Abstract

Agua Blanca fault is a major right-handed strike-slip fault at least 80 miles in length that cuts transversely across the peninsula of Baja California about 70 miles south of the international border. Its trend is anomalous in being more nearly parallel to the Transverse Ranges of southern California than to the San Andres fault system that elsewhere dominates the tectonic grain of the peninsula. Geographic features delineating the fault trace are, from east to west: Paso San Matías, Valle de La Trinidad, Cañon de Dolores, Valle de Agua Blanca, Valle de Santo Tomás, Bahía Soledad (south branch of fault), Punta Banda (north branch). Farther west, both branches of the fault control submarine topography, and possibly the fault system is continuous with the northwest-trending San Clemente fault off the southern California coast.

Physiographic expression of the Agua Blanca fault is remarkably similar to that of the San Andreas. Typical features are Recent scarps, offset streams, shutterridges, fault sags and saddles, side-hill ridges, and fault-controlled valleys. Most of these features are particularly well exhibited in Valle de Agua Blanca, which is designated as the type locality.

Rocks cut by the fault are mainly Cretaceous plutonic rocks of the southern California batholith and Lower Cretaceous (Albian) metavolcanic rocks. Along the Pacific Coast, the fault cuts Upper Cretaceous (Maestrichtian) post-batholithic sedimentary rocks that are otherwise surprisingly little deformed as compared to rocks of similar age in most of California.

Agua Blanca fault shows a history of right-lateral displacement throughout its length: Recent stream offsets occur from Valle de La Trinidad nearly to the Pacific Ocean; distinctive Quaternary(?) fan gravels in Valle de Agua Blanca are offset laterally 3 miles from their most likely source area across the fault; in the same area, a fault slice of distinctive antiperthitic granodiorite is best explained by 7 miles of lateral displacement, and a nearby slice of quartz diorite may indicate displacement as great as 14 miles. In general, evidence of both Recent activity and amount of total displacement appear to increase westward. The fault coincides in gross aspect with a broad east-west zone of seismic activity in a region elsewhere characterized by relative quiescence, but no large historical earthquakes can be positively correlated with this fault.

Despite its orientation athwart the regional tectonic grain, Agua Blanca fault does not appear to represent a deep-seated structural feature analogous to those of the Transverse Ranges. Instead, it is probably one of several paths by which the San Andreas fault tends to break around the “knot” caused by the great bend of the San Andreas in southern California.

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