Active tectonics research using trenching technique on the south-eastern section of the Sudetic Marginal Fault (NE Bohemian Massif, central Europe)
Abstract The NW–SE striking Sudetic Marginal Fault (SMF) is one of the most conspicuous tectonic structures in central Europe. It controls the pronounced morphotectonic escarpment of the Sudetic Mountains for a length of 130 km. This paper presents the results of trenching on the SMF, undertaken in order to assess activity of the fault. The trenching technique has not hitherto been applied on either this particular fault system or elsewhere in the Bohemian Massif. However, it is the most effective tool for near-surface fault investigation in areas that are well vegetated and therefore devoid of fault outcrops. Moreover, the study area is situated in an intraplate region with a low displacement rate, which is exceeded by an erosion rate and does not favor the preservation of fault scarps. The trench sites were selected from prior DEM analyses, geomorphological fieldwork, and geophysical sounding. The trenches exposed a range of lithologies of Variscan crystalline rocks and Caenozoic sediments. At least four phases of faulting have been distinguished based on structural data, succession and age of the deformed sediments. Reverse faulting (N160°E), which displaced the Miocene sediments over the crystalline rocks, post-dates their deposition (15 Ma) and pre-dates Late Glacial gelifluction. Horizontal movements that have affected the Miocene deposits along a structure striking N35°E have the same wide time constraint. Younger reverse faulting, which caused a co-seismic relief step, post-dates the Late Glacial gelifluction but preceded the early Holocene colluvium deposition (10,940 ± 140 cal yrs BP). Normal faults striking N145°E, which cut the Miocene unit, might have been reactivated after the early Holocene colluvial sedimentation but before the buried soil (430 ± 120 cal yrs BP). Moreover, based on the identified prehistoric earthquake, respective minimum moment magnitude M 6.3 and slip rate 0.03 mm/year were estimated.