Structural framework of the bashkirian anticlinoirum, SW Urals

The Bashkirian anticlinorium of the southwestern Urals shows a much more co mplex structural architecture and tectonic evolution than previously known. Pre-Uralian Proterozoic extensional and compressional structures controlle d significantly the Uralian tectonic convergence. A long-lasting Proterozoi c rift process created extensional basement structures and a Riphean basin topography which influenced the formation of the western fold-and-thrust-be lt with inversion structures during the Uralian deformation. A complete oro genic cycle during Cadomian times, including terrane accretion at the easte rn margin of the East European platform, resulted in a high-level Cadomian basement complex, which controlled the onset of Uralian deformation, and re sulted in intense imbrication and tectonic stacking in the subjacent footwa ll of the Main Uralian fault. The Uralian orogenic evolution can be subdivi ded into three deformation stages with differently oriented stress regimes. Tectonic convergence started in the Late Devonian with ophiolite obduction , tectonic accretion of basin and slope units and early flysch deposits (Zi lair flysch). The accretionary complex prograded from the SE to the NW. Con tinuous NW/SE-directed convergence resulted finally in the formation of an early orogenic wedge thrusting the Cadomian basement complex onto the East European platform. The main tectonic shortening was connected with these tw o stages and, although not well constrained, appears to be of Late Devonian to Carboniferous age. In the Permian a final stage of E-W compression is o bserved throughout the SW Urals. In the west the fold-and-thrust-belt progr aded to the west with reactivation of former extensional structures and min or shortening. In the east this phase was related to intense back thrusting . The East European platform was subducted beneath the Magnitogorsk magmati c are during the Late Paleozoic collision. The thick and cold East European platform reacted as a stable rigid block which resulted in a narrow zone o f intense crustal shortening, tectonic stacking and high strain at its east ern margin. Whereas the first orogenic wedge is of thick-skinned type with the involvement of crystalline basement, even the later west-directed wedge is not typically thin-skinned as the depth of the basal detachment appears below 15 km and the involvement of Archean basement can be assumed.