Fault-Related Folding in California's Northern Channel Islands Documented by Rapid-Static GPS Positioning

Rapid-static Global Positioning System positioning was used to measure late Quaternary deformation of uplifted coastal terraces on Santa Rosa and San Miguel Islands, which have accumulated regional warping and local faulting over 100+ k.y. Late Quaternary deformation on Santa Rosa is sharply partitioned, with several hundred meters of surfacerupturing left slip, but 15+ m of vertical motion taken up by folding, resulting in anticlinal growth of at least 0.12 m/k.y. Deformation on San Miguel Island to the west is consistent with warping on the north limb of the regional anticlinal structure. Deformation on both islands matches activity to the east, suggesting that the Santa Rosa Island fault represents an en echelon segment of the larger Transverse Ranges Boundary fault system and the Northern Channel Islands antiform. The Northern Channel Islands have been the focus of a debate over differing geometrical models of fault-related folding, and the deformation measurements presented here suggest that uplift of the islands is occurring over a smoothly curved thrust ramp rather than a fault-bend fold. This reinterpretation is important, as current earthquake-hazard assessments on this and other buried reverse faults depend entirely on assumptions unique to the fault-bend fold geometry. We suggest that alternative models should be considered where geologically reasonable, and that surface uplift measurements across fault-related folds can provide a crucial test of subsurface geometry.