During several campaigns, long-period electromagnetic variation measurements were carried out in the central Andes between 19.5 ¿S and 22 ¿S in the convergence zone of the oceanic Nazca Plate and the continental South American plate. A robust multiple-station analysis has been used to investigate local differences of geomagnetic variation as an indication of subsurface conductivity distribution. For this purpose, spatially interconnected arrays of simultaneously running field stations have been further combined to form a large synthetic array. The analysis reveals a pronounced NS-striking anomaly with a maximum at a period of about hundred seconds in the Chilean Precordillera, which is likely due to the presence of the rather complex Precordillera fault system. While this anomaly vanishes towards longer periods, strong anomalous currents in the Bolivian Altiplano can be observed within the whole period range, indicating a great body of enhanced conductivity underneath. ¿ 2001 American Geophysical Union