Break-off and stagnation of the subducting Nazca slab in the mantle transition zone beneath West-Central South America

Abstract

The eastward subduction of the Nazca Plate beneath the South American continent forms the Andean subduction zone, while deep mantle dynamics associated with slab-entrained mantle flow, thermal structure, and geometry of the subducted slab remain debated. In this study, we have utilized a total of 49,237 receiver functions recorded by 669 stations in West-Central South America to map the variations in topography of the mantle transition zone (MTZ) discontinuities and elucidate their causes. Simultaneous depressions in apparent depths of MTZ discontinuities beneath the northern Andean Orogen are attributed to subduction-driven entrained mantle flow that generates low seismic velocity anomalies within the upper mantle. In contrast, joint analysis of detected MTZ thinning and high-velocity anomalies under the Andean Orogen (south of 26o S) reveals a slab break-off segment within the MTZ. The uplift of the 410 km discontinuity confirms penetration of the subducting Nazca slab into the MTZ. Notably, significant MTZ thickening (primarily engendered by the depression of the 660 km discontinuity) is deciphered under the Central South American continent and indicates stagnation of subducted slab segments at the bottom of the MTZ.