THERMAL HISTORY AND EXHUMATION OF THE POLISH WESTERN OUTER CARPATHIANS: EVIDENCE FROM COMBINED APATITE FISSION TRACK AND ILLITE-SMECTITE DATA

Apatite fission track dating and track length analysis combined with X-ray diffraction studies of illite-smectite provide new data on burial, thermal history and uplift in the Western, Polish part of Outer Carpathians accretionary wedge. These analyses were performed for the Magura Nappe and deeper structurally Dukla and Silesian nappes as well as for hinterland (Pieniny Klippen Belt) and for foreland of the Outer Carpathians. Except small areas, the present erosional level in the studied part of the Western Outer Carpathians as well as in the PKB has reached the AFT total annealing zone. These data agree with maximum paleotemperatures interfered from smectite-illite data showing mostly temperatures higher than 110oC. Apatite fission tracks (AFT) analyses indicate uplift and exhumation between 16 Ma and 35 Ma. The obtained ages are: from 15.9 Ma to 31.9 Ma for the Magura Nappe, 20.8 Ma and 32.1 Ma for the Silesian Nappe, 19.9 Ma for the Dukla Nappe, from 16.2 to 35.8 Ma for the Pieniny Klippen Belt. Geological setting The Polish Western Carpathians (PWC) form the northernmost part of the Carpathian belt, which belongs to the Alpine-Carpathian orogenic system. They are subdivided into two main tectonic units: the Inner and the Outer Carpathians separated by the narrow Pieniny Klippen Belt (PKB), (Fig. 1). The Polish segment of the Western Outer Carpathians is an accretionary wedge composed of Upper Jurassic to Lower Miocene rocks overthrusting the Miocene strata of the Carpathian Foredeep. The Magura Nappe is the most internal part of the wedge formed at Eocene-Middle Miocene (Świerczewska, Tokarski, 1998; Nemčok et al. 2000). The nappe is trusted over more external part of the wedge composed of stacks of nappes: Dukla, Silesian, Subsilesian and Skole nappes. According to Nemčok et al. (2000) this part of wedge was formed at Oligocene Middle Miocene. In other interpretation, the Magura Nappe is a Late Oligocene/Early Miocene accretionary wedge thrusted over Early/Middle Miocene accretionary wedge composed of more external nappes of the Outer Carpathians (Oszczypko 2004). The PKB contains both strongly deformed Klippen successions of Jurassic to Cretaceous age as well as Jurassic to Lower Paleogene non-Klippen successions. The lattest were incorporated into the PKB as a result of Late Cretaceaus and post-Paleogene folding (Birkenmajer 2001). In order to constrain thermal history and exhumation of the PWC, apatite fission track analyses (AFT) were performed with correlation to illite-smectite paleotemperature data. In this paper only the results from the Outer Carpathians, the Carpathian Foredeep and the PKB are discussed. Thermal structure of the Outer Carpathians and adjoining areas The thermal structure of the Outer Carpathians was reconstructed based on the degree of the smectite-to-illite transformation (for methods see: Środoń 1984, Šucha et al. 1993). However only for the Magura Nappe structural data combined with illite/smectite studies were used for reconstruction the burial and thermal history as well as for reconstruction of tectonic activity of the nappe and its foreland (Świerczewska 2005). The observed thermal alteration of rocks on the presentday erosion surface of the Outer Carpathians, resulting from temperatures ranging from <75 C to 200C, is related to tectonic burial (Świerczewska 2005 and references therein). The thermal alternation of the strata in sedimentary basins prior to folding are poorly documented (Świerczewska 2005). The Miocene rocks of Carpathian Foredeep were never in temperatures higher than 75C (Dudek 1999). In the PKB the maximum paletemperatures always exceeded 110C (Świerczewska 2005). For the Magura Nappe two main stages of uplift and erosion can be distinguished (Świerczewska 2005). The first stage was related to uplift and erosion of accretionary wedge before Karpatian time (c. 17 Ma). The second stage was related to thrusting of the Magura Nappe over its foreland and to coeval internal deformation of the Nappe. The accretion-related thermal structure was strongly modified during the subsequent thrusting. This stage started in Early Miocene and was still in progress during Early Sarmatian times (c. 11.5 Ma). Materials for AFT analyses (Tab. 1) The AFT studies were performed on seven surface samples of Eocene-Oligocene sandstones collected over the central area of the Polish parts of the Magura Nappe. Two samples were collected from the Cretaceous and Oligocene rocks of the Silesian Nappe close to the frontal overthrust of the Magura Nappe, one sample from the Miocene strata of the Carpathian Foredeep in the front of the Outer Carpathians and three samples from the PKB. In the PKB samples were collected from Jurassic and Cretaceous strata of the Grajcarek Unit and from the Klippen succession. Two samples of Oligocene sandstone were collected from the basement of the Magura Nappe exposed in the tectonic windows (the Dukla Nappe and equivalent units). Results and discussion of AFT data Seven samples from the Magura Nappe show a rather wide scatter of ages from c. 15.9 Ma to 31.9 Ma (Fig. 1). Two samples from Silesian Nappe gave ages c. 20.8 Ma and 32.1 Ma, single sample from Carpathian Foredeep obtained age c. 25 Ma. Samples collected from the PKB are dated between c. 16.2 and 35.8 Ma. The dates for the Dukla Nappe are c. 19.9, 20.6 and 156 Ma.