STRUCTURE OF THE MIDDLE URALS, EAST OF THE MAIN URALIAN FAULT

In the Middle Urals, volcanic-are and back-are basin rocks of Ordovici an to Devonian age occur in the Tagil Synform. These outboard terranes were thrust westwards in the late Carboniferous onto continental marg in associations of late Proterozoic and Palaeozoic age, now exposed in the Central Uralian Uplift. The Main Uralian Fault coincides approxim ately with the suture separating the outboard terranes from the East E uropean Platform margin. New fieldwork in the hinterland of the Middle Urals in the area east of the Tagil Synform has found structural evid ence favouring E-directed thrusting of accreted terranes and eugeoclin al allochthons in the late Palaeozoic. The upper tectonic units are co mposed of ophiolite melange and volcano-sedimentary rocks of Ordovicia n to Devonian age; they are thrust onto high-grade gneisses, some of p ossible microcontinental affinities, extensively intruded by mid-Palae ozoic granitic plutons. The nappes in the hinterland are refolded by m ajor upright antiforms and synforms that fold the entire tectonostrati graphy. After thrust assembly, all tectonic units east of the Main Ura lian Fault were intruded by late Carboniferous to early Permian granit es. Reflection seismic profiles (recorded to 8 s TWT), recently reproc essed at Cornell University, image the major fold structures and demon strate that they are restricted to the upper crust, being underlain by an extensive zone of flat-lying middle crustal reflectivity. At 10-15 km depth the latter appears to truncate all structures, including the late-to post-tectonic granitoids and extensional faults, east of the Main Uralian Fault. Previous studies (potential-field, refraction-and wide-angle-reflection seismics) have identified an anomalously deep cr ust under the Tagil Synform and have concluded that the root zone of t he orogen is located beneath this belt. The new evidence presented her e supports this interpretation, with back-thrusting of the oceanic roc ks eastwards over Palaeozoic accreted terranes. (C) 1998 John Wiley & Sons, Ltd.