CRUSTAL CONTROL OF TRANSTENSION AND STRAIN PARTITIONING IN THE WESTERN GREAT BASIN

Northwest displacement of the Sierra Nevada block is accommodated by a system of kinematically linked structures localized along the western margin of the Great Basin. Active deformation is expressed by an east to west increase in regional velocity field, recorded by a dense array of GPS sites, and by belts of seismicity. The regional velocity increases from 2-3 mm/yr in the central Great Basin to ~14 mm in the Sierra Nevada and exhibits a clockwise rotation from WNW to NW. Incremental strain axes determined from earthquake focal mechanisms and fault-slip inversion show an east to west anticlockwise rotation of 50° from parallel to the velocity trajectory in the central Great Basin to nearly orthogonal to the velocity field along the eastern flank of the Sierra Nevada. The progressive divergence between strain- and velocity-field trajectories marks the transition from plane to nonplane strain conditions of deformation. The regional velocity field is oblique to the eastern margin of the Sierra Nevada block and many structures within a NW-trending boundary zone separating the Sierra Nevada and the central Great Basin and results in constrictional strain during transtensional deformation. East of the southern Sierra Nevada, the boundary zone forms the northern extent of the Eastern California Shear Zone, which has an eastern margin that coincides with an abrupt east to west increase in the regional velocity. Farther north, the belt of active deformation bifurcates into the NW-trending Walker Lane and NNE-trending Central Nevada Seismic Belt. From south to north, the velocity boundary steps east across a NNE-trending structural stepover that kinematically links faults of the Eastern California Shear Zone and central Walker Lane and continues northeasterly along a series of dog-leg steps and ultimately follows the eastern margin of the Central Nevada Seismic Belt. Interaction of the regional velocity field and pre-Tertiary crustal anisotropy produced displacement partitioning on NNW, ENE, NW, and NNE structures and the local development of nonplane strain conditions. The crustal anisotropy was developed during late Proterozoic continental rifting and formation of the lower Paleozoic miogeocline and during younger Mesozoic backarc and intra-arc deformation.