Transtensional tectonics along the south Scotia Ridge, Antarctica

Abstract

Multichannel seismic reflection profiles recorded aboard B/O Hespérides during the austral summer of 1991–1992 were used to identify the tectonic style of the South Scotia Ridge along the Scotia/Antarctica plate boundary. The ridge is composed of continental crustal fragments transported eastward from the South America-Antarctic isthmus 28 to 6 Ma during the opening of Drake Passage. It is made up of two highs (north and south branches of the South Scotia Ridge) separated by a central depression that contains four narrow deeps. Fragmentation of the ridge during and since its transport to its present position is due to transtensional sinistral motion along the Scotia-Antarctic plate boundary. This fragmentation of the ridge appears to have been in two phases. During an early phase of transtension, which probably took place in the Oligocene, a half graben fronted by a high along its northern edge was formed along the southern flank of the ridge. Concurrent with this transtension episode an extensive sediment prism was deposited north of the ridge. The second tectonic episode during which the present plate boundary was established in its current location along the central depression may have begun about 4 Ma. Transtensional tectonics along the sinistral transform fault plate boundary during this phase led to the creation of the present tectonic geomorphology of the South Scotia Ridge. Extension during this phase is characterized by listric faults dipping both north and south which root into a northerly dipping basal detachment surface. Motion along these faults caused the block above the detachment surface (upper block—north branch of the South Scotia Ridge) to undergo some degree of tilting. Differences in morphology along the north branch suggest that this block tilting varies along strike being the least on its eastern and western ends and maximum in the center. This suggests that continuity of the listric faults parallel to the plate boundary is disrupted by transverse structures, structures which may have been produced by bends along the plate boundary. As the blocks were transported laterally along the transtensional sinistral plate boundary they experienced some degree of counterclockwise rotation along the right lateral transverse structures creating local zones of compression and extension at their corners. It is this compression created by the rotating blocks which led to the deformation of the sediments north of the eastern end of the north branch of the South Scotia Ridge and within the central depression itself.