OBLIQUE STRIKE-SLIP FAULTING OF THE CENTRAL CASCADIA SUBMARINE FORE-ARC

At least nine WNW trending left-lateral strike-slip faults have been m apped on the Oregon-Washington continental margin using sidescan sonar , seismic reflection, and bathymetric data, augmented by submersible o bservations. The faults range in length from 33 to 115 km and cross mu ch of the continental slope. Five faults offset both the Juan de Fuca plate and North American plates and cross the plate boundary with litt le or no offset by the frontal thrust. Left-lateral separation of chan nels, folds, and Holocene sediments indicate active slip during the Ho locene and late Pleistocene. Offset of surficial features ranges from 120 to 900 m, and displaced subsurface piercing points at the seaward ends of the faults indicate a minimum of 2.2 to 5.5 km of total slip. Near their western tips, fault ages range from 300 ka to 650 ka, yield ing late Pleistocene-Holocene slip rates of 5.5 +/- 2 to 8.5 +/- 2 mm/ yr. The geometry and slip direction of these faults implies clockwise rotation of fault-bounded blocks about vertical axes within the Cascad ia forearc. Structural relationships indicate that some of the faults probably originate in the Juan de Fuca plate and propagate into the ov erlying forearc. The basement-involved faults may originate as shears antithetic to a dextral shear couple within the slab, as plate-couplin g forces are probably insufficient to rupture the oceanic lithosphere. The set of sinistral faults is consistent with a model of regional de formation of the submarine forearc (defined to include the deforming s lab) by right simple shear driven by oblique subduction of the Juan de Fuca plate.