LITHOSPHERIC STRENGTH AND PARTITIONING OF THE LOAD DURING FLEXURE AROUND ROSS ISLAND, WEST ANTARCTICA

Ross Island is a Neogene volcanic island located in the Ross Sea of West Antarctica. The island is surrounded by a sedimentary moat created by progressive loading of the lithosphere that occurred during four phases of volcanism on the island. Volcanism on Mt. Bird between 4.1-2.5 Ma led to the first topographic expression of Ross Island. Subsequent volcanic episodes between 2.5 – 1.7 Ma, 1.7 – 1 Ma and 1 Ma – present formed Mt. Terror, Hut Point Peninsula and Mt. Erebus, respectively. Periods of flexural subsidence associated with each of these volcanic loading events are evident in stratigraphy imaged on seismic reflection surveys in the area. We identified five unconformities in seismic surveys that bound distinct episodes of flexural subsidence. These unconformities were used to create isopach maps which were then decompacted to determine the thicknesses of strata at the time of subsidence and deposition. The maps show the flexural moat around Ross Island to be a composite of smaller flexural sub-basins formed at different times by loading at each of the four volcanic centers on the island. We modeled subsidence in these basins along 2-D profiles extracted from the decompacted isopach maps using an axisymmetric continuous plate, point load flexural model. The 2-D profiles originate at each of the four volcanic centers and trend radially outward from Ross Island. The models to north and east of the island, constrained by profiles originating at Mt. Bird and Mt. Terror, show flexural rigidity of 36 x 10¹⁸ N-m and 237x 10¹⁸ N-m. The flexure models to the south and west, constrained by profiles originating from HPP and Mt. Erebus, show flexural rigidities of 6 x 10¹⁸ N-m and 11 x 10¹⁸ N-m respectively. These estimates show that the flexural strength of lithosphere to the west and south of the island are lower than that to the east and north. The net load that produces flexure around Ross Island, estimated by adding loads determined from the flexure models from each of the four volcanic centers, is 6 ± 4 x 10¹⁶ N. This is less than the estimated load of 9 x 10¹⁶ N for the mass of the Ross Island. We attribute this deficit of load magnitude to the presence of a buoyant sub-lithospheric load beneath the Ross Island that results from low density upper mantle.