We report new high‐precision U/Pb ages and geochemical data from the Chaltén Plutonic Complex to better understand the link between magmatism and tectonics in Southern Patagonia. This small intrusion located in the back‐arc region east of the Patagonian Batholith provides important insights on the role of arc migration and subduction erosion. The Chaltén Plutonic Complex consists of a suite of calc‐alkaline gabbroic to granitic rocks, which were emplaced over 530 kyr between 16.90 ± 0.05 Ma and 16.37 ± 0.02 Ma. A synthesis of age and geochemical data from other intrusions in Patagonia reveals (a) striking similarities between the Chaltén Plutonic Complex and the Neogene intrusions of the batholith and differences to other back‐arc intrusions such as Torres del Paine (b) a distinct E‐W trend of calc‐alkaline magmatic activity between 20 and 17 Ma. We propose that this trend reflects the eastward migration of the magmatic arc, and the consistent age pattern between the subduction segments north and south of the Chile triple junction suggests a causal relation with a period of fast subduction of the Farallón‐Nazca plate during the Early Miocene. Previously proposed flat slab models are not consistent with the present location and morphology of the Southern Patagonian Batholith. We advocate, alternatively, that migration of the magmatic arc is caused by subduction erosion due to the increasing subduction velocities during the Early Miocene.