Quaternary neotectonic activity of the Sudetic Marginal Fault in Pieszyce area, Góry Sowie Massif (NE Bohemian Massif, SW Poland)

The Sudetic Marginal Fault (SMF) is a prominent tectonic structure, clearly expressed in the morphology of the NE part of the Bohemian Massif in SW Poland. The outcrop of the SMF core zone was recently exposed within the Góry Sowie Massif in the Pieszyce area (Poland) during earthworks carried out in 2022 and 2024. In this unique exposure, a distinct displacement of the contact between the underlying Sowie Góry gneisses and the overlying Quaternary sediments was recognized. The main fault zone steeply dips at 70°to the ENE. In the southern fault block, no sedimentary cover was observed, whereas Quaternary sediments attain a thickness of up to 4.5 m close to the main fault on the northern side.

Tectonically altered gneisses occur within the main fault zone, while the highly weathered crystalline basement beneath the sedimentary cover in the northern block is cut by numerous secondary tectonic zones filled with grayish fault gouge. Within these zones, we documented vertical veins of (up to 15 cm) filled with overlying deposits, including isolated gravel-sized clasts. Some of the observed veins penetrated bedrock to the depth of at least 1 m. Bedrock and fault-zone materials were systematically analyzed using XRF and XRD methods. Elevated concentrations of mercury and arsenic were observed in the fault zones. Micromorphological analysis of two oriented thin sections collected directly from the fault gouge, together with mineral phase identification based on XRD analyses of fault-gouge samples, reveals pervasive grain-size mixing and syn-deformational clay mineral realignment, indicating repeated brittle deformation under near-surface conditions.

Sedimentological studies, including facies and granulometric analyses, allowed to classify the sediments overlying the northern block as preglacial(?), fluvioglacial, and glacial origin. Petrographic analysis of clasts >10 mm revealed a dominance of locally derived material, with a minor contribution of Scandinavian clasts in the upper part of the profile. Clast imbrication measurements in preglacial sediments indicate transport in the WNW-ESE and NE-SW directions, interpreted as progradation of a locally sourced alluvial fan from the Sowie Góry Block. Measurements of cross-bedding and erosional channel axes within the fluvioglacial sediments indicate transport mainly towards the SSE, consistent with meltwater flow from the Scandinavian Ice Sheet margin and mixing with locally supplied Sudetic material derived from the crystalline basement. OSL dating of selected samples confirmed the Middle Quaternary deposition age of the fluvioglacial sediments in the Pieszyce area and provides direct evidence for Quaternary activity of the Sudetic Marginal Fault.

Keywords: Sudetic Marginal Fault, Sudetes, neotectonics, Quaternary sediments, OSL dating