HR: 1340h
AN: T33C-1478    [Abstracts]
TI: Pore Pressure Development in Sub-Décollement Sediments in Subduction Zones: Insights From Laboratory Data and Numerical Modeling
AU: * Skarbek, R M
EM: rskarbek@geosc.psu.edu
AF: Department of Geosciences, Penn State University, Deike Building, University Park, PA 16802, United States
AU: Saffer, D M
EM: dsaffer@geosc.psu.edu
AF: Department of Geosciences, Penn State University, Deike Building, University Park, PA 16802, United States
AB: Pore pressure in subduction zones is a primary control on fault strength and sliding stability. Rapid loading of sediments by tectonic loading and burial generally outpaces fluid diffusion, resulting in the generation of excess pore pressure. The limited drainage allows only a small amount of consolidation and deformation relative to a completely drained section. In addition to the development of excess pore pressure, it is important to understand the distribution of porosity reduction, because it partly controls deformation style. The sub-décollement (underthrust) sediments are especially important because they directly influence the frictional properties and stress state in the décollement zone. Previous studies have documented elevated sub- décollement fluid overpressures at several margins, which serve to reduce the frictional strength along the wedge base. Here, we use numerical methods to solve the one-dimensional equation for fluid diffusion through porous media, to analyze the development of overpressures within subducting sediments beneath a highly permeable décollement that acts as a drainage boundary. Our model allows the compressional and hydrological properties of consolidating sediments to vary in time and space in response to drainage and porosity reduction (e.g., Gamage and Screaton, 2006). We define sediment hydraulic and mechanical properties through extensive geotechnical testing of sediments gathered during ODP Legs 190 (Nankai margin) and 205 (Costa Rican margin). We use constant rate of strain (CRS) consolidation experiments to define coefficient of consolidation (c_v), coefficient of volume compressibility (m_v) and permeability (k), at effective stresses from 0 - 90 MPa, corresponding to porosities of 12 - 68 %. Permeabilities derived from the CRS tests are supplemented by flow through tests in which a hydraulic gradient is imposed across a sample. The experimental results define a log-linear relationship between permeability and porosity ( φ). All permeability and porosity data fall within an upper bound defined by logk= 8.8261 φ - 20.7370 and lower bound defined by logk= 6.2952 φ - 20.9633. We have used our model to study the buildup of pore pressures in the Nankai Trough, an actively accreting prism where the décollement is located approximately 600 mbsf at the trench, and at the non-accretionary Costa Rica margin. At both margins, we compare our modeled pore pressures and porosities with those observed by drilling within a few kilometers of the trench, and with values inferred from geophysical data up to ~20 km from the trench. Simulated excess pore pressures for the Nankai Through range from about 5.5 - 8 MPa at 20 km from the trench. These values correspond to the permeability bounds reported above and differ substantially from values of ~15 MPa inferred from seismic velocities 20 km landward of the trench (Tobin and Saffer, 2006) suggesting that décollement permeability and overpressure in the fault zone play an important role in affecting excess pore pressure in the sub-décollement zone. In both cases, we observe that drainage allows the zone of minimum effective stress to migrate down section, which may partly control downstepping of the décollement. Ongoing work includes continued testing of samples to further constrain the compressional and hydrological properties of subducting sediments, and incorporation of fluid production from clay dehydration, and evaluation of the effects of finite décollement permeability or overpressure into our models.
DE: 8100 TECTONOPHYSICS
SC: Tectonophysics [T]
MN: 2007 Fall Meeting