Paleogeographic evolution of the Southern Pannonian Basin: ⁴⁰Ar/³⁹Ar age constraints on the Miocene continental series of Northern Croatia

Abstract

The Pannonian Basin, originating during the Early Miocene, is a large extensional basin incorporated between Alpine, Carpathian and Dinaride fold-thrust belts. Back-arc extensional tectonics triggered deposition of up to 500-m-thick continental fluvio-lacustrine deposits distributed in numerous sub-basins of the Southern Pannonian Basin. Extensive andesitic and dacitic volcanism accompanied the syn-rift deposition and caused a number of pyroclastic intercalations. Here, we analyze two volcanic ash layers located at the base and top of the continental series. The lowermost ash from Mt. Kalnik yielded an ⁴⁰Ar/³⁹Ar age of 18.07 ± 0.07 Ma. This indicates that the marine-continental transition in the Slovenia-Zagorje Basin, coinciding with the onset of rifting tectonics in the Southern Pannonian Basin, occurs roughly at the Eggenburgian/Ottnangian boundary of the regional Paratethys time scale. This age proves the synchronicity of initial rifting in the Southern Pannonian Basin with the beginning of sedimentation in the Dinaride Lake System. Beside geodynamic evolution, the two regions also share a biotic evolutionary history: both belong to the same ecoregion, which we designate here as the Illyrian Bioprovince. The youngest volcanic ash level is sampled at the Glina and Karlovac sub-depressions, and both sites yield the same ⁴⁰Ar/³⁹Ar age of 15.91 ± 0.06 and 16.03 ± 0.06 Ma, respectively. This indicates that lacustrine sedimentation in the Southern Pannonian Basin continued at least until the earliest Badenian. The present results provide not only important bench marks on duration of initial synrift in the Pannonian Basin System, but also deliver substantial backbone data for paleogeographic reconstructions in Central and Southeastern Europe around the Early–Middle Miocene transition.

Appendix: Intercalibration of Fish Canyon Tuff and Drachenfels 40Ar/39Ar standards for irradiation batch VU69-A

⁴⁰Ar/³⁹Ar ages reported in this paper are calculated relative to the Vrije Universiteit Amsterdam in-house standard Drachenfels. The age for this standard was originally reported as 25.26 Ma relative to Taylor Creek Rhyolite sanidine of 27.92 Ma (Wijbrans et al. 1995). Recent updates of international standard ages require also an update for the age of Drachenfels sanidine. Drachenfels sanidine has been used as main flux monitor in irradiation batch VU69. Along with Drachenfels, the Fish Canyon Tuff sanidine standard has been loaded on several locations next to Drachenfels. Here we provide the intercalibration data set between these two standards for irradiation batch VU69-A, the same batch in which the Glogovnica, Sjeničak and Paripovac samples are irradiated (Table 3). A compilation of Drachenfels—FC over a series of irradiations batches is in progress. (⁴⁰Ar*/³⁹ArK)_Drachenfels/(⁴⁰Ar*/³⁹ArK)_{Fish Canyon} ratios (or R-values) are calculated for three pairs of Drachenfels—Fish Canyon Tuff standards (A1 and A2, A16 and A17 and A31 and A32; Table 2). In Table A1 (online supplementary material), all relevant analytical data are reported.

Table 3 Overview of relevant samples and standards in irradiation batch VU69A

Weighted mean of R(A16-A17) and R(A32-A31) yields 0.9028 ± 0.0008 (Table 4). This yields in an age of 25.477 ± 0.030 Ma for Drachenfels relative to FCs of 28.198 ± 0.022 Ma of Kuiper et al. (2008) when using Steiger and Jäger (1977) decay constants. This translates to 25.479 ± 0.030 Ma for Drachenfels relative to FCs of 28.201 ± 0.022 Ma of Kuiper et al. (2008) when using Min et al. (2000) decay constants compilation. It is unclear why the data pair A1 and A2 is an outlier, although the individual analyses of A1 and to a lesser extent A2 are very heterogeneous.

Table 4 R values of three Drachenfels/Fish Canyon Tuff pairs