A record of superimposed late- and post-Variscan regional-scale tectonic events at the NE margin of the Bohemian Massif: structural evolution of the Kamionki Graben (SW Poland, Sudetes)

Recent results of detailed geological mapping and structural analysis complemented by 2D Electrical Resistivity Tomography (ERT) and 2D Seismic P-wave Refraction Tomography (SRT-P) are used to propose a new structural model for the polyphase evolution of the Kamionki Graben (NE Bohemian Massif). This intriguing tectonic structure is composed of syn- to late-orogenic Mississippian („Culm”) strata, which represent an outlier surrounded entirely by the metamorphic rocks of the Góry Sowie Massif. The Kamionki Carboniferous succession was folded (and, locally, also probably thrusted over the gneissic basement) into the WNW–ESE to W–E and, less commonly, NW–SE oriented folds during the late Mississippian (early Namurian?) Epoch. The graben development only slightly postdated the folding of the Carboniferous and we correlate it with significant regional uplift and erosion, which was related to the late-orogenic gravitational collapse of the newly formed Variscan orogen. Hence, the main boundary faults of the graben may have been genetically linked with a late Carboniferous–early Permian extensional (transtensional?) episode at the NE Bohemian Massif. The structure of the graben was later reshaped during the Late Cretaceous–early Paleogene trans-regional tectonic shortening event, which likely led to the reactivation of the main boundary faults of the graben as well as large-scale, gentle folding of the strata, visible only in map-view, especially in the northern parts of the graben. The later, Neogene (?), NE–SW-oriented extensional regime resulted in the formation of normal faults and it was responsible for the distinct compartmentalization of the graben. Our study provides the first description of the internal structure, fault pattern and kinematic evolution of the Kamionki Graben. Strong lithological contrasts between the sedimentary infill of the graben and its crystalline shoulders and floor makes the seismic and resistivity geophysical methods a valuable tool for investigation of its internal structure.