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Abstract

Igneous rocks 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). Changing major and trace elements, and Sr- and Nd-isotopic characteristics of magmatic rocks from this paleo-arc system monitored gradual crustal thickening prior to and substantial crustal thickening during crustal shortening. Balanced structural cross-sections indicate that volume-constant transpression only accounts for 67 % (or 8 km) of thickening of late Eocene Andean crust. Temporal and spatial geochemical and isotopic changes diagnostic of crustal thickening indicate that the remainder was accommodated by underplating of mantle-derived basalts at or near the base of the arc crust prior to and during transpression. The ratio of tectonic to magmatic crustal thickening lies in the order of ~2:1. Whole magmatic crustal addition-rates during the ~12 m.y. duration of arc transpression are in the order of 35 km3/km model-length/m.y. Mafic underplating may have thickened the Andean crust considerably, but most pre-Neogene crustal thickening was due to discrete episodes of tectonic shortening.