The onset of orogenic shortening in the northern Andes Mountains coincided with latest Cretaceous accretion of the Caribbean oceanic plateau. We present isotopic data (εHf in zircon and εNd) coupled with arc position to test whether accretion led to abrupt crustal thickening in the northern Andes of Ecuador and Colombia. A rapid isotopic excursion toward more evolved crustal compositions was synchronous with ca. 75–70 Ma collision in Ecuador and preceded a similar deviation in Colombia at ca. 70–55 Ma. The rapid but diachronous shift to more evolved isotopic signatures is attributed to progressive northward accretion of the oceanic plateau and associated thickening of continental crust. We emphasize the effects of accretion on the magmatic evolution of Cordilleran-type margins, initially provoking shortening and crustal thickening, and ultimately providing a substrate for subsequent arcs.

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