New ⁴⁰Ar/³⁹Ar, major and trace element, and isotopic data for ca. 24–15 Ma backarc volcanic rocks from the Sierra de Huantraico, Sierra Negra, and Sierra de Chachahuén–La Matancilla regions (36°S–38°S) in the Neuquén Basin shed light on the early Miocene evolution of the south-central Andes. A model calling for incipient shallowing of the sub-ducting slab under the northern Neuquén Basin and an increase in the rate of westward motion of South America relative to the underlying mantle at ca. 20 Ma can explain many regional features. Early Miocene magmatism in the Neuquén Basin began with the eruption...