GEOMETRY AND MECHANICS OF INHERITED FOREARC FAULTS ON VANCOUVER ISLAND

Many subduction forearcs, like northern Cascadia, contain networks of inherited faults that accommodate deformation in response to changes in plate boundary tractions, over timescales ranging from that of tectonic reorganizations to the megathrust seismic cycle. Here we summarize recent observations from Vancouver Island that indicate Eocene or older inherited structures, including the Leech River (LRF), San Juan (SJF), and Beaufort Range (BRF) faults, have experienced temporal or spatial changes in fault kinematics through time, and may accommodate Quaternary crustal strain. The LRF, a boundary between the Pacific Rim and Crescent terranes, displays a complex set of brittle shears and Quaternary active scarps that likely accommodate a combination of right-lateral and dip slip along a subvertical to north-dipping fracture network. The SJF, which separates the Pacific Rim and Wrangellia terranes, shows evidence for brittle faults crosscutting older mylonites and serpentinized shear zones that indicate post-Oligocene extensional and right-lateral slip along portions of the fault. The range front of the BRF, part of the Cowichan fold and thrust system, contains potentially active fault scarps offsetting till and colluvium that appear to accommodate both right-lateral slip and transtension. We used Mohr Coulomb analyses to explore the stress states that could allow failure along these crustal faults. We find that, in the current stress field, late in the megathrust interseismic period, the BRF and eastern LRF are close to optimally oriented for right-lateral slip and right-lateral transpression, while the SJF and western LRF can only fail if fault dips are moderate, fault friction is low, and pore fluid pressure is high. We also find that modeled co-seismic stress changes generated by a ~M9 megathrust earthquake bring steeply-dipping faults closer to failure, with right-lateral transtensional slip promoted. These results suggest that forearc faults on Vancouver Island may have the potential to respond to either interseismic or coseismic stresses, and highlight the need for quantification of the geometry and slip histories of forearc faults.