TRANSTENSIONAL RIFTING IN THE LATE PROTO-GULF OF CALIFORNIA NEAR BAHíA KINO AND on ISLA TIBURóN, SONORA, MéXICO

We test the role of obliquity in promoting continental rupture in the Gulf of California rift through structural mapping, basin analysis, geochronology and paleomagnetism of pre- and syn-rift volcanic and sedimentary rocks exposed in coastal Sonora near Bahía Kino and on Isla Tiburón. This area is host to the northwest-striking dextral Sacrificio, Bahía Kino and La Cruz faults onshore that are likely linked to offshore transform faults, such as the De Mar fault and Tiburón fracture zone, that accommodated Gulf opening. In coastal Sonora tectonic basins formed after deposition of the 12.50 ± 0.08 Ma Tuff of San Felipe (TSF). Deposited near the base of one of these non-marine basins are the 6.53 ± 0.18 Ma Tuff of Cerro Tordillo and 6.39 ± 0.02 Ma Tuff of Mesa Cuadrada (TMC). These dates calibrate rapid conglomerate sedimentation and tilting rates that suggest faulting and basin subsidence commenced in coastal Sonora ca. 7 Ma. Paleomagnetic vectors from TSF and TMC show variable amounts of clockwise vertical-axis rotation that increase from 35° in the south to 53° in the north. Slip on the dextral Bahía Kino fault decreases from 10 km in the south to 0 km in the north, where ~6 km of slip is absorbed on the low-angle Punta Chueca fault. These tradeoffs in deformational style demonstrate the intimate link between normal faulting, strike-slip faulting and block rotation across this transtensional Coastal Sonora Fault Zone. Hanging-wall cutoff angles >90° require a poly-stage history of tilting on at least two generations of normal faults. From ~7 to 6 Ma, ~16 km of transtension at azimuth 294° occurred in coastal Sonora, which represents a significant portion (~28%) of Pacific-North America plate motion. On southwest Isla Tiburón, marine rocks were deposited in a tectonic basin formed by slip on the La Cruz fault. These strata are underlain by a 6.7 ± 0.8 Ma air-fall tuff and capped by a 4.13 ± 0.09 Ma rhyodacite flow. New mapping results indicate that a significant older slip history may be present on the La Cruz fault, the age of which is not yet constrained. Overall, these results show that transtensional strain may have initiated in latest proto-Gulf time, ca. 7 Ma. This finding is consistent with the hypothesis that a higher rate of strain in transtensional shear zones may have facilitated rift localization and eventual continental rupture.