Permeabilities of subduction zone sediments and their effect on pore pressure generation

Permeability is a fundamental sediment property influencing fluid flow, hence fluid pressures in the subsurface. Because elevated fluid pore pressures play a critical role in the development of accretionary complexes, including the development of the decollement zone, it is important to simulate pore pressures based on a systematic relationship of permeability and porosity. Based on laboratory permeability measurements of Northern Barbados, Costa Rica, Nankai and Peru subduction zones sediments, a high correlation between permeability and porosity was found for argillaceous sediments while little correlation was found for carbonate dominant sediments. Grain size distribution predicted more meaningful correlation between permeability and porosity than the depositional environment of the sediment alone. In the second part of the research, a one-dimensional loading and fluid flow model near the toe of the Nankai subduction zone was used to examine the effects of lower bulk permeability (sensitivity to a permeability-porosity relationship), lateral stress in the prism, and addition of a low-permeability barrier to the decollement. The results predicted significant increase in pore pressures below the decollement zone with lower bulk permeability, or when a low-permeability barrier is added at the decollement. Both simulations with lateral stress and a low-permeability barrier at the decollement resulted in sharp increases in porosity at the decollement, similar to that observed in measured porosities. In addition, these two scenarios predict maximum excess pore pressure ratios at the decollement suggesting that either of these factors would contribute to stable sliding along the decollement. In the third part of the research, results from a two-dimensional prism growth and flow model indicate pore pressures close to lithostatic pressures at the decollement when decollement was given the same permeability as the surrounding sediments. However, these pore pressures were unable to reach lithostatic pressures thus not allowing horizontal hydrofracture in the decollement zone. Addition of vertical hydrofractures in the underthrust sediments did not increase pore pressures to lithostatic in the decollement. When a bulk permeability-vertical stress relationship was assigned to the decollement, pore pressures reach values close to lithostatic pressures, suggesting that high pore pressures can be sustained at the base of the prism while fluid is expelled at the toe of the complex.