The Intra-Sudetic Basin, a ~ 12 km deep Variscan intramontane basin, has the best preserved post-orogenic sedimentary record available at the NE margin of the Bohemian Massif. Apatite fission track (AFT) analyses have been performed on 16 sedimentary and volcanic samples of Carboniferous to Cretaceous age from the Intra-Sudetic Basin to improve understanding of the post-Variscan thermal evolution. AFT central ages range from 50.1 ± 8.8 to 89.1 ± 7.1 Ma (Early Eocene to Coniacian), with 13 of them being Late Cretaceous. The mean track length values range from 12.5 ± 0.4 to 13.8 ± 0.5 (except for one sample 14.4 ± 0.2) µm. This relatively short mean track length together with the unimodal track length distributions and rather low standard deviation (0.8 to 1.7 µm) in most samples indicate a long stay in the partial annealing zone during slow cooling. However, in the northern part of the Intra-Sudetic Basin, samples show a wider track length distribution (standard deviation of 1.8 to 2.1 µm) that could indicate a more complex thermal evolution possibly related to Mesozoic reheating. Vitrinite reflectance data combined with thermal models based on the AFT results indicate that the Carboniferous strata reached maximum palaeotemperatures in the latest Carboniferous to Early Permian time, corresponding to a major coalification event. The second phase of temperature rise occurred due to Late Mesozoic sedimentary burial, but it had no influence on maturation of the Carboniferous organic matter. Final cooling phase in the Late Cretaceous-Paleogene was related to tectonic inversion of the Intra-Sudetic Basin, which occurred after deposition of a significant thickness of Cenomanian-Turonian sediments. Thermal modelling demonstrates that ~ 4 km thick cover of Upper Cretaceous sediments is required to obtain a good match between our AFT data and modelled time-temperature paths. This outcome supports a significant amount of Late Cretaceous-Paleogene inversion within the Variscan belt of Central Europe.
The low-temperature thermal history of the Holy Cross Mountains (HCM) is investigated by apatite fission track and apatite and zircon (U-Th)/He thermochronology.Our results provide constraints on the deformation history of Palaeozoic basement rocks in the transition area from Precambrian to Palaeozoic Europe that are exposed from beneath Permian-Mesozoic sediments within the HCM. Late to post-Variscan cooling of the Palaeozoic strata from maximum temperatures is shown to be a major feature of the HCM. This cooling likely followed a heating event related to burial and/or hot fluid circulation along the Holy Cross Fault in the late Carboniferous.The central part of the HCM shows a rapid cooling event caused by tectonic inversion, which started in the Late Cretaceous. However, this event was less pronounced in the western margin of the HCM, where slow cooling continued throughout the Mesozoic with only minor acceleration of the cooling rate since the latest Cretaceous.
In the present study, the thermal history of the Late Carboniferous (Stephanian) coal-bearing sediments of the Sabero Coalfield has been reconstructed in order to elucidate coal rank. The Sabero Coalfield is located in a small intramontane coal-bearing basin along the Sabero-Gordón fault zone, one of the major E-W trending strike-slip fault systems of the southern part of the Cantabrian Zone (NW Spain). The total thickness of the Stephanian succession is in excess of 2,000 m, and is composed of siliclastic rocks and intercalated coal seams with tonsteins. Mean vitrinite reflectance values in the Stephanian rocks in the Sabero Coalfield are in the range from 0.61 to 3.14% Rr, but most values are in the range from 0.8 to 1.5% Rr (based on 84 samples). Average vitrinite reflectance gradient is high (0.73% Rr/km), which suggests high value of average paleogeothermal gradient (52 o C/km). The maximum paleotemperatures calculated from vitrinite reflectance values for the Stephanian rocks range between 89 o C (top of the Stephanian) and 195 o C (bottom of the Stephanian). Coalification of the organic matter in the Stephanian rocks was achieved in the Early Permian, and was most likely related to several almost simultaneous related to magmatic and hydrothermal activity during high subsidence period in the pull-apart basin. The primary, burial-related maturity pattern, was probably slightly overprinted by fluid migration event, which is supposed to have occurred in Early Permian time.
The Lower Paleozoic basins of eastern Poland have recently been the focus of intensive exploration for shale gas. In the Lublin Basin potential unconventional play is related to Lower Silurian source rocks. In order to assess petroleum charge history of these shale gas reservoirs, 1-D maturity modeling has been performed. In the Łopiennik IG-1 well, which is the only well that penetrated Lower Paleozoic strata in the study area, the uniform vitrinite reflectance values within the Paleozoic section are interpreted as being mainly the result of higher heat flow in the Late Carboniferous to Early Permian times and ~3500 m thick overburden eroded due to the Variscan inversion. Moreover, our model has been supported by zircon helium and apatite fission track dating. The Lower Paleozoic strata in the study area reached maximum temperature in the Late Carboniferous time. Accomplished tectono-thermal model allowed establishing that petroleum generation in the Lower Silurian source rocks developed mainly in the Devonian – Carboniferous period. Whereas, during Mesozoic burial, hydrocarbon generation processes did not develop again. This has negative influence on potential durability of shale gas reservoirs.
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