An abandoned rift in the southwestern part of the South Scotia Ridge (Antarctica): Implications for the genesis of the Bransfield Strait

The western segment of the South Scotia Ridge is a narrow submarine ridge composed of continental crustal blocks, which hosts the left-lateral boundary between the Scotia and Antarctic plates. This plate boundary segment gradually merges westward with the Bransfield Strait, a young (<4 Ma) basin located between the South Shetland Islands and the northern segment of the Antarctic Peninsula. Here we present a series of multichannel seismic profiles that reveal the presence of a linear chain of submarine volcanic edifices running parallel to the present-day Scotia-Antarctic plate boundary. These edifices consist of alkali basalts (~4 Ma), as shown by published data from a dredge haul. The occurrence, distribution, and shape of these magmatic manifestations suggest that they were emplaced in a rift-related environment, associated with the activity of the former western segment of the Scotia-Antarctic plate boundary. When spreading centers of the Phoenix-Antarctic ridge NW of the South Shetland Islands became inactive at about 3.3 Ma, the former western Scotia-Antarctic plate boundary was abandoned, and it moved to the present-day location of the Bransfield Strait, where the motion was accommodated, and where active extension is ongoing. The proposed mechanism for the development of the Bransfield Strait, consistent with the temporal sequence of tectonic events, reconciles discrepancies in tectonic evolutionary models presented till now and does not require the presence of slab-pull and/or slab retreat forces. The Bransfield Strait is not a classic back-arc basin, but rather an actively extending marginal basin at the transition from mature rifting to incipient, punctiform spreading.