ISLA SAN PEDRO NOLASCO: A LATE MIOCENE INTRUSIVE RECORD AT THE EASTERN MARGIN OF THE GULF OF CALIFORNIA: INSIGHTS FROM GEOLOGICAL, GEOCHEMICAL, GEOCHRONOLOGICAL AND PALEOMAGNETIC STUDIES

Isla San Pedro Nolasco (ISPN) is a structural high bounded by inactive dextral oblique-slip faults in the east-central part of the Gulf of California rift zone and is composed of intrusive rocks not exposed on other Gulf of California islands. We present the results from geological mapping, geochemical, geochronological and paleomagnetic data for the ISPN intrusive complex. The intrusive rocks compose a sheet-like body of intermediate and felsic composition intruded by an intermediate and acidic dike swarm. All intrusive rocks (host and dikes) range in age from ca. 9 to 10 Ma ^(40Ar/39Ar) and show a hydrous ferromagnesian mineral association (amphibole and biotite) with a calc-alkalic and transitional affinity. This hydrated mineralogical association has not been recognized in the coeval rocks along the onshore western margin of the North American plate (coastal Sonora). However, such hydrous mineralogical association is found in the coeval rift transitional volcanic rocks from the Baja California Microplate. Moreover, the comparison of the paleomagnetic direction obtained from ISPN rocks, respect to the 10 Ma paleomagnetic pole for North America indicates that Isla San Pedro Nolasco has experienced no vertical-axis rotation but has displaced ~100 km to the northwest. These results, supported by detailed geological studies, indicate that the ISPN continental block has been pulled apart by transtensional faulting of the late Miocene Gulf of California shear zone before the westward migration of the North America-Pacific plate boundary at ca. 3-2 Ma. Eventually, ISPN became isolated as an island during the late Miocene flooding of the Gulf of California seaway.