The morphology and composition of subducted slabs in the upper mantle transition zone (400 to 700 km depth) have direct implications for the style of mantle convection. Here we investigate the seismic reflectivity structure at transition zone depths beneath South America by combining the arrival times and amplitudes of an updated data set of SS precursors. We observe broad regions with up to 20 km depressed 410- and 660-km discontinuities and diminished precursor amplitudes beneath the back-arc region of Nazca subduction, which are incompatible with temperature-dominated mineral phase boundary perturbations. A probabilistic inversion is applied to simultaneously determine mantle temperature and composition, and the outcomes suggest a mechanically mixed transition zone with basalt enrichment beneath the Amazon basin. This is corroborated by a novel amplitude-versus-offset inversion that measures the changes in density and shear velocity across the discontinuities. Our observations offer direct evidence for thermochemical anomalies within the mantle, which are largely associated with temporary slab stagnation. The slab-transition zone interactions suggest that the Nazca-South American convergence may represent a later-stage analog to the Pacific plate subduction in northeast Asia. •SS precursors resolve high-resolution transition zone structures in South America.•We image a thickened transition zone with diminished amplitudes beneath Amazonia.•North-south differences exist as the Nazca slab interacts with transition zone.•A thermochemical anomaly is detected, likely enriched in basalt.•Nazca subduction may represent a later-stage analog to Pacific plate subduction.