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 (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.