Late Pleistocene structural evolution of the Camarillo fold belt: Implications for lateral fault growth and seismic hazard in Southern California

The Camarillo fold belt in the Western Transverse Ranges poses a significant seismic hazard to nearly one million people living in Southern California, yet few published geologic or geochronological data from this fold belt exist. The Camarillo fold belt is composed of several south-verging, west-plunging anticlines that characterize the western extent of the Simi fault zone, which extends for 40 km through urbanized Ventura and Los Angeles Counties. Surface and subsurface geologic data are utilized to accurately construct five cross sections within discrete structural domains to assess the local style of deformation, and to quantify the magnitudes of fault slip, fault- and fold-related uplift, and percent shortening. Eight new optically stimulated luminescence (OSL) dates from three paleoseismic trenches and six numerical dates from a previous study were utilized to quantify the timing and rates of deformation on discrete faults and folds. The onset of deformation in the Camarillo fold belt is everywhere younger than ca. 125 ka and locally as young as ca. 25 ka. Quaternary deformation occurs on reactivated steeply dipping (70°) Miocene faults, with shortening being largely accommodated in a narrow zone (<2 km) of folding where fold-related uplift is typically twice as great as hanging-wall uplift in response to fault slip. A minimum fault slip rate between 0.8 mm/yr and 1.4 mm/yr, a recurrence interval of 715-1100 yr, and a maximum M _w 6.8 earthquake for faults within the Simi fault zone is estimated, which are comparable to other better-studied fold belts in Southern California. A model of punctuated lateral fault propagation is proposed to explain westward growth of the Simi fault, which occurs in discrete pulses that are separated by intervals of fault displacement accumulation and fold amplification during constant-fault-length conditions. Lateral fault growth is limited in space and time by an orthogonal north-striking fault set, which juxtaposes a series of west-plunging anticlines that decrease in structural relief and age toward the west.