Is subduction ongoing in the South Shetland Trench, Antarctic Peninsula?: new constraints from crustal structures of outer trench wall

Abstract

The South Shetland Trench (SST) northwest of the South Shetland Islands is the only trench along the Pacific margin of the Antarctic Peninsula. Multichannel seismic reflection profile shows very narrow normal faulting zone (about 20 km) on the outer wall of the SST and thick trench-fill sediments (up to 1000 m) on two distinct crustal horst and graben structures in the trench. These structures are rare in the active subduction trenches. On the basis of the implications from these seismic structures and known tectonic history, the following scenarios of subduction activities of the former Phoenix plate are proposed: (1) normal faulting has not occurred since subduction rate in the SST sharply decreased after cessation of seafloor spreading at the West Scotia Ridge at about 6 Ma; and (2) subduction almost stopped after the cessation of spreading in Drake Passage at about 3.3 Ma. Recent contractional structures around the SST are indicative of current crustal shortening (not subduction) accommodating trenchward movement of South Shetland Islands caused by the ongoing extension of Bransfield Basin behind the South Shetland Islands.