TIMING OF CANYON INCISION OF THE RIO PATIVILICA IN REPONSE TO UPLIFT OF THE ANDES IN NORTHERN PERU

Uplift of the Peruvian Andes has occurred since ~10-20 Ma, and since that time deep canyons have been incised (3-4 km of relief) and high-relief ranges have been exhumed along the crest of the uplift (peak elevations 5-7 km).  Two main erosional stages record the progressive uplift and exhumation of the Peruvian Andes, and both cut into the Puna surface, which is a flat erosional surface in the Andes.  The Vallé surface represents ~2-2.5 km of incision into the Puna Surface, and locally these valleys are filled with Late Miocene ignimbrites. The second phase of erosion is the Cañón Stage, which involves deep incision into the Puna and Valle surfaces.  The Cañón incision phase represents ~2.5-3 km of further incision that follows Vallé stage erosion, but the timing of these events is not well constrained.  Canyon incision has progressed deep enough to expose low-temperature isotherms that predate uplift. Using an average geothermal gradient of 25^oC/km for this area, only 3-5 km of rock needs to be removed to expose both the apatite helium (AHe) and fission track closure depths. The Rio Pativilca Valley cuts deeply into the edge of the uplift and its maximum incision is ~3.5 km.  Thirteen samples were collected from the Cretaceous Coastal Batholith in an up-river traverse at ~4 km spacing.  Apatite helium ages in this traverse get progressively younger from 30 to 6 Ma. Except for the sample closest to the coast, the AHe ages fall into two main groups, 16-11 Ma and 8-5 Ma.  We suspect that the 16-11 Ma ages represents the Valle stage of incision, while the second group represents the onset of the Cañón stage, which continues to the Recent.