CURVED FAULT TRACES, SURFACE UPLIFTS, AND EVOLVING LANDSCAPES: MORE THAN SIMPLE STRIKE SLIP FAULTING ON THE CHANNEL ISLANDS, SOUTHERN CALIFORNIA

Four of the five islands comprising Channel Islands National Park, southern CA, have an E-W alignment and structural grain expressed partly by a series of large E-W-striking faults located on and offshore of the islands. Our new Quaternary geologic mapping, field-based fault kinematic measurements, and LiDAR-based observations of landforms along these faults inform how tectonic deformation has evolved during the late Neogene and Quaternary--important to seismic hazard assessments--and elucidate possible linkages between the deformation and development of the landscapes imprinted on the islands. Large faults, spanning Santa Rosa (SRI), Santa Cruz (SCI), and Anacapa (ACI) Islands, experienced components of sinistral strike slip based mainly on kinematic observations on their principal slip surfaces and left-deflected stream channels. Faults on SRI and ACI have broadly arcuate, concave-north traces and form youthful south-facing escarpments, whereas the SCI fault has negligible concavity but similarly forms south-facing scarps. Dip-slip striae accompany and locally overprint strike-slip striae on the fault slip surfaces, and Pleistocene marine-terrace, alluvial, fluvial, and eolian deposits are typically displaced relatively down to the south. Youthful, incised, elevated landforms characterize the northern, concave sides, however, the apparent magnitude of surface uplift varies along the strike of each fault. The lateral variation in surface uplift along the ACI fault can be partly explained by localized crustal contraction resulting from its arcuate geometry combined with sinistral strike slip, but uplift variations along the SCI and SRI faults are mostly attributed to lateral gradients in south-directed reverse slip that accompanied the strike slip as well as local tectonic crowding due to interactions with secondary faults. Low topography at the ends of each of the island-spanning faults, presently submerged by narrow bodies of sea water, may correlate with releasing steps in the sinistral-slip faults which have an overall left-stepping en-echelon geometry. The arcuate geometry of the island-spanning faults may have helped accommodate previously recognized large (>90^o) clockwise rotations of crustal blocks of the coastal western Transverse Ranges that include the islands.