Field investigation of Cenozoic structures in the northern Cascadia forearc, southwestern British Columbia

Structural studies of fault systems in the northern Cascadia forearc document a transition from margin-normal shortening to dextral strike-slip faulting and associated margin-parallel extension some time in the Late Oligocene-Early Miocene. The transition from compression to extension in the northern forearc region is coeval with subduction and northeastward underplating along the Cascadia subduction zone, and with northward migration of forearc slivers and associated margin-parallel shortening in the southern Cascadia forearc of Washington State and Oregon. We present a working hypothesis for strain partitioning along the plate margin that involves both decoupling of the northern Cascadia forearc from the subduction-accretion complex to the west and the corresponding arc to the east, and internal deformation along linked systems of dextral strike-slip and normal dip-slip faults. The transition from compression to dextral transtension along the northern Cascadia forearc likely reflects changing boundary conditions associated with oblique subduction beneath an arcuate bend in the plate margin.