LATE MIOCENE (12-6 MA) TRANSTENSIONAL FAULTING, BLOCK ROTATIONS, AND VOLCANISM DURING INCEPTION OF THE GULF OF CALIFORNIA OBLIQUE RIFT, SOUTHWESTERN SONORA, MEXICO

The history of late Miocene (Proto-Gulf) deformation on the Sonoran margin of the Gulf of California is key to understanding how Baja California was captured by the Pacific plate and how strain was partitioned during early stages of this transtensional rift system. Geologic mapping and paleomagnetic investigations in southwestern Sonora and 40Ar/39Ar dating of pre-, syn-, and post-tectonic volcanic units indicate that late Miocene deformation was largely restricted to a NW-SE 100-120 km wide coastal belt. Extension inboard of this area is older (>15 Ma) and occurred in an intra-arc setting. In the coastal belt, deformation is profoundly transtensional, with NW-SE striking, dextral strike slip faults operating in concert with N-S and NNE-SSW striking normal and sinistral-normal faults, producing large-scale rotations about both horizontal and vertical axes, and yielding an overall NW to WNW extension direction. This fundamentally triaxial strain field produced clockwise vertical axis rotations of up to 112° and tilts of up to 90° on blocks bound by arrays of antithetic sinistral-normal faults. The total amount of mid- to late- Miocene NW directed extension is still poorly constrained, but likely exceeds several hundred km. Within this late Miocene Sonoran coastal belt, deformation and volcanic activity migrated NW. Inboard (Sierra Libre and Sierra El Bacatete) volcanism commenced at ~13.0 Ma and peaked at 12.0 Ma, with deformation largely bracketed between 12.0 and 10.6 Ma. In the Sierra El Aguaje, volcanic activity commenced at 11.0 Ma and peaked at 10.5 Ma, with most faulting and tilting tightly bracketed between 10.7 and 9.3 Ma. On the coastal mountains north of San Carlos, faulting and tilting continued after 8.5 Ma. Voluminous and compositionally diverse (basalt to rhyolite) late Miocene (13-8 Ma) volcanic rocks within the Sonoran coastal belt were erupted from several large centers (e.g. Sierra Libre, Sierra El Aguaje). This eruptive activity was closely associated in space and time with tectonic activity, with transtensional deformation apparently triggered and localized by pulses of magmatic activitiy. The Sonoran coastal belt represents a spectacular example of distributed transtension that ultimately led to rupturing of the continental lithosphere.