FOREARC FAULTING IN NORTHERN CASCADIA: WHERE, WHEN, AND WHY? (Invited Presentation)

Subduction forearcs contain networks of new and inherited faults that accommodate strain in response to both near- and far-field plate boundary tractions. However, our understanding of the conditions and processes that are most important in controlling the location and kinematics of forearc strain accommodation is limited. Here we summarize results of ongoing work in the northern Cascadia forearc on Vancouver Island to evaluate which faults accommodate active strain, the role of inherited structures in promoting localization of active deformation, and how the geometry and kinematics of active faults are related to the processes driving forearc deformation. Geomorphic, structural, paleoseismic, and geophysical data provide evidence for late Pleistocene to Holocene seismogenic slip along ~NW-SE striking segments of the Leech River, Beaufort Range, and Elk Lake faults. Field data suggest that these structures are high-angle faults that accommodate both right-lateral and dip slip up to ~230 km from the trench, in a corridor between the Strait of Juan de Fuca to the northern end of the Cascadia subduction zone. While the surface traces of these active forearc faults occur adjacent to and strike sub-parallel to pre-existing faults, active faults do not appear to re-occupy inherited fault planes, and subsurface projections of high-angle active faults appear to diverge at depth from moderately-dipping, inherited fault planes. Mohr Coulomb analyses suggest that high-angle, NW-SE striking active structures are near-optimally oriented for failure in the observed upper plate stress field, whereas inherited, moderately dipping, E-W to NE-SW striking faults are not. These data suggest that while crustal faults may opportunistically occupy portions of inherited structures when mechanical conditions for failure are favorable, they may diverge from these structures if they are poorly aligned with the driving stress field. Finally, boundary element modeling predicts strike-slip kinematics on upper plate faults that are inconsistent with field and focal mechanism data. These results suggest that plate coupling cannot be the only mechanism driving forearc deformation in northern Cascadia, and that far-field plate boundary tractions or crustal rotation associated with oroclinal bending likely play an important role.