Evidence for an active transtensional Beaufort Range fault in the northern Cascadia forearc

Abstract

Geologic records of fault slip in subduction forearcs provide critical data on stress and strain in the upper plate and the seismogenic potential of hazardous faults. However, few active upper-plate faults have been identified in the northern Cascadia forearc. Here we investigate the slip history of the Beaufort Range fault (BRF) on Vancouver Island, BC, Canada, a proposed source of the 1946 M 7.3 Vancouver Island earthquake, the largest recorded in Cascadia. We use recently-collected lidar data, field mapping, and surveying of offset landforms to map the extent of previously unidentified post-glacial (<14 ka) tectonic scarps and reconstruct 3D fault slip vectors. Post-glacial landforms show increasing displacement with age, suggesting at least three Mw~6.5-7.5 earthquakes since ~14 ka, the most recent <4 ka. These displacements suggest the BRF is one of the fastest-slipping faults in the northern Cascadia forearc (0.5-2 mm/yr). Kinematic slip inversions of offset geomorphic piercing lines are consistent with right-lateral transtension along a steeply NE-dipping fault. Because BRF fault geometry and kinematics are similar to the 1946 earthquake, it is a plausible source. The kinematic similarity of millennial and decadal slip data suggests the BRF has accommodated transtension over multiple earthquake cycles.