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

The Gulf of California is an oblique-rift basin that formed by Late Cenozoic dextral-oblique extension along the Pacific-North America plate boundary. The time at which plate boundary dextral strain became established in the Gulf region is debated. Two end-member models propose that late Miocene dextral strain was either: (1) accommodated on the Tosco-Abreojos fault west of the Baja California peninsula, or (2) distributed across the entire Gulf Extensional Province. The Sierra Bacha in western Sonora, Mexico, contains late Miocene volcanic and volcaniclastic rocks that accumulated during early rifting in the northern Gulf region, thus offering insight into proto-Gulf volcanism and deformation style. The volcanic section has a composite thickness of about 1,400 m and changes up-section from mainly andesitic lava flows at the base to rhyolitic flows and tuffs at the top. The volcanic matrix of a basaltic andesite near the base of the section was dated at 11.76 ± 0.08 Ma using the ⁴⁰Ar/³⁹Ar isochron method. The section dips moderately (25 to 40 degrees) to the NE and is capped unconformably by a nearly flat-lying basalt unit dated at 6.4 ± 1.9 Ma (K-Ar; Gastil and Krummenacher, 1977). The NE-dipping section is cut by NW-striking normal faults and minor N-striking left-lateral faults that record late Miocene NE-SW extension, consistent with preliminary fault-kinematic analysis that yields a tentative T-axis azimuth of ~230 degrees (N=17).

While a small component of dextral shear is possible, the lack of evidence for strong dextral shear in the Sierra Bacha supports a newly emerging, intermediate model for proto-Gulf strain evolution in which late Miocene dextral strain became localized in a narrow zone along coastal Sonora on the eastern margin of the future Gulf of California at ca. 7 Ma (Bennett, 2009). During late Miocene time the Sierra Bacha, located northeast of the coast and inboard from large-offset dextral faults in the coastal zone, experienced orthogonal NE-SW extension, suggesting a modified model for regional strain partitioning. Future work in the Sierra Bacha will focus on expanding the kinematic analysis, completing the geologic map, and conducting paleomagnetic studies to further test the hypothesis that this area experienced limited or no dextral shear during opening of the Gulf of California.