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Abstract

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 degrees 45'-22 degrees 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 approximately 10 km. This shortening produced approximately 5.4 km of tectonic crustal thickening and resulted in a minimum of approximately 42 km late Eocene Andean crustal thickness. Temporal and spatial geochemical changes diagnostic of crustal thickening indicate that the remainder ( approximately 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 approximately 2:1. Whole-crustal magmatic addition rates during the approximately 12 m.y. duration of arc transpression are approximately 35 km (super 3) 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