Resistivity cross section through the southern central Andes as inferred from magnetotelluric and geomagnetic deep soundings

Abstract

To investigate the upper lithospheric structure and subduction‐induced processes across an active continental margin, magnetotelluric (MT) and geomagnetic deep sounding (GDS) experiments were done in the southern central Andes (21°–25°S) of northern Chile, southern Bolivia and northwestern Argentina. Using two‐dimensional modeling, we have constructed an E‐W resistivity cross section at latitude 21°45′S that involves all the Andean units from the Pacific coast to the lowland plains. The crust of the Coastal Cordillera has high resistivities of more than 5000 ohm m with some conductive structures embedded. Increased electrical conductivities between depths of 8 and 40 km may be related to the rise of fluids released from the subducted Nazca plate. The electrical resistivity of the ‘normal Andean’ crust seems to be low, having values of only 200 ohm m. A wide zone of extremely high electrical conductivity with an E‐W extent of more than 330 km was detected. This high‐ conductivity zone (HCZ) is found at a depth of about 25 km under the Western Cordillera and has a total conductance of more than 23,000 S. Here, it may be caused by partially melted crust, an interpretation which would correlate with those from gravity and seismic data. The HCZ can be followed in the lower crust eastward below the Altiplano where it has a total conductance of about 15,000 S. In the western part of the Eastern Cordillera, total conductance first increases and then drops abruptly at about longitude 65°10′W. The latter HCZs can be explained in terms of thrust tectonics, where detachment zones may involve fluids as well as increased mineralizations. The lowlands of the sub‐Andean and the Chaco are characterized by a cover of anisotropic low resistivity. The resistivity of the crust and uppermost mantle increases from west to east to more than 1000 ohm m, while the deeper mantle gets unexpectedly much more conductive.