Velocity and inferred porosity model of the Oregon accretionary prism from multichannel seismic reflection data: Implications on sediment dewatering and overpressure

Abstract

A two‐dimensional model of seismic velocity derived from multichannel seismic data collected off Oregon in 1989 shows that as sediments are carried from Cascadia Basin into the accretionary prism, there are measurable changes in velocity‐depth profiles. In the seaward most area of the basin, where no thrust faults are observed, there is a landward (and downward) increase of velocity in the sedimentary section. We attribute the velocity increase in the basin to a reduction of porosity resulting from consolidation and cementation, accompanied by diffusive flow of pore water driven by lateral tectonic as well as gravitational stress. Near the base of the slope there is an area of incipient thrusting (the protothrust zone) where protothrusts sole out into a protodécollement. Synthetic seismogram modeling of the reverse‐polarity reflection from the protodécollement shows a 100‐m‐thick layer with a slightly lower velocity relative to the sediments above it. Above the protodécollement, velocity continues to increase landward. We suggest that in this area the diffusive flow of pore water out of the sediment is augmented above the protodécollement by fault‐focused flow. Below the protodécollement a reversal in velocity may be due to an increase in porosity resulting from overpressuring of pore fluid trapped by reduction of the permeability of the sediment above the protodécollement. Farther landward, where thrusting has formed a fault‐bend fold, velocity values are lower in the accreted section of sediments relative to the velocity at a comparable subbottom depth in the protothrust zone. The decrease in velocity is a result of microfracturing of the highly consolidated sediments accompanying uplift and folding and reflects the increasing role of fracturing and faulting in the control of dewatering of the sediments.