CRUSTAL ANISOTROPY, STRAIN LOCALIZATION, AND THE DEVELOPMENT OF TRANSTENSIONAL STRAIN PARTITIONING IN THE CENTRAL WALKER LANE, WESTERN GREAT BASIN

Late Cenozoic strain localization within the boundary zone between the Sierra Nevada block and the central Great Basin was controlled by a crustal architecture inherited from pre-Tertiary extensional and contractional tectonism. Crustal anisotropy localized late Cenozoic displacements on NNW, ENE, NW, and NNE trending structures and promoted development of nonplane strain conditions. The NW displacement of the Sierra Nevada block is accommodated by 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 12-14 mm in the Sierra Nevada and exhibits a clockwise rotation from WNW to NW. Extensional incremental strain axes determined from earthquake focal mechanisms and fault-slip inversion show 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 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. The obliquity of the velocity field and tectonic boundaries results in constrictional strain during transtensional deformation. East of the southern Sierra Nevada, the boundary zone 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 (CNSB). From south to north, the velocity boundary between the central Great Basin and the western Great Basin steps east across a NNE-trending structural stepover that kinematically links faults of the Eastern California Shear Zone and central Walker Lane. North of the stepover, the velocity boundary continues northeasterly along a series of dog-leg steps and ultimately follows the southeastern margin of the CNSB.