Character of the decollement in the Leg 131 area, Nankai Trough

Abstract

The decollement under the Nankai Trough accretionary prism in the Leg 131 area is imaged seismically as a high-amplitude, reversed-polarity reflection. The decollement is a 19m-thick zone of intense brecciation where it was penetrated on Leg 131. Physical properties measurements at Site 808 indicate that the strata beneath the decollement have lower velocities and densities than overlying strata. The seismic signature of the decollement varies considerably along both strike and dip. Synthetic seismic models show that the decollement signature can best be modeled by a sharp decrease in velocity and density beneath the decollement. Numerous models with a range of reasonable overlying structures were explored to examine the lateral amplitude variations. The thickness of the decollement zone is very close to its tuning thickness, so tuning effects can explain much of the amplitude variations. The changes in amplitude are therefore caused by lateral changes in velocity, density, and thickness of the decollement, which are in turn probably driven by fluid pressure variations within the decollement zone. -from Authors