Burial and exhumation history of the Polish Outer Carpathians: Discriminating the role of thrusting and post-thrusting extension

Abstract

Several tectonic processes have been proposed to be important in the Alpine to the recent evolution of the Polish Outer Carpathians, i.e., Paleogene–Neogene thrusting, post-thrusting extension, Quaternary reactivation of compression. This work tests the effect of these processes on exhumation of the wedge, in order to verify their timing and relative role in shaping the Polish Outer Carpathians. AFT, AHe and ZHe analyses were performed on samples collected along the study region. This allowed the reconstruction of their post-depositional minimum heating, depth of burial and timing of cooling and exhumation. After deposition samples from the innermost units were heated up to temperatures of ca. 60–120 °C (ca. 4–9 km), whereas samples from the outer units were heated to temperatures lower than 60 °C (ca. 4 km). Cooling and exhumation occurred progressively later from west to east (between ca. 25 and 10 Ma in the western sector, between ca. 15 and 5 Ma in the eastern sector). These results have been put in relationship with single structural features to understand the relative role of thrusts and normal faults. In the eastern sector of the study region samples at the footwall of significant normal faults show a higher degree of reset and younger ages than samples located at the hangingwall. This implies that, in this sector, extensional tectonics played an important role in exhumation. However, proper tectonic exhumation only had a minor role, the major exhumation mechanism being erosion, which is enhanced in the footwalls and reduced in the hangingwalls. In the western sector of the Polish Outer Carpathians only in few cases evidence of enhanced erosion to the footwall of major normal faults could be observed based on the present data. In this region the wedge exhumed mainly as a response to thrusting and normal faulting only played a minor role, locally modifying the pattern of erosion. Finally no major exhumation events occurred after Miocene, pointing to a minor relevance of recent compression. Unreset ZHe dates were used to estimate exhumation of source rocks through the ZHe PRZ and to outline the provenance of sediments forming the Polish Outer Carpathians. Exhumation occurred both in response to the Variscan (Late Devonian to Triassic) and Alpine (Late Jurassic to Early Paleocene) orogeneses, suggesting sedimentary provenance both from the southern margin and from the basement heights within the Outer Carpathian basin.