PALAEOZOIC TECTONIC EVOLUTION OF THE MIDDLE URALS IN THE LIGHT OF THE ESRU SEISMIC EXPERIMENTS

Abstract

A new compilation of deep seismic reflection data, in combination with the results of recent geological and isotopic investigations, provides the basis for significantly refined structural and evolutionary models of the Middle Urals. Man of the major structural boundaries observed at the surface (e.g. M a in Uralian Thrust, Main Uralian Normal Fault, Serov-Mauk Fault, Prianitchnikova Shear Zone) can be followed on the seismic reflection data to depths of 8-15 km. Only a few features can be traced through the horizontal to shallow dipping zones of middle-crustal reflectivity that extend beneath much Of the Middle Urals. Exceptions might be the Deevo Thrust and a newly discovered deep band of moderately dipping reflections beneath the West Siberian Basin that may represent the remnants of an ancient subduction zone. The reflectivity of the lower crust and Motto are similar beneath the hinterland of the Urals and below the western part of the West Siberian Basin, suggesting that in both places they have the same extensional origin. Accretion of various exotic terranes to the eastern margin of Baltica during Late Palaeozoic time involved considerable shortening of the continental margin. The accreted terranes (e.g. Tagil, Petrokamensk and Alapaevsk Arc Complexes) consist mostly of island-arc material formed during Mid- and Late Palaeozoic time. Closely associated with the island arcs are several high-grade metamorphic complexes (e.g. Salda and Murzinka-Adui Complexes) that were exhumed during and after Late Palaeozoic convergence. Towards the end of the orogeny, several north-south-trending strike-slip faults (e.z. Sisert Fault) were active. After the orogen, major periods of tectonism and magmatism in the Middle Urals resulted in the extrusion of the Siberian trap basalts, crustal stretching and/or underplating and the formation of the West Siberian Basin, the planet's largest intracontinental sedimentary basin. Finally, a period of renewed thrust faulting, often reactivating older faults, led to the inversion of the Mesozoic basins.