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The subducted Nasca plate produces anomalies of spectacular low electrical resistivity beneath the Andean knee. This is the result of two magnetotelluric field campaigns in 1982. The measurements were carried out on a profile from the Pacific coast to the Eastern Cordillera crossing northern Chile and southern Bolivia. Several anomalies may be distinguished. The strongest anomaly has been detected beneath the Pre- and West-Cordillera, with resistivities around 0.5 Ωm at depths greater than 5-10 km, strike direction roughly parallel to the main crest of the High-Cordillera. But also, the Altiplano is underlain – at a depth of 40-50 km – by very low-resistive material. In agreement with other geophysical and geological observations, we interpret these low resistivities as being due to the partially melted state. Further arguments identify the Pre-Cordillera anomaly, probably, with a plutonite which is still "alive", and the highly-conductive material 40 km beneath the Altiplano with the source of andesitic magma. A crucial point in this study is the distinction between the E- and B-polarization of the apparent resistivities. We distinguished both polarizations by the direction of the induction arrows and by the lateral continuity of the apparent resistivity.
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