COMPARISON OF LATE CENOZOIC STRUCTURES ACROSS THE SOUTHERN GULF OF CALIFORNIA

There have been only cursory examinations of the match of late Cenozoic structures across the southern Gulf of California. The database on both sides of the gulf has grown considerably in the past few years. Here I compare the two sides of the Gulf of California using two models for reconstruction of the southern gulf. The comparison involves the regions from Loreto to Los Cabos on the southern Baja California peninsula and from Los Mochis to Tepic on mainland Mexico. Model 1 (after Stock and Hodges, 1989) involves closure of the gulf by 300 km, results in an embayment at the mouth of the gulf, and does not account for proto-gulf extension. Model 2 (after Nieto-Samaniego et al, 1999) puts the southern tip of Baja California against Tepic and has no embayment at the mouth of the gulf. If proto-gulf extension was E-W, then 150-200 km of extension would modify Model 1 to closely match Model 2. If proto-gulf extension was ENE-WSW directed as most data suggest, then one can only produce Model 2 from Model 1 with ~100 km greater strike-slip motion than the 300 km normally assumed. On the Baja California peninsula, there may be four rift segments that alternate in dip direction of major faults. On the mainland, there appear to be three segments near Tepic, Mazatlan, and Culiacan, and a complex region near Los Mochis. Model 1 suggests that the La Paz and Mazatlan segments match and both have E-dipping faults; the Timbabichi and Loreto segments match across to the Culiacan segment, with dominantly E-dipping fault directions. Model 2 suggests that the La Paz - Tepic and Timbabichi - Mazatlan segments match with opposite dipping faults; the Loreto/Mulege segment lies against the Culiacan segment with both E and W dipping faults. In summary, neither model produces a simple pattern of faults and rift segments across the Gulf of California.