Abstract.As a contribution to Europrobe's seismic reflection profiling in the Urals (ESRU)project, three overlapping seismic reflection data sets were acquired in the Middle Urals. A 56 km long profile was registered over the Europe-Asia suture, two 25 km long intersecting profiles were collected over the Urals superdeep borehole (SG4), and an 80 km long profile was recorded eastward extending east toward
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. Many of the major structural boundaries observed at the surface (e.g. Main 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 Moho 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.g. Sisert Fault) were active. After the orogeny, 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.
Within the hinterland of Urals, there occur a variety of high-grade complexes that have been interpreted to be fragments of continents that were accreted to Baltica during Paleozoic orogeny. Some workers have inferred the high-grade complexes to have rifted off Baltica during the Early Paleozoic; others claim that they were derived from elsewhere (perhaps Siberia) and are truly exotic terranes. In the Middle Urals, the amphibolite-to granulite-facies complexes of ªmicro-continentalº affinities are generally composed of mafic to intermediate gneisses, overlain by Paleozoic mØlange and island-arc volcanics. New Pb±Pb single zircon dating provides evidence only of a Paleozoic history for one of these high-grade units, the Salda Metamorphic Complex. Eight intrusion ages have been obtained from these granulite-facies gneisses and younger intrusions. The protoliths of the gneisses, the Teliana and the Brodovo intrusive suites, were formed at around 350 Ma (359 5, 357 7 and 343 9 Ma) and at 393 5 Ma, respectively. They are cut by the East Emekh granite (334 4 Ma) and the Basianovo gabbro (336 2 Ma). The new single zircon data fit well with previously conventional K±Ar, Pb±Pb and U±Pb ages by A.P. Grevtsova et al. (unpublished data). It is proposed here that the hinterland of the Middle Urals is dominated by Devonian to Middle Carboniferous subduction-related magmatic complexes that were emplaced prior to and during collisional orogeny in the Urals. The evidence presented here and age-data from other hinterland high-grade complexes casts doubt on the ªmicro-continentº interpretation.
EUROPROBE-related seismic reflection surveys in the Middle Urals, Russia (latitude 56-62 ~ since 1993 have led to an increased understanding of the crustal structure and tectonic evolution of this region. A 400 km long profile now extends from the foreland basin in the west well into the West Siberian Basin in the east. Bivergent structures characterize the upper crust of the Uralide orogen, whereas the middle and lower crust generally contain gently west-dipping reflections. A crustal root is imaged down to almost 60 km beneath the exposed Urals. Below the foreland and the West Siberian Basin the lower crustal reflectivity is pronounced and the Moho lies at a depth of 40-45 km. Below the foreland on the recently acquired Serebrianka-Beriozovka profile, two sets of late arriving (20-25 s) reflections are present. One set reflects from a zone in the mantle at about 60-70 km depth that strikes ENE and dips about 45 ~ to the SSE. The other set may represent imbricated lower crust. Major events during the Palaeozoic tectonic evolution of the Middle Urals were: continental and oceanic rifting (Late Cambrian to Early Ordovician); development of a passive continental margin (Mid-Ordovician to Mid-Carboniferous); intra-oceanic subduction below the Tagil arc (Silurian to Devonian); east-dipping subduction of the Baltica plate (Silurian to Early Devonian); possible subduction reversal with formation of the Alapaevsk island arc and the Krasnoturjinsk-Petrokamensk active continental margin (Devonian to Early Carboniferous); active building of a mountain belt and intrusion of collision-related granitic plutons (Carboniferous to Permian).
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