The Polish sector of the Carpathian mountain belt constitutes the northernmost part of the whole orogen (Fig. 1a). This is traditionally subdivided into two main parts, known as Inner and Outer Carpathians, being separated by the Pieniny Klippen Belt, a narrow zone of intensely deformed and sheared Mesozoic to Paleogene rocks (Birkenmajer, 2001). The Polish segment of the Outer Carpathians is commonly interpreted as an accretionary wedge – composed of Upper Jurassic to Lower Miocene sedimentary successions – overthrusting Miocene siliciclastics of the Carpathian Foredeep. Stratigraphic evidence indicates that thrusting terminated diachronously along the Carpathian front – being progressively younger towards the east (Nemcok et al., 2006a) – while fault-controlled ‘intramontane’ basins developed on top of the orogenic wedge in Neogene-Quaternary times (Birkenmajer, 1978; Tokarski, 1978). In the study area (Fig. 1b), thrusting is commonly interpreted to have ceased at ca. 11 Ma (Nemcok et al., 2006a; Garecka and Jugowiec, 1999). South of the frontal thrust sheets (Skole and Borislav-Pokuttia Units), the Magura Unit constitutes the structurally uppermost tectonic element of the Outer Carpathian orogenic wedge. It forms a roof sequence overriding a series of tectonic units – known as Dukla, Silesian and Subsilesian Units – that are also exposed in a series of tectonic windows. The tectonic contacts separating these units are commonly portrayed as thrust faults. However, recent geological mapping (Jankowski, 2004), as well as our new field survey, point out the occurrence of younger-on-older, mainly south dipping tectonic contacts that are best interpreted as extensional faults (Fig. 1b–c) rather than out-of-sequence thrust contacts (Nemcok et al., 2000; Nemcok et al., 2006b). In most instances, poor exposure of main tectonic contacts effectively hinders detailed kinematic analyses, and information on shear senses is generally lacking in the existing literature. Our fieldwork allowed us to unravel significant reworking of the preexisting folds and thrusts by extensional structures associated with orogen-normal horizontal stretching (Fig. 2). This is shown, for instance, by the intensely deformed Oligocene strata of the Silesian Unit exposed in the footwall to the Magura Unit within the Mszana Dolna tectonic window (sites b and c in Fig. 1b). These strata (Krosno Fm) display extensional reworking – in the form of S dipping extensional shears – of a preexisting, well-developed S-C fabric associated with top-to-the-north sense of shear, the latter being consistent with the original thrust-related emplacement of the Magura Unit. Extensional structures are well developed also in the Kleczany area (site f in Fig. 1b). Here, a hard-linked extensional fault system, including SSW dipping normal faults and mainly NNE trending, oblique-slip transfer faults, is exposed in thick-bedded sandstones of the Krosno Fm (Silesian Unit) located in the footwall to the Magura Unit. Although most post-thrusting normal faults dip to the south, north dipping extensional shear zones also occur (Figs. 1b, 2). The horizontal extension pointed out in this study (Fig. 2) is consistent with NNE directed, orogen-normal extension representing the latest deformation event in the Silesian Unit according to (Rubinkiewicz, 2007).

Based on clay mineralogy, fluid inclusion and AFT data, exhumation of sedimentary units from depths in the range of 6–10 km has been inferred for a large part of the Polish Outer Carpathians (Anczkiewicz and Swierczewska, 2008; Swierczewska, 2005; Swierczewska et al., 1999; Swierczewska et al., 2000; Hurai et al., 2004). Although the recorded burial conditions resulted from both sedimentary and tectonic loads, the latter appear to have been largely dominant in controlling the thermal evolution of the analyzed units. In fact, complete stratigraphic successions are preserved and have been extensively studied (Oszczypko, 2004), their thickness being clearly insufficient to explain recorded burial conditions (Swierczewska, 2005). Furthermore, significant burial is recorded by the youngest strata (Krosno Fm) of the stratigraphic successions exposed in the footwall to the Magura Unit. The load on top of such strata could have been represented uniquely by overlying tectonic units.

