Rift propagation, detachment faulting, and associated magmatism in Bransfield Strait, Antarctic Peninsula

Abstract

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.