Neogene to Recent strain fields in the Peruvian Andes have been influenced by two fundamental plate boundary conditions. The first—oblique convergence between the Nazca and South American plates—has produced a component of sinistral deformation in the Andean region. Examples include counterclockwise rotation of late Cenozoic intercordilleran basins (e.g., Ayacucho, ca. 9 to 7 Ma), sinistral transtension that partially facilitated emplacement of the Cordillera Blanca batholith (ca. 8 Ma), and oblique sinistral slip along large-scale, active normal faults (e.g., Cordillera Blanca detachment fault, Quiches fault). The second boundary condition involved flattening of the Nazca slab, which at the latitudes studied here (8-13°S), was primarily caused by subduction of the aseismic (hotspot) Nazca Ridge. Plate reconstruction shows that the Nazca Ridge, the leading edge of the flat slab, subducted under the Cordillera Blanca region at ca. 5 Ma, a time coincident with both the cessation of arc magmatism and the onset of detachment faulting. We propose that lithospheric buoyancy from the subducted ridge triggered extensional collapse of pre-thickened continental crust. Detachment faulting nucleated in crust thermally weakened by emplacement of the Cordillera Blanca batholith, the penultimate magmatic event prior to slab flattening. The Andean region is thus dominated by transcurrent and extensional deformation, with contraction isolated to the trench and sub-Andean regions. Many questions remain, such as how slab flattening, and attendant plate coupling, might have affected the partitioning of oblique plate convergence in the overriding plate.