For the purposes of this book the term ‘thrustbelt’ is given a broad meaning to encompass any deformed belt in which contractional or transpressional brittle and brittle/ductile structural styles dominate over other types of structures, including conventional thrustbelts, transpressional ranges, toe thrusts and accretionary prisms (Figs. 1.1–1.6, Tables 1.1–1.6).

Conventional thrustbelts

Conventional thrustbelts evolve out of either passive margin or intracratonic rift systems and their consequent sedimentary basins (Fig. 1.7). Examples of passive margin sediments involved in a thrustbelt are seen in the Appalachians, Andes or Alps. Examples of orogenic belts evolved out of intracratonic rifts are the Atlas Mountains, Palmyrides or the northern Andes. The rift systems, whether of pure extensional or transtensional origin, form the fundamental crustal weaknesses that focus compressional stress and provide the volume of rocks that subsequently become incorporated into the thrustbelt. Nice examples of extensional and transtensional rifts later involved in thrusting come from the Urals. Their different geometries in relation to the direction of compression determined different structural styles in different parts of the Urals. Passive margin basins, with their broad post-rift sedimentary prisms tapering out onto the nonrifted cratons favour ‘thin-skin’ structural styles inwhich the sedimentary cover strata are detached and deformed independently of the underlying basement (Fig. 1.8). Intracratonic rift systems, on the other hand, tend to produce ‘thick-skin’, or basementinvolved thrustbelts in which inverted half-grabens or uplifted basement blocks are a dominant feature (Figs. 1.9 and 1.10). However, the distinctions between thinand thick-skin thrustbelt styles are not rigid. Even in the thin-skin variety, the basal thrust surfaces root within displaced basement elements, many of which can be demonstrated to have been older normal faults (Fig. 1.11).