The Upper Paleozoic Preflysch and Overthrusting in the Tuerkstan-Alay Ranges, Southern Fergana

摘要

The Upper Paleozoic preflysch sedimentary rocks in the Tuerkstan-Alay ranges are combined in a common section with limestone of autochthon and synsedimentation carbonate nappes, thus forming the uppermost layers of the stratigraphie section of the latter. By their lithology, relatively small thickness, fades variability, and position at the boundary between carbonate and flysch-olistostrome sequences (in the autochthon), these rocks correspond to a certain extent to preflysch of the Urals and the Mediterranean Alpine Bell. This association of clayey, carbonate, and terrigenous rocks is strictly constrained in stratigraphy (the upper portion of the lower Moscovian substage and the lower portion of the upper Moscovian substage) and localization (the southern slopes of carbonate platforms). The formation of this rock association immediately predated the Late Paleozoic overthrusting and deposition of terrigenous flysch. In paleotectonic terms, preflysch is an indicator of the initial stage of tectonic and magmatic activation that led to the replacement of carbonate sedimentation with deposition of terrigenous and clayey sediments, coeval volcanism, and stratiform ore formation. The following sequence of events has been outlined in the Early and Middle Carboniferous: (1) thrusting of volcanic-sedimentary rocks filling troughs over the northern margins of carbonate platforms, (2) lateritic weathering and deposition of marine bauxite in the Bashkirian and early Moscovian, (3) repeated overthrusting and deposition of preflysch on southern slopes of platforms, (4) invasion of the frontal flysch trough from the south, (5) scouring of preflysch and the underlying limestone, and (6) formation of flysch-olistostrome sequences and tectonic and gravity nappes in the late Moscovian time. This interpretation takes into account the southward vergence of thrust sheets and nappes, the structure and localization of allochthonous fragments of marginal zones of carbonate platforms, and the pre-Bashkirian thrusting of volcanic and sedimentary rocks over the condensed pelagic deposits of the Shalan Group. It is suggested that bauxite and preflysch materials had the same source and were deposited in the Middle Carboniferous on the offshore carbonate shoals.