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Details about the timing and kinematics of rifting are crucial to understand the conditions that led to strain localization, continental rupture, and formation of the Gulf of California ocean basin. We integrate detailed geologic and structural mapping, basin analysis, and geochronology to characterize transtensional rifting on northeastern Isla Tiburón, a proximal onshore exposure of the rifted North America margin, adjacent to the axis of the Gulf of California. Slip on the Kunkaak normal fault tilted its hanging wall down-to-the-east ~ 70° and formed the non-marine Tecomate basin, deposited across a ~ 20° angular unconformity. From 7.1–6.4 Ma, the hanging wall tilted at 35 ± 5°/Myr, while non-marine sandstone and conglomerate accumulated at 1.4 ± 0.2 mm/yr. At least 1.8 ± 0.1 km of sediments and pyroclastic deposits accumulated in the Tecomate basin concurrent with clockwise vertical-axis block rotation and ~ 2.8 km of total dip-slip motion on the Kunkaak fault. Linear extrapolation of tilting and sedimentation rates suggests that faulting and basin deposition initiated ~ 7.6–7.4 Ma, but an older history involving initially slower rates is permissible. 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. The Coastal Sonora fault zone on mainland Sonora, which accrued several tens of kilometers of late Miocene dextral offset, continues to the northwest, across northeastern Isla Tiburón and offshore into the Gulf of California. The establishment of rapid, latest Miocene transtension in the Coastal Sonora fault zone was synchronous with the 8–7 Ma onset of transform faulting and basin formation along the nascent Pacific-North America plate boundary throughout northwestern Mexico and southern California. Plate boundary strain localized into this Gulf of California shear zone, a narrow transtensional belt that subsequently hosted the marine incursion and continental rupture in the Gulf of California. [ABSTRACT FROM AUTHOR]

The Gulf of California rift has accommodated oblique divergence of the Pacific and North America plates in northwestern México since Miocene time. Due to its infancy, its rifted margins preserve a rare onshore record of early continental break-up processes and an opportunity to investigate the role of rift obliquity in strain localization. We map rift-related structures and syn -tectonic basins on southern Isla Tiburón, a proximal onshore exposure of the rifted North America margin. We integrate analysis and geochronology of syn -tectonic sedimentary basins and mapping of crosscutting relationships to characterize the style and timing of fault activity. On southern Isla Tiburón, an early phase of extension initiated between ~ 19–17 Ma and ~ 12.2 Ma. Subsequently, these normal faults and related basins were cut by the La Cruz strike-slip fault and buried by deposits of the La Cruz basin, an elongate, fault-controlled trough coextensive with the La Cruz fault. Crosscutting relationships show that the NW-striking La Cruz fault accrued 5 ± 2 km of dextral slip ~ 8–4 Ma. The La Cruz fault and parallel Tiburón transform were kinematically linked to detachment faulting that accommodated latest Miocene to Pliocene oblique opening of the offshore Upper Tiburón pull-apart basin. The onset of strike-slip faulting on Isla Tiburón was synchronous with the ~ 8–6 Ma onset of transform faulting and basin formation along > 1000 km of the reconstructed Pacific-North America plate boundary. This transition coincides with the commencement of a clockwise azimuthal shift in Pacific-North America relative plate motion that increased the obliquity of the Gulf of California rift and formed the Gulf of California shear zone. The record from the proto-Gulf of California illustrates how highly oblique rift geometries, where transform faults are kinematically linked to pull-apart basins, enhance the ability of continental lithosphere to rupture and, ultimately, hasten the formation of new oceanic rift basins. [ABSTRACT FROM AUTHOR]

Accurate information on the timing of earliest marine incursion into the Gulf of California (northwestern México) is critical for paleogeographic models and for understanding the spatial and temporal evolution of strain accommodation across the obliquely divergent Pacific-North America plate boundary. Marine strata exposed on southwest Isla Tiburón (SWIT) have been cited as evidence for a middle Miocene marine incursion into the Gulf of California at least 7 m.y. prior to plate boundary localization ca. 6 Ma. A middle Miocene interpretation for SWIT marine deposits has played a large role in subsequent interpretations of regional tectonics and rift evolution, the ages of marine basins containing similar fossil assemblages along ~1300 km of the plate boundary, and the timing of marine incursion into the Gulf of California. We report new detailed geologic mapping and geochronologic data from the SWIT basin, an elongate sedimentary basin associated with deformation along the dextral-oblique La Cruz fault. We integrate these results with previously published biostratigraphic and geochronologic data to bracket the age of marine deposits in the SWIT basin and show that they have a total maximum thickness of ~300 m. The 6.44 ± 0.05 Ma (Ar/Ar) tuff of Hast Pitzcal is an ash-flow tuff stratigraphically below the oldest marine strata, and the 6.01 ± 0.20 Ma (U/Pb) tuff of Oyster Amphitheater, also an ash-flow tuff, is interbedded with marine conglomerate near the base of the marine section. A dike-fed rhyodacite lava flow that caps all marine strata yields ages of 3.51 ± 0.05 Ma (Ar/Ar) and 4.13 ± 0.09 Ma (U/Pb) from the base of the flow, consistent with previously reported ages of 4.16 ± 1.81 Ma (K-Ar) from the flow top and (K-Ar) 3.7 ± 0.9 Ma from the feeder dike. Our new results confirm a latest Miocene to early Pliocene age for the SWIT marine basin, consistent with previously documented latest Miocene to early Pliocene (ca. 6.2-4.3 Ma) planktonic and benthic foraminifera from this section. Results from biostratigraphy and geochronology thus constrain earliest marine deposition on SWIT to ca. 6.2 ± 0.2 Ma, coincident with a regional-scale latest Miocene marine incursion into the northern proto-Gulf of California. This regional marine incursion flooded the northernmost, >500-km-long portion of the Gulf of California shear zone, a narrow belt of localized strike-slip faulting, clockwise block rotation, and subsiding pull-apart basins. Oblique Pacific-North America relative plate motion gradually localized in the >1000-km-long Gulf of California shear zone ca. 9-6 Ma, subsequently permitting the punctuated south to north flooding of the incipient Gulf of California seaway. [ABSTRACT FROM AUTHOR]