MECHANICAL MODEL FOR THE CASCADIA SUBDUCTION WEDGE, FOREARC BASINS AND MEGATHRUST EARTHQUAKES

The Cascadia forearc exhibits a characteristic structural morphology in common with most active margins. From trench to arc, the margin consists of margin-parallel domains including the (1) the trench slope, with piggy-back style slope basins; (2) the continental shelf, an region of little deformation, and deposition of up to several km of sediment in shelf basins; (3) a coastal mountain belt with net rock uplift over geologic time; and (4) a topographic and sometimes depositional basin. In Cascadia, the latter two correspond to the Coast Range, Olympics, and Insular Ranges and the Willamette Valley, Puget Sound, and Georgia Strait, respectively, We present a model that suggests the entire forearc system can be explained in terms of critical wedge theory. Critical wedge theory predicts a critical surface slope from a decollement dip, decollement yield stress and wedge yield stress. To obtain the morphology of the Cascadia margin, it is necessary to consider the effects of flexural compensation, the transition from frictional to viscous deformation and sedimentation. The four domains of the forearc are explained as follows. (1) The trench slope is a critical wedge with a shallowly-dipping decollement. (2) The shelf overlies the more steeply-dipping slab, so that the critical surface slope dips landward. This results in a structural high at the shelf edge, trapping sediments in shelf basins. The sedimentation increases the surface slope from negative to zero, causing the wedge to be stable rather than critical and thus prevents this segment of the wedge from deforming. (3) In our model, the coast overlies the frictional-viscous transition for the decollement, such that an effective detachment forms along a sub-horizontal isotherm in the upper plate. The coast ranges correspond to this region with ongoing deformation a consequence of the critical state of the wedge. (4) This wedge is backthrust over the arc basement, flexing it down, forming the forearc lowlands and basins. Sedimentation in these basins helps localize deformation in the coastal high. Megathrust earthquakes are confined to the shelf (our domain 2); this mechanical model suggests that great earthquakes are associated with a stable, not internally-deforming upper plate and are linked with structures that form over geologic timescales.