Paleozoic-Mesozoic thermal evolution along the East European Platform margin based on kerogen thermal maturity analysis combined with apatite and zircon low temperature thermochronology in NE Poland

Abstract. The Phanerozoic tectono-thermal evolution of the SW slope of the East European Platform (EEP) in Poland is reconstructed by means of thermal maturity, low temperature thermochronometry and thermal modelling. We provide a set of new thermochronometric data and integrate stratigraphic and thermal maturity information to constrain the burial and thermal history of sediments. Apatite fission track analysis (AFT) and zircon (U-Th)/He (ZHe) thermochronology have been carried out on samples of sandstones, bentonites, diabase and crystalline basement rocks collected from 17 boreholes located in central and NE Poland. They penetrated sedimentary cover of the EEP subdivided from the north to south into the Baltic, Podlasie and Lublin Basins. The average ZHe ages from Proterozoic basement rocks as well as Ordovician to Silurian bentonites and Cambrian to lower Carboniferous sandstones range from 848 ± 81 Ma to 255 ± 22 Ma with a single early Permian age of 288 Ma, corresponding to cooling after a thermal event. The remaining ZHe ages represent partial reset or source ages. The AFT ages of samples are dispersed in the range of 235.8 ± 17.3 (Middle Triassic) to 42.1 ± 11.1 (Paleogene) providing a record of Mesozoic and Cenozoic cooling. The highest frequency of the AFT ages is in the Jurassic and Early Cretaceous prior to Alpine basin inversion. Thermal maturity results are consistent with the SW-ward increase of the Palaeozoic and Mesozoic sediments thickness. An important break in a thermal maturity profile exists across the base Permian-Mesozoic unconformity. Thermal modelling showed that significant heating of Ediacaran to Carboniferous sedimentary successions occurred before the Permian with maximum paleotemperatures in the earliest and latest Carboniferous for Baltic-Podlasie and Lublin Basins, respectively. The results obtained suggest an important role of early Carboniferous uplift and exhumation at the SW margin of the EEP. The SW slope of the latter was afterward overridden in the Lublin Basin by the Variscan orogenic wedge. Its tectonic loading interrupted Carboniferous uplift and caused resumption of sedimentation in the late Viséan. Consequently, a thermal history of the Lublin Basin is different from that in the Podlasie and Baltic Basins, but similar to other sections of the Variscan foreland, characterised by maximum burial at the end of Carboniferous. The Mesozoic thermal history was characterised by gradual cooling from peak temperatures at the transition from Triassic to Jurassic due to decreasing heat flow. Burial caused maximum paleotemperatures in the SW part of the study area, where the EEP was covered by an extensive sedimentary pile. However, farther NE, due to low temperatures caused by shallow burial, the impact of fluids can be detected by VR, illite/smectite and thermochronological data.