The Cordillera Blanca Batholith and ignimbrites of the intermountain Callejon de Huaylas basin represent the last magmatic event in the Andean cycle of central-northern Peru. This most spectacular example of an extensional regime is characterised by acid magmatism spanning some 10 Ma, with both magma emplacement and subsequent deformation being fundamentally controlled by a ca. 300-km lineament known as the Cordillera Blanca fault complex. The western margin of the batholith has been strongly deformed by the Cordillera Blanca fault, resulting in a series of ductile to brittle structures that increase with intensity away from the relatively undeformed core towards the margin. The late-stage brittle fabrics are coeval with extensional structures in the Callejon de Huaylas basin, and relate to uplift and extension along the fault zone. The more ductile fabrics, as indicated by combined K-Ar and 40Ar-39Ar mineral dates, formed earlier in the intrusion history, and are consistent with emplacement into an active dextral (strike-slip) shear zone. The rapid uplift and high erosion rates seen from the Late Miocene onwards and the current state of stress in this region of thickened crust are due to gravitational instability (body forces) set up at high topographical levels, and buoyancy forces resulting from the intrusion of young, acid magmas at high crustal levels. © 1992.