The Ross–Delamerian Orogen in the southwest Pacific and Antarctica: an active plate boundary for Gondwana in the late Neoproterozoic and Cambrian

ABSTRACT Neoproterozoic and Cambrian tectonic and magmatic events in southeast Australia, Tasmania, New Zealand and the Transantarctic Mountains have been described as aspects of the Ross–Delamerian Orogeny. Summaries of the salient points suggest they are parts of the same active margin that developed along a paleo-Pacific rifted edge of the fragmented Rodinia supercontinent. Late Neoproterozoic to Cambrian subduction produced a major magmatic arc that cuts Neoproterozoic and early Cambrian rocks. The Ross–Delamerian Orogen includes two exotic blocks, the Beardmore and VanDieland microcontinents, which accreted in the early Cambrian and middle Cambrian respectively. Other possible exotic elements are the Bowers Terrane in northern Victoria Land and the Takaka Terrane in New Zealand, with both sharing a common middle to late Cambrian history. Compressive deformation occurred at poorly constrained intervals in the early Cambrian but is well constrained in the middle Cambrian. In Australia and Victoria Land of Antarctica, magma compositions changed in the late Cambrian reflecting a change to extension. Bi-modal volcanism and stratigraphy in the Pensacola Mountains of Antarctica are similarly interpreted. Subduction slowed and ceased towards the end of the Cambrian implying a change in relative motion along the Australian–Antarctica segment of the Gondwanan plate boundary.