CRITICAL TAPER MODEL OF FOLD-AND-THRUST BELTS AND ACCRETIONARY WEDGES

The fold-and-thrust belts and submarine accretionary wedges that lie along compressive plate boundaries are one of the best understood deformational features of the Earth's upper crust. Although there is considerable natural variation among the many fold-and-thrust belts and accretionary wedges that have been recognized and explored, several features appear to be universal . In cross section, fold-and-thrust belts and accretionary wedges occupy a wedge-shaped deformed region overlying a basal detachment or decollement fault; the rocks or sediments beneath this fault show very little deformation. The decollement fault characteristically dips toward the interior of the mountain belt or, in the case of a submarine wedge, toward the island arc; the topography, in contrast, slopes toward the toe or deformation front of the wedge. Deformation within the wedge is generally dominated by imbricate thrust faults verging toward the toe and related fault-bend folding. Two North American fold-and-thrust belts that exhibit these features are shown in Figure 1. Neither of these two examples is tectonically active today; the southern Canadian fold-and-thrust belt was active during the late Jurassic and Cretaceous (150-100 Ma), whereas the southern Appa­ lachians were deformed during the late Carboniferous to Permian Alle­ ghenian orogeny (300-250 Ma). Figure 2 shows two examples that are currently active: the Taiwan fold-and-thrust belt, produced by the sub­ duction of the Eurasian plate beneath the Philippine Sea plate (Suppe 1981 , 1987); and the Barbados accretionary wedge, produced by the sub-