The Ellsworth-Whitmore Mountains crustal block: Its role in the tectonic evolution of West Antarctica

The 1983–1984 season of the joint British Antarctic Survey‐U.S. Antarctic Research Program geology and geophysics project on the Ellsworth‐Whitmore Mountains crustal block (EWM) has yielded new observations and laboratory data relevant to the geological evolution of West Antarctica and its tectonic relationship to the rest of Gondwanaland. This is a synthesis of results presented in companion papers in this volume. New paleomagnetic data favor a Jurassic reconstruction in which there has been little or no relative displacement between the EWM and the Antarctic Peninsula. They may be restored together, with a 15°–20° counterclockwise rotation and a northward translation of approximately 10° of latitude, to a position on the Pacific side of the Falkland Plateau‐Cape Fold Belt‐Coats Land junction. Orthogneiss exposed at Haag Nunataks represents a Proterozoic cratonization event, and aeromagnetic data demonstrate that related rocks occur beneath the ice from the northeastern edge of the Ellsworth Mountains as far as the base of the Antarctic Peninsula. Although this basement does not demonstrably extend beneath the EWM, retention of the present‐day relative positions of the Antarctic Peninsula, Haag Nunataks, and the EWM is geologically compatible with the above reconstruction. To the south of the block, the igneous and sedimentary rocks of the Thiel Mountains are recognized as part of the Precambrian basement and Phanerozoic successions of the Transantarctic Mountains, geologically and geophysically distinct from the folded sedimentary succession of the EWM. The latter are mostly lithologically correlative with the lower part of the thick Cambrian‐Permian Ellsworth Mountains succession, and throughout much of the area, share the same simple structural style and trend related to post‐Permian folding. Discordant and more complex structures are observed at separate localities on the margins of the EWM. The youngest exposed rocks in the EWM are a suite of Middle Jurassic peraluminous “S‐type” granites. These are crustal anatectic (or at least highly contaminated) melts which point to the presence of a deep continental basement beneath the EWM succession. They signify an intracontinental thermal event which, like that associated with contemporaneous magmatism in various tectonic environments throughout Gondwanaland, seems to herald the breakup of the old supercontinent. The subsequent Mesozoic‐Cenozoic history of the EWM is recorded in geophysical evidence for crustal rifting and extension, some of which may be related to the migration of West Antarctic crustal blocks to their present positions, others of which are probably very young features related to Cenozoic alkali magmatism outside the EWM and the recent marked uplift of the Ellsworth Mountains