Bransfield Strait, Antarctic Peninsula Active Extension behind a Dead Arc

Abstract

Bransfield Strait is a marginal basin landward of the South Shetland Trench. It lies between the South Shetland Islands and the tip of the Antarctic Peninsula and is an example of an extensional basin formed by rifting within a continental volcanic arc. The Antarctic Peninsula is the product of at least 200 m.y. of subduction with the majority of the exposed rocks related to continental arc volcanism older than 20 Ma. Volcanism in Bransfield Strait started by 0.3 Ma and continues today. This new volcanism maintains some of the chemical signatures of the old arc volcanism but also has signatures transitional between arc rocks and backarc basin rocks. On the basis of high heat flow, active volcanism, extensional faulting, and earthquake fault plane mechanisms, Bransfield Strait is an active extensional basin forming within the Antarctic Peninsula. Seismic refraction work in Bransfield Strait indicates some thinning of continental crust, but the basin itself is underlain by as much as 30 km of anomalous crustal material. The observed extension seems to be confined to Bransfield Strait, which is bounded by the landward projections of the Hero and Shackleton fracture zones. The present extension in Bransfield Rift started less than 4 m.y. ago, and possibly less than 1.5 m.y. ago, following the demise of the Antarctic-Phoenix spreading center (ANT-PHO ridge), which ceased spreading about 4 Ma. Apparent, continued subduction at the South Shetland Trench after the ANT-PHO ridge stopped spreading may occur as trench rollback. The amount of trench rollback should be comparable to the amount of extension in Bransfield Strait.