SAN FERNANDO VALLEY AND EAST VENTURA BASIN: INTRODUCTION AND OVERVIEW

These basins are superimposed on varied basement terranes (San Gabriel, Verdugo, Santa Monica Mountains) and Upper Cretaceous-Paleogene sequences that responded to a different paleogeographic framework. The Miocene is marked by extension, rotation, and volcanism; major features include a volcanic complex in the western Santa Monica Mountains, a NW-SE rift in the east Ventura basin, and the SE-facing Chatsworth normal faults at the W edge of the San Fernando Valley with more than 2.5 km of separation. Another normal fault near the San Diego Freeway brings granitic rocks against Paleogene strata in the western San Fernando Valley. The San Gabriel fault accumulated most of its 60 km right slip during Mint Canyon, Modelo, and early Castaic deposition; the subsurface Castaic and younger strata correlate across the fault. The Simi Hills-Oak Ridge block diverted the late Miocene Tarzana fan around its southeastern edge (Chatsworth faults) and northern edge (ancestral Santa Susana and Oak Ridge faults). The modern basin framework began with Fernando deposition based on contrasts between the Towsley Fm., which stopped at the ancestral Santa Susana boundary fault, and the Fernando, which stopped farther south at the Frew fault in the Santa Susana footwall. The Fernando and Saugus follow an east-west configuration with a saddle at San Fernando Pass; the thickest deposits are in the Sylmar basin north of Mission Hills and in the Ventura basin from Piru to the sea. The Pico, Newhall-Potrero-Del Valle, and Holser anticlines were south-facing homoclines during Fernando deposition probably underlain by NNE-dipping blind thrusts. The active Verdugo-Mission Hills and Santa Susana faults began as Miocene extensional structures that were reactivated with an opposite sense of slip; they have thicker deposits in their hangingwalls than their footwalls. The Oak Ridge and San Cayetano faults may have had a similar origin. The south-side-up Holser and Del Valle faults reactivated the Fernando homoclines but in the opposite sense so that Fernando thicknesses are greatest in their hangingwalls. Reversals in structural vergence may account for the opposing vergences of the 1971 and 1994 earthquake faults.