Stress, porosity, and pore pressure in fold-and-thrust belt systems
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I use forward geomechanical modeling to study the mechanical and fluid flow behaviors in compressional regions such as fold-and-thrust belts and accretionary wedges. Under drained conditions, lateral tectonic loading increases the mean effective stress and deviatoric stress and drives the sediments to shear-failure as the sediment approaches the deformation front (or trench location). The shear-induced porosity-loss accounts for about one third of the total porosity-loss during tectonic loading under drained conditions. There are four characteristic stress regions in my model: far-field, transition, critical state wedge, and footwall. In the transition zone, the shear-stress ratio varies significantly and the stress state changes from uniaxial-strain compression state to critical state. Increasing the basal friction coefficient leads to a higher surface slope angle and more porosity loss in the footwall whereas increasing the sediment internal strength leads to a lower surface angle and more porosity loss in the hanging wall. Fluid flow has a great impact on stress and compression in fold-and-thrust belts. My models show that the hanging-wall overpressure is greater than the footwall near trench but lower than the footwall overpressure towards the inner wedge. The high hanging-wall overpressure near trench is cause by the rapid increase of total mean and deviatoric stress. A significant finding is that the high overpressure near trench reduces the vertical effective stress and causes the décollement to be weak near the frontal wedge. Low permeability and high convergence rate promote overpressure generation and enlarge the overpressure-weakened decollement region. This study has broad impacts on the earthquake studies, faults stability analysis, and topics associated fluid flow transport in compressional margins.