Santa Monica and Santa Ana Mountains—Relation to Oligocene Santa Barbara Basin

Abstract An analysis of the stratigraphy and structure of the Santa Monica and Santa Ana Mountains demonstrates a close affinity between the two, one that can be explained only by a much closer original geographic relation of the two ranges. The Santa Monica Mountain block, bounded by the Malibu Coast fault on the south and the Las Posas fault on the north, began a westward movement across the ancestral basin during the Luisian stage of the middle Miocene. Approximately 33 mi (53 km) of left-lateral displacement took place prior to the Mohnian stage of the upper Miocene, breaking the original basin into the separate Ventura and Los Angeles basins joined by a San Fernando subbasin. This displacement took place in no more than 2.5 m.y. at a minimum rate of movement of 0.8 in. (2 cm)/year. Four mi (6.4 km) of post-Mohnian left-lateral displacement subsequently occurred. The magnitude of displacement is determined by projecting the Santa Ana Mountains parallel with the San Andreas direction, to a point of intersection with the Raymond fault, and then by matching similar stratigraphy on opposite sides of the fault. Restoration of the Santa Monica Mountains block to its pre-Luisian location, combined with the addition of known thicknesses of sections of the Sespe Formation, permits the construction of original Sespe isopachs and the reconstruction of the Oligocene basin, herein called the Santa Barbara basin. This basin was bounded, south of the present Ventura basin and west of the present Los Angeles basin, by a positive Franciscan area, the Anacapia of Reed and Hollister. The Luisian orogeny, responsible for the westward rifting of the Santa Monica Mountains, also produced the breakup of Anacapia, and exerted the most profound influence on southern California geology of any Cenozoic event.