RESISTIVITY CROSS-SECTION THROUGH THE SOUTHERN CENTRAL ANDES AS INFERRED FROM MAGNETOTELLURIC AND GEOMAGNETIC DEEP SOUNDINGS

To investigate the upper lithospheric structure and subduction-induced processes across an active continental margin, magnetotelluric (MT) a nd geomagnetic deep sounding (GDS) experiments were done in the southe rn central Andes (21 degrees-25 degrees S) of northern Chile, southern Bolivia and northwestern Argentina. Using two-dimensional modeling, w e have constructed an E-W resistivity cross section at latitude 21 deg rees 45'S that involves al the Andean units from the Pacific coast to the lowland plains. The crust of the Coastal Cordillera has high resis tivities of more than 5000 ohm m with some conductive structures embed ded. Increased electrical conductivities between depths of 8 and 40 km may be related to the rise of fluids released from the subducted Nazc a 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 wa s detected. This high-conductivity zone (HCZ) is found at a depth of a bout 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 s eismic 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 in creases and then drops abruptly at about longitude 65 degrees 10' W. T he latter HCZs can be explained in terms of thrust tectonics, where de tachment 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.