TIMING AND MAGNITUDE OF TRANSFORM FAULTING IN THE NORTHERN GULF OF CALIFORNIA: IMPLICATIONS FOR OBLIQUE RIFT LOCALIZATION AND RECONSTRUCTIONS OF THE PACIFIC-NORTH AMERICA PLATE BOUNDARY

The Gulf of California oblique rift has accommodated dextral-oblique Pacific-North America (PAC-NAM) plate divergence since Miocene time. Its rifted margins preserve a rare onshore record of early continental break-up processes from which to investigate the role of rift obliquity in strain localization. We integrate structural mapping and basin analysis of syn-tectonic sedimentary basins with geochronology of pre- and syn-rift volcanic units to estimate the timing of rift-related fault activity and basin formation on Isla Tiburón, an exposure of the North America margin proximal to the rift axis. On southern Isla Tiburón, an early phase of east-west extension initiated sometime after ~19 Ma and was ongoing by ~12.3 Ma, when deposits of the Tuff of San Felipe were emplaced. Extensional faults and basins were subsequently buried by younger deposits of the La Cruz basin, which accumulated along the La Cruz fault ca. ~8 - 4 Ma due to 5 ± 2 km of dextral motion. On northeastern Isla Tiburón, transtension also commenced in the latest Miocene, recorded by <7.1 Ma Tecomate basin deposits that accumulated above the hanging wall of the Kunkaak normal fault. Strike-slip faulting along the Yawassag fault truncated Tecomate basin-fill and the Kunkaak fault via ≥8 km of post-6.4 Ma dextral offset. Onset of strike-slip faulting on Isla Tiburón was synchronous with the onset of transform faulting along a significant length of the PAC-NAM plate boundary ca. 9 - 7 Ma. This event was coeval with a 15 - 20° clockwise azimuthal shift in PAC-NAM relative motion that enhanced the obliquity of the rift, and immediately preceded strain localization ca. 6 Ma. The restored, latest Miocene positions of focused transtensional activity are restricted to a ~50 - 100 km-wide, NNW-trending belt, embedded within the western Mexican Basin and Range. A paleomagnetic transect across the plate boundary at the latitude of Isla Tiburón shows that only drill sites within this belt have experienced large magnitude clockwise vertical-axis block rotations. The proto-Gulf of California illustrates how highly oblique rift geometries, where transform faults develop and are kinematically linked to large-offset normal faults in pull-apart basins, enhance the ability for rupturing continental lithosphere and, ultimately, hasten the formation of new oceanic rift basins.