A new seismic velocity model for the south Central Andes is derived from
full waveform inversion, covering the Pampean flat and adjacent Payenia
steep subduction segments. Strong focused crustal low-velocity anomalies
indicate partial melts in the Payenia segment along the volcanic arc,
whereas weaker low-velocity anomalies covering a wide zone in Pampean
possibly indicates remnant melts in the past. Thinning and tearing of
the flat Nazca slab below the Pampean is inferred by gaps in the
high-velocity slab along the inland projection of the
Juan-Fernandez-Ridge. A high-velocity anomaly in the upper mantle below
the flat slab is interpreted as a relic Nazca slab segment, which
indicates an earlier slab break-off during the flattening process,
triggered by the buoyancy of the Juan-Fernandez-Ridge. In Payenia,
large-scale low-velocity anomalies atop and below the re-steepened Nazca
slab are associated with the re-opening of the mantle wedge and sub-slab
asthenospheric flow, respectively.