Eocene to Recent permanent forearc deformation in Northern Cascadia, southern Vancouver Island, British Columbia, Canada

Active deformation along upper crustal faults in the northern Cascadia forearc remains poorly understood because of low crustal strain rates, glacial erosion, and dense forest cover. We present new results from lidar surveys, paleoseismic trenching, geologic mapping, and fault kinematic analyses from southern Vancouver Island, British Columbia, that provide insight into how permanent strain has been accumulating in the crust of the North America plate since the Eocene Epoch ( 35 Ma). On the E-W striking, Leech River fault, which forms the northern boundary of the oceanic Siletz terrane accreted in the Eocene, new paleoseismic trenches show post-glacial surface rupture with a complex rupture pattern typical of strike-slip earthquakes. Trench 1, located 35 km west of the city of Victoria, British Columbia, exposed faulted and folded glacial sediments, and surface warping suggestive of a pressure ridge. However, trench 2, located 700 m west of trench 1, did not show evidence of late Quaternary surface rupture. Approximately 20 km north of the Leech River fault, the E-W striking San Juan fault, which bounds the Mesozoic Wrangellia and Pacific Rim terranes, and interpreted to be last active during Eocene Siletz terrane accretion, displaces sediments of the Oligocene Carmanah Group near its western extent. Therefore, slip along this segment of the San Juan fault must have occurred after the deposition of the Carmanah Group in the Oligocene, and several million years after Eocene accretion. Lineaments observed in recent lidar data suggest that slip on the San Juan fault may continue into late Quaternary time. Kinematic inversion of brittle slickensides crosscutting Mesozoic rocks shows that left-lateral transpression has dominated the San Juan fault. However, normal faults offsetting the Oligocene Carmanah group sediments suggest that the stress regime has changed either temporally or spatially. These results suggest that the terrane-bounding faults on southern Vancouver Island, which have traditionally been associated with Mesozoic and Eocene terrane accretion, have also aided in accommodating Eocene to Recent permanent strain in the northern Cascadia forearc.