Magnetotellurics in the Eger Rift: Regional and local three-dimensional subsurface imaging and modelling of fluid pathways from the crust-mantle boundary to the surface

The Bohemian Massif is part of the geodynamically active European Cenozoic Rift System and represents its easternmost termination. The study area is situated at the junction of three different Variscan tectonic units and hosts beside the Eger Rift a series of different fault systems. The entire region is characterized by ongoing magmatic processes in the intra-continental lithospheric mantle expressed by, e.g., the occurrence of repeated earthquake swarms, the presence of Quaternary volcanoes, and massive degassing of mantle-derived CO₂ in mineral springs, mofettes as well as. Several geoscientific studies suggest that fluid circulation along the deep-reaching faults seems to play an important role in explaining the underlying geodynamic processes. As part of an ICDP drilling programme, we applied the Magnetotelluric (MT) method with the goal to contribute to the understanding of the physical and chemical processes and interaction that led to the magma and fluid transport by mapping potential fluid pathways from the crust-mantle boundary up to the surface. Here, we present 3D inversion models of two different overlapping regional and one local MT experiments located in the Cheb basin close to the Hartoušov mofette field. The most prominent large-scale conductivity features of the regional models are several channels from the lower crust to the surface, possibly representing pathways for fluids into the earthquake swarm region, mofette fields, and known spas. However, such a conductive channel is absent in the local model beneath the surface expression of the mofettes. We will test two different hypotheses, namely a vertical ascending channel versus lateral fluid migration. Results from synthetic modelling studies and available geoscientific constraints hint that such a channel might exist directly beneath the mofette field, but due to the given data quality, station distribution, and the subsurface conductivity structure within a conductive sediment basin, it might be challenging to resolve.