Geometry and quaternary slip behavior of the San Juan de los Planes and Saltito fault zones, Baja California Sur, Mexico: Characterization of rift-margin normal faults

An array of north-striking, left-stepping, active normal faults cuts the southwest margin of the Gulf of California and across the southern tip of the Baja California peninsula. This is the gulf margin fault system of the oblique-divergent plate boundary within the Gulf of California. Detailed geologic and geomorphic mapping along the onshore San Juan de los Planes and Saltito fault zones allowed us to delineate geometric sections and to infer the tectonic history of thefault zones. To achieve a more complete understanding of these individual normal faults within a larger array, we mappedfaults to ̃10 km offshore using seismic CHIRP (compressed highintensity radar pulse) profiling. Both onshore faults slip at a low rate and have a low total offset. Along the San Juan de los Planes fault zone, which is entirely onshore, theyoung, scarp-forming fault reactivated older faults to rupture a broad, low-relief pediment surface with thin Quaternary cover, reflecting a two-stage slip history along this fault zone. The offshore study suggests a northward continuation of the onshore Saltito fault, and a complex fault array north of the La Gata fault on the east side of the San Juan delos Planes basin extending northward to the west Cerralvo fault. Our results suggest relatively low rates of active faulting of <1 mm/yr across the San Juan de los Planes system of faults compared to high rates on the active gulf-axis system,and relatively higher rates on earlier Neogene gulf margin faults in other areas along the southwest Gulf of California margin.