GEOMORPHIC MAPPING AND PALEOSEISMIC TRENCHING REVEAL MULTIPLE LATEST PLEISTOCENE TO HOLOCENE EARTHQUAKES ON THE RIGHT-LATERAL OBLIQUE BEAUFORT RANGE FAULT, NORTHERN CASCADIA FOREARC, BRITISH COLUMBIA, CANADA

Oblique plate convergence occurs in most subduction systems, but how oblique slip is partitioned between the plate interface and upper plate structures is poorly understood. Forearc faults can accommodate trench-parallel slip, but in many forearcs, the slip rates, kinematics, rupture histories, and even locations of these faults are poorly constrained. Here we present results from surficial mapping, topographic surveys, and paleoseismic trenching to characterize the timing, displacement, and kinematics of seismogenic slip along the Beaufort Range fault (BRF), a fault sub-parallel to the trench in the northern Cascadia forearc on central Vancouver Island, BC, Canada. Our mapping reveals ~35 km of discontinuous tectonic scarps along the BRF that systematically offset post-glacial (<13 ka) landforms and deposits both vertically and right-laterally, indicating that the BRF has been active in the latest Pleistocene to Holocene. Differential offset of late Pleistocene to Holocene channels and interfluves and the existence of both degraded scarps and locally well-preserved free-faces at the ground surface suggest multiple ruptures since the Last Glacial Maximum. Confirmation of this comes from a paleoseismic trench across the BRF, which exposed deposits we interpret to be related to five surface-rupturing earthquakes. Radiocarbon dates from 43 detrital charcoal samples collected from colluvial wedges and channel sediments ponded against the uphill-facing scarp suggest that these surface-rupturing events occurred over the past ~12 kyr with an average recurrence interval of ~2-5 kyr. Offsets determined from trenching and total-station-derived topographic surveys of interfluves and channel thalwegs in radiocarbon-dated deposits yield estimated minimum offset rates of ~0.4 mm/yr vertical and ~0.2 mm/yr right-lateral. Given that the right-lateral component of oblique convergence between the Juan de Fuca and North America plates is ~10 mm/yr at the latitude of central Vancouver Island, the right-lateral Holocene slip rate we estimate for the BRF is only 2% of the expected budget from total trench-parallel motion. This suggests that the remaining slip must either be accommodated as slip on other faults in the overriding plate, on the plate boundary interface, or as distributed upper plate deformation.