Abstract: Structural Geology in the Western Part of the Los Angeles 1:100,000-scale Sheet

R. H. Campbell, R. F. Yerkes

Structural elements involved in the Neogene evolution of the western Transverse Ranges are emphasized by a reduced-scale digital mosaic of 16 7.5-min. quadrangles that make up the west half of the Los Angeles 1:100,000 sheet. East-west trending, north-side-down tilted mountain blocks (Santa Monica Mountains, Simi Hills, Oak Ridge-Santa Susana Mountains) form upland areas from Santa Monica Bay on the south to the Santa Clara River valley on the north; intervening valleys include San Fernando, Conejo, and Simi.

North-dipping Cretaceous and/or lower Paleogene strata underlie the south flanks of the tilted blocks; upper Paleogene and Miocene strata predominate on the north flanks. The blocks are lapped unconformably by middle Miocene volcanic rocks and Miocene to Pliocene marine strata. Subsequently, they have been further deformed by north-over-south thrusting, especially in the Santa Susana Mountains and along the south flank of the Santa Monica Mountains.

Middle Miocene block faulting (tilting), submarine basaltic volcanism, widespread detachment faulting, and large strike-slip displacement on a high-angle ancestor of the Malibu Coast fault are all compatible with a middle Miocene episode of crustal extension contemporaneous with initial formation of the Los Angeles

and Ventura basins. The present episode of regional north-south compression, as indicated by recorded seismicity, measured geodetic changes, and field evidence for Quaternary thrust deformation at and near some margins of the older structural blocks, probably began in the late Pliocene.

AAPG Search and Discovery Article #90981©1994 AAPG Pacific Section Meeting, Ventura, California, April 27-29, 1994