Seismic reflection fabrics of continental collision and post-orogenic extension in the Middle Urals, central Russia

Deep seismic reflection surveys in the Middle Urals, central Russia, provide the basis for a revised structural and tectonic interpretation of late Paleozoic continental collision and early Mesozoic post-orogenic extension in this orogen. These profiles (ESRU '93 and ESRU '95) form a transect from the former East European platform margin in the west to accreted island-are and oceanic terrans in the east, crossing the Main Uralian fault (MUF), the principal suture zone in the Urals. The foreland of the Middle Urals is characterized by a fold-and-thrust belt developed above a gently E-dipping detachment at similar to 10 km depth, and a deeper, presumably older, and previously unrecognized detachment system of probable Late Proterozoic (Baikalian) age. A large (similar to 5 km) frontal ramp mark the axis of the regional Kvarkush anticlinorium which dominates the foreland structure. Reflectivity in the middle and lower crust of the East European platform is inferred to be primarily a Precambrian fabric. Based on correlation of the seismic data with surface geology, the MUF is redefined spatially, seismically and structurally as the top of an similar to 40 degrees E-dipping reflective package, placing it (1) at the boundary between upper greenschist to amphibolite facies rocks of continental affinity to the west and unmetamorphosed to lower greenschist facies rocks of the Tagil island are to the east, (2) at the primary topographic break in the Middle Urals, and (3) similar to 10 km further east at this latitude than recent interpretations, but consistent with previous Russian maps. At depth, the MUF appears to merge in a listric fashion with a zone of subhorizontal mid-crustal reflectivity (similar to 15-20 km depth) that truncates a series of large-scale antiforms and associated W-dipping (E-vergent?) shear zones of inferred Paleozoic age across the Uralian hinterland. These relationships suggest that the MUF was reactivated as a normal fault during late- to post-orogenic evolution of the orogen. Based on interpretation of the Moho as the base of lower-crustal reflectivity, crustal thickness varies from similar to 48 to 51 km (similar to 14-15 s) across the ESRU '93 profile, in concert with existing Russian studies showing a thickened crust beneath the Urals. To the east, on the ESRU '95 profile (beneath the Tagil island are), a strongly W-dipping reflective package in the lower crust suggests abrupt thinning of the crust to similar to 45 km (similar to 13 s), consistent-with nearby wide-angle reflection data. At the surface, this crustal thinning coincides with the Triassic Imenovskii graben. We interpret these collective observations as evidence for post-orogrenic extension of the Middle Urals in early Mesozoic time, in association with reactivation of the MUF as a breakaway normal fault at the westernmost boundary of the West Siberian basin extensional province. In contrast, results from the URSEIS '95 experiment indicate that the Southern Urals are largely unextended, implying that post-orogenic evolution varied along strike within the orogen. (C) 1998 Elsevier Science B.V. All rights reserved.