ELECTROMAGNETIC STUDY OF THE ACTIVE CONTINENTAL MARGIN IN NORTHERN CHILE

Abstract Magnetotelluric and geomagnetic deep sounding measurements were carried out in the magmatic arc and forearc regions of northern Chile between 19.5° and 22°S to study the electrical conductivity structures of this active continental margin. The instruments used covered a very broad period range from 10 −4 s to approx. 2 × 10 4 s and thus enabled a resolution of deep as well as shallow structures. In this paper we focus on the interpretation of data from an east-west profile crossing Chile from the Pacific coast to the Western Cordillera at 20.5°S. A decomposition of the impedance tensors using the Groom-Bailey decomposition scheme shows that a two-dimensional interpretation is possible. The resulting regional strike direction is N9°W. Two-dimensional models were calculated in this coordinate frame and include the significant bathymetry of the trench as well as the topography of the Andes. The final model shows a generally high resistivity in the forearc and a very good conductor below the Precordillera. Unlike earlier models from areas further south, a good conductor is not observed below the magmatic arc itself. This correlates with the so-called Pica gap in the volcanic chain and a higher age of volcanic activity compared with adjacent areas.