Subduction-accretion complexes can be approximated as wedge-shaped continua with a rigid buttress behind and a subducting litho-spheric slab beneath. Thick wedges undergoing prograde metamorphism have a negligible long-term yield strength and are likely to exhibit a complex nonlinear viscous rheology. Such a wedge will tend to deform internally until it reaches a stable configuration, in which the gravitational forces generated by the wedge geometry balance the traction exerted on its underside by the subducting slab. Accretion of material at the wedge front will lengthen the wedge and cause it to shorten internally to regain the stable geometry. This shortening will be expressed as late (out-of-sequence) thrusting, backthrusting, and folding. Conversely, underplating of sediment or crustal slices will thicken the wedge, which may need to extend internally to regain stability. Extension will cause listric normal faults that may merge downward into zones of ductile extension. Continued underplating at depth and compensating extension above provides a mechanism for bringing high-P/low-T metamorphic rocks to upper levels in the rear of the wedge, where they are commonly observed. Many major tectonic boundaries in convergent orogens (such as the Coast Range thrust in the Franciscan Complex, major nappe contacts in the Alps, and the contact between the Nevado-Filabride and Higher Betic nappe complexes in the Betic Cordillera) show abrupt increases in metamorphic grade downward across them. This is consistent with their origin or reactivation as uplift-related, extensional structures.

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