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Wedges and buffers: some new structural observations from the Dnieper–Donets Basin, onshore Ukraine

Abstract The onshore Ukraine Dnieper–Donets Basin (DDB) is a large partially inverted late Palaeozoic intracratonic basin. Rifting began in the late Devonian when two evaporite sequences were deposited, and Carboniferous post-rift thermal sag was accompanied in some areas by some extension. Although important quantities of Devonian salt are calibrated by wells and diapirs have been documented, classic salt diapirs are probably not as common as suggested by the literature. Indeed, in several parts of the basin salt movement was stopped and buried by up to 5 km of overburden. Four important tectonic events related to salt movement have been documented over the DDB: Tournaisian–Early Visean extension, Middle Serpukhovian extension, Late Carboniferous–Early Permian movements mostly documented recently as extension and uplift and Alpine compression. Seismic imaging of the basin has not traditionally been of the highest quality, but the reprocessed seismic data made available over several parts of the DDB as part of a Ferrexpo/RDS multi-licence project has enabled three new structural observations of the basin to be made. Firstly, it is proposed that basin margin extension, facilitated by listric faulting with thin-skinned décollement on Devonian salt and exemplified by well-imaged Tournaisian syntectonic wedges, is linked to thick-skinned partial inversion of some basement rift faults. Secondly, it is proposed that these thick-skinned faults acted as obstacles or buffers, where monoclines began to develop. These folds were exaggerated by the later tectonic movements. Thirdly, we propose that these linked tectonic events may have been caused by pulses of sag which punctuated the Lower Carboniferous post-rift thermal sag process. These three observations fit neatly with published interpretations of Tournaisian–Lower Visean and Middle Serpukhovian extension. Finally, however, a literature review and new seismic interpretation also clearly show that Late Carboniferous–Early Permian movements inverted marginal listric faults and their Tournaisian wedges, and also inverted the downdip thick-skinned faults causing the monoclines to further develop prior to the Alpine overprint. This, contrary to many published papers over the last 15 years or so, suggests that this Late Carboniferous–Early Permian event brought shortening rather than extension to the DDB.