Multichannel seismic data from Bransfield Strait, a young (<4 Ma?) marginal basin located at the NW tip of the Antarctic Peninsula, reveal a complex array of rifting styles short of clearly defined seafloor spreading. The strait is undergoing basinwide extension. While enriched mid-ocean ridge basalts have been dredged from topographic highs in the NE part of the basin, seismic evidence does not support the formation of oceanic crust there: Acoustic signatures of volcanic crust at known axial deep highs extend laterally subseafloor; their distribution and style support propagation of volcanism and associated extension from NE to SW. Active extensional structures along both margins control a strong physiographic asymmetry. The South Shetland Islands margin is composed of a few, closely spaced, large-offset normal faults; the gentler Antarctic Peninsula margin exhibits broader, distributed extension. To the NE, normal fault polarity reversal structures (NFPRs) dominate; listric faults sole into NW dipping, low-angle reflectors interpreted as detachments. To the SW, larger NFPRs and convex-up geometries up to ~20 km across suggest broad uplift rather than detachment-based extension. Such uplift may precedes seafloor volcanism and rapid associated deflation along detachments. NW-SE trending accommodation zones also segment the basin along-strike. Bransfield Strait extension may result both from plate boundary tectonics at the South Shetland Trench and at the evolving Antarctic-Scotia plate boundary. Extension was probably initiated by onset of subduction rollback at the trench, as a slab window opened from the SW. Augmented extension in the NE part of the basin may be a response to left-lateral transtension associated with reorganization of the triple junction at the Shackleton Fracture Zone-South Scotia Ridge intersection. ¿ 1998 American Geophysical Union