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Justin Wilgus
Public Documents
2
A Middle Crustal Channel of Radial Anisotropy Beneath the Northeastern Basin and Rang...
Justin Wilgus
and 2 more
February 23, 2020
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.
Shear velocity evidence of upper crustal magma storage beneath Valles Caldera
Justin Wilgus
and 4 more
October 06, 2022
Valles Caldera was formed by large rhyolitic eruptions at ~1.6 and 1.23 Ma and it hosts post-caldera rhyolitic deposits as young as ~70 ka, but the contemporary state of the magmatic system is unclear. Local seismicity beneath Valles Caldera is rare and shear-velocity (Vs) structure has not been previously imaged. Here, we present the first local Vs tomography beneath Valles Caldera using ambient noise Rayleigh dispersion from a ~71 km transect of nodal seismographs with mean spacing of ~750 m. An ~6 km wide low-Vs anomaly (Vs<2.1 km/s) is located at ~3-10 km depth within the 1.23 Ma caldera’s ring fracture. Assuming magma in textural equilibrium, the new tomography suggests that melt fractions up to ~17-22% may be present within the upper crustal depth range where previously erupted rhyolites were stored.