Tectonics of Miocene–Pliocene fresh-water molasses in the Carpathian Foredeep (Witów Series, South Poland)

Abstract

Normal faults of different orientations appear to be the youngest manifestations of faulting in the Polish Outer Carpathians, composed of the Lower Cretaceous trough Lower Miocene strata, and the related Carpathian Foredeep, which is filled with the Lower to Middle Miocene sediments. In the Outer Carpathians, the folds and thrusts produced by accretion-related shortening were formed between the Paleocene and early Late Miocene. The origin of normal faults is still debatable, since it is not known whether these faults were a result of multidirectional extension produced in a single collapse event, or differently oriented extension proceeding in a series of successive events. Structural studies of the Late Miocene–Pliocene(?) fresh-water molasses of the Witów Series provide a possibility to reconstruct the Late Neogene stress field in the central part of the Polish Carpathian Foredeep and, indirectly, in the central part of the Polish Outer Carpathians. The strata of such an age are unique features in the Polish Carpathian Foredeep, proving thereby a key record of structural deformation during the latest stages of orogenic evolution of the Carpathian orogen. The molasses are cut by joints, and normal and strike-slip faults which were formed in two successive events: (1) a syn-depositional one, proceeding under NNW-SSE to N-S oriented horizontal compression, possibly coeval with reactivation of a NE-striking sinistral fault of the Kurdwanów-Zawichost Fault Zone in the basement; and (2) a post-depositional one, during N-S to NE-SW-oriented extension. In the first event, reactivation of the NE-striking sinistral fault led to formation of N-S-oriented joints, as well as NW-striking dextral, and NNW-trending normal faults. This event was probably contemporaneous with sinistral reactivation of some thrusts in the Western Outer Carpathians, induced by eastward-directed extrusion of crustal blocks in the Carpathian internides. In the second event, both W-E and NW-SE-oriented joints and WNW-striking normal faults were formed. The latter most probably originated due to reactivation of the Early Paleocene WNW- and NW-striking normal faults in the basement. Therefore, normal faults detected in the Outer Carpathians and Carpathian Foredeep appear to be a result of not a single collapse event but of different successive events. This extensional episode lasted at least to the late Pleistocene. No evidence for the recent NNE-directed tectonic compressive stress, typical for that segment of the Carpathian arc, has been found yet.