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The Cordillera Blanca and Cordillera Huayhuash contain some of the highest topography in the Andes and provide insight into tectonomagmatic processes associated with the onset of flat-slab subduction. These adjacent ranges shared a similar history of deformation and exhumation prior to the late Miocene, when synconvergent extension began in the Cordillera Blanca. Magmatism in the Cordillera Huayhuash has been inferred as coeval with magmatism in the Cordillera Blanca. Yet, extension, which has been correlated with magmatic heat flow, is limited to the Cordillera Blanca. New zircon U-Pb dates and trace and rare earth element concentrations from the Cordillera Blanca batholith and the Huayllapa pluton in the Cordillera Huayhuash and reassessment of existing zircon data help to characterize regional magmatic processes prior to the establishment of flat-slab subduction. Two compositionally distinct samples of the Huayllapa pluton yielded mean ages of 24.8 ± 0.4 Ma and 25.4 ± 0.8 Ma. In contrast, the Cordillera Blanca batholith has a protracted crystallization history postdating that of the Cordillera Huayhuash by up to 20 m.y. Miocene magmatism in the Cordillera Blanca began at 19 Ma and ended with injection of large volumes of geochemically distinct, mantle-derived magma from 10 to 5 Ma. We suggest that 6–5 Ma magmatism in the Cordillera Blanca promoted elevated heat flow and reduced shear strength, which facilitated extensional shearing along the western slopes of the range, whereas colder amagmatic crust in the Cordillera Huayhuash inhibited southward propagation of faulting. Our data demonstrate that the linkages between magmatism and elevated heat flow identified in the Cordillera Blanca are important driving processes in initiating extension in cordilleran-style orogenies. [ABSTRACT FROM AUTHOR]