Recent tectonic activity of the Polish Western Outer Carpathians: Geomorphic and gravimetric constraints

Two geodynamic test transects across the Polish segment of the Western Carpathians, crossing the Orava Basin in the west (KO) and the Pieniny Klippen Belt and Magura Nappe along the Dunajec River valley in the east (DD) have recently been analysed. Multidisciplinary studies conducted along these transects included gravimetric, geodetic, geologic and morphostructural investigations. Gravimetric and geodetic results appear to suggest recent subsidence of the Orava Basin, particularly intensive in the Wróblówka Graben, confirming conclusions derived from geomorphic analyses. Data obtained for the Dunajec River transect do not show any particular differentiation among individual benchmarks, what can point to either minor uplift of the entire area (already suggested by the results of geomorphic and morphotectonic studies), minimal differences between successive slices of the Magura Nappe and the Pieniny Klippen Belt, or both. Horizontal displacements of benchmarks, different for the KO and DD transects, towards the west and SW as well east and SE, respectively, can result from general uplift of the area comprised between these transects, i.e. the Gorce Mts. A new geodynamic transect DS, running along the Soła River valley cuts several units of the Outer Western Carpathians of Poland. These are, from the south, the Magura Nappe, Fore-Magura group of nappes, as well as the Godula and Cieszyn units of the Silesian Nappe, sub-Silesian Nappe, Skole Nappe, and the Carpathian Foredeep. Within the Magura Nappe, thrust faults of subordinate units (slices) are of the order of a few kilometres, and individual slices are composed of strongly imbricated anticlines and synclines striking SW-NE. The Fore-Magura group of nappes is composed of thrust sheets including both Magura- and Silesian-type lithostratigraphic members that build strongly imbricated folds of northern vergence. The Silesian Nappe is subdivided in this portion of the Western Carpathians into the Cieszyn (northern part) and Godula (southern part) units. In a tectonic window close to Żywiec, the Godula unit is underlain by the Cieszyn unit which overlies the sub-Silesian Nappe. Strata belonging to the latter nappe are exposed farther north in a number of small-scale outliers in front of the Silesian Nappe, north of Bielsko-Biała and close to Kęty-Wadowice. The nappe is composed of several north-verging imbricated folds, thrust one over another. All these units are cut by strike-slip and oblique-slip faults oriented roughly NS. One of the most prominent fault zones accompanies the Soła River valley, dextrally offsetting the Carpathian frontal thrust. These faults were mainly formed during final stages of thrusting of the flysch nappes, postdating Burdigalian time. The discussed western portion of the Outer Carpathians is traversed by several sets of regional and local photolineaments, coinciding to a large extent with the Soła River fault and associated subordinate faults. In the Pliocene and Quaternary, the area witnessed differential vertical and some remnant horizontal movements resulting in the formation of elevated and subsided areas. In the study area, the Soła River valley separates two prominent elevated regions that were uplifted in the Pliocene and early Pleistocene. A probably younger episode of Pleistocene and Holocene uplift is marked by the presence of two, nearly E-W trending, zones of abnormally high river bed gradients: one associated with the Jabłonków Depression in the south, and another one situated north of Żywiec, in the Beskid Śląski and Beskid Mały Mts. dissected by the Soła River water-gap. Recent uplift is usually observed in frontal parts of nappes, slices and imbricated folds and probably results from buckling induced by the ongoing thrusting.