Petrological modeling of the flat and steep subduction zone in Central Chile, based on seismological, thermo-mechanical and mineralogical data

The Central Chilean subduction zone (27-35°S) is host to a multitude of unexplained phenomena, all likely linked to one another. The region displays a well developed, highly seismically active flat subduction, which correlates very well with the Juan Fernandez aseismic Ridge (JFR) track, subducting with the Nazca plate beneath South America. The nature of deformation back to normal subduction (30°) to the north and south of the flat slab is still debated. The continental crust above it is one of the thickest in the world (>70 km), and arc volcanism has ceased after having progressively migrated eastward at the inception of slab flattening. We notice the presence of a Double Benioff Zone (DBZ) with unexpectedly large interplane distance (30 km) than a conventional distance linked with the oceanic's plate age (~35 Ma). We also interpret a sequence of intermediate-depth earthquakes to occur along a reactivated pre-existing fault plane, probably indicating a link with outer rise faults. In order to better understand the interaction between the subducting and overriding lithospheres, the role of the JFR on the flat slab and the microseismicity, we have aimed at modeling the petrological composition of the area, along the flat slab (31.5°S) and the normally subducting slab (33.5°S). We have been looking for domains of plate hydration (and to which plausible amounts), and for the location of buoyant material in the slab, which is thought to be the main cause for its flattening. To do so, we combined seismic data with instantaneous thermo-mechanical models and mineralogical rock property databases. Here, we present seismic tomography images of first wave arrivals for the region from temporary seismic campaigns. Our seismic velocities associated to the ~100 km depth flat slab are best modeled with eclogitized oceanic crust, with a density > 3400 kg/m3. Non-eclogitized MORB material leads to much too low Vp and Vs values to explain our data in the flat slab. The continental mantle is characterized by spinel and garnet peridotites and is probably unusually cold, and the slab mantle is comprised dominantly of garnet peridotite. The lower continental crust appears to be eclogitized, in accordance with independent seismic studies for the area. What we find also is a moderately hydrated slab (2-12 wt% H2O) and continental mantle corner wedge (1-8 wt% H2O) down to a maximum of 70 km depth. Other domains are characterized by rocks with less than 0.1%wt H20.