Restraining segments and reactivation of the Santa Monica–Dume–Malibu Coast fault system, offshore Los Angeles, California

Abstract

A regional west striking system of surface and blind faults transects northern metropolitan Los Angeles, separating the Santa Monica Mountains from two deep sedimentary basins. The surface faults include the Santa Monica, Dume, and Malibu Coast faults. The three‐dimensional (3‐D) geometries of these faults and deformed dated strata were examined in order to determine how oblique shortening is accommodated, how structural relief grows along a mountain front, and how block translation is related to block rotation. Industry seismic reflection, well, and outcrop data were used to construct digital structure‐contour maps of several of these faults and three stratigraphic horizons that intersect them. These maps, swath bathymetry, digital elevation models, and seismicity were incorporated into a 3‐D digital database. Modeling included new approaches to determining strike‐slip displacement by separate analyses of shortening and structural relief in a restraining double bend. Stratigraphic thicknesses indicate Miocene extension across the Santa Monica and Dume faults, which have been reactivated as a single or linked moderately dipping arcuate fault that accommodates one coherent block motion via left‐lateral and left‐reverse displacement. Modeling indicates 5 km (+8/−1 km) of left‐lateral displacement on part of the Santa Monica–Dume fault and ∼11° of clockwise rotation of the Santa Monica Mountains during the last ∼4 ± 1 m.y. Modern displacement rates modeled from GPS data are similar to our modeled post‐∼4 Ma rates of westward escape and clockwise rotation of the Santa Monica Mountains.