Oblique strike‐slip faulting of the central Cascadia submarine forearc

Abstract

At least nine WNW trending left‐lateral strike‐slip faults have been mapped on the Oregon‐Washington continental margin using sidescan sonar, seismic reflection, and bathymetric data, augmented by submersible observations. The faults range in length from 33 to 115 km and cross much of the continental slope. Five faults offset both the Juan de Fuca plate and North American plates and cross the plate boundary with little or no offset by the frontal thrust. Left‐lateral separation of channels, folds, and Holocene sediments indicate active slip during the Holocene 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, yielding 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 Cascadia forearc. Structural relationships indicate that some of the faults probably originate in the Juan de Fuca plate and propagate into the overlying forearc. The basement‐involved faults may originate as shears antithetic to a dextral shear couple within the slab, as plate‐coupling forces are probably insufficient to rupture the oceanic lithosphere. The set of sinistral faults is consistent with a model of regional deformation of the submarine forearc (defined to include the deforming slab) by right simple shear driven by oblique subduction of the Juan de Fuca plate.