THE TECTONO-METAMORPHIC EVOLUTION OF GNEISS COMPLEXES IN THE MIDDLE URALS, RUSSIA - A REAPPRAISAL

The Middle Urals are characterized by a major virgation in the linear trend of the Urals orogen, and represent the most highly contracted pa rt of the late Palaeozoic collisional belt, This part of the orogen is dominated by metamorphic complexes and major fault and shear zones. T he Main Uralian Fault zone (MUF), the east-dipping suture of the oroge n containing low-grade metamorphic rocks, separates the Sysert Complex in the east from the Ufaley Complex in the west. The Sysert Complex i n the hanging wall of the MUF consists of intensely deformed gneisses, granitic intrusions and a metamorphosed melange zone, Tectonic and is otopic investigations suggest the following stages for the evolution o f the Sysert Complex: (a) pre-orogenic rifting and magmatism during Or dovician and Silurian times; (b) oceanic closure, island are formation related to convergence and subduction during Devonian times; (c) majo r ductile deformation under amphibolite facies conditions related to N W-directed thrusting associated with crustal stacking during collision in Carboniferous times; (d) exhumation and contractional intracontine ntal tectonics during Permian times; and (e) closing of isotope system s related to cooling and the end of orogenic shortening through Triass ic times. The Ufaley Complex, in the footwall of the MUF, is interpret ed 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 Pa laeozoic metasediments and amphibolites (East Ufaley). High-pressure r elies in the East Ufaley Complex are interpreted to be the result of s ubduction, whereas intense ductile deformation is related to overthrus ting 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 sign ificant sinistral component, The MUF is interpreted as a major normal fault that developed congruent with continental subduction and that co mpensated lithospheric thickening and the rapid exhumation of subducte d crust in the footwall.