The post-depositional history of the sedimentary units is particularly well constrained by AFT data (Tables 1 and 2; Fig. 1b). This methodology is a useful tool to unravel the cooling histories experienced by rocks in their motion toward the surface (see (Donelick et al., 2005) for a review). Samples have been collected from medium- to coarse-grained siliciclastic sandstones. Most of the apatite grains are well rounded, ranging in size between 50 and 100 μm. In few cases, euhedral crystals also occur. Sample preparation and analysis followed the procedures outlined in (Zattin et al., 2002). Unfortunately, the low number of measured track lengths does not allow any inference about the rates of cooling for the new samples reported in this study. Moreover, it is not possible to exclude any re-heating event after maximum burial. However, the relationships between stratigraphic and AFT ages suggest that most samples were affected, during the Miocene, by maximum temperatures higher than total annealing temperature (ca. 125 °C (Reiners and Brandon, 2006)). Within the study area, AFT cooling ages range between 32.1 ± 4.8 and 7.0 ± 0.8 Ma. For the uppermost tectonic unit (Magura Unit), cooling ages are in the range of 31.9 ± 4.1 to 15.9 ± 1.9 Ma. In the footwall to the latter unit, cooling ages tend to become younger to the east, reaching values in the range of 10.0 ± 1.1 to 7.0 ± 0.8 Ma. Only one of the analyzed samples (PL7) did not reach burial conditions sufficient to produce total annealing of apatite. This sample records Late Cretaceous exhumation of the source rock.

An offset of AFT cooling ages in excess of 10 M.y. is recorded across the tectonic contact between Magura Unit hanging-wall rocks (displaying a cooling age of 21.3 ± 2.0 Ma) and the units exposed immediately in the footwall in the Ropa tectonic window (Fig. 1b).

Timing of exhumation partially overlaps with thrust activity within the study area. However, our new AFT data from the eastern sector unraveled a recent exhumation stage, taking place in the last 10 Ma. Exhumation in this sector largely post-dated thrusting, as the sampled units display AFT cooling ages in the range of 10–7 Ma (Table 2). These values may imply relatively fast exhumation in Late Miocene to Present times, depending on applied paleogeothermal gradients. Paleogeothermal gradients inferred from methane–water fluid inclusions in quartz-calcite veins from the Magura Unit and from tectonically underlying footwall units (exposed in the Mszana Dolna and Szczawa tectonic windows) are 20 °C km⁻¹ and 17 °C km⁻¹, respectively (Hurai et al., 2004). Assuming a mean paleogeothermal gradient of 18 °C km⁻¹ (Swierczewska, 2005), a surface temperature of 10 °C and an AFT closure temperature of 110 °C, a time-averaged exhumation rate of ca. 0.8 mm y⁻¹ is obtained for samples PL1, PL4 and PL8 (all showing AFT cooling ages around 7 Ma). Even assuming a paleogeothermal gradient of 25 °C km⁻¹ (i.e., present-day mean geothermal gradient in the Magura Unit; (Swierczewska, 2005) and references therein), a time-averaged exhumation rate of ca. 0.6 mm y⁻¹ is obtained for the same samples. These values are significantly higher with respect to reconstructed erosion rates. Although these are available only for the Quaternary, a meaningful comparison with time-averaged exhumation rates may be carried out, taking into account the overall lithological homogeneity of the tectonic units forming the thrust belt (i.e., the rocks exposed during the Quaternary are likely to have similar characteristics, in terms of resistance to erosion, with respect to those exposed in Late Miocene and Pliocene times). Quaternary stages of ‘increased erosional activity’, as defined by Zuchiewicz (Zuchiewicz, 1991) based on the analysis of river terraces, are characterized by rates of 0.15–0.21 mm y⁻¹ (800–472 ka) and 0.18–0.40 mm y⁻¹ (130–90 ka). Only for a negligible time span – at the scale of the exhumation processes considered here – of 15 ka (Latest Pleistocene-Holocene), erosion rates in the range of 0.2–2.0 mm y⁻¹ have been inferred by the latter author. Furthermore, the evidence that the youngest beds of the highest stratigraphic unit (Krosno Fm) are generally preserved not only within the tectonic windows – where they occur in the footwall to the Magura Unit – but also ahead (i.e., north) of the Magura Unit front, suggests that the overall amount of erosion has not been large.

