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Magnetotelluric (MT) data were collected at the Laguna del Maule volcanic field (LdMVF), located in central Chile (36°S, 70.5°W), which has been experiencing unprecedented upward ground deformation since 2007. These data were used to create the first detailed three-dimensional electrical resistivity model of the LdMVF and surrounding area. The resulting model was spatially complex with several major conductive features imaged at different depths and locations around Laguna del Maule (LdM). A near-surface conductor (C1; 0.5 Ωm) approximately 100 m beneath the lake is interpreted as a conductive smectite clay cap related to a shallow hydrothermal reservoir. At 4 km depth, a strong conductor (C3; 0.3 Ωm) is located beneath the western edge of LdM. The proximity of C3 to the recent Pleistocene-to-Holocene vents in the northwest LdMVF and nearby hot springs suggests that C3 is a hydrous (>5 wt% H 2 O), rhyolitic partial melt with melt fraction >35% and a free-water hydrothermal component. C3 dips towards, and is connected to, a deeper conductor (C4; 1 Ωm). C4 is located to the north of LdM at >8 km depth below surface and is interpreted as a long-lived, rhyolitic-to-andesitic magma reservoir with melt fractions less than 35%. It is hypothesized that the deeper magma reservoir (C4) is providing melt and hydrothermal fluids to the shallower magma reservoir (C3). A large conductor directly beneath the LdMVF is not imaged with MT suggesting that any mush volume beneath LdM must be anhydrous (<2 wt% H 2 O), low temperature and low melt fraction (<25%) in order to go undetected. The presence of large conductors to the north has important implications for magma dynamics as it suggests that material may have a significant lateral component (>10 km) as it moves from the deep magma reservoir (C4) to create small, ephemeral volumes of eruptible melt (C3). It is hypothesized that there may be a north–south contrast in physical processes affecting the growth of melt-rich zones since major conductors are imaged in the northern LdMVF while no major conductors are detected beneath the southern vents. The analysis and interpretation of features directly beneath the lake is complicated by the surface conductor C1 which attenuates low-frequency signals. The attenuation from C1 does not affect C3 or C4. At 1 km depth directly beneath LdM, a weak conductor (C2; <10 Ωm) is imaged but is not required by the data. Forward modeling tests show that a relatively large (30 km 3 ), high melt fraction (>50%), silicic reservoir with 5 wt% H 2 O at 2 to 5 km depth beneath the inflation center is not supported by the MT data. However, a smaller (10 km 3 ) eruptible volume could go undetected even with relatively high melt fraction (>50%). The location of large melt regions to the north has important implications for long-term volcanic hazards at LdMVF as well as other volcanoes as it raises the possibility that the vent distribution is not always indicative of the location of deeper source regions of melt. [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]