Complex patterns of fluid and melt transport in the central Andean subduction zone revealed by attenuation tomography

Abstract

We present a high resolution 3-D model of P-wave attenuation (Qp−1) for the central Andean subduction zone. Data from 1500, mostly intermediate depth (60–250 km) earthquakes recorded at three temporary seismic networks covering the forearc, arc, and backarc around 23°S were used for tomographic inversion. The forearc is characterised by uniformly high Qp values, indicating low temperature rocks, in accordance with low surface heat flow values. Prominent low Qp anomalies are found beneath the magmatic arc and the backarc in the crust and mantle. Continuous regions of low Qp connect earthquake clusters at 100 km and 200 km depth with zones of active volcanism in the arc and backarc. Fluids fluxed from the subducted oceanic lithosphere into the overlying mantle wedge, where they induce melting, explain our observations. We propose that low Qp regions indicate source and ascent pathways of metamorphic fluids and partial melts. Ascent of fluids and melts, as imaged by seismic Qp, are not vertical, as is often implicitely assumed. Instead, sources of fluids are located at different depth levels, and ascent paths are complex and exhibit significant variation within the study area. The largest Quaternary backarc volcano Cerro Tuzgle is fed by mantle melts which are imaged as a plume of low Qp material that reaches to the strong earthquake cluster at 200 km depth.