Active crustal fragmentation along the Scotia-Antarctic plate boundary east of the South Orkney Microcontinent (Antarctica)

The structure of the Scotia-Antarctic plate boundary is poorly known east of the South Orkney Microcontinent. New multichannel seismic profiles, together with magnetic, gravity and swath bathymetry data obtained during the SCAN97 cruise, show a complex relief of raised blocks and elongated depressions that may reach more than 6000 m in depth. These depressions develop in relation with extensional active structures and constitute an uncommon feature in the oceans, where most of the trenches are formed in subduction contexts. The main crustal elements of the area include the oceanic crust of the Scotia Plate, the Discovery Bank composed of continental crust, a tectonic domain with intermediate features between continental and oceanic crusts that includes the Southern Bank, and the oceanic crust of the northern Weddell Sea, representing the Antarctic Plate. The Intermediate Domain was probably developed during the Late Cenozoic subduction of the Weddell Sea oceanic crust below the Discovery Bank. The fault zone associated with the plate boundary is characterized at present by sinistral transcurrent and transtensional slips, which develop a NE-SW elongated deep pull-apart basin with extreme crustal thinning and mantle uplift. The complex bathymetry and structure of the plate boundary are consequences of the presence of continental and intermediate crusts - where the deformations are concentrated - between the two stable oceanic domains. The location of a major part of the plate boundaries around the Scotia Arc is probably determined by the position of the continental and intermediate crustal fragments surrounded by oceanic crust, due to the differential behavior experienced during deformation. (C) 2002 Elsevier Science B.V. All rights reserved.