Tarasy rzeczne Nysy Kłodzkiej oraz drobne plejstoceńskie struktury tektoniczne w strefie sudeckiego uskoku brzeżnego i przełomu bardzkiego w Janowcu

Abstract

Terraces of Nysa Kłodzka river and small-scale tectonic structures close to the Sudetic Marginal Fault at Janowiec (Bardo Mts.).S u m m a r y . New research in the N part of the Bardo Gorge (close to the Sudetic Marginal Fault) evidenced the occurrence of two hitherto unknown Pleistocene river terraces. The terraces were interpreted basing on digital elevation model (DEM) 1:10,000 and radars sweeping of the Earth’s surfaces (DETM level 2). Moreover, the results of petrographical analysis of deposits exposed in the 22 high river terrace show two-tier structure of the terrace. This suggest a polygenetic age of the terrace. Within the last terrace fractured clasts and small-scale faults were observed. Fractured clasts were studied in the bottom part of the 22-m-high river terrace where numerous fractured clasts occur. This part of the terrace is composed of gravels containing clasts up to 80 cm across. The matrix is sandy-clayey. The fractures are usually restricted to particular clasts. The fractures compose two sets striking NW-SE and N-S. Gravels exposed inthe uppermost part of the terrace are cut by few strike-slip, N-S striking, sub-vertical faults which can be traced along a vertical distance up to 50 cm. These faults cut the gravel matrix but omit clasts. Fractures cutting the clasts do show a well organized architecture which is independent of orientation of a-b surfaces of the host clasts. These features indicate that the fractures were formed in situ within the studied gravels. Furthermore, for the majority of fractures, the angle between the fracture and the a-b surface of the host clast is less than 80º, confirming the in situ origin of fractures. The fractures of one set are parallel to the strike of the Sudetic Marginal Fault. This suggests that the origin of the fractures is related to that of the fault. The fractures of the second set may represent Riedel shears associated with the activity of the Sudetic Marginal Fault. The orientations of minor faults confirm this supposition. Summing up, we believe that during the Pleistocene, the Sudetic Marginal Fault was a dextral strike-slip fault, at least in the discussed area.