Abstract
AbstractA new seismic model for crust and upper mantle of the south Central Andes is derived from full waveform inversion, covering the Pampean flat subduction and adjacent Payenia steep subduction segments. 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 the Pampean segment are interpreted as remnant partial melts. Thinning and tearing of the flat Nazca slab is inferred from gaps in the slab along the inland projection of the Juan Fernandez Ridge. A high‐velocity anomaly in the mantle below the flat slab is interpreted as relic Nazca slab segment, which indicates an earlier slab break‐off triggered by the buoyancy of the Juan Fernandez Ridge during the flattening process. 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.
Highlights
AAThe temporal and spatial appearances of flat subduction segments along the South American western margin have been extensively debated (e.g., Antonijevic et al, 2015; Gutscher et al, 2000; Marot et al, 2014; Ramos & Folguera, 2009)
A new seismic model for crust and upper mantle of the south Central Andes is derived from full waveform inversion, covering the Pampean flat subduction and adjacent Payenia steep subduction segments
In contrast to the vigorous partial melting represented by strong low-velocity anomalies in the middle-crust beneath the Altiplano-Puna Volcanic Complex and volcanic arc for the northern Chile steep subduction zone (Gao et al, 2021; Ward et al, 2014; Yuan et al, 2000), the middle crust in the Pampean flat subduction AzAoneAA(28◦ –33◦ S) exhibits only moderately low to normal velocities along the volcanic arc (Figure 2a)
Summary
The temporal and spatial appearances of flat subduction segments along the South American western margin have been extensively debated (e.g., Antonijevic et al, 2015; Gutscher et al, 2000; Marot et al, 2014; Ramos & Folguera, 2009). In the Pampean flat subduction zone (Kay & Mpodozis, 2002; Ramos et al, 2002), the Nazca slab propagates horizontally for 200–300 km beneath the southern Central Andes (Figure 1a) whereas the Payenia segment was dominated by flat subduction period from 15 to 5 Ma but has been re-steepening since 4–5 Ma (Ramos & Folguera, 2009, 2011). The JFR arrived at the current position beneath the Sierras Pampeanas around 12 Ma (Figure 1) and the related flat subduction of the Nazca slab has again triggered inland migration and cessation of the subduction-related volcanism (Kay & Mpodozis, 2002), uplift of the main Andes, thick-skinned deformation, crustal thickening and basement uplift over a broad zone in the overriding plate (Cristallini & Ramos, 2000; Ramos et al, 2002). We employ seismic full waveform inversion (FWI) to better constrain the seismic structure in the crust and upper mantle beneath the south Central Andes, which would facilitate to further investigate the slab configuration and the crustal melt distributions in response to the subduction of the JFR in the Pampean and the mantle wedge evolution in response to the slab re-steepening in the Payenia
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