The central Andes in South America is an ideal location to investigate the interaction between a subducting slab and the surrounding mantle to the base of the mantle transition zone. We used finite-frequency teleseismic P-wave tomography to image velocity anomalies in the mantle from 100 to 700 km depth between 18°S and 28°S in the central Andes by combining data from 11 separate networks deployed in the region between 1994 and 2009. Deformation of the subducting Nazca slab is observed in the mantle transition zone, with regions of both thinning and thickening of the slab that we suggest are related to a temporary stagnation of the slab in the mantle transition zone. Our study also images a strong low-velocity anomaly beneath the Nazca slab in the mantle transition zone, which is consistent with either a local thermal anomaly or a region of hydrated material. The shallow mantle (<165 km) under the Eastern Cordillera is generally fast, consistent with proposed underthrusting of the Brazilian cratonic lithosphere or a string of localized lithospheric foundering. Several discontinuous low-velocity anomalies are observed beneath parts of the Altiplano and Puna Plateau, including two strong low-velocity anomalies in the upper mantle under the Los Frailes volcanic field and the southern Puna Plateau, consistent with proposed asthenospheric influx following lithospheric delamination.