Tectonic variation in the Dniepr-Donets Basin from automated modelling of backstripped subsidence curves

The multiphase rift evolution of the Dniepr-Donets (DD) Basin is analysed by 1D backstripping and tectonic modelling of 66 stratigraphic sections, mostly from well data, distributed throughout the basin. The backstripping results illustrate strong temporal and lateral variations in tectonic subsidence. It is minor during an 'inceptive' pre-rift phase (Givetian-early Frasnian; 380.8-370 Ma) and does not show clear spatial correlations within the DD Basin framework. The main rift phase (Frasnian-Famennian; 370-362.5 Ma) corresponds to an acceleration in subsidence rates with tectonic subsidence up to 1500 m along the basin axis, displaying a slight increase from the northwest to the southeast. Strong lateral variations perpendicular to the basin axis reflect the effects of major basin border faults. A rift phase in the Visean (345-340 Ma) is characterised by relatively minor subsidence (up to about 500 m, except for some wells in the southeast), following a stage of extremely low rates of deposition in the earliest Carboniferous. The end of this rift phase can be taken to mark the onset of convex subsidence patterns in the later Carboniferous and Permian, typical in shape of post-rift thermal subsidence, but significantly increasing in magnitude from northwest to southeast in the basin. This trend shows no one-to-one relationship with Late Devonian syn-rift subsidence.

The tectonic subsidence curves have been modelled using lithospheric stretching assumptions allowing different stretching factors for the crustal and sub-crustal lithosphere and the incorporation of finite and multiple stretching phases. A numerical technique was developed that automatically finds best fit stretching parameters for part or all of the tectonic subsidence data, given the specification of onset and duration of the various rifting events. Maps of the 1D modelling results display intrabasinal variations in terms of timing and magnitude of rifting. The results indicate that the observed spatial and temporal variations of tectonic subsidence in the DD Basin, specifically the increasing ratio of Carboniferous to Late Devonian subsidence from northwest to southeast, cannot be explained by either a uniform or depth-dependent stretching model adopting only a single Frasnian-Famennian rift phase. A two-phase model, in which a significant part of Carboniferous tectonic subsidence is related to Visean rift activity, strongly dominated by sub-crustal lithospheric attenuation, provides a satisfactory fit to the data although the tectonic implications of such a model are not easily reconcilable by independent constraints. A semi-quantitative investigation of the possible role of withdrawal of underlying Devonian salt during the Carboniferous to produce additional post-Devonian rift phase sediment accommodation space suggests that the effects of such a mechanism could be significant but that they are unlikely to preclude the necessity of the Visean rifting event to explain the Carboniferous basin evolution.