Electromagnetic Survey on a Crystalline Nappe Pile: Implications for the Existance of Graphite on Shear Planes

Extremly high electrical conductivities were found by electromagnetic and direct current measurements on the crystalline nappe complex of the Muenchberg Gneiss Massif in Southern Germany. Typically the resistivities reach values in the order of 1 Ω m and less and the conductance exeeds 10000 Siemens in places. In contrast the metamorphic rocks of this nappe are supposed to be very resistive. 3D forward modelling for both different data sets revealed a very high conductivity zone at shallow depth, which correlates with the lowermost part of the nappe pile. It is speculated that the high conductivity is caused by graphitic accumulations forming an extensive electronic network of regional extent, which is already known from different places within the Variscan crust in Central Europe. In addition strong self-potential anomalies were found at the margin of the nappe pile, which indicate a vertical dipping high conductivity structure and supports the hypothesis of graphite accumulated on shearplanes. This field study supports the observation of the formation of natural graphite during the forward thrusting of nappes, in which pressure, shear stress or strain facilitates graphitization.