CRUSTAL-SCALE STRUCTURE AND EVOLUTION OF AN ARC-CONTINENT COLLISION ZONE IN THE SOUTHERN URALS, RUSSIA

The outcropping geology of the southern Urals contains a well-preserve d accretionary complex related to the Paleozoic collision that took pl ace between the Magnitogorsk are and the former East European Craton. The crustal-scale structure of the accretionary complex has been deter mined from outcropping field geology that is integrated with three ref lection seismic profiles. The reflection profiles show the accretionar y complex to be highly reflective, allowing direct comparison of many reflections with surface geological features. We interpret the accreti onary complex to be a thrust stack that is composed of shallowly subdu cted continental shelf and rise material, syncollisional sediments der ived from the are, deeply subducted high-pressure gneisses that are in tercalated with eclogites and blueschist, and, at the highest structur al level, ophiolite complexes. It is bound at the base by a thrust and at the rear by a highly deformed zone (the Main Uralian fault) adjace nt to the backstop (the Magnitogorsk are). Deposition of the Late Devo nian volcaniclastic sediments of the Zilair Formation appears to be re lated to collision, uplift, and erosion of the are, possibly following the arrival of the full thickness of the East European Craton contine ntal crust at the subduction zone. With the arrival of the continental crust at the subduction zone, offscraping and underplating of Paleozo ic slope and platform material took place at the base of the accretion ary complex. Uplift of the are was followed by its collapse and the un conformable deposition of Lower Carboniferous shallow water carbonates on top of it. A time lag of 10 - 15 Myr occurred between the high-pre ssure metamorphism and the subsequent arrival of the East European Cra ton at the subduction zone.