COLD, DRY, DEPLETED LITHOSPHERE ABOVE THE FLAT SLAB IN CENTRAL CHILE AND ARGENTINA

While the geographic association between flat-slab subduction and both the cessation of arc volcanism and the presence of basement-cored uplifts has long been recognized, an understanding of their relationship has been elusive. Of particular interest with regards to this question in the central Chilean flat-slab region is the nature of the mantle material trapped between the top of the horizontal subducted Nazca plate at 100 km depth, and the base of the overriding continental crust at 40-60 km depth. We investigate the seismic velocities (Vp and Vs) and the Vp/Vs ratio of this material by inverting regional body wave travel times from local slab events recorded at stations deployed by the Chile Argentina Geophysical Experiment (CHARGE), and find low P-wave velocities, high S-wave velocities, and unusually low Vp/Vs ratios. This is inconsistent with the presence of hydration or melt in this material, but is consistent with unusually low temperatures. However, a normal cold dry mantle peridotite would not have such high shear wave velocities or such low Vp/Vs ratios. In order to get high shear velocities and low Vp/Vs ratios, it is necessary for the material above the flat slab to have a markedly depleted composition. This level of depletion is consistent with those found in xenoliths from cratonic lithosphere, and the high shear velocities and low Vp/Vs ratios are consistent with those observed beneath cratons. Because the source of cratonic lithosphere in this area is uncertain, we investigate the possibility that such a depleted composition could be found in oceanic lithosphere. This might indicate that the material above the current flat slab is the remnant oceanic lithosphere left behind after an earlier, shallower period of flat-slab subduction.