Abstract Detailed marine geophysical surveys of the inner California Continental Borderland west of northern Baja California show that the region is underlain by two major northwest-trending Quaternary dextral wrench fault systems. The San Clemente fault system lies along the western part of the inner borderland and comprises the San Clemente and San Isidro fault zones. Together, these fault zones connect to form a long (>300 km), narrow (<5 to 10 km), continuous zone of shear similar to the longer San Andreas transform system onshore. The Agua Blanca fault system is a complex northwest-trending zone of dextral shear delineated by three or more subparallel wrench fault zones in the eastern part of the inner borderland. The westernmost, San Diego Trough-Bahia Soledad fault zone, consists of relatively long (-50 km), continuous main fault traces which cut the Quaternary sediments of the nearshore basin trough. The Coronado Bank-Agua Blanca fault zone is more complicated, with numerous discontinuous, subparallel, right- and-left-stepping, en echelon and anastomosing fault traces which are associated with substantial structural relief. A nearshore zone of faulting, marked by the Estero-Descanso fault zone in the south and the Newport-Inglewood-Rose Canyon fault zone in the north, parallels the coast and defines the eastern boundary of the California Continental Borderland structural province. All of these eastern fault zones merge into the transpeninsular Agua Blanca fault, and their N30°W trend differs significantly (>20°) from the trend of the major Peninsular Ranges fault zones. The ridge-and-basin physiography of the inner borderland and transtension evident along the Agua Blanca fault system show that dextral oblique rifting has dominated the late Cenozoic tectonic style of the inner California Continental Borderland. Systematic differences in the post-Miocene style of deformation, with extension in the south, right-slip in the center, and convergence in the north, imply that Transverse Ranges convergence is affecting inner borderland tectonism. Historical earthquake activity shows that the inner borderland is an active part of the present-day Pacific-North American plate boundary, with focal mechanisms generally consistent with the tectonism inferred from the geologic structure. Counterclockwise rotation of a semirigid “Southern California Shear Zone,” which is the splintered northern end of the long, narrow, Baja California microplate, explains the observed patterns of deformation within the region. Because significant right-slip may occur to the west of Baja California along the San Clemente fault system, and does not cross the peninsula to the Gulf of California transform system, estimates of Pacific-North American relative plate motion based on sea-floor-spreading rates at the mouth of the Gulf of California may be in error.

Abstract Sea Beam provides a new tool for accurately mapping sea-floor morphology. Recent maps compiled from Sea Beam data collected in the California Continental Borderland show many sea-floor geomorphic features that are similar to tectonic landforms observed along subaerial fault zones. In particular, numerous features associated with recently active strike-slip faults have been mapped along the offshore San Clemente-San Isidro, San Diego Trough-Bahia Soledad, and Coronado Bank-Agua Blanca fault zones. Linear scarps, trenches and ridges, hillside benches and valleys, and small closed depressions are aligned along what are inferred to be active traces of these major offshore fault zones. Other nontectonic sea-floor geomorphic features observed include submarine canyons and channels; these may be truncated, offset, or deviated by tectonic movements in the area. Detailed study of such features may provide data necessary to determine the displacement and slip-rates on submarine faults.

Abstract This chapter summarizes the results of the writer’s work on the paleogeography and facies changes of the Monterrey Formation in the San Hilario area of Baja California, as part of the exploration for phosphate rock prospects carried out in the area by the Mineral Resources Council (Consejo de Recursos Minerales). Field work from March 1975 to January 1976 covered detailed mapping on l:10,000-scale vertical airphotos of the Monterrey Formation outcrop belt, an area 70 km long and 4 to 5 km wide, together with the upper and lower portions of the respectively under- and overlying Tepetates and San Isidro Formations. Additional information includes six cross sections along the principal stream beds, eight lithologic columns (scale 1:100), and a stratigraphic correlation table of these columns (1:2,000). As an aid to correlation and future detailed geological work, a collection of Polaroid photographs describes virtually all the Monterrey outcrops exposed at the streambeds, focusing on the section’s geoeconomic features. Part of this material is included here.