The past ∼25 m.y. of geologic history in the northern ∼300 km (∼33°–36°S) of the Andean Southern Volcanic Zone has seen waxing and waning magmatic production rates and episodic eastward relocation of arc segments accompanied by abrupt chemical changes in the magmas. These changes can be linked to episodes of crustal thickening at times of backarc thrusting and to peaks of subduction erosion of forearc crust and mantle lithosphere at times of frontal-arc migration to the east. The magmatictectonic coupling is well seen in the history—enhanced by 28 new K-Ar ages, >160 major and trace element analyses, and Sr, Nd, and Pb isotope analyses—of a west to east transect through the El Teniente copper district near 34°S. The temporal trends in magmatic chemistry in this transect are like the well-documented south to north trends in Pleistocene to Holocene volcanic centers of the Southern Volcanic Zone, and both can be linked to the same events. The magmatic changes require differences in magma source regions as shown by isotopic data and in depths of crustal magma generation/fractionation as shown by pressure-sensitive trace element distributions. “Adakitic” magmas in the region are attributed to a combination of melting the base of thickened lower crust and crust entering the mantle through subduction erosion. Subduction erosion is argued to peak in episodes of frontal-arc migration at ca. 19–16 Ma and ca. 7–4 Ma. The combined effects of crustal shortening and forearc truncation in the past 20 m.y. near 34°S have led to the loss of ∼170 km of crustal width. The timing and arc length over which these events occurred show that subduction of the Juan Fernández Ridge on the Nazca plate cannot have been the major driving force. The history of the region shows the importance of non-steady-state processes in arc-magma production and the necessity of studying arc systems over millions, not tens of thousands, of years.