LATEST NEOPROTEROZOIC BASIN INVERSION OF THE BEARDMORE GROUP, CENTRALTRANSANTARCTIC MOUNTAINS, ANTARCTICA

Structural and age relationships in Beardmore Group rocks in the centr al Transantarctic Mountains of Antarctica indicate that they experienc ed a single deformation in latest Neoproterozoic to early Paleozoic ti me. New structural data contrast with earlier suggestions that Beardmo re rocks record two orogenic deformations, one of the early Paleozoic Boss orogeny and a distinct earlier tectonic event of presumed Neoprot erozoic age referred to as the Beardmore orogeny. In the Nimrod Glacie r area, Beardmore metasedimentary rocks contain only a single set of g eometrically related regional structures associated with the developme nt of upright, large-and small-scale flexural-slip folds. Deformation of Beardmore strata involved west directed contraction of modest regio nal strain at relatively high crustal levels. Existing ages of detrita l zircons from the Cobham and Goldie formations constrain Beardmore Gr oup deposition to be younger than similar to 600 Ma. This is significa ntly younger than previous age estimates and suggests that Beardmore d eposition may be closely linked to a latest Neoproterozoic East Antarc tic rift margin. The lack of structural evidence for polyphase deforma tion and the relatively young depositional age for the Beardmore Group thus raises the question of a temporally and/or tectonically unique B eardmore orogeny. Here I suggest that Beardmore shortening may be rela ted to tectonic inversion of East Antarctic marginal-basin strata beca use of localized compression during proto-Pacific seafloor spreading. Basin inversion is but one stage in a protracted Ross tectonic cycle o f rifting, tectonic inversion, subduction initiation, and development of a mature convergent continental margin during latest Neoproterozoic and early Paleozoic time. The term ''Beardmore orogeny'' has little m eaning as an event of orogenic status, and it should be abandoned. Rec ognition of this latest Neoproterozoic history reinforces the view tha t the broader Ross orogeny was not a single event but rather was a lon g-lived postrifting tectonic process along the East Antarctic margin o f Gondwanaland.