LATE CENOZOIC TECTONICS OF THE EAST VENTURA BASIN, TRANSVERSE RANGES,CALIFORNIA

The east Ventura basin originated in the middle Miocene as a rift syst em bounded on one side by the Oak Ridge-Simi Hills structural shelf an d on the other side by a granitic ridge parallel to the San Gabriel fa ult. This fault began accumulating right slip 10-12 m.y. ago at a rate of 4.5-9 mm/yr (depending on whether total slip is 45 or 60 km), slow ing to about 1 mm/yr in the Quaternary. North of the Santa Clara River , rifting ended prior to deposition of the uppermost Miocene-lower Pli ocene Towsley Formation. South of the Santa Clara River, the rift axis shifted southwest toward the Oak Ridge-Simi Hills shelf as the Towsle y Formation accumulated against a normal-fault ancestor of the Santa S usana fault. A change to contractile tectonics occurred in the Pliocen e with deposition of the Fernando Formation, when the Newhall-Potrero anticline developed as a monocline above a blind reverse fault; the Pi co anticline to the southeast and the Temescal and Hopper Ranch-Modelo anticlines to the northwest may have a similar origin. Tectonic inver sion and displacement on the southwest-verging Santa Susana fault bega n about 0.5 Ma based on appearance of locally-derived clasts in the up per Saugus Formation and its equivalents, and continues today, along w ith the southwest-verging San Cayetano fault farther west. Also active are northeast-verging backthrusts occurring in the east Ventura basin fold belt, and a segment of the San Gabriel fault which now acts as a northeast-dipping oblique-slip reverse fault. Northeast-trending disc ontinuities and structures divide the present deformation zone into fo ur segments. In the Hopper Canyon segment at the west end of the area in the west Ventura basin, the San Cayetano fault places Miocene Model o Formation over Pliocene-Pleistocene strata more than 5 km thick, lar gely at maximum burial. To the southeast, the Newhall-Potrero segment is characterized by north-vergent backthrusts within the east Ventura fold belt and by southward thrusting of the basin sequence (tectonic i nversion) over the structural shelf on the Santa Susana fault. Farther southeast, the Placerita segment is marked by reverse faulting on bot h the Santa Susana fault and the San Gabriel fault. Southeast of the b asin in the San Fernando Valley, the Sylmar segment contains a thick P liocene-Pleistocene sequence overridden by basement rocks of the San G abriel Mountains as well as the south-verging Mission Hills-Granada Hi lls and Northridge Hills fault zones.