Dating thermal events in the Donbas Foldbelt (Ukraine)

The Donbas Foldbelt (Ukraine) is a thrust-faulted and folded, inverted, segment of the Dnjepr-Donets Basin. It comprises Devonian and Carboniferous sediments. The latter host one of the major coalfields in the world. The mechanisms causing formation and inversion of the basin are being studied in the frame of the European Science Foundation EUROPROBE "GeoRift" project. Nevertheless, the age of the compressional structures is still a matter of controversy. Studies of the thermal history of the Donbas Foldbelt using the basin modelling approach were performed by the applicant (Sachsenhofer et al., 2000) and showed a moderate heat flow during Permian maximum burial (40-70 mW/m 2 ). An additional heating event with high heat flow (100-200 mW/m 2 ) occurred either before or after maximum burial. Main goal of the proposed project is to date this "additional heat flow event" using apatite and zircon fission track analysis. The age is of importance for different scientific and applied aspects of the evolution of the basin. Knowledge of this age will result in: * A better understanding of the thermal and geodynamic evolution of the basin 1. Only the knowledge of the exact age of the heat flow event allows a profound discussion on its cause. 2. Any age younger than Early Permian would provide definitive proof for a Cretaceous age of the main compressional event. 3. The rheological behaviour of the lithosphere is strongly controlled by temperatures. 4. Knowledge of the age of coalification will result in better estimates of the thickness of eroded sediments. * A better understanding of the coal-bed methane potential of the Donbas Foldbelt. * A better understanding of epigenetic gold deposits, which occur in areas characterised by the "additional heat flow event".

The Donbas Foldbelt is a folded and inverted segment of the Dnjepr-Donets Basin. The basin fill comprises Devonian and Carboniferous sediments. The latter host one of the major coalfields of the world. Significant parts of the Carboniferous sediments were eroded during an early Permian uplift phase. The age of the compressional structures (Permian or late Cretaceous) is a matter of discussion. Previous investigations have shown that paleo-heat flows during Permian maximum burial were in the range 40 to 75 mW/m and provided evidence for an additional thermal event (100 - 200 mW/m) with an unknown age. Within the frame of the present project, apatite fission track (FT) data were used to determine the age of the "additional heat flow event" and to study the Mesozoic thermal history. Main study results include: The heating event in the Krasnoarmeisk area occurred during early/middle Triassic times (240-230 Ma). The areal extent of the event is larger than presumed earlier. The Permo-Triassic(?) event also occurred in the Donetsk-Makeevka region, but cannot be dated with the FT data. This is because of a relatively strong (90-100C) Jurassic/Cretaceous thermal overprint. The latter is probably related to intraplate extension and magmatic activity during late Jurassic times. Although the southern Donbas area was affected by Permian alkaline magmatism, the thermal overprint is mild. This suggests that the Permian magmatic phase had only little influence on regional heat flow. Rocks in the southern and westernmost study area cooled below 60C already during the Jurassic (140-160 Ma), whereas rocks in the Donets-Makeevka region remained at temperatures above 60C till Cretaceous/Tertiary times (125-45 Ma). In the latter area, a correlation exists between the present-day depth of the samples and their cooling ages. This suggests that folding occurred before late Cretaceous times. The results have implications for different scientific and applied aspects of the evolution of the basin including (1) the geodynamic evolution of the basin, (2) the timing of coal-bed methane generation, (3) the timing of epigenetic mercury, antimony and gold deposits in the Donbas Foldbelt.