PROGRESSIVE LOCALIZATION OF DEXTRAL SHEAR IN THE LATE PROTO-GULF OF CALIFORNIA

Shear deformation during the early history of a rift may profoundly affect the efficiency and success of lithospheric rupture and formation of a new ocean basin. The Gulf of California (GOC) rift is well suited to study the role of rift obliquity in continental rupture. Transtensional strain in the GOC is accommodated along en-echelon pull-apart basins bounded by dip-slip and oblique-slip faults and linked by strike-slip faults and accommodation zones. Lithospheric rupture is well documented at ca. 6 Ma when >90% of Pacific-North American relative plate motion localized into the GOC. In the northern GOC, the eastern rift margin of the Upper Delfín-Upper Tiburón rift segment preserves an onshore record of the earliest phase of this localization process. Two NW-striking shear zones bound this 37-km-wide rift segment. Our geologic mapping, paleomagnetic measurements, and geochronology of pre-rift and syn-rift volcanic and sedimentary rocks provide timing and displacement constraints for these shear zones. The Coastal Sonora Fault Zone, exposed on northeast Isla Tiburón and in adjacent coastal Sonora, formed and then truncated transtensional non-marine basins beginning ca. 7 Ma. In coastal Sonora, we document ~12 km of discrete dextral displacement, clockwise block rotations up to 53°, and up to 122% extension that together accommodated ~14 km of transtensional strain over a 1 Myr period. The southern of the two shear zones is the La Cruz fault, which transects southern Isla Tiburón. Associated dextral transpression and transtension formed the elongate Southwest Isla Tiburón-Sauzal basin, where fossil-rich marine sandstone and conglomerate is underlain by a 6.7 ± 0.8 Ma tuff. The base of the marine basin displays ~1 km of dextral displacement, while Early Miocene volcanic rocks are offset >15 km. This displacement history supports significant proto-Gulf shear along the La Cruz fault. Overall, our results suggest that significant shearing along strike-slip faults initiated by ca. 7 Ma, at least 1 Myr prior and proximal to the locus of continental rupture. We hypothesize that progressive localization of dextral shear into a broader region of extension may have allowed kinematically linked normal faults to localize crustal thinning and acted as a catalyst for lithospheric rupture in the Gulf of California.