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

Abstract

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-arc and oceanic terranes 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 ∼10 km depth, and a deeper, presumably older, and previously unrecognized detachment system of probable Late Proterozoic (Baikalian) age. A large (∼5 km) frontal ramp marks 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 ∼40°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 arc to the east, (2) at the primary topographic break in the Middle Urals, and (3) ∼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 (∼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 ∼48 to 51 km (∼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 arc), a strongly W-dipping reflective package in the lower crust suggests abrupt thinning of the crust to ∼45 km (∼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-orogenic 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.