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

• Published in: Geochemistry, geophysics, geosystems : G3 geophysics, geosystems : G3, 2019-11, Vol.20 (11), p.5014-5037
• Main Authors: O'Donnell, J. P., Brisbourne, A. M., Stuart, G. W., Dunham, C. K., Yang, Y., Nield, G. A., Whitehouse, P. L., Nyblade, A. A., Wiens, D. A., Anandakrishnan, S., Aster, R. C., Huerta, A. D., Lloyd, A. J., Wilson, T., Winberry, J. P.
• Format: Article
• Language: English
• Subjects: Ambient noise; Anisotropy; Antarctica; Cenozoic; Continental crust; crust; Crustal thickness; Deformation; Glaciation; Ice sheets; Magma; Minerals; Mountains; Orientation; P-waves; Phase velocity; Precambrian; Provenance; Rifting; Sea level; Seismic activity; Seismic velocities; Seismic waves; seismology; Shear wave velocities; tectonics; Wave phase; Wave velocity
• ISSN: 1525-2027; 1525-2027
• DOI: 10.1029/2019GC008459

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 ∼26–30 km thick extending south from Marie Byrd Land separates domains of more extended crust (∼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 ∼35–40 km is modeled beneath the Haag Nunataks‐Ellsworth Mountains, decreasing 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 (VSH>VSV) 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 ∼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. Plain Language Summary The landmasses we recognize today as South America, Africa, Madagascar, India, Australia, and Antarctica were once joined in a supercontinent called Gondwana. West Antarctica is key to accurately recreating the Gondwana jigsaw puzzle, but the ice sheet limits access to the telling rock record. Here we map West Antarctica's crustal thickness using seismic waves to gauge where tectonic stretching has occurred. The thickest crust is found beneath the southern Transantarctic Mountains and Haag Nunataks‐Ellsworth Mountains. Thinner crust characterizes the West Antarctic Rift System (WARS), being thinnest in the Ross and Amundsen Sea Embayments. The crustal thickness variations we map along the WARS suggest a complex history involving several rifting episodes since the