Lateral Shortening in the Transverse Ranges Minimizes the Roles of the San Gabriel and San Andreas as Strike-Slip Faults
R. H. Paschall
Regional and detailed surface and subsurface geology by several authors indicate that the San Gabriel fault does not meet the San Andreas at its northwest and southeast ends. Also, the Castaic and Mint Canyon formations compose a strati-structural unit (i.e., the Mint Canyon cannot have been displaced without the Castaic also being displaced). Consequently, claims of 50–80 km of strike-slip displacement based on correlations in the “Soledad” Basin and Lockwood Valley are invalid, because no Castaic Formation occurs in the latter.
Hypotheses proposing displacement of the San Gabriel Mountains and “Soledad” Basin formations to the Orocopia Mountains require movement first on the San Gabriel fault and later on the San Andreas. Because these faults were never connected, and horizontal displacement, if any, on the San Gabriel is minimal, those hypotheses are without foundation. The dominant tectonics in the “Big Bend” region of the San Andreas has been lateral shortening of ~50%, which has produced several mountain ranges, thrust faults, folds at oil fields from Elwood to Castaic Junction, and the San Fernando, Whittier, and Northridge earthquakes. Also, anorthosite clasts in Oligocene to Miocene formations in the area demonstrate periodic uplift of the San Gabriel Mountains, which sourced oil-productive Upper Miocene and Pliocene sediments in the Los Angeles and Ventura basins. Furthermore, the Ventura basin has no continental margin unless it is the “Soledad” Basin. One oddity, Oligocene redbeds called “Vasquez” lie only 13 km east of similar beds called “Sespe” up Piru Creek, yet rocks 52 km south of Piru Creek are called “Sespe.” The San Gabriel fault has thus posed a psychological barrier to logical thought for 75 years.
AAPG Search and Discovery Article #90904©2001 AAPG Pacific Section Meeting, Universal City, California