Mantle flow pattern associated with the Patagonian slab window determined from azimuthal anisotropy

Abstract

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 SummarySlab 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 PointsShear 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.