Abstract

The Northern Channel Islands demonstrate pervasive Quaternary deformation, including faulting, regional warping, and localized folding. Field mapping on Santa Cruz Island, interpretation of seismic-reflection profiles north of the island, measurement of fault-zone striations, and study of uplifted and deformed coastal terraces show which structures have been active in the late Quaternary and the pattern and rates of deformation during that time. The Santa Cruz Island fault is the largest and most active ground-rupturing fault on the island. The fault is predominantly left-lateral, although striations and displaced landforms demonstrate that it has a smaller, but significant component of reverse slip. The Poso Beach fault is an active reverse structure that cuts a 125 ka coastal terrace on southwest Santa Cruz Island. Other major faults include the “south branch” of the Santa Cruz Island fault, the Potato Harbor fault, and “Fault C”, but conclusive evidence of late Quaternary slip on these faults is lacking at this time. In addition to brittle deformation, regional warping is recorded by deformation of Pleistocene coastal terraces and progressive submergence and tilting on the island’s northern continental shelf. North of the Santa Cruz Island fault, warping takes the form of subsidence of the shelf and uplift of the island, with uplift reaching a maximum near the fault. On the south half of the island, deformation consists of a broad, low-amplitude south tilt, upon which localized folding associated with the Poso Beach fault and the Christi anticline is superimposed. These secondary folds, as well as southeast-striking right-lateral faults near Valley Anchorage, probably represent local accommodation of right-lateral structures of the California Borderland as they meet the east-west-trending western Transverse Ranges to the north. Overall, the pattern of late Quaternary deformation on Santa Cruz Island is consistent with a model in which the island, and probably the entire Northern Channel Islands chain, is part of a regional anticline, the north limb of which is progressively tilting above a concave-up listric thrust fault.