TECTONIC SEDIMENT THICKENING, FLUID EXPULSION, AND THE THERMAL REGIME OF SUBDUCTION ZONE ACCRETIONARY PRISMS - THE CASCADIA MARGIN OFF VANCOUVER-ISLAND

Tectonic sediment thickening, fluid expulsion, and the resulting thermal regime of the northern Cascadia subduction zone accretionary prism have been described by numerical models. Both the sediment thickening and the fluid expulsion are found to have important thermal effects. Constraints on the models are provided by (1) detailed heat flow profiles across the continental slope accretion zone from variations in the thermally controlled depth to a gas hydrate bottom-simulating reflector (BSR), and (2) porosity changes across the accretion zone inferred from the landward variation in the P wave velocity-depth profiles. The heat flow in the region of rapid prism thickening centered 15 km landward of the deformation front is 20% below that predicted by a larger scale thermal model that ignores the thermal effects of the sediment accretion processes. The porosity also is substantially higher in this region compared to at the same depths in the adjacent deep sea Cascadia basin. The model results are in agreement with both the thermal and velocity data if the incoming sediment section is initially simply tectonically shortened horizontally and thickened vertically, generating a high porosity underconsolidated section. Reconsolidation and fluid expulsion in response to the tectonic thickening is slow, occurring mainly 20-30 km landward of the deformation front. The maximum predicted fluid expulsion rate is about 4 x 10(-11) m s-1 (1.3 nun a-1). The pore pressure distribution across the deformation front region has been estiniated from the velocity data using the simplifying assumption that velocity is only a function of effective pressure; pore pressures are high, reaching 80% of lithostatic 15 to 20 km landward of the deformation front. An estimate of the bulk permeability in the seaward tens of kilometers of the prism ranging from 10(-16) m2 near the top to about 10(-18) m2 near the base has been obtained through matching the pore pressure distribution in the numerical model to that obtained from the velocity data. The prism sediments are inferred to be substantially underconsolidated and pore pressures to be high for at least the seaward 30 km of the accretionary prism.