Geology and sandbox analogue modelling of the Karatau structural complex (Southern Urals)

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The Urals is an example of a Paleozoic orogen which extends for nearly 2,500 km and represents the geographic border between Europe and Asia. From north to south, it is divided into Polar, Cis-Polar, Northern, Middle and Southern Urals. One of the most geologically complicated structures of the Southern Urals is the Karatau structural complex (KSC). Its complexity is confirmed by the diversity of points of view on the origin: 1) the complex arose due to vertical movements; 2) the complex is thrusted as a result of horizontal compression; 3) the complex is a flexural bulge resulted of a load on the lithosphere. The KSC belongs to the Bashkirian Megazone, part of the shortened paleomargin mainly composed of Riphean-Vendian deposits (1650–540 Ma) above an Archean basement. It appears on the map as a nearly square-shaped structure bounded by two strike-slip faults where the western deformation front of the Ural is apparently offset by 20 to 30 km to the northwest. We follow Kamaletdinov’s and Brown’s ideas, where a shortening is applied parallel to a former elongated extensional sedimentary basin perpendicular to the margin. The result being a farther propagation of the deformation front within the deep basin bounded by normal faults, which are reactivated as transfer strike-slip faults. The geodynamic setting thus corresponds to a rift-rift-rift triple junction reactivated during the orogenic shortening. We reviewed and compared the most recently available geological data and seismic lines. Subsequently, we constructed a series of new geological sections to refine the depth of the former rift sequences and the décollements both within and outside the KSC. Using this precise model of décollement depth, we use sandbox analogue modelling to better understand the kinematic and mechanic evolution of the KSC and test our hypothesis. Sandbox analogue modelling made it possible to recreate the general features of the KSC: approximative number of thrusts, their forward propagation to the west with respect to the front of deformation outside the KSC, and the differential shortenings with models respecting the constraints on the variations of décollements depths within and outside the KSC.