INFLUENCE OF PRE-TERTIARY UPPER-CRUSTAL STRUCTURE on LATE NEOGENE AND HOLOCENE DISPLACEMENT AND STRAIN WITHIN THE CENTRAL WALKER LANE AND NORTHERN EASTERN CALIFORNIA SHEAR ZONE

Late Neogene and Holocene displacement and nonplane strain within the central Walker Lane (WL) and northern Eastern California Shear Zone (ECSZ) are localized and controlled by persistent variations in upper-crustal strength imparted by late Proterozoic and Mesozoic structures. East of the central Sierra Nevada, the transition between the central WL and northern ECSZ is dominated by a major eastward deflection of the north-trending continental margin formed during late Precambrian rifting. Paleomagnetic signatures of Mesozoic plutonic rocks and superposed Phanerozoic structures indicate a primary origin for the morphology of juxtaposed continental and non-continental crust, whose strength differential localized Paleozoic and Mesozoic contractional structures. This crustal feature was subsequently modified during early Mesozoic truncation of the continental margin by NW trending, orogen parallel transcurrent faults initiated during inception of the Mesozoic fringing arc system. The contemporary system of transcurrent and extensional faults separating the Sierra Nevada from the central Great Basin follows this pre-existing structural grain. From south to north, the NNW to NW trending belt of faults and seismicity of the ECSZ narrows from 125 to 75 km to the latitude of the central Sierra Nevada where they merge with the central WL and abruptly widen to 150 km. The kinematics of the structures were initiated in the mid Pliocene (~3 Ma) and are characterized by fault slip consistent with earthquakes with normal, right-oblique, and left-oblique first motions. Campaign GNSS measurements from over 100 sites spanning the central WL and northern ESCS yield a velocity field that increases from 2-3 mm/yr with an azimuth of WNW in the central Great Basin to 12-14 mm/yr with a NW azimuth in the Sierra Nevada. The velocity trajectory is oblique to the major crustal boundaries, resulting in nonplane strain conditions during transtensional deformation. The northern ESCZ exhibits extension-dominated transtension whereas the central WL deformation is partitioned into two spatial domains. In the east, the central WL undergoes wrench-dominated transtension that, to the west, abruptly changes to extension-dominated transition across the NW trending Mesozoic boundary now located along the eastern Wassuk Range.