Abstract

New multichannel seismic reflection profiles, acoustic imagery, and swath bathymetric data from the Vancouver Island continental margin reveal the surface morphology, cross-sectional geometry, and deformation style of this part of the Cascadia accretionary prism. The incoming 3- to 5-km-thick sediment section fails most commonly along landward-dipping thrusts that penetrate to or very close to the top of the underlying oceanic crust. The faults are listric with approximately circular cross sections, and hanging-wall sediments are back-tilted in a planar manner. High, near-lithostatic pore pressures in the deeper part of the section are inferred from the flattening of the faults near the décollement, and from the long distances over which the décollement has propagated seaward. The faults dip more steeply at 40°-45° near the surface where Pleistocene turbidites are displaced. Similar folds have developed where the turbidites are particularly thick (>4 km), and near-hydrostatic pore pressures are inferred to occur in the turbidite part of the section. An increase in pore pressure with depth is expected as a consequence of sedimentation and tectonic loading, particularly because the deeper hemipelagic sediments are probably far less permeable than are the overlying turbidites.

The top of the downgoing Juan de Fuca plate, which defines the base of the accreted sedimentary wedge beneath the margin, is well imaged in the new seismic reflection profiles and is further defined by land reflection data, seismic refraction, and earthquake hypocenters. The landward-dipping continental backstop to the prism is also well located. If these constraints are used for the geometry of the accretionary prism, a general balance is found between the present prism volume and the estimated total sediment supply over the approximately 42-m.y. accretion history. This result, along with the observation that deformation-front faults penetrate most or all of the sediment section, suggests that little or no sediment has been subducted since initiation of the present phase of subduction in the Eocene. Application of the critically tapered wedge model of Davis and others to the observed prism geometry suggests that high average pore pressures occur in the prism, particularly the seawardmost part beneath the lower slope. Beneath the continental shelf, the dip of the subducting slab increases to greater than 11°, a dip that according to the critically tapered wedge model, is too steep to allow upward growth of an accretionary prism. This provides a simple explanation for ongoing subsidence of the Tofino sedimentary basin over the older part of the accretionary complex. To the southeast, the Juan de Fuca plate dips much more gently beneath the Olympic Peninsula of northern Washington, allowing the accretionary prism to grow upward well above sea level to form the Olympic Mountains.

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