ACTIVE STRAIN PARTITIONING IN THE CENTRAL WALKER LANE, WESTERN GREAT BASIN: COMPATIBILITY OF GPS VELOCITIES AND NONPLANE INCREMENTAL STRAIN ESTIMATED FROM SEISMICITY AND FAULT-SLIP INVERSION

The central Walker Lane is part of a transtensional boundary zone separating plane-strain extension in the Great Basin and NW block motion of the Sierra Nevada. Active displacement is accommodated by a complex array of ENE to NW trending faults forming a 100 km wide stepover between NW-trending transcurrent faults of the Eastern California shear zone and northern Walker Lane. Earthquake focal mechanisms are consistent with geologic offsets and have right-lateral, left-lateral, and dip-slip motions. GPS velocites record a complex but systematic westward change in direction from WNW to NW and an increase in magnitude from 2-4 mm/yr to ~14 mm/yr. Incremental strain estimated from earthquake focal mechanisms and fault slip inversion records an east to west transition from plane to nonplane conditions accompanied by a 55° anticlockwise rotation of the axis of maximum horizontal extension. Along the western margin of the central Great Basin, extension is N50°W and progressively changes to N80-75°E along the eastern margin of the Sierra Nevada. The west-central Great Basin deforms in plane strain and has a NW-trending extension axis consistent with NW-directed GPS velocities. In the central Walker Lane, progressive divergence between extension axis orientation and the GPS velocity field indicates nonplane strain (constriction) associated with transtensional deformation. West from the eastern margin of the central Walker Lane, NW-directed extension varies from plane strain to wrench dominated transtension with a strain vorticity of 1.0-0.8. Farther west, as the Sierra Nevada is approached from the east, the extensional component of bulk strain increases and deformation shows a progressive change to extension dominated transtension with a corresponding change in strain vorticity of 0.7 to 0.2. Constrictional strain formed during active deformation in the central Walker Lane is partitioned into spatial domains that vary from wrench dominated to extension dominated transtension from the central Great Basin westward toward the Sierra Nevada.