Abstract

The geologic history of the Santa Monica Mountains-Los Angeles basin area indicates a sequence of changing regional stresses that is in remarkable agreement with those predicted by a constant-motion plate-tectonic model (Atwater, 1970) relating movements along the San Andreas fault to the Cenozoic history of sea floor spreading in the northeastern Pacific. Relatively quiescent conditions in early Tertiary were disrupted beginning in middle Miocene time. The style of that deformation indicates a dominantly extensional stress field, with important north-south and northeast-southwest components, manifested by: a) outbreak of basaltic volcanism, b) uplift of the east-trending Simi Hills and associated detachment faulting emplacing thrust sheets in the area to the south, now occupied by the Santa Monica Mountains, and c) major left-lateral slip on the Malibu Coast fault and its continuation to the east. The beginning of volcanism (at about 16 my ago in the Santa Monica Mountains) apparently marks the inception of the middle Miocene deformation. This correlates very closely with the inferred beginning of renewed movement on the northern San Andreas fault as inferred from the data plot of Nilsen and Link (1975) and follows closely after the subduction of the trailing (western) edge of the Farallon plate at the continental margin between the Mendocino and Murray fracture zones about 21 my ago. The renewal of movement on the San Andreas marks the attachment of a triangular piece of the North American plate, bounded on the northeast by the San Andreas fault on the west by the continental rise, and on the south by old east-west structures approximating the southern end of the Salinian block. Interplate right-lateral slip, previously accomodated along the western margin of the continent, was then transferred, at least partly, to the northern San Andreas fault while to the north and south of the triangle, it continued to be localized along the continental margin. The geometric results of continued right slip along this interplate boundary included tension and left-lateral shear characteristic of the middle Miocene deformation in the Santa Monica Mountains and the basin-inception phase of the Los Angeles basin. The extensional stress field persisted until at least as recently as 10 my ago, when spreading ceased off Baja California. Subsequent attachment of the Continental borderland to the Pacific plate accounts for southward overthrusting of early Pliocene and younger strata on the Malibu Coast fault and right-lateral displacement on the Newport-Inglewood and Whittier-Elsinore zones.