Experimental Investigation of Decollement Zone Mechanics and Consolidation State

Studies on accretionary prism decollement fault zones show them to be very weak structures, with pore pressures close to the lithostatic overburden stress. We have designed a series of experiments to address whether high strains and retained high porosity can coexist in such decollement zones. The experiments will also document the stress paths to obtain these conditions, as well as the microstructures that are developed. We are deforming artificial analogs of decollement lithologies (35% silt sized quartz, 50% montmorillonite, 15% kaolinite) in a high-pressure ring shear device mounted on an MTS torsion-testing device. Experiments were conducted with a normal stress of 9 MPa, a shear strain rate of ca. 7 x 10^-4s^-1 to a shear strain of ca. 40 (360\deg rotation in 16 days). Fluid pressures of 7 MPa and 8.8 MPa were used. Experiments were conducted on samples which were normally consolidated and overconsolidated (compacted and then subject to a decrease in effective stress). Experimental conditions, are such, that they simulate in situ conditions from currently sampled decollement zones (e.g. Barbados and Nankai). A normally consolidated sample shows a shear stress of ca. 2.5 MPa (normal stress - 9 MPa, fluid pressure 7 MPa, normally consolidated); the overconsolidated samples are expected to be stronger and the samples with a higher fluid pressure are expected to be weaker. We are also able to measure fault-parallel and fault-perpendicular P and S wave seismic velocity. These results will allow comparison with seismic observations. Preliminary results indicate a reduction of Vp and Vs on the order of 5-8% with the reduction occurring with a shear stress of less than 15. The results of these experiments will allow us to explore the fundamental processes of fault mechanics and the significance of the seismic response. These results can be directly compared to existing geophysical and geologic information from decollement zones.