Terra Australis Orogen: Rodinia breakup and development of the Pacific and Iapetus margins of Gondwana during the Neoproterozoic and Paleozoic
The Pacific Ocean formed through Neoproterozoic rifting of Rodinia and despite a long history of plate convergence, this ocean has never subsequently closed. The record of ocean opening through continental rifting and the inception of ocean convergence through the initiation of subduction are preserved in the Neoproterozoic to late Paleozoic Terra Australis Orogen. The orogen had a pre-dispersal length along the Gondwana margin of approximately 18,000 km and was up to 1600 km wide. It incorporates the Tasman, Ross and Tuhua orogens of Australia, Antarctica and New Zealand, respectively, the Cape Basin of Southern Africa, and Neoproterozoic to Paleozoic orogenic elements along the Andean Cordillera of South America. The Terra Australis Orogen can be divided into a series of basement blocks of either continental or oceanic character that can be further subdivided on the basis of pre-orogenic geographic affinity (Laurentian vs. Gondwanan) and proximity to inferred continental margin sequences (peri-Gondwanan vs. intra-oceanic). These divisions reflect initial tectonic setting and provide an insight into the character of the orogen through time. The orogen incorporates elements that are inferred to have lain outboard of both West and East Laurentia within Rodinia. Subduction of the Pacific Ocean was established at, or close to, the Gondwana margin by around 570 Ma and occurred at about the same time as major global plate reorganization associated with final assembly of Gondwana and the opening of the Iapetus Ocean. The termination of the Terra Australis Orogen at around 300-230 Ma was associated with the assembly of Pangea. It is represented by the Pan-Pacific Gondwanide Orogeny and is marked in east Gondwana by a stepping out in the position of the plate boundary and commencement of the classic late Paleozoic to Mesozoic Gondwanide Orogen. The Pacific has been cited as an example of the declining stage of the Wilson cycle of ocean basins. However, its protracted history of ongoing subduction and the absence of any indication of major continental collisions contrasts with the clear evidence for opening and closing of oceans preserved in the Iapetus/Atlantic and Tethyan realms. The Terra Australis and other orogens that bound the Pacific are accretionary orogens and did not form through the classic Wilson cycle.