[1] A long-period magnetotelluric study was carried out in the central Andes between latitudes 19.5°S and 21°S along two almost parallel profiles of 220 and 380 km length, respectively. The investigation area extends from the Pacific coast to the southern Altiplano Plateau in the back arc of the South American subduction zone. The main geoelectrical structure resolved is a broad and probably deep-reaching highly conductive zone in the middle and deeper crust beneath the high plateau. Although the data show deviations from two-dimensionality, a two-dimensional approach is justified for large parts of the profiles. Sensitivity studies were carried out in order to constrain the depth extent. Another electrically conductive structure was resolved in the middle crust of the Chilean forearc, thought to be connected with the Precordillera fault system. The Andean Continental Research Program (ANCORP) seismic reflection profile, carried out along the same line at 21°S, revealed highly reflective zones below the Altiplano, in good correlation with the upper boundary of the Altiplano conductor. This highly conductive domain also coincides with low seismic velocities and a zone of an elevated v p /v s ratio and, although not well resolved, with low Q p seismic quality factors. Taking into account the enhanced heat flow and a derived temperature model, the most probable explanation lies in the assumption of granitic partial melts. The good conductor below the volcanic arc which was found in regions farther south at 22°S gradually vanishes toward the north; this is consistent with the results of seismic tomography concerning Q p values and a gap of recent volcanism.

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[1] A 400-km-long seismic reflection profile (Andean Continental Research Project 1996 (ANCORP'96)) and integrated geophysical experiments (wide-angle seismology, passive seismology, gravity, and magnetotelluric depth sounding) across the central Andes (21°S) observed subduction of the Nazca plate under the South American continent. An east dipping reflector (Nazca Reflector) is linked to the down going oceanic crust and shows increasing downdip intensity before gradual breakdown below 80 km. We interpret parts of the Nazca Reflector as a fluid trap located at the front of recent hydration and shearing of the mantle, the fluids being supplied by dehydration of the oceanic plate. Patches of bright (Quebrada Blanca Bright Spot) to more diffuse reflectivity underlie the plateau domain at 15-30 km depth. This reflectivity is associated with a low-velocity zone, P to S wave conversions, the upper limits of high conductivity and high V p /V s ratios, and to the occurrence of Neogene volcanic rocks at surface. We interpret this feature as evidence of widespread partial melting of the plateau crust causing decoupling of the upper and lower crust during Neogene shortening and plateau growth. The imaging properties of the continental Moho beneath the Andes indicate a broad transitional character of the crust-mantle boundary owing to active processes like hydration of mantle rocks (in the cooler parts of the plate margin system), magmatic underplating and intraplating under and into the lowermost crust, mechanical instability at Moho, etc. Hence all first-order features appear to be related to fluid-assisted processes in a subduction setting.

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