U-Pb and 40Ar/39Ar isotopic evidence for coeval Neogene pluton emplacement and fault-induced exhumation in the Cordillera Blanca, Peru

Thermochronology has been used to decipher the history of fault slip and footwall uplift along major normal faults. The Cordillera Blanca is the highest single range in the Peruvian Andes, with average elevations exceeding 5000 m. The western boundary of the range is well-defined by the active, 170-km-long Cordillera Blanca normal fault, which exhibits a range of dips along strike, from 36° WSW in the north to 19° WSW in the south. The footwall is defined by high topography, variable relief along strike from north to south (from >2500 m to 500 m), and has likely accommodated > 10 km of dip slip since the late Miocene. The footwall of the Cordillera Blanca normal fault is a granodioritic batholith with U-Pb zircon crystallization ages that suggest emplacement between 5 and 8 Ma. Based on a basal tuff in the associated hanging-wall supradetachment basin, normal faulting had initiated by 5.4 Ma. The goal of this study is to constrain the timing of fault initiation, age of intrusive activity, fault slip rates, and history of unroofing events along the Cordillera Blanca normal fault. In this study, multiple thermochronometers are applied to the footwall of the Cordillera Blanca normal fault. Spatial patterns from mica and feldspar ⁴⁰Ar/³⁹Ar ages and apatite (U-Th)/He ages indicate that the Cordillera Blanca detachment fault can be described as a flexural rolling hinge that has exhumed the footwall since 3-6 Ma. Deeply incised glacial valleys indicate that erosion has played a significant role during footwall exhumation. Provided that simplified assumptions are made about the geothermal gradient, fault dip, and mineral closure temperatures, calculations of fault slip rates, vertical exhumation rates, and horizontal extension rates are derived from ⁴⁰Ar/³⁹Ar, apatite (U-Th)/He, and apatite fission-track data. When combined with published Quaternary slip rates, it is apparent that these rates have been decreasing since the late Miocene. From this study, it is clear that pluton emplacement and footwall exhumation along the Cordillera Blanca normal fault overlapped temporally. Therefore, cooling ages record elevated temperatures due to both magmatic cooling and fault slip. New analyses of zircon (U-Pb) and potassium feldspar (⁴⁰Ar/³⁹Ar) constrain the initiation of exhumation between 5.5 and 7.5 Ma.