The tectono-metamorphic evolution of gneiss complexes in the Middle Urals, Russia: a reappraisal

Abstract

The Middle Urals are characterized by a major virgation in the linear trend of the Urals orogen, and represent the most highly contracted part of the late Palaeozoic collisional belt. This part of the orogen is dominated by metamorphic complexes and major fault and shear zones. The Main Uralian Fault zone (MUF), the east-dipping suture of the orogen containing low-grade metamorphic rocks, separates the Sysert Complex in the east from the Ufaley Complex in the west. The Sysert Complex in the hanging wall of the MUF consists of intensely deformed gneisses, granitic intrusions and a metamorphosed mélange zone. Tectonic and isotopic investigations suggest the following stages for the evolution of the Sysert Complex: (a) pre-orogenic rifting and magmatism during Ordovician and Silurian times; (b) oceanic closure, island arc formation related to convergence and subduction during Devonian times; (c) major ductile deformation under amphibolite facies conditions related to NW-directed thrusting associated with crustal stacking during collision in Carboniferous times; (d) exhumation and contractional intracontinental tectonics during Permian times; and (e) closing of isotope systems related to cooling and the end of orogenic shortening through Triassic times. The Ufaley Complex, in the footwall of the MUF, is interpreted as an east-dipping crustal stack that records an amphibolite facies Uralian metamorphism. Lithologically the complex can be divided into pre-orogenic European basement (West Ufaley) and intensely deformed Palaeozoic metasediments and amphibolites (East Ufaley). High-pressure relics in the East Ufaley Complex are interpreted to be the result of subduction, whereas intense ductile deformation is related to overthrusting onto West Ufaley. The West Ufaley Complex is composed of gneisses, amphibolites, migmatites and granitic intrusions and has been thrust onto Devonian limestones along a major shear zone. In both Sysert and Ufaley Complexes, NW-trending stretching lineations and top-to-the-NW kinematic indicators suggest an oblique plate convergence with a significant sinistral component. The MUF is interpreted as a major normal fault that developed congruent with continental subduction and that compensated lithospheric thickening and the rapid exhumation of subducted crust in the footwall.