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JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 111, B04101, doi:10.1029/2005JB003990, 2006

An evaluation of factors influencing pore pressure in accretionary complexes: Implications for taper angle and wedge mechanics

Demian M. Saffer

Department of Geosciences, Pennsylvania State University, University Park, Pennsylvania, USA


Barbara A. Bekins

U.S. Geological Survey, Menlo Park, California, USA


Abstract

At many subduction zones, accretionary complexes form as sediment is off-scraped from the subducting plate. Mechanical models that treat accretionary complexes as critically tapered wedges of sediment demonstrate that pore pressure controls their taper angle by modifying basal and internal shear strength. Here, we combine a numerical model of groundwater flow with critical taper theory to quantify the effects of sediment and décollement permeability, sediment thickness, sediment partitioning between accretion and underthrusting, and plate convergence rate on steady state pore pressure. Our results show that pore pressure in accretionary wedges can be viewed as a dynamically maintained response to factors which drive pore pressure (source terms) and those that limit flow (permeability and drainage path length). We find that sediment permeability and incoming sediment thickness are the most important factors, whereas fault permeability and the partitioning of sediment have a small effect. For our base case model scenario, as sediment permeability is increased, pore pressure decreases from near-lithostatic to hydrostatic values and allows stable taper angles to increase from ∼2.5° to 8°–12.5°. With increased sediment thickness in our models (from 100 to 8000 m), increased pore pressure drives a decrease in stable taper angle from 8.4°–12.5° to <2.5–5°. In general, low-permeability and thick incoming sediment sustain high pore pressures consistent with shallowly tapered geometry, whereas high-permeability and thin incoming sediment should result in steep geometry. Our model results compare favorably with available data from active accretionary complexes. Active margins characterized by a significant proportion of fine-grained sediment within the incoming section, such as northern Antilles and eastern Nankai, exhibit thin taper angles, whereas those characterized by a higher proportion of sandy turbidites, such as Cascadia, Chile, and Mexico, have steep taper angles. Observations from active margins also indicate a strong trend of decreasing taper angle (from >15° to <4°) with increased sediment thickness (from <1 to 7 km). One key implication is that hydrologic properties may strongly influence the strength of the crust in a wide range of geologic settings.

Received 10 August 2005; accepted 13 December 2005; published 4 April 2006.

Keywords: pore pressure; subduction zones; critical taper.

Index Terms: 3021 Marine Geology and Geophysics: Marine hydrogeology; 3060 Marine Geology and Geophysics: Subduction zone processes (1031, 3613, 8170, 8413); 8020 Structural Geology: Mechanics, theory, and modeling; 8045 Structural Geology: Role of fluids; 8120 Tectonophysics: Dynamics of lithosphere and mantle: general (1213).

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Citation: Saffer, D. M., and B. A. Bekins (2006), An evaluation of factors influencing pore pressure in accretionary complexes: Implications for taper angle and wedge mechanics, J. Geophys. Res., 111, B04101, doi:10.1029/2005JB003990.