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Shear faults in Tertiary clay formation of the Eger Basin ( Czech Republic ) and their role for the degassing of the uppermost mantle

The NW corner of the Bohemian Massif is characterized by the ENE-WNW trending Eger Graben (with lignite), the NNW-SSE trending Eger (Cheb) Basin, mafic volcanics of Tertiary to Quarternary age, mineral water springs, and mofettes of a high CO2 degassing flow rate, and by swarm earthquakes. The most recent swarm event lasted from August to November 2000 with more than 10 000 single events (Fischer & al. 2000). The hypocentres depth is between 5 and 11 km. From isotope geochemical (C, N, He) investigations it was determined that the total amount of gas (> 99 vol.% CO2) entering the atmosphere is about 9 Mio m p.a. (Weinlich & al. 1999). The delta CCO2 value (–1.8 to –4.0‰), the He/He ratios (R/Ra = 5), features from upper mantle xenolith studies and deep seismic profiling hints at an active magmatic reservoir in the uppermost mantle beneath the NW-Bohemia/ Vogtland area (Kämpf & al. 1999, Weinlich & al. 1999). The morphology along the western boundary of the Eger Basin (Fig. 1) is dominated by the down dipping Fichtelgebirge Granite (which was drilled beneath the basin filling), and towards east the basin is bordered by the escarpment of the NW-SE-running Marianske Lazne fault. There the basement is offset for more than 200 meters. To the north and south the bottom of the basin rises slowly to the surface.