SCARPE DIEM: LATE QUATERNARY RUPTURES ON THE BEAUFORT RANGE FAULT, BRITISH COLUMBIA, CANADA

The distribution, kinematics, and magnitude of active strain accumulation along crustal faults in the northern Cascadia forearc remain poorly understood. Here we provide new data from field observations, analysis of high-resolution lidar digital elevation models, and total station topographic surveys that document evidence for recent (late Quaternary) surface ruptures along 35 km of the NW-striking Beaufort Range fault in the forearc of the northern Cascadia subduction zone. At the southern end of the Beaufort Range near Port Alberni, multiple strands of 1-6 m tall uphill-facing scarps are developed in Quaternary alluvium. These surface offsets must postdate the deglaciation of central Vancouver Island (approx. 14 ka). Total station surveys of these scarps show ~5 to 15 m of vertical separation of the Quaternary channels and interfluves, which likely represents the cumulative displacement of multiple paleoseismic events. A significant right-lateral component is indicated by both the geometry of stepovers and by systematic ~1-5 m right-lateral displacements of channel thalwegs and interfluves. At the northern end of the Beaufort Range, channels are sheared right laterally across a series of en échelon scarps with ~2-4 m of vertical separation. Quaternary surface ruptures are subparallel to basement-involved bedrock faults that originally formed as part of the Cowichan fold and thrust system during the Eocene, suggesting recent ruptures may result from reactivation of the ancient bedrock fault system. Rupture length-magnitude scaling relationships suggest that a single event rupturing both sites would have a moment magnitude of ~7. These observations, combined with recent documentation of Holocene paleoseismicity along the Leech River fault, and geodetic and paleomagnetic rotations of the northern Cascadia forearc, suggest that the Beaufort Range fault is part of a network of seismogenic crustal faults that accommodate right-lateral oblique slip in the northern Cascadia forearc. Further quantification of the kinematics of slip, and the magnitude and recurrence of paleoseismic events is necessary to evaluate both the seismic hazard posed by these shallow fault sources on Vancouver Island, and the mechanisms driving this deformation.