A Middle Crustal Channel of Radial Anisotropy Beneath the Northeastern Basin and Range

A challenge in interpreting the origins of seismic anisotropy in deformed continental crust is that composition and rheology vary with depth. We investigated anisotropy in the northeastern Basin and Range where prior studies found prevalent depth-averaged positive radial anisotropy (V_SH > V_SV). This study focuses on depth-dependence of anisotropy and potentially distinct structures beneath three metamorphic core complexes (MCCs). Rayleigh and Love wave dispersion were measured using ambient noise interferometry, and Bayesian Markov chain Monte Carlo inversions for V_S structure were tested with several (an)isotropic parameterizations. Acceptable data fits with minimal introduction of anisotropy are achieved by models with anisotropy concentrated in the middle crust. The peak magnitude of anisotropy from the mean of the posterior distributions ranges from 3.5–5% and is concentrated at 8–20 km depth. Synthetic tests with one uniform layer of anisotropy best reproduce the regional mean results with 9% anisotropy at 6–22 km depth. Both magnitudes are plausible based on exhumed middle crustal rocks. The three MCCs exhibit ~5% higher isotropic upper crustal V_S, likely due to their anomalous levels of exhumation, but no distinctive (an)isotropic structures at deeper depths. Regionally pervasive middle crustal positive radial anisotropy is interpreted as a result of subhorizontal foliation of mica-bearing rocks deformed near the top of the ductile deformation regime. Decreasing mica content with depth and more broadly distributed deformation at lower stress levels may explain diminished lower crustal anisotropy. Absence of distinctive deep crustal V_S beneath the MCCs suggests overprinting by ductile deformation since the middle Miocene.