DEXTRAL SHEAR AS A CATALYST FOR LITHOSPHERIC RUPTURE IN THE NORTHERN GULF OF CALIFORNIA: KINEMATIC AND PALEOMAGNETIC EVIDENCE FROM COASTAL SONORA, MÉXICO

Dextral shear in the Gulf Extensional Province may have played an integral role in lithospheric rupture that formed the Gulf of California. To test this hypothesis, we measured fault kinematic indicators and paleomagnetic vectors from middle Miocene to Pliocene rocks near Bahía de Kino, coastal Sonora. Here dextral faults onshore may be directly linked to the transform faults offshore that accommodated Gulf opening. We mapped extensional basins overlying crystalline basement and middle Miocene andesitic rocks. The basins are floored by the 12.50 ±0.08Ma Tuff of San Felipe (Tsf). The 6.39 ±0.02Ma Tuff of Mesa Cuadrada (Tmc) separates proto-Gulf strata (12.5-6Ma) from modern-Gulf strata (post-6Ma). All sedimentary rocks mapped thus far are non-marine conglomerate, despite proximity to the Gulf of California. The basins are cut by sub-vertical NW-striking dextral faults, NNE-striking sinistral faults, and N-striking normal faults. Packages of syntectonic rocks are tilted down to the east up to 90° and are bounded by low-angle west-dipping detachment faults. Fault kinematic indicators support a transition from E-W extension pre-6Ma to NW-directed transtension post-6Ma. Paleomagnetic remanence directions measured on pyroclastic flows indicate clockwise vertical axis rotation up to 46°. At Bahía de Kino, Tsf is rotated 25.0° ±5.0°, but 5km to the northwest at Cerro Tordillos, Tsf is essentially not rotated. 9km further to the northwest at Punta Chueca, Tsf and Tmc display clockwise vertical axis rotations of 42.4° ±4.0° and 46.1° ±11.7° respectively. These indistinguishable rotation values indicate that dextral shear commenced after 6.4Ma, during the modern-Gulf period. The onset of transtension, determined from both kinematic data and paleomagnetic vectors, coincides with localization of lithospheric rupture into the northern Gulf of California.