Tracing Geophysical Indicators of Fluid-Induced Serpentinization in the Pampean Flat Slab of Central Chile

An apparent gap in the Andean volcanic arc in the Pampean section of the subduction zone in Chile (~28°–33°S) marks a section of flat slab subduction. In this tectonic environment, the fate of fluids released from the subducting Nazca slab remains uncertain and the degree of their interaction with the basal layer of the continental lithosphere is poorly understood. Results of a RF investigation and forward modeling effort at three long-running stations of the Chilean National Seismic Network allow us to constrain the position of the subducting Nazca slab and to address the physical properties of the interplate contact zone in Central Chile. Our observations suggest a transition in seismic character, from a weakly anisotropic contact in the normally subducting section north of the flat slab region to a strongly anisotropic plate contact within the flat slab region. We attribute this change to a transition from sheared olivine to serpentinized peridotite generated as a result of fluid release across the flat slab. This interpretation is supported by forward modeling synthetic RFs at each of the stations. We propose that the identified layer extends across the flat slab region, acting as a mineral reservoir that captures and, possibly, transports fluids from the dehydrating Nazca Plate as it subducts below South America. We note that the Ps converted phase at the slab interface at southernmost station GO04 suffers a 2-s discontinuity at 180° back azimuth, consistent with a 15–20-km scarp or kink in the Nazca slab to the south of the station.