THERMOTECTONIC EVOLUTION OF THE UKRAINIAN DONBAS FOLDBELT: EVIDENCE FROM ZIRCON AND APATITE FISSION TRACK DATA

Abstract

The Donbas Foldbelt forms part of a large Devonian rift cross-cutting the southern part of the Eastern European Craton. It comprises a 20-km-thick Devonian and Carboniferous sedimentary succession. Maximum burial occurred during early Permian time and was followed by a major exhumation phase. In this study we use zircon and apatite fission track dating to reconstruct the post-depositional thermal evolution of the inverted basin. Modelling of the fission track data, combined with modelling of vitrinite reflectance data, reveals that large parts of the basin were affected by a Permo-Triassic (∼250 Ma) heat flow event, which was presumably related to Permo-Triassic andesitic magmatism. This Permo-Triassic thermal event was predicted by previous modelling of vitrinite reflectance data, but only the fission track data indicates its wide areal distribution. Probably large parts of the southern margin of the Eastern European Craton were affected by this event. Whereas rocks west of the city of Donetsk (Krasnoarmeisk Monocline) experienced Permo-Triassic temperatures in the range of 90–105 °C, rocks northwest of Donetsk were heated to up to more than 240 °C. Jurassic temperatures northeast of Donetsk were in the order of 90–100 °C. These relatively high temperatures imply that a significant part of the Carboniferous sequence became eroded only during early Cretaceous times and/or that the Jurassic heat flow was significantly increased (∼90 mW/m2). Elevated heat flows may be related to Jurassic magmatic activity. Sediments in the southern Donbas Foldbelt (Yuzhno–Donbassky region) and the westernmost Krasnoarmeisk Monocline record cooling below 60 °C during Jurassic times, whereas samples northeast of Donetsk cooled below 60 °C during Cretaceous times. A correlation between this last cooling and the present-day depths of the samples suggests that the main episode of folding along the South Syncline and the South Anticline pre-dates Cretaceous cooling.