SEISMIC-REFLECTION FABRICS OF CONTINENTAL COLLISION AND POSTOROGENIC EXTENSION IN THE MIDDLE URALS, CENTRAL RUSSIA

Deep seismic reflection surveys in the Middle Urals, central Russia, p rovide the basis for a revised structural and tectonic interpretation of late Paleozoic continental collision and early Mesozoic post-orogen ic extension in this orogen. These profiles (ESRU '93 and ESRU '95) fo rm a transect from the former East European platform margin in the wes t 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 be lt developed above a gently E-dipping detachment at similar to 10 km d epth, and a deeper, presumably older, and previously unrecognized deta chment system of probable Late Proterozoic (Baikalian) age. A large (s imilar to 5 km) frontal ramp mark the axis of the regional Kvarkush an ticlinorium which dominates the foreland structure. Reflectivity in th e 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, seismicall y and structurally as the top of an similar to 40 degrees E-dipping re flective package, placing it (1) at the boundary between upper greensc hist to amphibolite facies rocks of continental affinity to the west a nd unmetamorphosed to lower greenschist facies rocks of the Tagil isla nd 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 re cent interpretations, but consistent with previous Russian maps. At de pth, the MUF appears to merge in a listric fashion with a zone of subh orizontal mid-crustal reflectivity (similar to 15-20 km depth) that tr uncates a series of large-scale antiforms and associated W-dipping (E- vergent?) shear zones of inferred Paleozoic age across the Uralian hin terland. These relationships suggest that the MUF was reactivated as a normal fault during late- to post-orogenic evolution of the orogen. B ased on interpretation of the Moho as the base of lower-crustal reflec tivity, crustal thickness varies from similar to 48 to 51 km (similar to 14-15 s) across the ESRU '93 profile, in concert with existing Russ ian studies showing a thickened crust beneath the Urals. To the east, on the ESRU '95 profile (beneath the Tagil island are), a strongly W-d ipping reflective package in the lower crust suggests abrupt thinning of the crust to similar to 45 km (similar to 13 s), consistent-with ne arby wide-angle reflection data. At the surface, this crustal thinning coincides with the Triassic Imenovskii graben. We interpret these col lective observations as evidence for post-orogrenic extension of the M iddle Urals in early Mesozoic time, in association with reactivation o f the MUF as a breakaway normal fault at the westernmost boundary of t he West Siberian basin extensional province. In contrast, results from the URSEIS '95 experiment indicate that the Southern Urals are largel y unextended, implying that post-orogenic evolution varied along strik e within the orogen. (C) 1998 Elsevier Science B.V. All rights reserve d.