Evolution of the Cordillera Blanca Normal Fault, Central Peruvian Andes: Evidence From Basin Analysis and 40Ar/39Ar Thermochronology

The Cordillera Blanca in the Andes of central Peru is located along a segment of modern flat-slab subduction. The range is bounded to the west by a NNW-striking, moderately WSW-dipping normal fault that persists for approximately 100 kilometers along strike. The fault has been active for the past 4-8 million years and has resulted in footwall exhumation of the Cordillera Blanca batholith, producing some of the highest peaks in the Andes. Miocene to Quaternary extension has been contemporaneous with subparallel shortening to the east in the Subandean fold-thrust belt. Initiation of extension may be related to the onset of flat-slab subduction or gravitational collapse. Sedimentary basin analysis and 40Ar/39Ar thermochronology have been undertaken in an attempt to reconstruct the evolution of the Cordillera Blanca normal fault. The Callejon de Huaylas extensional (supradetachment) basin forms the hanging wall of the Cordillera Blanca fault. Evidence of the unroofing history of the Cordillera Blanca is preserved within the late Miocene-Pliocene Lloclla Formation. A tuff at the base of the Lloclla Formation yields a 40Ar/39Ar age of 6.4 Ma. Measured stratigraphic sections in the Lloclla Formation have thicknesses of ~1300 meters and are interpreted to have been proximal alluvial and fluvial environments and distal lacustrine facies. We attribute initial basin subsidence to slip on the Cordillera Blanca normal fault. However, the only exposure of the Lloclla Formation extends approximately 20 kilometers along strike of the fault while the fault itself is over 100 kilometers long. We have analyzed over 25 potassium feldspar, biotite, and muscovite samples from the central and southern Cordillera Blanca batholith that were collected along two transects perpendicular to the Cordillera Blanca normal fault. Mica results along both traverses yield consistent bulk cooling ages from ~5 Ma next to the fault to ~7 Ma in the core of the batholith. Preliminary analysis of potassium feldspar 40Ar/39Ar step-heating results reveal that rocks proximal to the Cordillera Blanca normal fault cooled from >300° C at 5-6 Ma to below ~150° C by 2-3 Ma. Rocks far-removed from the Cordillera Blanca normal fault exhibited a similar cooling history but began cooling about 1-2 Ma earlier. We interpret these results to reflect basement exhumation along the Cordillera Blanca normal fault and are in the process of constraining its slip history with the new thermal history results and with samples from a northern transect of the batholith.