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All orogens have thick-skinned and thin-skinned-style provinces. Thick-skinned belts form in a variety of crustal settings common in upper plates of Andean-type orogens and in both plates of aging collisional orogens. Thin-skinned belts specifically require transport at one transcrustal sole thrust. In collision belts, the sole thrust is the suture. In belts antithetic to Andean-type subduction, the sole thrust departs upward from the point of the asthenospheric wedge.
Arrays of strike belts with different styles signal the thin-skinned scenario and its distortion in thick-skinned settings. A proposed zonation includes the following. (1) Detached fold belt with 1 to 5 km (0.6 to 3 mi) deep sole sloping at 0 to 2°, without topographic slope, and with strain increasing coreward to 80%. (2) Imbricate belt with 3 to 10 km (1.8 to 6 mi) deep sole sloping at 1.5 to 3.5°, and with bulk strain increasing to 100% or more. (3) Allochthonous belt consisting of polyphase (typical: thick-skinned pre-transport strain) crust and cover rocks above a subthrust sediment complex showing 100 to 500% strain. Base of subthrust complex is 5 to 20 km (3 to 12 mi) deep and slopes at 3 to 15°. (4) Root zone with polyphase and progressive strain, steep dips, re rocharriage, and late shallow detachment. (5) Late stage thick-skinned structures including foreland upthrusts, massifs (Helvetic and Penninic), and core complexes (cordilleran) occur anywhere between root zone and foreland. Their detachment, rise, and transport distort the thin-skinned features but cause more thin-skinned detachment in the external belts.
Presence and width of strike belts and their style elements (e.g. folds, ramps, imbrications, back thrusts) can be explained as interplay between gravity spreading and thrust loading of an elastic foreland crust. Style-determining factors include (1) time spent within thrust wedge, (2) rigidity of foreland crust, (3) topographic slope, and (4) distribution of competent and incompetent rock units.
Polyphase kinematic sequences reflect the numerical divergence between the elastic foreland wedge and the transport requirement of "optimum taper;" we distinguish polyphase wedge thickening and fold-discordant wedge transport.
Seismic work and drilling in all five zones provide the data base for the zonation. Hydrocarbons are produced from all zones except zone 3. Conventional production comes from zones 1, 2, and 5. Most ongoing overthrust plays are made in zone 2. Zone 3 has potential where the allochthon is thin (such as ophiolites or detached shelf sediments), where the foreland sediment series is complete (as in collision belts), and where collision is too recent for thermal stabilization.
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