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. We present results of a receiver-function investigation and forward-modeling effort at 3 long-running stations of the Chilean National Seismic Network. We constrain the position of the subducting Nazca slab and address the physical properties of the interplate contact zone in the area of transition between flat- and higher-angle subduction in Central Chile. Our observations suggest presence of highly-anisotropic low-velocity layer directly above the subducting Nazca slab. This interpretation is further supported by a forward-modeling effort to generate synthetic receiver functions at station GO03 and GO04 with a velocity model that incorporates a serpentinized interplate layer. We further report that receiver functions calculated at station GO04 exhibit an apparent positive Ps polarity signal at the nominal depth of the subducting oceanic Moho, but with a twist. The delay-time moveout with back azimuth varies continuously with back azimuth, but has a 2-second tear at 180° back azimuth (from the south). This behavior is consistent with a kink in the slab interface south of the station that may be an incipient slab tear. This kink represents a roughly 20 km depth change, which is consistent with the depth variation in seismicity south of the station.