TRANSTENSIONAL TECTONICS ALONG THE SOUTH SCOTIA RIDGE, ANTARCTICA

Multichannel seismic reflection profiles recorded aboard B/O Hesperide s during the austral summer of 1991-1992 were used to identify the tec tonic style of the South Scotia Ridge along the Scotia/Antarctica plat e boundary. The ridge is composed of continental crustal fragments tra nsported 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 tran stensional sinistral motion along the Scotia-Antarctic plate boundary. This fragmentation of the ridge appears to have been in two phases. D uring an early phase of transtension, which probably took place in the Oligocene, a half graben fronted by a high along its northern edge wa s formed along the southern flank of the ridge. Concurrent with this t ranstension 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 de pression 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 Ri dge. Extension during this phase is characterized by listric faults di pping both north and south which root into a northerly dipping basal d etachment surface. Motion along these faults caused the block above th e detachment surface (upper block-north branch of the South Scotia Rid ge) 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 cen ter. This suggests that continuity of the listric faults parallel to t he plate boundary is disrupted by transverse structures, structures wh ich may have been produced by bends along the plate boundary. As the b locks were transported laterally along the transtensional sinistral pl ate 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 cr eated by the rotating blocks which led to the deformation of the sedim ents north of the eastern end of the north branch of the South Scotia Ridge and within the central depression itself.