Anomalous Complex Electrical Conductivity of a Graphitic Black Schist From the Himalayas of Central Nepal

International audienceWe analyzed in the laboratory the frequency-dependent, complex-valued, electrical conductivity of a graphitic black schist and an augen gneiss, both collected in the Main Central Thrust shear zone in the Himalayas of central Nepal, which was heavily affected by the deadly Mw7.8 Gorkha earthquake in 2015. We focused on anisotropy and salinity dependence of both cores and crushed material as well as the impact of CO2 on conductivity. This black schist possesses an extraordinarily high polarizability and a highly frequency-dependent conductivity. Its anisotropy is very pronounced. The investigations can relate the main polarization feature to disseminated, aligned plates of graphite. By contrast, the augen gneiss shows low polarizability and a moderately anisotropic conductivity dominated by the pore-filling brine. We further demonstrate that neglecting the complex and frequency-dependent nature of conductivity can lead to serious misinterpretation of magnetotelluric data during inversion if highly polarizable rocks are present