Crustal and upper mantle structure beneath Antarctica and surrounding oceans

Abstract

We present and discuss a new model of the crust and upper mantle at high southern latitudes that is produced from a large, new data set of fundamental mode surface wave dispersion measurements. The inversion for a 2°×2° shear velocity model breaks into two principal steps: first, surface wave tomography in which dispersion maps are produced for a discrete set of periods for each wave type (Rayleigh group velocity, 18–175 s; Love group velocity, 20–150 s; Rayleigh and Love phase velocity, 40–150 s) and, second, inversion for a shear velocity model. In the first step, we estimate average resolution at high southern latitudes to be about 600 km for Rayleigh waves and 700 km for Love waves. The second step is a multistage process that culminates in a Monte Carlo inversion yielding an ensemble of acceptable models at each spatial node. The middle of the ensemble (median model) together with the half width of the corridor defined by the ensemble summarize the results of the inversion. The median model fits the dispersion maps at about the measurement error (group velocities, 20–25 m/s; phase velocities, 10–15 m/s) and the dispersion data themselves at about twice the measurement error. We refer to the features that appear in every member of the ensemble as “persistent.” Some of persistent features are the following: (1) Crustal thickness averages ∼27 km in West Antarctica and ∼40 km in East Antarctica, with maximum thicknesses approaching 45 km. (2) Although the East Antarctic craton displays variations in both maximum velocity and thickness, it appears to be a more or less average craton. (3) The upper mantle beneath much of West Antarctica is slow and beneath the West Antarctic Rift is nearly indistinguishable from currently dormant extensional regions such as the western Mediterranean and the Sea of Japan. Our model is therefore consistent with evidence of active volcanism underlying the West Antarctic ice sheet, and we hypothesize that the West Antarctic Rift is the remnant of events of lithospheric rejuvenation in the recent past that are now quiescent. (4) The Australian‐Antarctic Discordance is characterized by a moderately high velocity lid to a depth of 70–80 km with low velocities wrapping around the discordance to the south. There is a weak trend of relatively high velocities dipping to the west at greater depths that requires further concentrated efforts to resolve. (5) The strength of radial anisotropy (vsh − vsv)/vsv in the uppermost mantle across the Southern Hemisphere averages ∼4%, similar to the Preliminary Reference Earth Model. Radial anisotropy appears to be slightly stronger in West Antarctica than in East Antarctica and in the thinner rather than the thicker regions of the East Antarctic craton.