Rifting and lower crustal reflectivity: A case study of the intracratonic Dniepr‐Donets rift zone, Ukraine

Abstract

Intracratonic rifting, caused by late Devonian extensional stresses in the East European Craton, created the largest rift zone in Europe, the Pripyat‐Dniepr‐Donets rift (southeast Ukraine). The rift basin is approximately 2000 km long, up to 170 km wide, and 22 km deep. Wide‐angle refraction and reflection seismic data from the Donbas Basin deep seismic Refraction and Reflection Experiments (DOBRE'99) project together with gravity and magnetic data are analyzed for the structure and evolution of the Donbas Fold Belt, which is the uplifted and deformed part of the Dniepr‐Donets Basin. The seismic data are used for identification of large‐scale crustal structures and modeling of the seismic velocities of the crust and uppermost mantle. A ray‐trace‐based velocity and density model is derived by joint inversion of gravity and traveltime data. The inversion result reveals a zone of high density and velocity beneath the basin at middle to lower crustal levels, slightly offset to the NE of the rift axis. Full waveform synthetic seismograms, matching the observed data, show high‐amplitude and low‐frequency arrivals from this high‐density body as well as from the Moho. We interpret the high‐amplitude, low‐frequency signals as reflections from layered magmatic rocks, which intruded into the ductile lower crust during the main rift phase and subsequently were sorted by fractional crystallization. The intrusive material thickened the lower crust by approximately 50%. This may explain the enigmatic flat Moho topography across the rift zone which has been significantly stretched (β = 1.3). The rifting initiated in the late Devonian (Frasnien) as a consequence of back‐arc extension in relation to subduction of the Paleo‐Tethys Ocean. The subducting oceanic slab may have enriched the mantle with volatiles and created convection, leading to strong partial melting, upwelling, and continued rifting in the Famennien. We interpret the asymmetrical rift geometry as a combination of pure and simple shear tectonic processes.