TECTONOSTRATIGRAPHY OF THE SANTA MONICA MOUNTAINS, SOUTHERN CALIFORNIA

Southern California has experienced the following stages of development, which are recorded in corresponding tectonostratigraphic sequences in the Santa Monica Mountains: Late Jurassic remnant-ocean basin (Santa Monica Slate); Cretaceous continental-margin dissected arc (quartz diorite of Hollywood Hills); Cretaceous-Eocene forearc basin (Tuna Canyon/Chatsworth-Santa Susana Fms.); Oligocene-Early Miocene forearc basin and triple-junction transition (Sespe-Vaqueros Fms.); 18-12 Ma Transrotational basin (Topanga Fm.); 12-6 Ma Transtensional basin (Modelo Fm.); and 6-0 Ma Transpressional basin (Fernando Fm.). Transrotation of the Western Transverse Ranges (WTR) occurred as they were transferred to the Pacific plate during capture of the Monterey and Arguello mircoplates. Extension along low-angle detachment faults exhumed mid- and lower-crustal rocks (accretionary prism) in the continental borderland. The magnitude of extension decreased toward the northeastern pivot point; thus, the Middle Miocene Topanga basin deepened to the south and west as the WTR rotated. The Santa Ynez Canyon transfer zone (SYCTZ) separated the highly extended western Santa Monica Mountains (WSMM) from the slightly extended eastern Santa Monica Mountains (ESMM). The breakaway zone, along which the rotating Western Transverse Ranges separated from the northern Peninsular Ranges, formed along the south side of the ESMM and the north side of the WSMM, with the SYCTZ connecting the two segments of the breakaway. As a result, all of the Sespe-Vaqueros Fms. and much of the Santa Susana Fm. were eroded from the ESMM due to footwall uplift prior to deposition of the Topanga Fm., in contrast to the WSMM. North of the WSMM, the Cretaceous-Paleogene sequence was uplifted in the footwall of the breakaway to form the Chatsworth Hills, in contrast to north of the ESMM, where this forearc sequence remains buried below the San Fernando Valley. Transpression since 6 Ma has uplifted the SMM along north-dipping thrusts.