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Description
The rifting of Baja California away from mainland Mexico began in the late MiocenePliocene when the Pacific-North American plate boundary moved east into the Gulf of California. This plate boundary jump attached the Baja California Peninsula to the Pacific Plate and initiated opening of the Gulf The patterns of faulting, basement denudation, and rift basin evolution preserved on both sides of the Gulf represent the primary record of rifting and are the basis for evaluating different dynamical models for rifting. [sla Cerralvo is located in the lower Gulf of California region just east of La Paz on the Baja California part of the rift margin. It represents the last exposure of land at the edge of the Gulf rift. The island is dominated by Cretaceous granodiorite plutons intruded into quartzofeldspathic schist. Cenozoic normal faults bound the west side of the island and strike ~N20W parallel to the coastline. In the west-central part of the island a down-to-the-west normal fault (La Gringa Fault) was mapped which places a Pliocene(?) fossiliferous sandstonelimestone hanging wall sequence on footwall granodiorite and amphibolite facies schist. Along the southwest coast of the island a higher angle normal fault (Punta Sur fault) paralleling the coast places Pliocene(?) limestone/sandstone terrace deposits and underlying greenschist facies metasedimentary rock against amphibolite facies schist and metaplutonic rock. Recent apatite fission track thermochronology from Isla Cerralvo (Kohn et al., in preparation) yield young cooling ages from Cretaceous basement on the west side of the island ( ~ 7-8 Ma) and much older ages from the east side of the island (~54 Ma). The apatite data ret1ects a strong contrast in denudation across the island and are consistent with eastwards block tilting of the island in conjunction with mid-Miocene footwa11 uplift along the La Gringa Fault. Strike and dip of foliation in schist and metaplutonic rock in footwall rocks along the west side of the island consistently strike ~N20°W parallel to the trend of Cenozoic faulting suggesting transposition of fabrics. The thermochronology and structural data suggest that down-to-the-west normal faulting on the west side ofisla Cerralvo may have accommodated substantial dip slip separation. The La Gringa and Punta Sur faults on Isla Cerralvo may correlate to west vergent normal faults along the west side of the Sierra La Gata on mainland Baja across the Cerralvo channel to the south. The La Gringa, Punta Sur and La Gata thus may comprise a single integrated west vergent fault system. The west vergent character of this system contrasts with down-to-the-east vergence along the San Jose del Cabo fault farther south which accommodated ~5-7 kilometers of dip slip separation. The regional pattern of normal faulting along this segment of the Baja California rift margin is consistent with along strike vergence reversal of structures, as documented in the East African Rift zone extensional province and elsewhere.
