Late Cenozoic Tectonics of the East Ventura Basin, Transverse Ranges, California

The east Ventura basin originated in the middle Miocene as a rift system bounded on one side by the Oak Ridge-Simi Hills structural shelf and on the other side by a granitic ridge parallel to the San Gabriel fault. 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), slowing to about 1 mm/yr in the Quaternary. North of the Santa Clara River, rifting ended prior to deposition of the uppermost Miocene-lower Pliocene Towsley Formation. South of the Santa Clara River, the rift axis shifted southwest toward the Oak Ridge-Simi Hills shelf as the Towsley Formation accumulated against a normal-fault ancestor of the Santa Susana fault. A change to contractile tectonics occurred in the Pliocene with depos tion of the Fernando Formation, when the Newhall-Potrero anticline developed as a monocline above a blind reverse fault; the Pico anticline to the southeast and the Temescal and Hopper Ranch-Modelo anticlines to the northwest may have a similar origin. Tectonic inversion and displacement on the southwest-verging Santa Susana fault began about 0.5 Ma based on appearance of locally-derived clasts in the upper Saugus Formation and its equivalents, and continues today, along with 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 discontinuities and structures divide the present deformation zone into four segments. In the Hopper Canyon segment at the west end of the area in the west Ventura basin, the San Cayetano fault places Miocene Modelo Formation over Pliocene-Pleistocene strata more than 5 km thick, largely 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 inversion) over the structural shelf on the Santa Susana fault. Farther southeast, the Placerita segment is marked by reverse faulting on both the Santa Susana fault and the San Gabriel fault. Southeast of the basin in the San Fernando Valley, the Sylmar segment contains a thick Pli cene-Pleistocene sequence overridden by basement rocks of the San Gabriel Mountains as well as the south-verging Mission Hills-Granada Hills and Northridge Hills fault zones.

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