Structural Paragenesis and Geological Conditions of Formation of the Frontal Allochthon of the Southern Cis-Urals

The article presents the results of a structural and geological study of the Ik–Sakmara segment of the western zone of frontal deformations of the Urals. Within this tectonic zone, at the late stages of the evolution of the Hercynian structure of the Urals, a thick complex of Upper Paleozoic sedimentary rocks making up the eastern slope of the Pre-Ural Trough was deformed. The studies, including field study of structures, detailed structural mapping, and construction of principal cross sections, made it possible to refine ideas about the tectonic structure of the Southern Urals in this region—the most controversial in terms of structure. Based on the results of the study, multirank tectonic structures were demonstrated on the compiled maps and in cross sections, the morphological interrelation of which makes it possible to combine them into a single fold–fault paragenesis formed in the regional horizontal compression. The formation of tectonic structures began immediately after the deposition of the evaporite strata of the Kungur stage, accompanied by an orogenic rise during the deformation area up to 6–8 km. Calculations and graphical constructions performed in accordance with the basic provisions of the methods of structural reconstructions of fold–thrust belts and the concept of balanced sections made it possible to substantiate the allochthon structure of the zone of frontal deformations, to estimate the thickness of the allochthonous plate, and to determine the amplitudes of displacement (decreasing from east to west) along the subhorizontal detachment, as well as the value of the horizontal deformation of layers in the allochthon.