Seismic reflection and borehole data are used to create structure maps of four regional and three local unconformities that constrain the post middle Miocene evolution of the Victoria Land Basin (VLB), which is located in the western Ross Sea within the Late Cretaceous through Quaternary West Antarctic Rift System. Isochore maps of the strata between unconformities show that rifting was mostly amagmatic between 12 to 7.6 Ma, with subsidence controlled by faults bordering the northwest margin of the basin and in a tectonic zone along the southern basin axis known as the Terror Rift. Depocenters surrounding volcanic features in strata younger than 4.3 Ma indicate an increasing influence of flexure due to volcanic loading on the subsidence pattern in the southern VLB after this time. The intervening period, from 7.6 to 4.3 Ma, was a transitional period during which both extensional tectonism and magmatism exerted strong influences on basin morphology. Since 4.3 Ma, a series of flexural subbasins formed successively at different times and positions as the different volcanic centers that built Ross Island erupted. In composite, these subbasins form a flexural moat surrounding Ross Island and smaller volcanic centers immediately to the north. The widths of these basins indicate that the flexural rigidity of the lithosphere ranges from 0.20 × 1019 to 12.96 × 1019 N‐m (elastic thickness 0.6 to 2.4 km).

Ross Island is located in the southern Victoria Land Basin along the western margin of the West Antarctic Rift System. Episodic volcanism since ca. 4.6 Ma produced a discontinuous sedimentary moat around the island, coeval with regional extension. The moat is a composite of four smaller sub‐basins created during distinct episodes of volcanism. Subsidence within each sub‐basin is modeled as bending of a continuous elastic plate, first considering subsidence that occurred only during the time in which the associated volcano was active, and then considering cumulative subsidence since the onset of volcanism on Ross Island. Models based on strata deposited in each sub‐basin during the time interval in which the respective volcano was active yield flexural rigidities ranging from 6–36 × 1018 N‐m, with the lowest values associated with the youngest volcanoes on the south and southwest sides of the island. Models based on the entire stratigraphic interval deposited since the onset of volcanism yield flexural rigidities up to 20 times greater than models that consider only strata deposited within each sub‐basin when the associated volcano was active. Models of the composite basin overestimate the strength of the lithosphere due to inclusion of strata deposited during periods in which regional extension rather than local flexure dominated subsidence. The models indicate that Ross Island is in near flexural isostatic equilibrium with moderately low‐density (3,260 kg m−3) upper mantle, although an additional buoyant load equivalent to a mantle temperature anomaly of up to 200°C is permissible.

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