From Rodinia to Gondwana : supercontinent evolution in the Transantarctic Mountains

The Transantarctic Mountains provide a cryptic but important record of Proterozoic and Early Paleozoic supercontinent history, including Rodinian assembly, Rodinian breakup, transition from a drifting to subducting margin, and active plate-margin activity during Gondwanan assembly. A linkage between Laurentia and East Antarctica as part of Neoproterozoic Rodinia is plausible, based on isotopic data from rare exposures of crystalline basement in the Transantarctic Mountains. However, testing of paleogeographic details is difficult because the crustal structure of the East Antarctic shield is poorly known along much of its perimeter and because we lack well-dated Proterozoic paleomagnetic poles. The timing of Rodinian breakup is poorly constrained globally, yet local mafic magmatism of 800-650 Ma age provides the best evidence in the Transantarctic Mountains for Late Neoproterozoic crustal extension and possible rifting. Still uncertain are the position of the rift margin, the geometry of rifting, the extent of crustal thinning, the extent of rift-margin sedimentation, the location of possible transform offset, and the influence of these structural patterns on later orogenesis. A transformation from drifting to active subducting mode is inferred for the Late Neoproterozoic, but the nature and specific timing of this event are unknown. The Vendian-Early Paleozoic Ross Orogen reflects convergent-margin activity associated temporally, if not causally, with the consolidation of Gondwana. Inception of a convergent Gondwana margin is signalled by the earliest Ross granitoids at c. 560 Ma and may be reflected in early structural inversion of craton-margin sedimentary succession. Protracted Ross tectonism between 560 and 480 Ma involved episodic deformation, calc-alkaline magmatism, and syn-orogenic deposition of arc-derived detritus in a sinistral-transpressive, continental-margin arc setting. Sedimentary provenance in siliciclastic rocks appears to have shifted by late Early Cambrian time from cratonic to chiefly arc-derived material. Diachronous Ross magmatism and deformation may reflect progressive, hinged opening of paleo-Pacific seafloor from north to south (present-day co-ordinates) between Laurentia and East Antarctica-Australia. (author). 104 refs., 6 figs