A TECTONO-STRATIGRAPHIC RECORD OF THE LATEST MIOCENE GULF OF CALIFORNIA SHEAR ZONE FROM NORTHEASTERN ISLA TIBURON, SONORA, MEXICO

Details about the timing and kinematics of rifting are crucial to understand the conditions that led to the formation of the Gulf of California (GOC) ocean basin. The late Miocene Pacific-North American plate boundary localized into the Gulf of California shear zone, a narrow transtensional belt in northwestern Mexico, which was kinematically linked to the San Andreas fault system and hosted the marine incursion and continental rupture of the GOC rift. On northeastern Isla Tiburón, we integrate geologic and structural mapping of the Coastal Sonora fault zone with basin analysis, paleomagnetic data, and radiometric ages to characterize deformation of the rifted North America margin proximal to the rift axis. Slip on the Kunkaak normal fault tilted its hanging wall down-to-the-east ~70°. From 7.1–6.4 Ma, fanning dips indicate that tilting occurred at 35 ± 5°/Myr, while non-marine sandstone and conglomerate accumulated at 1.4 ± 0.2 mm/yr, deposited above a ~20° angular unconformity at the base of this Tecomate basin. At least 1.8 ± 0.1 km of sediments and pyroclastic deposits accumulated in the basin concurrent with ~2.8 km of dip-slip on the Kunkaak fault. Linear extrapolation of tilting and sedimentation rates suggests that faulting and basin deposition initiated ~7.6–7.4 Ma. The Kunkaak fault and Tecomate basin are truncated by NW-striking, dextral-oblique structures, including the Yawassag fault, which accrued >8 km of post-6.4 Ma dextral displacement and projects offshore into the GOC. Paleomagnetic data from regional ignimbrites mapped below and within the basin suggest ~70% of 29° of clockwise vertical-axis block rotation occurred sometime between 12.5 Ma and 6.4 Ma, but likely initiated shortly after 8 Ma at the onset of normal fault activity. Initial dextral transtension in the Coastal Sonora fault zone occurred via tilting and clockwise block rotation, but evolved with time as new strike-slip faults developed, crosscut previously rotated and tilted fault blocks, and accrued tens of kilometers of latest Miocene to earliest Pliocene dextral offset. The establishment of rapid, transtension in the Coastal Sonora fault zone was synchronous with the 8–7 Ma onset of transform faulting and basin formation along >500 km of the larger Gulf of California shear zone throughout northwestern Mexico and southern California.