TECTONIC EVOLUTION OF THE WESTERN CARPATHIANS THRUST BELT–FORELAND BASIN SYSTEM: NEW STRUCTURAL AND THERMOCHRONOMETRIC CONSTRAINTS

The Carpathians mountain belt, forming part of the Alpine orogenic system, is a more than 1500 km long, curved thrust belt. The Western Carpathians are the northernmost, W-E–trending branch of this belt. Traditionally, the Western Carpathians have been divided into two distinct parts, namely the Inner and the Outer Carpathians, separated by the so-called ‘Pieniny Klippen Belt’, a narrow zone of intensely deformed and sheared Mesozoic to Palaeogene rocks. Our reappraisal of the ‘Pieniny Klippen Belt’ suggests it originally represented a ‘wildflysch’ including olistostromes and olistoliths of extremely variable size, later deformed and sheared. These sediments are best interpreted as originally deposited in a foreland basin that developed during the Late Cretaceous in front of the Inner Carpathian orogen, which included tectonic units made of continental crust of Variscan age and its younger sedimentary cover. Such a Pieniny foreland basin succession was later deformed and partly thrust over the Magura Unit, which constitutes the structurally uppermost tectonic element of the Outer Carpathian orogenic wedge. The Magura Unit forms a roof sequence overriding a series of thrust sheets that are also exposed in a series of tectonic windows. Our balanced cross-sections, showing a mix of thin-skinned thrusting and thick-skinned tectonic inversion involving the reactivation of pre-existing basement normal faults, have been sequentially restored in order to unravel the tectonic evolution of the thrust belt–foreland basin system. The cross-sections, integrated with a large thermochronometric dataset (including apatite fission tracks and apatite and zircon (U-Th-(Sm))/He ages), allowed us to unravel the various stages of the tectonic development of the thrust belt–foreland basin system of Slovakia, Poland and Ukraine. In this area, the Carpathian orogen appears to be characterized by an original lack of oceanic lithosphere, which is instead commonly envisaged to have been interposed between the basement units exposed to the south (Inner Carpathians) and foreland-derived sedimentary units presently cropping out to the north (Outer Carpathians). These results imply a deep revision of the original paleogeographic setting and of the geodynamic evolution of the Western Carpathians.