Flexural Subsidence Around Ross Island, West Antarctica

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. Plain Language Summary Ross Island, located in the southern Ross Sea of West Antarctica, was created during the past 4.6 m.y. by eruptions from four major volcanic centers. These eruptions occurred during a time in which the western Ross Sea, including the region surrounding Ross Island, was experiencing slow rifting. Seismic surveys show thickening of sedimentary strata toward Ross Island, which is attributed to subsidence that resulted from both regional extension and from flexure of the lithosphere under the weight of the volcanoes. Geodynamic models simulating subsidence in the sedimentary moat surrounding Ross Island show that the strength of the lithosphere during the period in which each of the four main volcanoes was active is up to 20 times lower than the strength estimated from the total thickness of strata deposited in the moat over the 4.6 m.y. lifespan of the islan