SAN JOSÉ ISLAND ACCOMMODATION ZONE, BAJA CALIFORNIA SUR, MEXICO: IMPLICATIONS FOR RIFT SEGMENTATION AND STRAIN PARTITIONING IN THE SOUTHERN GULF OF CALIFORNIA

The Gulf of California is an active, oblique-divergent plate boundary. In the southern Gulf of California, a two-stage evolution took place wherein orthogonal rifting from ~12 to ~6 Ma was followed by overprinting from 6 to 0 Ma by the highly oblique-divergent, modern plate boundary. Recent findings confirmed the existence of ~50 to 100 km long rift segments, separated by accommodation zones, along the eastern side of the southern Baja California peninsula. The San José Island accommodation zone is the best-studied, and perhaps a representative example of, accommodation zones along the western edge of the southern Gulf of California. It is located approximately 70 km north-northwest of La Paz and separates the Timbabichi rift segment to the north from the La Paz rift segment to the south. The Timbabichi segment is approximately 50 km long, and the La Paz rift segment, characterized by an arcuate east-dipping master fault system, is approximately 100 km long. The San José Island accommodation zone is 45 km long, making it unusually long relative to the lengths of the adjacent rift segments. Rather than the topographic low expected between rift segments, the accommodation zone, including San José and San Francisquito Islands, is a relative topographic high and creates a salient prominently extending into the Gulf of California. Erosional retreat of the rift escarpment away from the main faults has occurred in the rift segments, but not in the accommodation zone. In contrast to the rift segments, the accommodation zone is characterized by complexly tilted strata and alternating sets of dominantly northeast- and northwest-striking normal faults with offsets ranging from 10’s to a few 100’s of meters. Kinematic analysis suggests a greater degree of overprinting by transtensional structures than south of the accommodation zone. Thus, the topographic expression of the accommodation zone may be due to Pliocene-Quaternary faulting that overprints mid- to late Miocene faulting. Recent earthquake and fault data in the southernmost Gulf of California suggest a high degree of strike-slip partitioning in the La Paz segment and no partitioning in the Loreto segment. The San José Island accommodation zone likely represents the transition between the areas with contrasting amounts of strain partitioning in the southern Gulf of California.