The application of electrical resistivity tomography and gravimetric survey as useful tools in an active tectonics study of the Sudetic Marginal Fault (Bohemian Massif, central Europe)

This paper presents the results of two separate geophysical investigations undertaken across the Sudetic Marginal Fault zone in the Bohemian Massif. This fault zone represents one of the most important tectonic features in central Europe. The first, preliminary, investigation used electrical resistivity tomography (ERT) to define the exact position of the main fault at two localities (Kamenička & Bílá Voda). Both profiles revealed a resistivity gradient that divided a complex of crystalline rocks with high resistivities from sedimentary deposits with low resistivities. From these data, sites were selected for palaeoseismic trenching. This trenching provided detailed information regarding the surveyed structures and lithologies. The second, more extensive, investigation used electrical resistivity tomography and gravimetric survey in order to extend the previously recognised features both laterally and to depth. The fault is displayed by the presence of an expressive resistivity gradient, even on profiles undertaken in more homogeneous lithologies. Finally, the most intensive investigations were conducted at Bílá Voda, where highly detailed ERT was undertaken on the floor of the trench. From this, it was possible to trace approximately logged geological structures to far greater depths and assess sedimentary thicknesses. A gravimetric survey was also undertaken adjacent to the trench. From modelling the gravitational effects of the fault zone, it is shown that the fault dips about 75° NE and that the vertical offset of Miocene vs. bedrock on the fault is approximately 200 m. The presented study has demonstrated the value of undertaking two phases of geophysical exploration separated by a trenching investigation. It is considered that this methodology is particularly suitable for the study of active tectonics in analogous intraplate settings where fault slip-rate is low.

Research Highlights
► ERT & gravimetric survey determined precise position of fault trace.
► Geophysical survey combined with trenching maximises effectiveness of methodology.
► Used research design enables to follow fault trace both laterally and to the depth.
► High-resolution ERT on trench floor gives more accurate geological interpretation.
► Gravity modelling suggests fault dip and vertical offset on Sudetic Marginal Fault.