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Sedimentologic, provenance, oxygen isotope, and 40Ar/39Ar results provide insights into late Cenozoic evolution of an extensional hinterland basin in the Peruvian Andes. The 5–6.5 km Cordillera Blanca composes the glaciated footwall of a low‐angle normal fault parallel to active contractional structures in the Andean fold‐thrust belt. The ∼200 km long, WSW dipping (19°–36°) Cordillera Blanca detachment fault accommodated >12–15 km of dip‐slip displacement, inducing subsidence of the hanging‐wall supradetachment basin, which is filled by ∼1300 m of conglomerate, sandstone, siltstone, and limited carbonate of the upper Miocene‐Pliocene Lloclla Formation. Lithofacies associations are attributed to lacustrine fan‐delta to proximal, stream‐dominated alluvial fan sedimentation. Provenance data record footwall unroofing of Jurassic through Miocene volcanic and sedimentary rocks during NW directed, axial dispersal of sediment. Provenance data record the appearance of footwall‐derived granite clasts from the upper Miocene Cordillera Blanca batholith and a change to transverse, WSW directed transport. Variations in the character and preservation of basin fill suggest along‐strike propagation of the fault through time. Initial extension and subsidence is constrained to the latest Miocene by an 40Ar/39Ar biotite age of 5.4 ± 0.1 Ma for a basal tuff in the Lloclla Formation. Estimation of paleolake water composition shows very negative δ18O(VSMOW) values (−13.6‰ to−18.2‰), indicating that high elevations comparable to modern were already attained in the Cordillera Blanca during earliest normal faulting and basin evolution. These results lend support to models suggesting that substantial shortening, crustal thickening, and surface uplift were necessary precursor conditions for the generation of the Cordillera Blanca detachment fault.

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