TECTONIC AND THERMAL EVOLUTION OF THE CORDILLERA BLANCA DETACHMENT SYSTEM, PERUVIAN ANDES: IMPLICATIONS FOR NORMAL FAULTING IN A CONTRACTIONAL OROGEN

The Cordillera Blanca, Peru provides a unique opportunity to study a large active normal fault in the hinterland of a compressional orogen. Detailed geomorphology, sedimentology, and thermochronology allow comparison to large normal faults in classic extensional settings, such as the Basin and Range Province. The Cordillera Blanca normal fault measures 170 km in length and dips west-southwest at low to moderate angles (19-36°). The fault bounds the western side of the Cordillera Blanca, the highest single mountain range in the Peruvian Andes, where average elevations exceed 5000 meters. The footwall is composed of late Miocene granodiorite, which has been deeply incised by glacial activity. New U-Pb zircon crystallization ages indicate that the footwall batholith cooled ~5-8 Ma. Cooling ages from ⁴⁰Ar/³⁹Ar studies of biotite and potassium feldspar reveal coeval exhumation ~4-6 Ma. The Cordillera Blanca lies directly above a shallowly-subducting segment of the Nazca Plate. Extension along the Cordillera Blanca normal fault may be a result of viscous coupling between the flat slab and overriding plate. Vertical exhumation rates have slowed since 3 Ma, likely caused by strengthening of the upper plate due to refrigeration.

The high relief, mappable fault trace, modern tectonic activity, and uniform footwall lithology make the Cordillera Blanca an ideal location to test models for the evolution of topography related to normal faulting. Classic normal fault models predict a tectonic and geomorphic symmetry in which maximum slip and surface uplift occur near the central fault segments and decrease toward the fault tips. Extraction of drainage network parameters from digital elevation models of the Cordillera Blanca reveal a pronounced asymmetry toward the northern fault tip. Initiation of faulting and basin subsidence is constrained at 5.4 ± 0.1 Ma by ⁴⁰Ar/³⁹Ar dating of a basal tuff. Oxygen isotopic compositions of modern waters and lacustrine carbonates are consistent with deposition at high altitude. Studies of fault propagation suggest that the geometry of sedimentary basin fill is related to fault growth. Distinct sedimentation patterns suggest that the Cordillera Blanca normal fault has behaved as a single segment and has propagated in a single, southward direction.