Latest Neoproterozoic to Mid-Cambrian age for the main deformation phases of the Transantarctic Mountains: new stratigraphic and isotopic constraintsfrom the Pensacola Mountains, Antarctica

New isotopic ages and a fresh understanding of stratigraphic relations amon g siliciclastic strata in the Pensacola Mountains along the northern margin of the East Antarctic craton result in removal of some constraints for the Proterozoic break-up of Rodinia and necessitate revision of the subsequent history of the East Antarctic margin. These rocks, formerly all included i n the Patuxent Formation, were thought to be of mid-Neoproterozoic age, to have formed as a consequence of Rodinia rifting, and to have been deformed during a Neoproterozoic orogenic event. Our data show, in contrast, that th ese siliciclastic strata were deposited in two chronologically distinct bas ins. The older basin, in which the Hannah Ridge Formation (new name) accumu lated, received sediment that contains detrital zircons of latest Neoproter ozoic or Early Cambrian age. It was deformed and its contents uplifted and eroded prior to the late Mid-Cambrian in an orogenic event that we interpre t as the early stage(s) of the Ross orogeny. The second basin formed later, accumulated turbidite-rich sediments of the redefined Patuxent Formation o f Mid- and probably Late Cambrian age, and was subsequently deformed, possi bly in Ordovician time. Review of both biostratigraphic and isotopic ages a long the length of the Transantarctic Mountains indicates that almost every where the main episodes of deformation predate 500 Ma and are thus older th an latest Mid-Cambrian, rather than Ordovician, as they are commonly consid ered to be. Only in the accreted Bowers and Robertson Bay terranes of north ern Victoria land, which reveal no clear record of pre-latest Mid-Cambrian or older folding, is the principal episode of Ross orogenic deformation dem onstrably younger than Late Cambrian.