Fig. 1. Left: P-T diagrams of metamorphic facies down to 70 km depth determined from laboratory measurements to explain different hydrous rocks characteristics of subduction zones (Peacok, 1996). P-T paths taken from a geotherm model at latitude 21 S (Springer & Foerster, 1998; right figure) are superimposed into the diagram as dashed lines for: the Coastal Cordillera mountain belt (CC; black), Precordillera (red) and the region between the Western Cordillera (the volcanic arc) and Altiplano (WC-AP; orange).EB: Epidote blueschist, EA: epidote amphibolite.
Right: W-E cross section at latitude 20.5 S of the 3- D resistivity model (in Ohm m) by Lezaeta (2001). Red to yellow colors is indicative of high conductivity zones (0.1-5 Ohm m): At upper crustal levels or at temperatures below the melting point, the enhanced conductivity is mostly due to ionic conduction from the presence of fluids within interconnected porous material or fractures. At mid-to lower crust levels where temperatures are in the melting point range of silicate rocks, only a 3 to 10% partial melt fraction can produce bulk conductivities in the values described by the model beneath the Altiplano high plateau, from 20 km down to about 60 km depth.
WF: West fissure, AF: Atacama fault. QBBS: Quebrada Blanca Bright Spot, a seismic reflector with no sign of high conductivity, suggesting that this is either a side effect due to 3-D structures or that there is a fluid saturated film with poor porous interconnection. In the Precordillera thrust fold system (west of WF at the surface) at depths within 20 to 30 km, temperatures of 300-500 C would allow the phase transformation of greenschist to epidote amphibolite, producing water to release. This mid-crustal zone has high conductivity values, suggesting that metamorphic reactions are the source for fluid production, ascending to upper crustal levels through the interconnected fractures of the Precordillera fault system.