SUBSURFACE MAPPING OF THE ROSS ISLAND FLEXURAL BASIN, WEST ANTARCTICA

Marine seismic reflection data collected in the Ross Sea region of southwest Antarctica over the past 30+ years has been tied to the ANDRILL and CIROS boreholes to develop a seismic stratigraphic model that constrains the spatial and temporal evolution of the flexural basin surrounding Ross Island. Ross Island is located at the southern end of the Terror Rift in the Ross Sea and formed from 4.6 Ma to present by extrusive volcanism. Preliminary mapping has identified a flexural moat on the west and north-northwest sides of the island that is roughly 40-45 km wide with sediment fill thickness of approximately 1100 m. The flexural moat is not imaged on seismic lines located to the east and northeast of the island. There, the thickness of strata that is time-equivalent to the infill of the flexural moat on the west side of the island remains constant from the Coulman High westward to within ~28 km of Ross Island (which is the landward extent of the seismic data coverage). A hinge zone is inferred to be present trending northeastward from Mt. Bird, separating the well-developed flexural moat on the west side of the island from sub-horizontal strata on the northeast and east sides. The concordant post-Miocene strata on the east and northeast sides of Ross Island imply either that the flexural basin on that side of the island does not extend beyond ~28 km from the Ross Island shoreline, or that the flexural basin is not present on that side of the island. The first scenario requires that the elastic strength of the lithosphere differ on either side of Ross Island. The latter scenario can be accounted for if the lithosphere beneath Ross Island is mechanically ruptured, with Ross Island acting as an end-load on the edge of a mechanical half-plate that forms the lithosphere beneath Ross Island and westward. In this model, the lithosphere east of Ross Island forms a second half-plate, bearing little or none of the Ross Island volcanic load.