The Structure and Tectonic Evolution of the East European Platform and the Southern Urals Junction Zone

摘要

A new model of tectonic evolution of the southern Urals is suggested based on data pertaining to the junction of paleocontinental and paleoceanic segments where various rock associations differing in their ages, compositions, and structure are tectonically juxtaposed. It is suggested that an oceanic basin existed along the Ural margin of the East European paleocontinent during at least 250-270 Ma from the Vendian to the Late Devonian. A marginal volcanic belt was located at the boundary between the East European paleocontinent and the proto-Ural ocean in Vendian-Early Cambrian time, and an accretionary wedge was formed at its eastern periphery. At the Cambrian-Ordovician boundary, the Uraltau uplift (borderland) originated on their spot and separated the oceanic region from the Sakmara riftogenic trough, which had been formed to the west of it. The second, Middle Ordovician to Middle Devonian, (paleo-Ural) pulse in the evolution of the active continental margin was related to the buildup of the Guberlya island arc on the borderland's margin. The Sakmara backarc basin was formed behind the arc and reached its maximum width in the Late Silurian-Early Devonian. Within the oceanic domain, the Magnitogorsk island-arc system facing the East European paleocontinent evolved at that time. The geodynamic environment changed in the Middle Devonian as a result of an eastward thrusting of the East European paleocontinent under the Magnitogorsk island arc. The tectonic stacking gave rise to the formation of intrabasinal cordilleras and, as a consequence, to the accumulation of thick mixtite-olistostrome units. The tectonic juxtaposition of various geological complexes initially occurred within the Sakmara basin, and then encompassed the Guberlya island arc and the western part of the paleo-Ural oceanic region. The tectonic stacking culminated in the Late Devonian and Carboniferous and brought about the formation of an asymmetric fanlike multilayer nappe-and-fold edifice with its axial ophiolitic suture extending along the Main Ural Fault. The divergent orogenic structure was reworked during the final stage of the East European paleocontinent underthrusting in the Late Paleozoic with the westward squeezing of the paleo-Ural complexes upon the margin of the paleocontinent and their variable strike-slip displacements.