ABSTRACT

Most foreland fold and thrust belts are linear or arcuate belts of folds and thrust faults that form a marginal part of an orogenic belt between an undeformed craton and a more intensely deformed inner zone. They are characterized by an assemblage of structures that include low-angle thrust faults, folds, tear faults, and ramped and folded thrust faults that deform a wedge-shaped sedimentary sequence. In some belts it can be proved, and in others it is suspected, that these structures have been detached along one or more decollement zones, producing a shortened structural cover and an unshortened structural basement. Vergence of thrusts and folds is predominantly toward the craton, and in many belts it can be shown that there is a general decreasing age of structural development toward the craton. AJn external foredeep filled with molassic sediments is an integral part of these belts, and the history of thrust movements can often be read from these rocks. Foreland fold and thrust belts are known in orogenic belts of early Proterozoic to recent age.

There is, however, great variation in the general geometry, composition, and evolution of these belts, as well as numerous exceptions to the characteristics outlined above. The sedimentary wedge may consist of rocks that range from continental margin to deep sea fan environments. Variation in rock sequence, particularly the variation in thickness of ductile units such as evaporites, greatly controls the geometry of structures. Some foreland belts involve crystalline basement rocks both locally or extensively, and in one belt (High Atlas, Morocco) basement shortening occurs directly beneath the belt.

Kinematic and dynamic evolution of foreland belts are diverse, but they are an integral part of orogenes and are related to convergent boundary dynamic systems coupled with varying amounts of transform motions. Foreland belts can be shown to have evolved from the following convergent settings: both synthetic and antithetic to B- and A-type subduction, continent and arc collisions, and convergence within transform systems or combinations of such systems. The dynamics of foreland belts has been related to crustal convergence, lateral spreading of plutonic and metamorphic rocks in orogene cores, and a variety of gravity-induced instabilities. Palinspastic reconstructions of foreland belts suggest that while all of these mechanisms may contribute in different degrees to the formation of foreland belts, crustal convergence is the dominant process.

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