The southern end of the Baja California peninsula is cut by a north-striking, left-stepping, active, normal-fault system-the marginal fault system of the oblique-divergent plate boundary within the Gulf of California. We conducted gravity surveys across the normal-fault-bounded basins, and, along with optically stimulated luminescence dating of offset piedmont surfaces and geologic data, we estimated fault-slip rates and assessed fault patterns across basins, gaining insight into basin evolution to better understand the role of upper-crustal processes during development of an obliquely rifted plate margin. Gravity surveys across the La Paz, San Juan de los Planes, and San José del Cabo basins revealed basin depths ranging from ~500 to 3000 m. The La Paz basin is a half graben with two smaller basins that refl ect the two main eastdipping splays of the Carrizal fault. Within the San Juan de los Planes and San José del Cabo basins, there are buried faults, indicating that during the early stages of basin formation, strain was distributed across these smaller intrabasin faults prior to development of the basinbounding faults. Slip rates coupled with basin depths suggest that the La Paz and San Juan de los Planes basins began forming ca. 2-5 Ma, overlapping in time with the formation of the main plate boundary at this latitude. The San José del Cabo basin has the greatest depth to bedrock (1.6-2.7 km), signifying that it accommodates a greater slip rate or a longer duration of slip than the other faults within this system.
The southwest margin of the Gulf of California has an array of active normal faults despite this being an oblique-divergent plate boundary with spreading centers that localized deformation along the plate boundary 2-3 million years ago. The Carrizal and Centenario faults form the western border fault of the Gulf of California marginal fault system within and south of La Paz Bay, and ~20-30 km west of the capital city of La Paz, Baja California Sur, Mexico. Geologic and geomorphic mapping, optically stimulated luminescence (OSL) geochronology, and paleoseismic investigations onshore, compressed high-intensity radar pulse (CHIRP) profi ling offshore, and analysis of uplifted marine terraces in the footwall of the offshore Carrizal fault provide some of the fi rst numerical and geometrical constraints on late Pleistocene-Holocene faulting along the Carrizal fault. The onshore Carrizal fault has ruptured with up to ~1-2 m of vertical displacement per event, likely producing ~M 6.3-6.9 earthquakes, and at least two to three surface rupturing earthquakes have occurred since 22 ka. Onshore paleoseismic excavations and uplifted marine terraces on the western side of La Paz Bay both suggest offset rates of 0.1-0.2 mm/yr, with a footwall uplift rate of 0.13 mm/yr since 128 ka, and an approximately constant rate since marine oxygen-isotope stage (MIS) 11 terraces (420 ka). A CHIRP survey identifi ed underwater fault scarps with heights ranging from 21 to 86 m on the Carrizal fault in La Paz Bay and from 3 to 5 m along the Centenario fault. The offshore Carrizal fault lies 8-10 km east of the western edge of La Paz Bay, forming a right step from the onshore Carrizal fault. The offshore Carrizal fault is the oldest fault of the fault system, and the fault likely grew in the latest Miocene to Pliocene in a complex way to the south toward the onshore Centenario and Carrizal faults. When the Alarcon spreading center started its modern rates at 2.4 Ma, the Carrizal fault likely slowed to the 0.1-0.2 mm/yr rates of the late Quaternary determined in this study.
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