Crustal deformation in the Sierra Nevada and Walker Lane region inferred from P-wave azimuthal anisotropy

Abstract

Crustal deformation in the Sierra Nevada and Walker Lane is mainly investigated by geologic and geodetic constraints which suffer from a lack of depth information. In this study, we construct a new depth-dependent azimuthally anisotropic P-wave velocity (Vp) model of this area to investigate regional deformation regimes in the upper and middle crust. The model is built based on adjoint-state traveltime tomography of ∼650,000 local direct P arrivals collected from 1967 to 2021. The average Vp structure agrees well with previous tomographic models with refined velocity features benefiting from the ray-free adjoint-state tomography and more arrival time data. The azimuthal anisotropy of Vp reveals distinct differences between the rigid Sierra Nevada block and the Walker Lane shear zone: (a) The western Sierra Nevada shows weak anisotropy (<2%) and NW-SE oriented fast velocity directions that are parallel to the Pacific-North American plate boundary, mainly reflecting preserved paleofabrics developed in past subduction processes. (b) In the shallow Walker Lane (<4 km), the orientation of fast-velocity directions is NNE controlled by regional compressive stress, and changes to NNW at greater depths in the north under shear deformation regimes. (c) In the central Walker Lane, strong anisotropy with ENE-oriented P fast axes is imaged at 6–16 km from the area south of Lake Tahoe (2%–4%) to the Long Valley volcanic area (4%–8%) because of clockwise crustal block rotations and crustal fluids. The proposed anisotropic Vp model provides new insight into how past and present-day deformation is accommodated in this tectonic complex plate boundary region.