Abstract
Abstract Magnetotelluric data were collected in the late 1990s in the Central Andes of Chile and Bolivia, with the aim to delineate the electrical conductivity distribution in the subsurface and its relations to subduction processes. In previous studies, these data were interpreted based on 2-D models. The principal result was a vast conductivity zone beneath the Altiplano high plateau at mid and lower crustal depths and a much smaller, though significant conductor associated with the Precordillera Fault System. However, there are some significant 3-D effects in the investigation area, in particular near the coast and on the eastern Altiplano. The aim of this work is to give a reinterpretation based on new 3-D inversion of these data. The 3-D inversion not only provides a better fit to the data compared to 2-D results but furthermore allows to include sites with strong telluric distortion which were ignored in previous studies. We are now able to explain anomalous phases above 90° and induction arrows pointing subparallel to the coast as observed at several sites in the Coastal Cordillera. These strongly distorted data are caused by highly conductive near-surface structures that are partly connected to the Pacific Ocean, forcing currents to flow around the sites. The lower crust beneath the Coastal Cordillera resembles a poorly conductive, nearly homogeneous half-space and is electrically unremarkable. Besides, we can now image the Precordillera conductor as a continuous, elongated feature. The volcanic arc of the Western Cordillera is highly resistive with the exception of a few conductive spots which may be associated with certain individual volcanoes or geothermal resources, respectively. The Altiplano conductor is again the dominant electrical feature in the Central Andes, indicating widespread melting of the middle and lower back-arc crust.
Highlights
In the late 1990s long-period magnetotelluric (MT) data were collected along two profiles at 19.9°S and 21.4°S in Northern Chile and Southwestern Bolivia
We further reduced the data set by only considering every second period, resulting in four periods per decade
The site density is by no means perfect in many parts of the Central Andes, we believe that the foregoing results are a significant improvement over the older 2-D models
Summary
In the late 1990s long-period magnetotelluric (MT) data were collected along two profiles at 19.9°S and 21.4°S in Northern Chile and Southwestern Bolivia (see Figure 1). The Precordillera developed as a volcanic arc in the Late Cretaceous and Early Paleogene (Scheuber et al 1994). This area is characterized by a large cover of rhyolitic ignimbrites and domes associated with basaltic and andesitic lavas and intruded by granitoid plutons. A large shear zone, the West Fissure or Falla Oeste, is located here. Along this zone are encountered some of the largest porphyry copper deposits in the world, e.g., Chuquicamata
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