Hydrogeologic processes in the Oregon‐Washington Accretionary Complex

Abstract

Rapid, subduction‐induced venting of fluids was recently discovered at the lower slopes of several convergent margins. In this study we examine, by numerical modeling, the sources and pathways for the expelled fluids at the lower slope of the Oregon‐Washington margin. Models are constructed on the basis of existing multichannel seismic records, Deep Sea Drilling Project drilling data, and regional tectonic history. We show that the hydrogeologic processes at this margin may be cyclic and episodic. An important fluid source may be the underthrust turbidites beneath the frontal thrust, which dewater under the increasing overburden of the advancing thrust sheet The underlain hemipelagic unit, on the other hand, is a poor source of fluid, even though it is highly pressurized. Since the dewatering turbidites have not been subjected to temperatures greater than 100°C, the above conclusion is consistent with the low‐temperature signatures in the carbon isotope data for vented fluids and carbonate cementation at the lower slope. Cyclic but episodic seaward migration of the frontal thrust of the accretionary complex leads to corresponding seaward migration of the fluid sources. Between the distinct thrusting episodes, the fluid sources grow both in size and pore pressure in response to the increasing overburden of the advancing frontal thrust Other factors being equal, thrusting at the landward vergent margin is more effective in creating fluid sources than thrusting at the seaward vergent margin. Some aspects of these inferences may be common to other similar convergent margins and are testable with future Ocean Drilling Program drilling.