Abstract

Venting of pore fluid was recently reported near the front of the Oregon accretionary prism and was directly measured at one of the venting sites during Alvin dives. In this study, we model the flow of pore fluid in the Oregon accretionary prism. We use seismic reflection sections to constrain subsurface structures, drill-hole results from Deep Sea Drilling Project Leg 18 to constrain sediment properties, and regional tectonic information to constrain the timing and the rate of plate convergence and sediment imbrication and thickening. The results of two-dimensional, time-dependent, finite-element modeling show that high pore pressures are generated beneath the decollement due to the load of imbricated and thickened sediments. Diffusive flow at a rate of several millimetres per year is predicted within the accre-tionary prism if fault zone permeability is discounted. The presence of a decollement or permeable fault penetrating the accretionary prism, as imaged on the seismic reflection section, can drastically alter the patterns of fluid flow and produce focused venting of fluid, as observed. On the other hand, fluid venting along faults may not significantly alter the high pore pressures below the decollement.

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