Overthrust Nappe

a sheet of rock that has moved a distance of a few or 100 or more kilometers in a horizontal direction from the place of initial occurrence. An overthrust nappe may be sedimentary, volcanic or, less frequently, igneous or metamorphic. Ranging in thickness from a few hundred meters to several kilometers, it is underlain by a gently rolling or almost flat surface.

Overthrust nappes are found primarily within geosynclinal systems of different ages. The rocks that lie in the base of the nappe and do not experience significant horizontal displacement are called the autochthon, and the rocks of the overthrust nappe itself are called the allochthon. The allochthon and the autochthon, although they are equal in age, often differ quite markedly in composition and conditions of formation and belong to different paleogeographic zones of folded structures. As a rule, alloch-thonous rocks originate from the innermost zones of a geosynclinal system. In this case, the overthrust nappe is frequently composed of rock that is older than the autochthon. Autochthonous rocks sometimes outcrop from beneath the allochthon in low-lying relief sectors, forming windows, whereas the rocks of the allochthon are preserved in the form of erosion remnants in elevated areas. The layers of the allochthon may form overtuned and recumbent folds or occur in the form of synclinoria, experiencing large, gentle uplifts and downwarpings together with the base.

Overthrust nappes are believed to be caused by transverse horizontal compression that occurs in geosynclinal systems and by the gravitational creep of the rocks that make up the mountain structures which have arisen from the geosynclines. Both factors may operate together, first compression and heaving and then gravitational creep.

Overthrust nappes were first described in the late 19th century in the Alps, the Canadian Rockies, and the mountains of Scandinavia. It was later established that the nappes have played a large part in the formation of some mountains (the Alps, the Carpathians, and the Himalayas), whereas in others they are not significant (for example, the Andes). Major overthrust nappes in the USSR have been identified in the Carpathians, the Caucasus, the Urals, the Tien-Shan, and the Koriak Highland.

V. E. KHAIN