Bransfield Basin Tectonic Evolution

Abstract

Bransfield Basin is located at the Pacific margin of the Antarctic Peninsula and constitutes an incipient oceanic back-arc basin. This basin developed as a consequence of the separation of the South Shetland Blockfrom the Antarctic Peninsula. Analysis of multichannel seismic profiles from Brazilian, Spanish, Japanese and Chinese cruises allows the shallow structure of the Bransfield Basin and its eastward prolongation through the South Scotia Ridge to be studied. The Bransfield Basin is asymmetrical and taking into account the shallow structures, its opening may be interpreted as related to a low angle normal fault that dips NW, with the South Shetland Block constituting the hanging wall. The margin adjacent to the Antarctic Peninsula exhibits all the features associated with a lower plate passive margin, such as the development of landward tilted half-grabens, the presence of a break-up unconformity, and the deposition of an oceanward dipping "drift" sequence. However, the margin near the South Shetland Islands is typical of an upper-plate margin: poorly nourished, sharp and with high angle faults. Extension is more developed in the Central Bransfield Basin, where a volcanic axis is recognized as the expression of a young spreading center, and there is possibly incipient oceanic crust. In the Bransfield Basin extremities, present-day deposits represent the synrift sequence,and extension continues. The Bransfield Basin probably develops as a consequence of two interacting processes. The main one is rollback of the trench hinge related to the continued sinking of the subducted slab of the former Phoenix Plate. A second process is related to the westwards propagation of the deformations associated with the Scotia-Antarctic plate boundary along the South Scotia Ridge and up to the Bransfield Basin.