Thick-skinned and thin-skinned styles of continental contraction

Continental crust subjected to horizontal contraction in convergent settings deforms in a variety of styles. In many instances, it is useful to consider the deforming crustal sections in terms of crystalline basement rocks underlying incipiently undeformed sedimentary strata. Three deformation styles are commonly found in such settings. The structural style referred to as thin-skinned tectonics encompasses a stack of thrust sheets composed of non- or weakly metamorphic sedimentary rocks. The associated thrust faults usually level off in a mechanically weak decollement horizon along which a substantial amount of displacement occurs in the course of the formation of the fold-and-thrust belt. Thrust faults may also cut down into the crystalline basement and level off a few kilometers beneath the basement-cover interface. The term basementinvolved thin-skinned tectonics is proposed to describe this style of continental contraction. This style, too, is characterized by stacks of thrust sheets. In many cases however, such nappe stacks are overprinted by pervasive folding of a crust thermally weakened by magmatic activity or regional burial metamorphism. Thick-skinned tectonics seems less common. This style implies that thrust faults cut across the entire upper crust (and possibly the lower crust). The associated continental contraction is smaller, and the ensuing deformation is characterized by warping of the basement-cover interface. Displacements accumulated in a major basal detachment horizon may connect into mantle by means of a subduction zone. However, under elevated temperatures, pervasive deformation of the hanging wall and footwall rocks may compensate large displacements over relatively short distances. Thin-skinned fold-and-thrust belts are common on both sides of collisional orogens. Noncollisional orogens tend to be more asymmetric.