The Evolution of Pore Pressure, Stress, and Physical Properties During Sediment Accretion at Subduction Zones

• Published in: Journal of geophysical research. Solid earth Vol. 128; no. 6
• Main Authors: Nikolinakou, M. A., Flemings, P. B., Gao, B., Saffer, D. M.
• Format: Journal Article
• Language: English
• Published: 01.06.2023
• Subjects: accretionary wedge; conceptual framework; coupled geomechanical modeling; shear‐induced overpressures; stress and overpressure evolution; weakened décollement
• ISSN: 2169-9313; 2169-9356
• DOI: 10.1029/2022JB025504

We study stress, pressure, and rock properties in evolving accretionary wedges using analytical formulations and geomechanical models. The evolution of the stress state from that imposed by uniaxial burial seaward of the trench to Coulomb failure within the wedge generates overpressure and drives compaction above the décollement. Changes in both mean and shear stress generate overpressure and shear‐induced pressures play a particularly important role in the trench area. In the transition zone between uniaxial burial and Coulomb failure, shear‐induced overpressures increase more than overburden and are higher than footwall pressures. This rapid increase in overpressure reduces the effective normal stress and weakens the plate interface along a zone that onsets ahead of the trench and persists well into the subduction zone. It also drives dewatering at the trench, which enables compaction of the hanging‐wall sediments and a porosity offset at the décollement. Within the accretionary wedge, sediments are at Coulomb failure and the pore pressure response is proportional to changes in mean stress. Low permeability and high convergence rates promote overpressure generation in the wedge, which limits sediment strength. Our results may provide a hydromechanical explanation for a wide range of observed behaviors, including the development of protothrust zones, widespread occurrence of shallow slow earthquake phenomena, and the propagation of large shallow coseismic slip. Plain Language Summary Earth's subduction zones form where two tectonic plates converge and one plate descends, or subducts, beneath the other (overriding plate). Overriding sediments are plowed onto the continent the way dirt piles up in front of a bulldozer. Some of the largest, most destructive, and tsunami‐generating earthquakes are produced along plate boundaries during subduction. The sediment behavior of both plates depends on changes in fluid pressure and stress, which are caused by tectonic forces (analogous to the bulldozer push). The strength of the plate boundary—which controls earthquake mechanics—also depends on fluid pressure, stress, and sediment rock properties. We use analytical and numerical models to simulate a subduction zone's evolution through space and time. The models use sediment‐behavior laws that account for the interaction between fluids and sediments, as the plates deform. We show that stress changes associated with the piling up of sediments generate abnormally high fluid pressures in the shallow parts of the subduction. These high pressures weaken the plate boundary, and limit powerful earthquakes to deeper in the crust. The stress changes also result in more compacted sediments and focused dewatering at the seafloor in the shallow areas of the subduction zone. Key Points Stress evolution from uniaxial to compressional state results in overpressure generation, fluid expulsion, and compaction Both mean and shear stress changes contribute to pressure generation; shear‐induced pressures are significant at and outboard of the trench High overpressures result in a weakened décollement that onsets ahead of the trench and persists tens of km into the subduction zone