The fact that erosion alone cannot explain exhumation rates recorded by sampled rock units cropping out in the eastern part of the study area, together with the evidence of normal faulting post-dating and dissecting fold and thrust structures, all suggest a significant role of extension-related tectonic exhumation. A fundamental role of tectonic exhumation is confirmed by the offset of AFT cooling ages across the tectonic contact separating the Magura Unit from the footwall units in the Ropa tectonic window (area of Ropa 1 well in Fig. 1b). Such a large offset, in excess of 10 M.y., suggests that the tectonic contact at the base of the Magura Unit has been active in this area – probably as a low-angle extensional fault partly reactivating a preexisting thrust – during the last 10 Ma (AFT cooling age of footwall rocks; sample PL6), long after exhumation of the Magura Unit rocks presently exposed in the adjacent hanging-wall block immediately to the south (sample PL5). As the extensional detachment reworked the pre-existing Magura Unit thrust, it is not possible to quantify exactly the magnitude of the extensional displacement, which anyway must have been of the order of a few kilometers (Fig. 1b).

According to (Zuchiewicz, 1998), the spatial arrangement of zones showing Quaternary uplift/subsidence suggests that the main mechanism controlling recent tectonic deformation in the Western Outer Carpathians is the relaxation of remnant horizontal stresses built up during Neogene thrusting. This is consistent with recent, post-thrusting thinning of the Outer Carpathian accretionary wedge as a result of gravity disequilibria. This process was accompanied by both orogen-parallel and orogen-normal extension (Zuchiewicz, 2001). The latter, being dominant, represents the youngest deformation event and also involves low- to moderate-angle normal faults (Rubinkiewicz, 2007). Age and style of extension, being mainly accommodated by roughly east-west striking structures within the study area (Figs. 1 and 2), are compatible with the regional geodynamic framework involving extensional collapse of Alpine belt segments and development of the Pannonian Basin (Horváth et al., 2006). Readjustments within the orogenic wedge also led to the reactivation of preexisting thrusts as low-angle, south dipping extensional faults in the study area. Furthermore, gravitational readjustments appear to have generated rootless, top-to-the-north low-angle faults by denudation of the tilted roof sequence (Magura Unit). The latter process, besides producing disrupted, rotated and structurally ‘chaotic’ frontal bodies of Magura Unit rocks (Konon, 2001; Jankowski, 2007), also enhanced tectonic exhumation of footwall units.

Morphotectonic evidence indicates that neotectonically uplifted areas become more numerous moving from west to east within the Polish Outer Carpathians, thus suggesting an eastward-increasing recent tectonic activity (Zuchiewicz, 1998). This is consistent with young tectonic activity controlling recent cooling ages unraveled by AFT data from the eastern sector of the study area.

New AFT data confirm that a significant part of the sedimentary rocks exposed in the Outer Western Carpathians experienced substantial tectonic burial (generally in excess of 6 km). Although part of the exhumation was coeval with shortening, our new data also point out a previously undetected, ‘young’ (i.e., post-thrusting) exhumation stage in the eastern sector of the study area. This, in turn, appears to be associated with a new tectonic regime affecting the orogenic wedge once convergence had ceased. Based on our results, we suggest that recent (< 10 Ma) tectonic evolution and exhumation in the Polish Outer Carpathians were mainly controlled by extension and gravitational readjustments within the orogenic wedge.