STRIKE-SLIP RATES FOR BOUNDING FAULTS IN THE OLYMPIC PENINSULA OF WASHINGTON STATE

Active transcurrent faults bounding the northern and southern margins of the Olympic Mountains in Washington state accommodate permanent strain in the upper plate of the Cascadia subduction zone. Constraints on the kinematics and slip rate for these structures result primarily from lidar analysis of displaced debris flow channels and luminescence and radiocarbon dating of buried channel fills. Despite the distinct and relatively fresh appearance of these geomorphic features apparent in airborne lidar, challenges resulting from their degradational nature and uncertainties in channel abandonment timing limit their fidelity in recording fault slip-rate. We instead rely on the well-dated glacial retreat of the Juan de Fuca lobe of the Cordilleran ice sheet at ca. 14 ka as a maximum bound on the age of these features. Individual channel offsets on the North Olympic fault zone range between 4 – 30 m, suggesting dextral strike-slip at rates upward of ~2 mm/yr since glacial retreat, as well as the persistence of both cumulative and probable single-event earthquake displacements. In the southern Olympic Mountains, a less well-constrained, older alpine glacial advance across the Canyon River fault may indicate comparable rates of sinistral slip over time. There, geomorphic offsets exceeding ~60 m record a longer history of strike slip displacement, despite a lack of constraints on individual channel ages. Taken together, the conjugate nature and similar slip rates for these bounding faults (dextral in the north, sinistral in the south) suggest that transcurrent faults accommodate a significant component of margin-parallel shortening, as well as ongoing oroclinal bending and rotation against a relatively rigid buttress of the Olympic Mountains. This style of deformation contrasts with structures within and east of Puget Sound, where margin-perpendicular upper-plate reverse faults and associated folds dominate the response to active upper-plate shortening.