LATE MIOCENE EXTENSION IN THE SIERRA BACHA, COASTAL SONORA, MEXICO: IMPLICATIONS FOR THE KINEMATIC EVOLUTION OF THE PROTO-GULF OF CALIFORNIA

The conditions and processes that led to plate boundary localization in the Gulf of California ca. 6 Ma, and in particular, the time at which dextral strain became established in the Gulf region, remain poorly understood and debated. Competing end member models propose that “proto-Gulf” (12-6 Ma) strain was: (1) partitioned between dextral strain on offshore transforms and NE-SW continental extension, or (2) distributed throughout the Gulf Extensional Province in a diffuse zone of integrated extension and dextral shear. Well exposed volcanic rocks in the Sierra Bacha, coastal Sonora, Mexico, preserve a rich record of proto-Gulf deformation and offer new insights into oblique-rift kinematics. The 2,000 m-thick volcanic section contains intermediate lava flows and subordinate basalts, rhyolites, and welded tuffs deposited between ~15 and 6 Ma. Domino-style fault blocks are uniformly tilted to the NE by a series of NW-striking normal faults that may sole into a concealed SW-dipping regional detachment fault inferred to be NE of the study area. Geologic mapping, fault-kinematic analysis, and preliminary paleomagnetic data show that the Sierra Bacha experienced 55-60% ENE-WSW extension between ~10 and 6 Ma, with little to no vertical-axis rotation. Kinematic data indicate that NW-striking normal faults were reactivated with minor oblique slip, likely after ~6 Ma, in a modified transtensional stress field. These results, along with data from the nearby Coastal Sonora fault zone to the south, support an emerging model for proto-Gulf deformation in which dextral strain was not ubiquitous across Sonora, but instead became progressively localized during latest Miocene time in a narrow coastal shear zone following more widespread, large-magnitude extensional deformation. According to this model, proto-Gulf strain evolved from NE-SW extension to shear-dominated transtension by ca. 7 Ma. Progressive localization of dextral shear in coastal Sonora during latest Miocene time likely weakened the lithosphere and played an important role in facilitating continental rupture and opening of the northern Gulf of California ca. 6 Ma.