STRUCTURE AND GEODYNAMIC EVOLUTION OF THE CENTRAL BRANSFIELD BASIN (NW ANTARCTICA) FROM SEISMIC-REFLECTION DATA

The Bransfield Basin is a young active rift basin located between the northern margin of the Antarctic Peninsula and the South Shetland Isla nds margin. Deception and Bridgeman islands divide the Bransfield Basi n in three subbasins, the western, central and eastern. Specific morph o-tectonic features and sediment fill differentiate each subbasin. The structure and geodynamic evolution of the Central Bransfield Basin, w hich is in a stage of incipient seafloor spreading, have been investig ated in detail from a dense grid of single-channel seismic reflection data. The Central Bransfield Basin is dominated by two families of nor mal faults which are oriented northeast and northwest. The NE-trending faults define three graben systems that are roughly parallel to the b asin axis. In an across-basin direction, the mean trend of this family of faults ranges from N71 (the graben system nearest to the Antarctic Peninsula) over N64 (the intermediate graben system), to N53 (the gra ben system nearest to the South Shetland Islands). The NW-trending fam ily of faults is responsible for the deepening of the basin from south west to northeast. Both families of faults define the overall Central Bransfield Basin structure, resulting in a complex division of the bas in floor. Additionally, tens of volcanic edifices are located on the b asin floor, the larger ones being associated to the NW-trending faults . Interaction of tectonics and sedimentation give place to the differe ntiation of three tectonostratigraphic units, TUI, TU2 and TU3 (from o ldest to youngest). TU1 occupies the southernmost graben system, and i t is affected by the NE-trending bounding normal faults. TU2 extends f urther northwestwards than TU1 and essentially fills the intermediate graben system. TU3 represents a further extension of the sediment infi ll over most of the Central Bransfield Basin, and marks the initiation of the infill of the northernmost graben system. Faults bounding this graben also determine the straight and abrupt morphology of the South Shetland Islands margin. The observed arrangement of the graben syste ms and the filling tectonostratigraphic units reveal a migration of ex tensional tectonics and associated depocentres from the Antarctic Peni nsula margin to the South Shetland Island margin. The left-lateral rot ation from N71 to N53 in the mean trend of the three successive graben systems could have been produced by the oblique subduction of the Pho enix plate and the effect of the sinistral strike-slip movement of the South Scotia Ridge, 200 km northeastwards of the Central Bransfield B asin. (C) 1998 Elsevier Science B.V. All rights reserved.