Characterization of excess pore pressures at the toe of the Nankai accretionary complex, Ocean Drilling Program sites 1173, 1174, and 808: Results of one‐dimensional modeling

Abstract

Elevated fluid pore pressures play a critical role in the development of accretionary complexes, including the development of the décollement zone. In this study, we used measured permeabilities of core samples from Ocean Drilling Program (ODP) Leg 190 to develop a permeability‐porosity relationship for hemipelagic sediments at the toe of the Nankai accretionary complex. This permeability‐porosity relationship was used in a one‐dimensional loading and fluid flow model to simulate excess pore pressures and porosities. Simulated excess pore pressure ratios (as a fraction of lithostatic pressure‐hydrostatic pressure) using the best fit permeability‐porosity relationship were lower than predicted from previous studies. We then tested sensitivity of excess pore pressure ratios in the underthrust sediments to bulk permeability, lateral stress in the prism, and a hypothetical low‐permeability barrier at the décollement. Our results demonstrated significant increase in pore pressures below the décollement with lower bulk permeability, such as obtained by using the lower boundary of permeability‐porosity data, or when a low‐permeability barrier is added at the décollement. In contrast, pore pressures in the underthrust sediments demonstrated less sensitivity to added lateral stresses in the prism, although the profile of the excess pore pressure ratio is affected. Both simulations with lateral stress and a low‐permeability barrier at the décollement resulted in sharp increases in porosity at the décollement, similar to that observed in measured porosities. Furthermore, in both scenarios, maximum excess pore pressure ratios were found at the décollement, suggesting that either of these factors would contribute to stable sliding along the décollement.