Uplift within accretionary foreland wedges has been explained by the development of duplexes, which provides a mechanism for “underplating” in submarine accretionary wedges with little deformation in the upper part of the wedge. We interpret duplex development beneath the Costa Rica forearc, a zone widely considered to be a prime example of nonaccretion because of the apparent absence of an imbricate fan of thrusts at the toe of the wedge. Large-scale structural geometry of accretionary wedges is compatible with low-angle, trenchward-dipping backstops in centrist to reverse-angle backstops used commonly in models of return flow within wedges, which have been invoked to explain the presence of high-P/T metamorphic rocks. We suggest that emplacement of high-P/T metamorphic rocks are better explained in a collisional setting where known uplift rates are high than by flow within noncollisional accretionary wedges.

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