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.

We present a new seismic tomography model for the crust and upper-mantle beneath the Central Andes based on multi-scale full seismic waveform inversion, proceeding from long periods (40–80~s) over several steps down to 12–60~s. The spatial resolution and trade-offs among inversion parameters are estimated through the multi-parameter point-spread functions. P and S wave velocity structures with a spatial resolution of 30–40 km for the upper mantle and 20 km for the crust could be resolved in the central study region. In our study, the subducting Nazca slab is clearly imaged in the upper mantle, with dip-angle variations from the north to the south. Bands of low velocities in the crust and mantle wedge indicate intense crustal partial melting and hydration of the mantle wedge beneath the frontal volcanic arc, respectively and they are linked to the vigorous dehydration from the subducting Nazca plate and intermediate depth seismicity within the slab. These low velocity bands are interrupted at 19.8º–21°S, both in the crust and uppermost mantle, hinting at the lower extent of crustal partial melting and hydration of the mantle wedge. The variation of lithospheic high velocity anomalies below the backarc from North to South allows insight into the evolutionary foundering stages of the Central Andean margin. A high velocity layer beneath the southern Altiplano suggests underthrusting of the leading edge of the Brazilian Shield. In contrast, a steeply westward dipping high velocity block and low velocity lithospheric uppermost mantle beneath the southern Puna plateau hints at the ongoing lithospheric delamination.