PHANEROZOIC FOLD-THRUST BELTS: GEOMETRIC AND KINEMATIC STYLE ELEMENTS

Fold-thrust belts (FTB) are the shallow, compressional, and ephemeral fringes of moving terranes with a life span of 1 to 10*10^6 * a. There are three geodynamic settings: (a) orogenic belts near subduction zones, (b) topographically sloping flanks of orogenic plateaus, (c) bathymetrically sloping extensional or passive sediment prisms. In settings (a) and (b), the FTB develops synthetic or parallel to subduction, or antithetic and indenting or conjugate to subduction. Style elements and shapes of FTB are not setting-specific, but they all comply with critical taper. Most orogenic belts are kinematic successions of discreet FTB units. In cross section, FTB units are wedge-shaped and 100 km wide. In map view, they are crescent-shaped and 1000 km long. They form by compressional wedge thickening and by toe addition, and at strain rates of between 1 and 10 * 10^-15 * s^-1. Modified by foreland load flexure, the wedge base is a thrust fault and/or detachment emerging from the ductile crust to the surface or to a blind supracrustal level. This limits the unroofing in FTB to 25 km or 7 kb. The wedge top slopes outward, is largely erosional, and may be depositional at the toe. Universal and bulkstrain- dependent structural styles in FTB are determined by covariant quantities of basement upthrusts, thrust imbrications, and detached folds. Kinematically, FTB styles reflect frontal and basal wedge growth.