Modeling of large‐scale accretionary wedge deformation

Abstract

Instantaneous stress and velocity fields within an active accretionary wedge can be calculated using plastic slip line theory, given the assumptions of a perfectly plastic rheology and plane strain conditions. After selecting a plastic yield stress and an average density, a stress field (slip line field) can be computed for a wedge of known cross‐sectional configuration from known stress boundary conditions on the upper surface. Basal surface tractions are derived without basal boundary conditions. The geometry of the computed slip line field can then be used to constrain the lower limit of the yield stress. The known velocity boundary conditions are insufficient for calculation of a velocity field because the coupling between the wedge and the subducting plate is unknown. Consequently, basal velocity distributions must be assumed to permit the construction of velocity fields. These assumed distributions are constrained by known rates of surface uplift and rates of tilt as well as inferred patterns of instantaneous strain rates within a wedge. The Sunda accretionary wedge west of central Sumatra is used to illustrate the plasticity approach. A minimum average yield stress of 20–30 MPa and a weaker basal layer of variable effective strength are indicated for this example. Surface uplift rates may be affected significantly by regions of high basal strain rate, finite velocity discontinuities, and underplating. The known pattern of uplift rates near Nias Island, located on the outer arc ridge, is consistent with the constraint that 90% of the total incremental shortening of the wedge is concentrated within 20 km of the trench. Given a sufficient number of uplift and tilt rate observations across a particular wedge, the relative influence of underplating, tectonic erosion, and basal strain rate variations could be assessed using the perfectly plastic model.