We use a large strain geomechanical model and critical state soil mechanics to study the evolution of stress and deformation in an evolving fold-and-thrust belt and its underlying footwall sediments. Both mean effective stress and deviatoric stress contribute to porosity loss within the wedge with 35% of the porosity loss resulting from increased shear. As a result, porosity increases abruptly across the décollement because both mean-effective and shear stresses are much higher inside the wedge than in the footwall. As the basal friction coefficient (μ b ) increases, more shear stress is transmitted across the décollement, resulting in additional compaction of the footwall sediment and decrease in the porosity contrast across the décollement. As the internal friction coefficient (μ s ) increases, the wedge sediment is more compacted because it can withstand higher mean-effective and deviatoric stresses. Inside the wedge, the sediment experiences subhorizontal shortening strain and subvertical elongation strain. We predict a 10-30 km wide "transition zone" in which the shear-stress ratios and compaction curves change rapidly between compressional critical state failure and uniaxial strain (K 0 ) state. Our model results agree with the taper angles and the stress orientations predicted by critical taper theory. This large-strain drained modeling approach provides first-order insights into the mechanical processes of loading and compaction in fold-and-thrust belts and a foundation for understanding field observations of pressure, stress, and deformation in thrust belt systems.While critical taper theory provides insight into the relations among pore pressure, material strength, and wedge geometry, it does not address the mechanics of sediment deformation within and beneath the wedge. Sediments in the wedge experience great horizontal shortening and vertical thickening caused by lateral tectonic loading. As a result, systematic changes in porosity occur within and below wedges. For GAO ET AL. 4454

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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.

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