Abstract Seismic anisotropy provides essential information for characterizing the orientation of deformation and flow in the crust and mantle. The isotropic structure of the Antarctic crust and upper mantle has been determined by previous studies, but the azimuthal anisotropy structure has only been constrained by mantle core phase (SKS) splitting observations. This study determines the azimuthal anisotropic structure of the crust and mantle beneath the central and West Antarctica based on 8—55 s Rayleigh wave phase velocities from ambient noise cross‐correlation. An anisotropic Rayleigh wave phase velocity map was created using a ray—based tomography method. These data are inverted using a Bayesian Monte Carlo method to obtain an azimuthal anisotropy model with uncertainties. The azimuthal anisotropy structure in most of the study region can be fit by a two‐layer structure, with one layer at depths of 0–15 km in the shallow crust and the other layer in the uppermost mantle. The azimuthal anisotropic layer in the shallow crust of West Antarctica, where it coincides with strong positive radial anisotropy quantified by the previous study, shows a fast direction that is subparallel to the inferred extension direction of the West Antarctic Rift System. Fast directions of upper mantle azimuthal anisotropy generally align with teleseismic shear wave splitting fast directions, suggesting a thin lithosphere or similar lithosphere‐asthenosphere deformation. However, inconsistencies in this exist in Marie Byrd Land, indicating differing ancient deformation patterns in the shallow mantle lithosphere sampled by the surface waves and deformation in the deeper mantle and asthenosphere sampled more strongly by splitting measurements.
Abstract Many recent Antarctic seismic structure studies use Rayleigh wave data and thus determine only the SV structure. Love waves provide greater resolution for shallow structure, and coupled with Rayleigh waves, can constrain radial anisotropy by comparing vertically ( V SV ) and horizontally ( V SH ) polarized shear velocities. In this study, we jointly analyze Rayleigh and Love wave phase and group velocities from ambient noise to develop a new radially anisotropic velocity model for West and Central Antarctica with an improved shallow crustal resolution using all broadband data collected in Antarctica over the past 20 years. Group and phase velocity maps for Rayleigh and Love waves are estimated and inverted for shear wave velocity structure using a Monte Carlo method. We determine a new sediment distribution map that reveals a thick sedimentary basin (∼4 km) beneath the Southeastern Ross Embayment. Sediment thicknesses at interior basins such as the Polar Subglacial Basin and Bentley Subglacial Trench are modest (<1.5 km), suggesting that these basins are sediment‐starved. The shallow crust as well as the mid‐to‐lower crust in several places shows strong positive anisotropy ( V SH > V SV ), likely due to lattice preferred orientation of mica‐bearing rocks. However, large regions of the mid‐to‐lower crust show negative anisotropy, likely due to lattice preferred orientation of plagioclase. The uppermost mantle is characterized by strong positive radial anisotropy (4%–8%) in West Antarctica, with the largest anisotropy beneath the Transantarctic and Whitmore Mountains, likely resulting from horizontal olivine preferred orientation due to tectonic activity.
<p>The Antarctic continent with its large ice sheets provides a unique environment to investigate the response of the solid Earth to ice mass change. A key requirement of such studies is high-resolution seismic images of the crust and upper mantle, which can be used to estimate the region&#8217;s viscous structure. Likewise, these images are key to understanding the region&#8217;s geologic history and underlying geodynamic processes. Although the existing transverse isotropic seismic model ANT-20(Lloyd et al., 2020) and azimuthally anisotropic seismic model ANT-30 (Lloyd et al., in prep) have regional-scale resolution from the upper mantle to the transition zone, there is a need for higher resolution within the uppermost mantle (< 75 km) and crust of Antarctica. In this study, we use the ANT-30 model (Lloyd et al., in prep), a 3D seismic model from earthquake data, as a starting model. We seek to improve its resolution within the upper ~100 km of the Antarctic mantle by fitting three-component ambient noise correlograms computed from broadband records collected in Antarctica over the past 20 years. This includes data from recent temporary arrays such as TAMSEIS, AGAP, TAMNNET, RIS, POLENET/ANET, and UKANET. The three-component cross-correlations of station pairs are calculated and properly rotated to extract ambient noise surface waves that include both Rayleigh and Love waves, which show excellent signal-to-noise ratio between 15 to 70 seconds. The benefit of including this data is twofold: (1) it provides surface wave observations down to 15 s, as opposed to 25 s and (2) it provides shorter intercontinental paths, which were absent due to the region&#8217;s earthquake distribution. We then use the software package SPECFEM3D_GLOBE to iteratively improve the 3-D earth model, minimizing the nondimensionalized traveltime phase misfit between the observed and synthetic waveforms. The preliminary results indicate a stronger positive radial anisotropy (V<sub>SH</sub> > V<sub>SV</sub>) in the lower crust and uppermost mantle for West Antarctica and part of East Antarctica.&#160; With more iterations, smaller-scale detail can be revealed by the new ambient noise data, resulting in a more reliable uppermost mantle and crustal structure.</p>
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