Late Cretaceous-early Palaeogene inversion-related tectonic structures at the northeastern margin of the Bohemian Massif (southwestern Poland and northern Czechia)

A brief, regional-scale review of the Late Cretaceous-early Palaeogene inversion-related tectonic structures affecting the Sudety Mountains and their foreland at the NE margin of the Bohemian Massif is presented and complemented with results of new seismic studies. The Sudetes expose Variscan-deformed basement, partly overlain by post-orogenic Permo-Mesozoic cover, containing a wide spectrum of tectonic structures, both brittle and ductile, in the past in this area referred to as young Saxonian or Laramide. We have used newly reprocessed legacy seismics to study these structures in the two main post-Variscan structural units of the area: the North Sudetic and Intra-Sudetic synclinoria. The results were discussed together with regionally distributed examples of tectonic structures from quarries and underground mines as well as those known from the literature. The Late Cretaceous-early Palaeogene tectonic structures in consecutively reviewed Sudetic tectonic units, from the north to the south, typically include gentle to moderately tight buckle folds, locally of detachment type or fault-related and high-angle reverse and normal faults, as well as low-angle thrusts - often rooted in the crystalline basement. The structures termed grabens in the local literature are at the same time frequently interpreted as bounded by reverse faults (hence we use here the term "reverse grabens") and typically reveal a strongly synclinal pattern of their sedimentary fill. The top of the crystalline basement, as imaged by seismic data in the North Sudetic Synclinorium below the faulted and folded cover, is synformally down-warped with a wavelength of up to 30 km, whereas on the elevated areas, where the basement top is exposed at the surface, it is tectonically up-warped (i.e. antiformally buckled). The compressional structures typically show an orientation that fits the regionally known Late Cretaceous-early Palaeogene tectonic shortening direction of NE-SW to NNE-SSW. The same applies to the regional jointing pattern, typically comprising an orthogonal system of steep joints of ca. NW-SE and NE-SW strikes. All the reviewed structures are regarded as being due to the Late Cretaceous-early Palaeogene tectonic shortening episode, although some of the discussed faults with a strike-slip component of motion may have been modified, or even produced, by late Cenozoic tectonism.