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Geological processes in Southern Patagonia are affected by the Patagonian slab window, formed by the subduction of the Chile Ridge and subsequent northward migration of the Chile Triple Junction. Using shear wave splitting analysis, we observe strong splitting of up to 2.5 s with an E‐W fast direction just south of the triple junction and the edge of the subducting Nazca slab. This region of strong anisotropy is coincident with low uppermost mantle shear velocities and an absence of mantle lithosphere, indicating that the mantle flow occurs in a warm, low‐viscosity, 200–300 km wide shallow mantle channel just to the south of the Nazca slab. The region of flow corresponds to a volcanic gap caused by depleted mantle compositions and absence of slab‐derived water. In most of Patagonia to the south of this channel, splitting fast directions trend NE‐SW consistent with large‐scale asthenospheric flow. Plain Language Summary: Slab windows represent openings or gaps in the downgoing slab, allowing the mantle to flow through the plane of the slab from one side of the subduction zone to the other. The subduction of a spreading ridge beneath South America forms a gap in the subducting slab below Patagonia, presenting an opportunity to investigate the influence of slab windows on mantle flow and geological processes. Although this region has been poorly instrumented in the past, the deployment of new seismic instruments and available data allow us to study how the mantle seismic velocity varies with direction in the region. From the directional dependence of seismic velocity, we can infer the direction of mantle flow. We observe a change from N‐S to E‐W mantle flow in the northern part of the slab window, near the edge of the subducting Nazca plate. The flow occurs in a warm, low viscosity shallow mantle channel corresponding to a gap in activity along the volcanic arc. Key Points: Shear wave splitting indicates strong anisotropy with an E‐W fast direction just south of the Chile Triple Junction and the edge of the subducting Nazca slabThe splitting and shear wave velocity structure suggest eastward shallow mantle flow in a 200–300 km wide channel around the edge of the Nazca slabIn most of southernmost Patagonia, splitting shows NE‐SW fast directions consistent with large‐scale asthenospheric flow [ABSTRACT FROM AUTHOR]

Subduction of the Nazca plate results in the uneven distributions of earthquakes and arc volcanoes along the South America's western margin. Here, we construct a high‐resolution shear‐wave velocity model from immediately offshore to the backarc in South America, using advanced full‐wave ambient noise tomography. Our new model confirms and provides further constraints on three major features, including (a) extensive low‐velocity anomalies within the continental crust, (b) two high‐velocity flat slab segments located beneath southern Peru and central Chile, and (c) complex slab geometry at flat‐to‐normal transitional subduction. The flat slab segments roughly correlate with the volcanic gaps but not with the seismicity gaps. We suggest that variations of slab geometry along strike and down dip have significantly modified the flow patterns within the mantle wedge. Subduction of oceanic ridges has altered the slab dehydration processes, which can influence the distribution of arc volcanism and the occurrence of intermediate‐depth earthquakes. Plain Language Summary: The subduction of the oceanic Nazca plate beneath the South American continent results in abundant earthquakes and arc volcanoes, which are unevenly distributed along the margin. There is a lack of intermediate‐depth earthquakes and arc volcanoes beneath Peru and central Chile/Argentina. How the properties of the subducted slab influence the distribution patterns of earthquakes and arc volcanoes remains as a fundamental question to be answered. In this study, we use an advanced seismological imaging method to construct a high‐resolution seismic velocity model within the South American margin. Our new model reveals two flat subduction regions beneath southern Peru and central Chile, which roughly correlate with the observed gaps of arc volcanoes but not of the earthquakes. Our model suggests that the slab could be either torn or distorted when transiting from flat to normal subduction. We propose that the variations of slab geometry have significantly modified the mantle flow patterns within the mantle wedge, resulting in the uneven distributions of arc volcanoes. The subduction of the Nazca Ridge and the Juan Fernandez Ridge has played a critical role in controlling the occurrence of the intermediate‐depth earthquakes. Key Points: Strong low‐velocity anomalies distribute extensively within the continental crust along latitudes of 12°–28°S at 15–35 km depthsThe subducting slab is divided into four segments along the margin including two normal sections and two nearly flat sectionsThe two flat slab segments beneath southern Peru and central Chile correlate well with the volcanic gaps but not with the seismic gaps [ABSTRACT FROM AUTHOR]

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. 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. Plain Language Summary: Taking advantage of the abundant information recorded in seismic waveforms, we imaged the seismic structure of the crust and upper mantle beneath central Chile and western Argentina, where the oceanic Nazca slab is subducting beneath the South American plate. The subducted Nazca slab is almost flat at a depth of 100–150 km in the north of the study area below the Pampean region, where the Juan Fernandez seamount ridge is subducting as part of the Nazca slab. The slab steepens again in the south in the Payenia region. Our model reveals pronounced low‐velocity anomalies within the Pampean flat slab along the inland projection of the Juan Fernandez Ridge, indicating that the Pampean flat slab is thinned or even torn apart. A high‐velocity anomaly is imaged beneath the flat slab, representing a former slab segment that was broken off during the slab flattening process and was overridden by the advancing young slab. Our model suggests a causal relationship between the oceanic ridge subduction and the flat slab formation. In the Payenia region, the slab re‐steepening resulted in the re‐establishment of the mantle wedge and induced hot mantle flow below the slab, which are characterized by low‐velocity anomalies in the model. Key Points: A new seismic model for the crust and upper mantle beneath central Chile and western Argentina is presentedThinning and tearing within the Pampean flat slab is detected along the inland projection of the Juan Fernandez RidgeA relic slab is imaged beneath the Pampean flat slab, reflecting slab break‐off during the flattening process [ABSTRACT FROM AUTHOR]