Regional P and S wave travel time data were used to obtain three-dimensional seismic tomography models for Vp, Vs, and Vp/Vs above the subducting slab in central Chile and Argentina. In this region, there is an abrupt change from a normal subduction geometry south of 33¿S to a flat subduction geometry to the north. We find low Vp, low Vs, and high Vp/Vs ratios in the southern half of our study area directly beneath the modern active volcanic arc, which we interpret as localized pockets of melt. In the northern half of our study area, above where the subducting Nazca plate flattens at 100 km depth, we find low Vp, high Vs, and low Vp/Vs ratios. These unusual results point to a lack of melt or hydrated mineralogies such as serpentine, both of which are characterized by high Vp/Vs values. The only mantle rocks that have low Vp/Vs and high Vs are Mg-rich compositions, such as dehydrated serpentinite or orthopyroxenite. We suggest that significant portions of the mantle overlying the flat slab consist of orthopyroxenite, formed by a transient fluxing of silica-rich fluids. Such fluids may have come from sediments that were subducted during the initiation of flat subduction at this latitude at ~10 Ma. This would imply that the hydration of mantle material above a flat slab can be a transient phenomenon, which leaves little residual-free water behind but significantly alters the mantle chemistry.