Deformations in the thrust zone between the Zelemianka and Parashka Skybas of the Ukrainian Carpathian Skyba Nappe (Hrebeniv quarry)

Abstract

In the article, based on own field observations, the deformation structures into the thrust zone of the Zelemianka Skyba (=thrust-sheet) onto the Parashka Skyba (the Ukrainian Carpathian Skyba Nappe) are described. They are exposed in the Hrebeniv quarry located in the Opir River basin (Lviv Region, Skole district). The tectonically disintegrated Stryi Formation (Upper-Cretaceous–Paleocene flysch) characterized by the rigid sandstone blocks (formed mainly as a result of boudinage) placed in a ductile clay matrix is represented in the Hrebeniv quarry. This formation is proposed to be classified as a “broken formation”, which is strongly tectonized, but retain their lithological and stratigraphic identity. It is a transitional element between the weakly deformed strata and tectonic mélange. Their characteristic features are: linearity of the distribution zone; significant monomictic composition and absence of exotic formations; the presence of an intensively tectonized matrix with clastolites (blocks) of less tectonized rocks of the same lithostratum (formation, series); the presence of newly formed hydrothermal minerals in clastolite cracks; the upper and lower contacts limiting the broken formation have a tectonic nature. An intensely tectonized matrix and a weakly tectonized rigid blocks belong to the same stratigraphic unit in a broken formation. The structural features suggest a brittle deformations in the studied rocks. Tectonic processes occurred in the completely lithified deposits. Thrust processes were accompanied by the formation of the duplexes, including antiformal stack duplexes, which are well expressed in the quarry. Duplexes are observed in different parts of the quarry, and the most representative ones are developed in its central section. Horses in the duplexes are characterized by a size of 1 to 3 meters here. They are generally subparallel, which is consistent with monoclinal bedding, however, they sometimes acquire an antiform appearance as a result of tectonic thrusting.