Mapping Crustal Shear Wave Velocity Structure and Radial Anisotropy Beneath West Antarctica Using Seismic Ambient Noise

Using 8- to 25-s-period Rayleigh and Love wave phase velocity dispersion data extracted from seismic ambient noise, we (i) model the 3-D shear wave velocity structure of the West Antarctic crust and (ii) map variations in crustal radial anisotropy. Enhanced regional resolution is offered by the UK Antarctic Seismic Network. In the West Antarctic Rift System (WARS), a ridge of crust similar to 26-30 km thick extending south from Marie Byrd Land separates domains of more extended crust (similar to 22 km thick) in the Ross and Amundsen Sea Embayments, suggesting along-strike variability in the Cenozoic evolution of the WARS. The southern margin of the WARS is defined along the southern Transantarctic Mountains and Haag-Ellsworth Whitmore Mountains (HEW) block by a sharp crustal thickness gradient. Crust similar to 35-40 km is modeled beneath the Haag Nunataks-Ellsworth Mountains, decreasing to similar to 30-32 km thick beneath the Whitmore Mountains, reflecting distinct structural domains within the composite HEW block. Our analysis suggests that the lower crust and potentially the middle crust is positively radially anisotropic (V-SH>V-SV) across West Antarctica. The strongest anisotropic signature is observed in the HEW block, emphasizing its unique provenance among West Antarctica's crustal units, and conceivably reflects a similar to 13-km-thick metasedimentary succession atop Precambrian metamorphic basement. Positive radial anisotropy in the WARS crust is consistent with observations in extensional settings and likely reflects the lattice-preferred orientation of minerals such as mica and amphibole by extensional deformation. Our observations support a contention that anisotropy may be ubiquitous in the continental crust.