Igneous intrusions were emplaced prior to and contemporaneous with horizontal shortening of the crust in the Late Cretaceous to late Eocene magmatic arc in north Chile (21°45′–22°30′S). Temporally changing major and trace elements of magmatic rocks from this paleo–arc system chronicled gradual crustal thickening prior to and substantial crustal thickening contemporaneously with crustal shortening. Balanced structural cross sections indicate a minimum of 9 km of arc-normal shortening that occurred simultaneously with dextral arc-parallel movements accounting for orogen-parallel lengthening of ∼10 km. This shortening produced ∼5.4 km of tectonic crustal thickening and resulted in a minimum of ∼42 km late Eocene Andean crustal thickness. Temporal and spatial geochemical changes diagnostic of crustal thickening indicate that the remainder (∼2.6 km) was accommodated by basaltic underplating at or near the base of the arc crust prior to and during transpression. The ratio of tectonic to magmatic crustal thickening is ∼2:1. Whole-crustal magmatic addition rates during the ∼12 m.y. duration of arc transpression are ∼35 km3 per kilometer of model arc length per million years. Mafic underplating may have thickened the Andean crust considerably, but most pre-Neogene crustal thickening was due to discrete episodes of tectonic shortening.