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 of ca. 24–20 Ma alkali olivine basalts from monogenetic and simple polygenetic centers located up to 500 km east of the trench. Their characteristics (Ta/Hf > 0.45, ε_Nd ⁼ +3.6–+4.2; La/Ta < 14; Ba/La < 16; ⁸⁷Sr/⁸⁶Sr = 0.7037–0.7040) indicate a backarc mantle devoid of arc-like components. These basalts erupted at a time of extension all along the margin during a period of rapid, near-normal Nazca–South America plate convergence when spreading ridges between the Pacific, Nazca, and Antarctic plates were becoming more parallel to the Chile Trench. Ridge rotation along with slab roll-back in response to slow relative motion between South America and the underlying mantle can explain why isotopically enriched magmas erupted far to the east of the trench in a generally extensional regime. Subsequently, 19–15 Ma basaltic to trachyandesitic backarc lavas with weak arc-like La/Ta (15–26), Ba/La (15–32), and Ta/Hf (0.2–4.5) ratios and a more depleted isotopic signature (ε_Nd = +3.9–+4.7; 87Sr/86Sr = 0.7033–0.7037) erupted in a contractional regime. Their chemical features fit with incipient shallowing of the Nazca plate under the northern Neuquén Basin. A contractional regime that extended all along the margin can be explained by westward acceleration of South America over the underlying mantle as Nazca–South America plate convergence slowed.