GEOMORPHIC EVIDENCE FOR RECENT RUPTURES ON THE BEAUFORT RANGE FAULT IN THE NORTHERN CASCADIA FOREARC OF BRITISH COLUMBIA, CANADA

Field observations and analysis of high-resolution lidar topography provide the first evidence for Quaternary ruptures on the Beaufort Range fault, in the forearc of the Northern Cascadia subduction zone, British Columbia, Canada. The Beaufort Range fault is a basement-involved thrust fault originally formed as part of the Cowichan Valley Fold and Thrust System during Eocene accretion of the Crescent/Siletz terrane, and no fault activity since then has been previously reported. We, however, document evidence for recent (Quaternary) ruptures along 35 km of the Beaufort Range fault at three sites. At the southern end of the Beaufort Range near Port Alberni, four strands of uphill facing scarps are developed in Quaternary alluvium, the formation of which must postdate deglaciation of central Vancouver Island (approx.14 ka). Vertical separation of the Quaternary channels and interfluves ranges from ~9 to 25 m, and likely represents the cumulative displacement of multiple paleoseismic events. Lateral displacements of channel thalwegs by 1-2 m indicate a component of right lateral slip at this site. At the northern end of the Beaufort Range, two additional sites record both vertical and lateral separation of Quaternary deposits and landforms. At the first site, channels developed in Quaternary alluvium are right laterally offset by 2-3 m across a 200 m long uphill facing scarp. At the second site, channels are sheared right laterally across four topographic benches. Although active faults have not been widely documented in the northern Cascadia forearc of British Columbia, abundant microseismicity beneath southern Vancouver Island and counterclockwise rotation of the Cascadia forearc north of the Olympic mountains indicated by paleomagnetic and geodetic data suggest permanent deformation is accommodated, at least in part, along active forearc faults. These observations suggest that the Beaufort Range fault is seismogenic, and part of a network of reactivated basement-involved faults that accommodate right lateral transpression associated with the counterclockwise rotation of the northern Cascadia forearc. Quantification of the magnitude, and recurrence of paleoseismic events along these structures is necessary to evaluate seismic hazard posed by these shallow fault sources on Vancouver Island.