The present bathymetry, basin geometries, and spatial earthquake distribution in the inner California borderlands reflect complex basin inversion processes that reactivated two low-angle Miocene extensional detachments as blind thrust faults during the Pliocene to Holocene. The Oceanside and the Thirtymile Bank detachments comprise the inner California blind thrust system. These low-angle detachments originated during Neogene crustal extension that opened the inner California borderlands, creating a rift system that controlled the deposition of early to late Miocene sedimentary units and the exhumation of the metamorphic Catalina schist. During the Pliocene, a transpressional regime induced by oblique convergence between the Pacific and the North American plates reactivated the Oceanside and the Thirtymile Bank detachments as blind thrust faults. This reactivation generated regional structural wedges cored by faulted basement blocks that inverted the sedimentary basins in the hanging wall of the Miocene extensional detachments and induced contractional fold trends along the coastal plain of Orange and San Diego counties. Favorably oriented high-angle normal faults were also reactivated, creating zones of oblique and strike-slip faulting and folding such as the offshore segments of the Rose Canyon, San Diego, and the Newport-Inglewood fault zones. We evaluate several different styles of geometric and kinematic interactions between these high-angle strike-slip faults and the low-angle detachments, and favor interpretations where deep oblique slip is partitioned at shallow crustal levels into thrusting and right-lateral strike-slip faulting.

Analyses of seismic reflection profiles, well data, earthquake information, and sea-floor geology indicate that the Oceanside and the Thirtymile Bank blind thrust faults are active and represent important sources of earthquakes in this region. Restored balanced cross sections provide a minimum southwest-directed slip of 2.2–2.7 km (1.4–1.8 mi) on the Oceanside thrust and illustrate the function of this detachment in controlling the processes of basin inversion and the development of the overlying fold and thrust belt.