CUMULATIVE DEXTRAL STRAIN ACROSS THE MIOCENE-PRESENT PACIFIC-NORTH AMERICA PLATE BOUNDARY: EASTERN CALIFORNIA SHEAR ZONE TO THE NORTHERN GULF OF CALIFORNIA

Late Cenozoic Pacific-North America (PAC-NAM) plate motion has been accommodated by a combination of localized and distributed shear, transtension, transpression and rotation of crustal-scale faults blocks. At the latitude of the Gulf of California (GOC), a global plate circuit model requires 600-650 km of cumulative northwestward displacement of the Pacific plate relative to the North America plate since 12.3 Ma, distributed in an uncertain way on faults both west and east of the stable Baja California microplate. Existing kinematic models estimate from as little as 300 km to as much as 450-500 km of cumulative dextral shear east of the Baja California microplate, with transtensional structures west of Baja California accommodating the residual component of PAC-NAM dextral shear. We compile late Cenozoic fault displacement data from the northern GOC, Salton Trough and Eastern California Shear Zone to test the compatibility of these kinematic models with known dextral displacements. Because no large-offset, trans-peninsular dextral faults cut the stable Baja California microplate, cumulative dextral shear across the northern GOC should be consistent with cumulative dextral shear along plate boundary strike across southern California and Arizona. Summation of dextral displacements across several onshore transects, perpendicular to the PAC-NAM plate boundary, consistently reveal ~320-380 km of total intracontinental shear. These findings suggest that previous models for relatively small offset (300 km) underestimate known geologic offsets, while larger offset models (450-500 km) require an additional 70-180 km of dextral plate motion that is currently undocumented in the northern GOC and southwestern United States. Thus, we propose a moderate-offset model in which ~320-380 km of dextral shear occurred across the northern GOC and southwestern United States, while ~220-330 km of shear occurred offshore southern California and northern Baja California. Discrepancies with larger post-12.3 Ma estimates across the southern GOC could be partially resolved if some PAC-NAM motion occurred west of the Magdalena fan. Additionally, several tens of km more dextral shear likely occurred along transform faults in the southern GOC than in the north because the southern GOC is farther from the PAC-NAM Euler pole.