Crustal Seismic Anisotropy of the Ruby Mountains Core Complex and Surrounding Northern Basin and Range

Metamorphic core complexes (MCC) are distinctive uplifts that expose deeply exhumed and deformed crustal rocks due to localized extensional deformation. Consequently, their detailed structure can provide a window into deep crustal mechanics. The North American Cordillera contains numerous MCC, one of which is the Ruby Mountain Core Complex (RMCC) located in the highly extended northern Basin and Range. To constrain the extent to which anisotropy below the RMCC deviates from the regional Basin and Range average and test the depth dependence of crustal anisotropy we conduct a radial anisotropy investigation below the RMCC and surrounding northern Basin and Range. Data from the Ruby Mountain Seismic Experiment (RMSE) and surrounding networks are used to cross correlate ambient noise and obtain Rayleigh and Love wave signals. Using the Frequency-Time Analysis (FTAN) method we compute inter-station Rayleigh and Love wave dispersion curves and invert for phase velocity maps for periods 5-40 s. A Markov chain Monte Carlo probabilistic inversion technique is used to obtain 3D Vs structure and estimate radial anisotropy as a function of depth and geographic location. Results show a relatively simple but pervasive, 5%, positive (V_SH > V_SV) anisotropy signal distributed throughout the middle crust ( 5-20 km) of the study region required at 95 % confidence. In contrast, there is an absence of lower crustal radial anisotropy. Interestingly, the volume directly below the RMCC does not strongly deviate from this trend indicating mechanisms of core complex exhumation either never produced a resolvable radial anisotropy pattern or anisotropy associated with RMCC exhumation has subsequently been overprinted. Multiple mechanisms have the potential to explain the observed depth distribution of anisotropy, including decreasing prevalence of mica from the middle-to-lower crust and a temperature-controlled rheological transition from localized ductile deformation in the middle crust to distributed ductile deformation in the lower crust.