been surrounded by the single, Pan-Rodinian Mirovoi Ocean (McMenamin and McMenamin, 1990, Hoffman, 1991, Meert and Powell, 2001). The breakout of Laurentia from the core of Rodinia resulted in the opening of the Pacific and Iapetus oceans along the western and eastern margins of Laurentia, respectively, and closure of the remnants of the Mirovoi Ocean, termed in part the Mozambique Ocean by Dalziel, 1991, Dalziel, 1997, leading to amalgamation of Gondwana by the end-Neoproterozoic (Collins and Windley, 2002). Major cratonic blocks that broke off Rodinia (e.g., the constituent fragments of West and East Gondwana and Baltica) were themselves fragmented through the formation (and ultimate closure) of additional oceanic tracts (e.g., Brasiliano and Adamastor oceans, and Tornquist's Sea).
The Pacific and Iapetus oceans formed through Neoproterozoic rifting of Rodinia. The Iapetus Ocean provides the type example of the Wilson cycle with formation of the Appalachian–Caledonian Orogen through ocean closure and continental collision (Wilson, 1966). In contrast, the Pacific Ocean has never completely closed and is bounded by accretionary orogens formed through ongoing cycles of plate convergence. The ocean has been bounded throughout its history by West Laurentian and East Gondwanan continental margins (Bell and Jefferson, 1987, Dalziel, 1991, Hoffman, 1991, Moores, 1991) and although the original relationship between these continental masses is uncertain (compare Borg and DePaolo, 1991, Moores, 1991, Li et al., 1995, Li et al., 1996, Burrett and Berry, 2000, Karlstrom et al., 2001, Wingate et al., 2002, Kröner and Cordani, 2003, Pisarevsky et al., 2003), they provide a remarkable record of the ocean's development from the Neoproterozoic to the Recent (Coney et al., 1990, Coney, 1992, Dickinson, 2004).
The record of initiation of the Pacific and Iapetus margins of Gondwana and the subsequent inception of convergent plate interaction is preserved in a Neoproterozoic to late Paleozoic orogenic belt here termed the Terra Australis Orogen (Fig. 2). The orogen forms a fundamental crustal element that extends along the margin of Gondwana.
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, South Africa and South America rather than the geologic reality of the intercontinental distribution and original continuity of related tectonostratigraphic rock units. 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 in the late Neoproterozoic to early Paleozoic interval, synthesizing along- and across-strike comparison of rock units and discussing termination of the orogen at the end of the Paleozoic.
Section snippets
Definition and tectonic framework
The Terra Australis Orogen extends from the northeast1 coast of Australia, south through Tasmania, New Zealand, the Transantarctic Mountains and the Antarctic Peninsula, across the tip of southern Africa and into South America (Fig. 2). The orogen commenced with the establishment of continental margin sequences along the Australian/East Antarctic segment of East Gondwana in the mid-Neoproterozoic, through opening of the Pacific Ocean, and
Lithotectonic subdivision
Traditional divisions of specific segments of the Terra Australis Orogen have generally been based on structural overprints related to late orogenic events, for example, individual fold belts/orogens of eastern Australia, New Zealand and Antarctica (Leitch, 1974, Stump, 1995, Scheibner, 1996). Preiss (see also Drexel et al., 1993, Drexel and Preiss, 1995) recognized the importance of differentiating depositional and orogenic belts with the Neoproterozoic depositional basin of the Adelaidean
Continental margin sequences
Continental margin sequences of the Terra Australis Orogen developed on continental lithosphere stabilized within Rodinia, and are divisible into East and West Gondwana sequences, reflecting their contrasting location and the timing of breakup within Rodinia.
Gondwana margin igneous assemblages
Igneous rocks of predominantly convergent margin character occur associated with the continental margin successions, as well as outboard, but proximal, to the margin (Fig. 4). They are predominantly Cambro-Ordovician in age and are generally associated with shallow marine or terrestrial siliciclastic strata. They include the Mount Windsor province of northeast Queensland (Henderson, 1986, Stolz, 1995), the Mt. Wright Volcanics of western New South Wales (Crawford et al., 1997), the Mount
Parautochthonous and allochthonous assemblages
Outboard of the Gondwana continental margin sequences are a series of parautochthonous to allochthonous assemblages comprising Gondwana- and Laurentian-derived continental lithosphere, oceanic lithosphere that formed at, or near, the Gondwana margin, and oceanic lithosphere that lay in an intra-oceanic setting removed from either the Gondwanan or Laurentian margins.
Subduction initiation
The earliest record for lithospheric convergence and subduction preserved within orogenic systems is provided by the oldest record of one or more of the following: supra-subduction zone magmatic rocks or derived products (e.g., volcaniclastic strata), ophiolitic rocks formed in a supra-subduction zone environment (e.g., back arc, forearc or proto-arc basin) and/or material metamorphosed in a subduction zone environment (e.g., eclogite). Evidence from these sources for the East Gondwana segment
Conclusions
The Terra Australis Orogen lies along the Pacific and Iapetus margins of Gondwana, forming a fundamental lithospheric element within Gondwana. Prior to breakup of Gondwana/Pangea, 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 1600 km. The orogen comprises continental margin sequences recording the breakup of the East and West
Acknowledgements
I am grateful to Evan Leitch for discussions over a number of years which have help develop the concepts outlined in this paper. Craig Buchan, Alan Collins, Ian Fitzsimons, Jim Hibbard, Zheng Xiang Li, Brendan Murphy, Sergei Pisarevsky, Carlos Rapela, Rob Strachan, John Veevers and Michael Wingate, and journal reviewer Alfred Kröner are thanked for discussion and comments on the manuscript. This is TSRC publication No. 295 and contribution to IGCP projects 440 and 453.
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