Heterogeneous deformation in the Cascadia convergent margin and its relation to thermal gradient (Washington, NW USA)

Abstract

We combine structural balancing with thermal and strength-envelope analysis of the Cascadia accretionary wedge to determine the influence thermal gradient has on the structure of the prism. BSR-derived heat flow in the Cascadia accretionary margin decreases from 90–110 mW/m2 at the deformation front to 45–70 mW/m2 in the upper slope. Extension of the thermal gradient to the top of the oceanic crust shows that the base of the prism reaches temperatures between 150–200°C and 250–300°C at the deformation front and the base of the upper slope, respectively. This high thermal gradient favors the development of a vertical strain gradient, which is accommodated by heterogeneous deformation of the accretionary prism. This process produces two overlying thrust wedges, a basal duplex and an overlying landward- or seaward-vergent imbricate stack. The thermal structure also influences the deformation distribution and structural style along the shortening direction. Initiation of plastic deformation at the base of the prism below the Cascadia upper slope affects the wedge geometry, changing its taper angle and favoring the development of a midcrustal duplex structure that propagates seaward as a dynamic backstop. Uplift related with this underplating process is accompanied with deep incision of submarine canyons, sliding and normal faulting in the upper slope. Heterogeneous deformation accommodated by the development of transfer faults separating landward-vergent from seaward-vergent domains is also observed along the margin. Landward-vergent areas accommodate 30–40% shortening at the front of the wedge, while in the narrower and thicker seaward-vergent segments shortening occurs mostly by underplating below the upper slope.