Sandbox model simulation of forearc evolution and noncritical wedges

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Abstract
Convergent plate boundaries exhibit a wide range of behaviors, ranging from sediment accretion and underplating to large‐scale erosion of the forearc. We have used sandbox modeling to simulate the growth of thin‐skinned contractional wedges over a strong backstop at an accretionary margin. The backstop in our model causes the growth of two distinct wedges with quite different growth histories, analogous to the accretionary wedge and the inner deformation belt (retrowedge) found in at least some forearcs. After starting with an initially steep slope, the surface flattens as the accretionary wedge grows by frontal accretion, until the wedge attains and maintains a minimum possible critical taper. The retrowedge, driven by uplift of the outer arc high, increases its surface slope as it grows until a kinematic steady state is attained. It exhibits a stable taper with slope larger than the minimum critical taper but never larger than the theoretical maximum critical taper when a basal décollement is present above the backstop. In models in which the retrowedge has not developed a décollement, it has a taper with a slope at the angle of repose. When a basal décollement is present above the backstop, the main body of the retrowedge acts as a bulldozer or a backstop, pushing the sediments in front of it, which then form an additional wedge with a small (minimum) critical taper. A broad outer arc high stands above the toe of the backstop, where maximum principal stresses rotate to meet the stress conditions in the two contractional belts on either side of it. As these belts grow in opposite directions, the accretionary wedge accretes large quantities of sediment at its front, but the retrowedge does not. Because the retrowedge can exhibit a non‐critical, stable taper in the absence of rapid erosion or accretion, it is misleading to assume that active thin‐skinned wedges are necessarily at maximum or minimum critical taper.