Understanding the effects of flat slab subduction on volcanic products

Abstract:

An apparent gap in the Andean volcanic arc in the Chilean-Pampean section of the subduction zone in Chile (~28°-33°S) marks a section of flat-slab subduction. Here, volcanic activity ceased ~5 Ma, counter to the commonly accepted mechanism of flux-induced melting in the mantle wedge. In this tectonic environment, the fate of fluids released from the subducting Nazca slab remains uncertain, the degree of their interaction with the basal layer of the continental lithosphere and how the presence of subduction-derived fluids may impact magmatic evolution of the area in the future is poorly understood. We present results of a multidisciplinary study combining geophysical and petrologic observations, focused on the processes influencing subduction zone geometry in Central Chile and their impact on regional seismic and volcanic activity. A broad-scale geophysical survey revealed evidence of a highly anisotropic layer above the subducting slab beneath stations within the transition zone and flat slab region, possibly indicating an area of extensive hydration triggered by fluid release from the subducting plate.

Here, we present a detailed compilation of available geochemical data to understand any cross- or along-arc variations that could be attributed to the geometry of the subducting slab over time, focused on trace element trends that are indicative of interactions with hydrated mantle. We include new analyses for recent eruptions at volcanoes north and south of the flat slab. There is an apparent correlation between anisotropic signatures and changes in geochemical patterns north of the flat slab region, which has been influenced by the subducting Juan Fernandez Ridge, and south of the current position of the flat slab. By combining the geophysical results with petrologic observations, it is our goal to further constrain the inherent relationship between flat slab subduction and its effects on surrounding volcanism.