The uppermost mantle seismic velocity structure of West Antarctica from Rayleigh wave tomography: insights into tectonic structure and geothermal heat flow
We present a shear wave model of the West Antarctic upper mantle to ∼200 km depth with enhanced regional resolution from the 2016-2018 UK Antarctic Seismic Network. The model is constructed from the combination of fundamental mode Rayleigh wave phase velocities extracted from ambient noise (periods 8-25 s) and earthquake data by two-plane wave analysis (periods 20-143 s). We seek to (i) image and interpret structures against the tectonic evolution of West Antarctica, and (ii) extract information from the seismic model that can serve as boundary conditions in ice sheet and glacial isostatic adjustment modelling efforts. The distribution of low velocity anomalies in the uppermost mantle suggests that recent tectonism in the West Antarctic Rift System (WARS) is mainly concentrated beneath the rift margins and largely confined to the uppermost mantle (<180 km). On the northern margin of the WARS, a pronounced low velocity anomaly extends eastward from beneath the Marie Byrd Land dome toward Pine Island Bay, underlying Thwaites Glacier, but not Pine Island Glacier. If of plume-related thermal origin, the velocity contrast of ∼5% between this anomaly and the inner WARS translates to a temperature difference of ∼125-200 . However, the strike of the anomaly parallels the paleo-Pacific convergent margin of Gondwana, so it may reflect subduction-related melt and volatiles rather than anomalously elevated temperatures, or a combination thereof. Motivated by xenolith analyses, we speculate that high velocity zones imaged south of the Marie Byrd Land dome and in the eastern Ross Sea Embayment might reflect the compositional signature of ancient continental fragments. A pronounced low velocity anomaly underlying the southern Transantarctic Mountains (TAM) is consistent with a published lithospheric foundering hypothesis. Taken together with a magnetotelluric study advocating flexural support of the central TAM by thick, stable lithosphere, this points to along-strike variation in the tectonic history of the TAM. A high velocity anomaly located in the southern Weddell Sea Rift System might reflect depleted mantle lithosphere following the extraction of voluminous melt related to Gondwana fragmentation. Lithospheric thickness estimates extracted from 1D shear wave velocity profiles representative of tectonic domains in West Antarctica indicate an average lithospheric thickness of ∼85 km for the WARS, Marie Byrd Land, and Thurston Island block. This increases to ∼96 km in the Ellsworth Mountains. A surface heat flow of and attendant geotherm best explains lithospheric mantle shear wave velocities in the central WARS and in the Thurston Island block adjacent to Pine Island Glacier; a geotherm best explains the velocities in the Ellsworth Mountains, and a geotherm best explains a less well-constrained velocity profile on the southern Antarctic Peninsula. We emphasise that these are regional average (many hundreds of km) heat flow estimates constrained by seismic data with limited sensitivity to upper crustal composition.