REGIONAL PLATE MOTION PARTITIONING IN THE SOUTHERN GULF OF CALIFORNIA: EVIDENCE FROM ISLA ESPIRITU SANTO, BAJA CALIFORNIA SUR, MEXICO

The southern Baja California peninsula, Mexico, lies along an oblique-divergent plate boundary, which occupies the Gulf of California. Plate motion partitioning has been recognized as an important process in many oblique plate margins, but is still poorly understood in oblique-divergent settings. Patterns of active faulting and modern seismicity along the southern portion of the Baja California plate boundary suggest strain partitioning on a regional scale. Fault plane solutions for earthquakes along transform faults in the deep gulf indicate strike-slip motion. Conversely, large earthquakes within the plate margin indicate normal motion. Therefore, east-west extensional faulting is located in the margin along Baja California, while northwest-directed transform motion occurs along the plate boundary in the Gulf of California.

Based on the above criteria, the Baja California peninsula may be divided into three domains. The southern domain, from ~25°N latitude to the southern tip of the peninsula, includes the study area and is characterized by an array of multiple normal faults that cut across the narrow marine shelf and peninsula and have facilitated several moderate to large earthquakes this century.

Mapping on Isla Espirítu Santo has demonstrated that major faults trend N-NNW and are steeply dipping (70°-90°). These faults accommodated tens to a few tens (rarely 100's) of meters of offset. Total displacement accommodated by these faults is ~300 m and dominantly dip slip, with fault plane straie raking between 60°-120°. This confirms the idea that extensional faulting appears to be located along the margin of the Baja California peninsula.