Geometry and evolution of rift-margin, normal-fault-bounded basins from gravity and geology, La Paz-Los Cabos region, Baja California Sur, Mexico

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 reflect the two main east-dipping 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 basin-bounding 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.