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A Middle Crustal Channel of Radial Anisotropy Beneath the Northeastern Basin and Range
  • Justin Wilgus,
  • Chengxin Jiang,
  • Brandon Schmandt
Justin Wilgus
University of New Mexico

Corresponding Author:[email protected]

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Chengxin Jiang
University of New Mexico
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Brandon Schmandt
University of New Mexico
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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 (MCC’s). 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 feasible based on exhumed middle crustal rocks. The three MCC’s 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 sub-horizontal 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 MCC’s suggests over-printing by ductile deformation since the middle Miocene.
Jul 2020Published in Tectonics volume 39 issue 7. 10.1029/2020TC006140