engleski

Thermal history and exhumation of the central part of the Karst Dinarides, Croatia

This study was undertaken to obtain the thermochronological data to unravel the thermal history of the central part of the Karst Dinarides, exposed along the NE Adriatic coast in the Velebit Mt. and neighboring areas. The lower part of the sedimentary section (Upper Carboniferous to Triassic), covered by a succession of Mesozoic carbonates has been studied by an apatite fission track (AFT) and apatite and zircon (U-Th- Sm)/He thermochronology. The ZHe ages are between 80 and 70 Ma (full range is from 257 to 41 Ma), while the actinide content ranges from 133 to 874 ppm eU. No correlation between the ZHe ages and the eU content can be detected. Reproducibility of replicates is good for the majority of samples. The ZHe ages increase in the SE structural domains. AFT ages are younger than the corresponding stratigraphic ages and yield a single population of grain ages. Central ages range from 81 to 24 Ma1 with the vast majority clustering between 50 and 30 Ma. The mean track length varies from 14.4 to 11.9 pm, with the vast majority around 13 pm. Due to the rela¬tively low uranium content only six samples yielded the track numbers > 50. Considering samples, where the track number is sufficient and the track length > 12.9 pm with narrow distribution (SD < 1.6) we can assume that they experienced after the burial a slow cooling through the apatite partial annealing zone (PAZ ~60-120 0C). AFT ages plotted against their mean track lengths show a classic 3/4 boomerang trend, representing the progressive overprinting of an older component of tracks by a heating in a single dominant palaeothermal event. Older samples (> 60 Ma) have longer mean track lengths ; these decrease with a de¬creasing age (60-55 Ma), increase again in samples of 55-30 Ma. This trend may suggest rapid cooling at 50-30 Ma. The measured Dpars range from 2.5 to 1.5 pm, indicating that samples have a variable annealing kinetics. There is no correlation between the AFT ages and the elevation of the samples in general. AHe ages corrected for the alpha ejection showed the age range from 53 to 28 Ma, with eU contents that range from 7 to 88 ppm and the majority of ages are clustered around 30 Ma. No correlation between the AHe ages and the eU content can be detected. All AFT ages are younger than the corresponding ZHe ages and overlap within an uncertainty with the AHe ages, which is in agreement with the closure temperatures of these thermochronometers. Rapid exhu-mation started in the studied structural domains between 80 and 35 Ma, Le. between the Campanian and the end of Eocene, followed in some domains by a younger exhumation and cooling pulse. Acknowledgments. This study was funded by the project no. N307 475238 granted to Marek Lewandowski by the Polish Ministry of Science and Higher Education, and in part by the Croatian Science Foundation project IP-2014-09-9666. We thank Marta Mileusnic for her help during the field work.

exhumation, thermal hiostory, Karst Dinarides, Croatia

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