Neotectonics of the San Cayetano fault, Transverse Ranges, California

Abstract

Study of late Quaternary alluvial deposits cut by the San Cayetano reverse fault indicates that the dip-slip rate of faulting increases from 1.05 ± 0.2 mm/yr at Sisar Creek near the western end of the fault eastward to 1.35 ± 0.4 mm/ yr at Bear Canyon and 2.35 ± 0.55 to 4.15 ± 0.85 mm/yr at Mud Creek. At Timber Canyon near the central part of the fault, a minimum slip rate of 3.6 ± 0.4 mm/yr is suggested by fanhead segmentation; however, a higher slip rate of 8.75 ± 1.95 mm/yr is suggested by stratigraphic evidence. Stratigraphic separation of 7.5 km in less than 1 m.y. across the fault between Fillmore and Piru indicates a slip rate of at least 7.5 mm/yr for the eastern section of the fault. Recent activity is shown by faulted Holocene or latest Pleistocene fan alluvium and colluvium at Bear Canyon and the Silverthread oil field, respectively, and inferred at Timber Canyon by fanhead deposition. Active folding of the footwall is demonstrated by tilted and faulted Holocene and Pleistocene alluvial fans: present tilt rates approach 1°/10 ka yr, and associated bedding plane faulting rates are about 0.5 to 1.5 mm/yr. The hanging wall, however, does not appear to be involved in active deformation, inasmuch as Pleistocene deposits as old as several hundred thousand years are not tilted or affected by bedding-plane faulting. The only exception is in the Silverthread field area, where hanging-wall deformation is directly relatable to fault geometry, not folding of the hanging-wall rocks. The lack of documented moderate- to large-magnitude historical earthquakes on this fault is in strong contrast with its demonstrated late Quaternary activity. Based on its length, slip rate, overall geometry, and segmentation into two parts, a recurrence interval of between 200 and 600 yr for the western segment and about 230 yr for the eastern segment is suggested.