EMPLACEMENT AND EXHUMATION OF THE CORDILLERA BLANCA BATHOLITH, PERU

The Cordillera Blanca is the highest range in the Peruvian Andes and is underlain by a late Miocene trondjhemitic batholith derived from a deep, garnet-bearing source region.  Although previous studies had determined batholith intrusion was at 8 Ma, new U-Pb zircon ages indicate that crystallization occurred over several m.y. with some crystallization taking place as late as 6 Ma. The processes by which the batholith was exhumed poses a fundamental geologic problem in that significant extension occurred in a regional stress field characterized by ENE-WSW compression related to flat-slab subduction of the Nazca plate beneath South America.  Pliocene exhumation of the Cordillera Blanca occurred along a still active low-angle normal fault, the Cordillera Blanca detachment.  The fault dips 25-40° WSW and features a ~1-km-thick mylonite zone within batholithic footwall rocks. Fault scarps cutting glacial moraines show that the fault has been active during Quaternary time but the Miocene history of exhumation and fault slip remain poorly constrained.  The hanging wall of the normal fault includes the Callejon de Huaylas supradetachment basin containing 8.7-7.5 Ma volcanic rocks and an Upper Miocene-Pliocene succession of nonmarine sedimentary rocks.  The late Mio-Pliocene sedimentary record constrains early motion on the southern fault zone at 5.4 Ma.  Muscovite, biotite, and potassium feldspar ⁴⁰Ar/³⁹Ar thermal history results in the central portion of the range, indicate that the normal faulting was active between 6-3 Ma with apatite (U-Th)/He closure occurring as late as 2 Ma.  We are currently addressing along-strike variations in exhumation, fault slip, and basin development along the Cordillera Blanca normal fault by performing additional U-Pb zircon and ⁴⁰Ar/³⁹Ar thermal history measurements from seven new traverses distributed along the entire 100-km length of the batholith.  Similarly, numerical simulations that take concomitant pluton emplacement and normal faulting into account are in progress.  Initial results indicate denudation rates between 1-2 mm and ~ 12 km overall normal slip along the detachment.