Abstract

The subduction zone of central Chile (36 degrees S) has produced some of the world's largest earthquakes and significant volcanic eruptions. Understanding the fluid fluxes and structure of the subducting slab and overriding plate can provide insight into the tectonic processes responsible for both seismicity and magmatism. Broadband and long-period magnetotelluric data were collected along a 350-km profile in central Chile and Argentina and show a regional geoelectric strike of 15 +/- 19 degrees east of north. The preferred two-dimensional inversion model included the geometry of the subducting Nazca plate as a constraint. On the upper surface of the Nazca plate, conductors were interpreted as fluids expelled from the downgoing slab via compaction at shallow depth (C1) and metamorphic reactions at depths of 40-90 km (C2 and C3). At greater depths (130 km), a conductor (C7) is interpreted as a region of partial melt related to deserpentinization in the backarc. A resistor on the slab interface (R1) is coincident with a high-velocity anomaly which was interpreted as a strong asperity which may affect the coseismic slip behavior of large megathrust earthquakes at this latitude. Correlations with seismicity suggest slab fluids alter the forearc mantle and define the downdip limit of the seismogenic zone. Beneath the volcanic arc, several upper crustal conductors (C4 and C5) represent partial melt beneath the Tatara-San Pedro Volcano and the Laguna del Maule Volcanic Field. A deeper lower crustal conductor (C6) underlies both volcanoes and suggests a connected network of melt in a thermally mature lower crust. Plain Language Summary Central Chile has experienced some of the world's largest earthquakes and many large volcanic eruptions. These disasters are caused by the tectonic processes in this region as the oceanic Nazca plate subducts beneath the South American plate. As the oceanic plate subducts, water is released, which can increase pressure, lubricate the plates, and lower rock melting points to produce magma. As such, fluids are an important control on where earthquakes and volcanoes occur. We image the electrical conductivity of this region using the magnetotelluric method since saline water and/or partial melt both have a high conductivity. Several conductors are imaged near the plate interface that can be related to presence of water or partial melt. Correlations between the locations of conductors and zones of low seismic velocity reported in previous seismic studies support the idea that there is an anomalous block of rock at 30-km depth, which increases friction along the subducting plate interface. This could explain why the 2010 Maule earthquake did not rupture far inland. A conductor at 40-km depth correlates with a gap between two clusters of earthquakes, which suggests that fluids may chemically alter the continental plate and reduce friction in this gap.