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Magnetotelluric (MT) data were used to create a three-dimensional electrical resistivity model of the Altiplano-Puna magma body (APMB) in the area surrounding Volcán Uturuncu in southern Bolivia. This volcano is at the center of a zone of surface deformation with a diameter of 150 km and persistent inflation of ∼10 mm/yr. Low electrical resistivities (<3 Ωm) at a depth of 14 + 1/-3 km below sea level (16-20 km below surface) are interpreted as being due to the presence of andesite melts in the APMB, and require a minimum melt fraction of 15%. The upper crustal resistivity structure is characterized by finite-length, dike-shaped conductors, oriented approximately east-west near sea level. A combination of dacite partial melts and aqueous fluids is required to explain the observed low-resistivity values. Geodetic data do not require any deformation in these shallow regions. The geometry of the upper surface of the APMB beneath Volcán Uturuncu is consistent with that predicted by geodynamic models that suggest that the APMB bulges upward directly beneath Volcán Uturuncu, near the measured inflation center (∼3 km west of Volcán Uturuncu). Viscosity estimates from the MT-derived resistivity model gives a maximum value of 1016 Pa·s and is consistent with models that propose diapir-like ascent of magma above the APMB. Resistivity models are compared and quantitatively correlated to seismic velocity models, showing good agreement on the spatial extent and depth of the APMB. A forward modeling study shows that the small differences in the depth to the top of the APMB between the different geophysical methods could be explained by variations in the composition of the magma body. [ABSTRACT FROM AUTHOR]

The Salar de Atacama region (23°-24°S) in the Central Andes of northern Chile represents an anomalous segment of the Andes, where a very large forearc basin is present (Salar de Atacama) and the axis of the active volcanic chain is deflected to the east from its regional trend. Here we used magnetotelluric (MT) data to infer the distribution of partial melts and hydrothermal fluids in order to better understand the mechanism controlling the magmatism in this subduction zone. Our 3-D inversion model shows a complex electrical resistivity structure, with several mid crustal conductors (<5 Ωm) imaged around the Salar de Atacama basin (SdAb). Although the forearc is dominated by high resistivities (>1000 Ωm), conductors were imaged below the Precordillera and in the northern SdAb, where major trench-parallel and transverse fault systems are present. We interpret these forearc conductors as evidence of active and fossil hydrothermal systems hosted by the Precordillera Fault System and the Calama-Olacapato-El Toro transverse fault. Conductors found along the arc are spatially coincident with active volcanoes likely representing partial melting in the continental crust. A large conductor was imaged in the backarc, indicating the presence of a significant volume of partial melts below the Altiplano-Puna plateau, as many previous geophysical studies have suggested. The model shows that the mid-lower crust and subcontinental mantle (i.e., >15 km depth) below the SdAb is dominated by resistivity values >100 Ωm., suggesting the presence of a relatively poorly hydrated continental crust and mantle wedge in this segment of the northern Chile subduction zone. These results, together with petrophysical interpretations derived from previous seismic and gravimetric studies, suggest that the lithosphere below the SdAb represents an impermeable block, which inhibits the upward flow of fluids and melts derived from deeper parts of the subduction system. [Display omitted] • We present a 3-D electrical resistivity model of Central Andes lithosphere (23°–24°S). • The Salar de Atacama basin (SdAb) lithosphere represents a high-resistivity block. • Petrophysical properties suggest that the SdAb block is an impermeable region. • The SdAb impermeable block may explain the eastward deflection of the volcanic arc. [ABSTRACT FROM AUTHOR]