Tectonics of the middle Ural

Sammanfattning: This thesis concerns the crustal structure and tectonic evolution of the Middle Urals. Theresults are based on near-vertical reflection seismic profiling, geological fieldobservations and single zircon geochronology,The Ural Mountains delimit. the eastern margin of Baltica. Prior to PalaeozoicUralian orogeny, this margin was bounded by an ocean. The closing of this ocean wassimultaneous with the formation of a series of intra-oceanic island-arcs during theSilurian and Devonian. In the Middle Urals, these arcs collided with Baltica during theLate Carboniferous and Early Permian. New isotope-age data show that high-grademetaigneous complexes in the hinterland are not microcontinents, as previouslythought, but rather the roots of the island-arcs. The reflection seismic data reveal that thearcs are separated from the continent and each other by major thrusts, that are E-dippingin the west and W-dipping in the east. The suture between Baltica and the accretedterranes is the Main Uralian Fault Zone (MUFZ), a complex E-dipping deformationzone that records both early suturing and later extension. Normal displacements are alsoobserved on faults in the hinterland, some normal movement probably occurring at thesame time as the collision, and some related to Triassic opening of the West SiberianBasin (WSB). The deposits of the WSB now cover large parts of the eastern MiddleUrals. The seismic data show prominent, sub-horizontal reflections from the lower crust(38 to 43 km depth) under the WSB and the Middle Urals, east of the MUFZ. The Moho is, in most places, interpreted to be at the base of this reflective band. However, directly east of the MUFZ and for 50 km eastwards, the Moho, as identified from other geophysical methods, is at c. 50 km depth, i.e. below the base of the sub-horizontal reflectivity. The features observed in the lower crust, based on their truncation of the Palaeozoic structures and by comparison with seismic data from other areas, are interpreted to have formed from crustal stretching and to be related to the opening of the WSB.