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

Abstract

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 (VSH > VSV). 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 VS 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 VS, 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 VS beneath the MCCs suggests overprinting by ductile deformation since the middle Miocene.

Radial Anisotropy and Sediment Thickness of West and Central Antarctica Estimated From Rayleigh and Love Wave Velocities

“…In our imaging, the EM shows 5 ± 2% positive anisotropy throughout the crust, but the adjacent Whitmore Mountains (WM) only show strong positive radial anisotropy in the upper crust, and the middle to lower crust does not have a clear anisotropic pattern. Strong positive radial anisotropy in the middle to lower crust is broadly observed in other regions that have undergone extensional deformation, such as the North American Basin and Range, California, Tibet, Central North China, and Madagascar (Ai et al, 2020;Dreiling et al, 2018;Moschetti et al, 2010b;Wilgus et al, 2020;Xie et al, 2013). Positive crustal anisotropy is usually ascribed to highly anisotropic mica or amphibole minerals with a preferential orientation from horizontal compression or extension (Brownlee et al, 2017;Erdman et al, 2013;Lloyd et al, 2009).…”

Segmentation and Radial Anisotropy of the Deep Crustal Magmatic System Beneath the Cascades Arc

“…(2015), in which our study region is the largest area showing negative anisotropy. The rest of the tectonically active western U.S. crust is dominated by positive radial anisotropy (Moschetti et al., 2010), which may be focused at middle crustal depths (Wilgus et al., 2020). The exact reason for the negative anisotropic feature in the crust beneath our study region remains to be explored, but indicates subvertical foliations or cracks.…”

Crustal Deformation in Southern California Constrained by Radial Anisotropy From Ambient Noise Adjoint Tomography

“…The origin of the negative anisotropy is intriguing, as it is rarely observed at middle/lower crustal depths in previous anisotropic tomography studies (e.g., Luo et al, 2013;Wilgus et al, 2020;Xie et al, 2013). For example, a large-scale model covering the western United States (Xie et al, 2015) only exhibits small patches of negative anisotropy in the front of the Cascadia arc, with underlying mechanisms remaining to be explored.…”