HR: 14:50h
AN: T43C-05 INVITED     [Abstracts]
TI: Modeling Fluid Sources and Their Role in Controlling Subduction Zones Pore Pressures
AU: * Bekins, B A
EM: babekins@usgs.gov
AF: U. S. Geological Survey, MS 496 345 Middlefield Rd., Menlo Park, CA 94025 United States
AB: In the last two decades there has been substantial progress understanding the origin and magnitude of pore pressure along subduction megathrusts. In the early 1980's steady-state force balances of fold and thrust belts provided predictions of pore pressure for a range of taper angles and friction coefficients. These predictions set the stage for subsequent studies examining factors that generate and maintain fluid pressures over geologic time. Most central to recent understanding of pore pressure maintenance in subduction zones has been the concept of fluid sources. These are fluids and volatiles in pores, fractures, mineral structures, or organic material that are mobilized during burial and heating. It is the dynamic balance of the rate of release of fluids against permeability and flow path length that controls the magnitude of pressure. Examples that compare data with model results for Barbados, Nankai, and Costa Rica show that modeling of fluid sources and pore pressure in these settings has reached the level of predictive capability. Barbados data acquired from two long-term borehole observatories indicate that pore pressures in the decollement increase sharply with distance from the trench, reaching a significant fraction of lithostatic pressure after 4 km of subduction. Pressure results from a steady-state hydrogeologic model using laboratory permeability data and accounting for fluid sources from consolidation and clay dehydration are in excellent agreement with the measured observatory pressures. In Nankai, modeling of consolidation and clay dehydration shows that both fluid sources become unimportant only ~30 km from the trench leading to a drop in predicted fluid pressures. This same location has been identified as the updip limit of the inferred locked zone on the megathrust. In Costa Rica, pressures obtained from consolidation modeling using laboratory permeabilities match in situ fluid pressure values estimated from consolidation tests at shallow depths. Deeper in the system, dehydration provides an exceptionally large source of fluids due to the high smectite content and rapid burial of the subducted sediments. The location of peak smectite dehydration corresponds to the location of mud volcanoes venting freshened fluids at the seafloor and appears to be related to the distribution of seismicity on the megathrust. These successes indicate that modeling results are beginning to reach a level where they can be extended to other subduction zones that are less well-characterized and to depths below those reached by drilling.
DE: 8045 Role of fluids
DE: 8104 Continental margins: convergent
SC: Tectonophysics [T]
MN: Fall Meeting 2005