GENERAL GEOLOGIC HISTORY OF ROCKS ALONG THE LARSE II SEISMIC LINE

The LARSE II seismic line in Los Angeles County, California traverses three geologic provinces. Rocks in the northern province (Antelope Valley) were deposited far to the north in nonmarine environments during the late Tertiary and were brought into juxtaposition with the middle province by 315 km of right slip along the San Andreas fault, most of the slip occurring since 5 Ma.

The middle province (San Gabriel Mountains/Santa Clara Valley/Sierra Pelona) contains Precambrian and Mesozoic metamorphic and plutonic basement rocks. Cretaceous through Eocene sedimentary deposition occurred in a forearc basin, late Oligocene through medial Miocene volcanism and deposition in the DiSoCuMa depositional basin, late Miocene deposition in the Soledad and Ventura depositional basins, and Pliocene and early Pleistocene deposition in the Ridge and Ventura depositional basins. Miocene clockwise rotation in this province averages 37°. The southern boundary of the middle province, the San Gabriel fault zone, has sustained 60 km of right slip, which brought the middle and southern provinces into final juxtaposition about 5 Ma.

In the southern province (Santa Monica Mountains/San Fernando Valley/Santa Susana Mountains), Upper Cretaceous through lower Oligocene forearc basin deposits unconformably overlie Upper Jurassic Santa Monica Formation that was intruded by Cretaceous plutons. Following early Oligocene uplift and basin-wide erosion, deposition was renewed during the late Oligocene through early Miocene in the AnaVent depositional basin. Initiation of transrotation and extension in the medial Miocene brought subsidence, widespread volcanism, and deep-water sedimentation to the AnaVent basin. Continued transrotation and extension created the Ventura depositional basin at the north end of the AnaVent basin, and deposition continued here through the early Pleistocene. The amount of clockwise rotation in this province varies from ~56° in the east to ~77° in the west. The difference in amount of rotation between the middle and southern provinces is because the San Gabriel fault was originally curved and as the fault moved laterally, the amount of rotation increased to the west.