Continental fragmentation along the South Scotia Ridge transcurrent plate boundary (NE Antarctic Peninsula)

Abstract

The study of the South Scotia Ridge on the basis of swath bathymetry, multichannel seismic and magnetometry profiles, obtained during the HESANT92/93 cruise and complemented with satellite gravimetry and seismicity data illustrates the tectonics of the region. The thinned continental crust fragments of the ridge are bounded by oceanic crust of the Scotia Sea to the North and Powell Basin to the South. The northern boundary represents the contact between the Scotia and Antarctic plates. This boundary is a sinistral transpressional fault with transtensional segments and moderate recent tectonic activity. Another fault located at the southern boundary appears inactive and does not reveal any features that would enable the kinematics to be determined. Both faults have associated steep scarps since they separate oceanic and continental crust types. The most significant active deformation lies in the axial depression of the ridge, within a band delineated by fault systems with WSW-ENE and SW-NE strikes. These faults develop pull-apart basins, which separate the northern and southern blocks of the ridge. The northern block is being fragmented from the Antarctic Plate by a zone of transtensive faults, and is probably a crustal element independent of the Antarctic Plate. The axial depression, which crosses the ridge slightly obliquely, is characterized by deep basins locally more than 5000 m deep and associated high seismicity. The fault geometry and earthquake focal mechanisms indicate an active sinistral transtensive regime for the fault system, although it may locally have transpressive regimes depending on the fault plane and the stress field orientations. The internal basins are characterized by an asymmetric development showing itself as depositional wedges generally thickening northward. Deposits onlap the southern margins and are affected by normal faults in the northern margins. The seismicity around the Scotia Plate shows that the present stresses are compressive along the northern boundary with the South American Plate (σ 1 SW-NE and subhorizontal) and along the western boundary with the Antarctic Plate (σ 1 WNW-ESE and subhorizontal). For the South Scotia Ridge, however,σ 1 is steeply inclined andσ 3 is subhorizontal with a NW-SE trend. The stress distribution in Bransfield Strait is similar to the ridge and the recent extension could be partially explained by the westward continuation of the active fault system of the central South Scotia Ridge. The fragmentation of continental crustal blocks, due to the tectonic activity along the transcurrent plate boundaries, is a mechanism that contributes to the deformation of the northeastern end of the Antarctic Peninsula. This area appears appropriate for the analysis of continental plate fragmentation processes.