THERMOCHRONOLOGICAL EVIDENCE FOR KILOMETER-SCALE INCISION OF THE NORTHERN PERUVIAN ANDES

The Rio Pativilca, which deeply incises the western flank of the Peruvian Andes, has up to ~3.5 km of modern relief. High-standing geomorphologic surfaces suggest two phases of uplift. The Puna erosional surface formed near sea level before and now has a modern elevation of ~4500 m. The Vallé phase is characterized by broad, shallow sloping valleys formed by ~2-2.5 km of incision, and in some locations are filled by 5-6 Ma ignimbrites. The Cañón phase is characterized by steeply sloping valleys that represent another ~2-2.5 km of incision. Thermochronometry was used to date the timing of canyon incision. Twenty-four samples were taken from a transect up the Rio Pativilca and into the crest of the Andes. Thirteen are from the Cretaceous Coastal Batholith exposed by the incision of the Rio Pativilca. AHe ages decrease up valley from 30 to 5.5 Ma and AFT ages decrease from 60 to 14 Ma. AFT/AHe and AHe/Surface pairs yield changes in erosion rates at ~18 Ma and again at ~6 Ma. ZFT ages remain steady at 35 Ma up into the upper valley where they decrease to 25 Ma. These later ages most likely represent the last stages of plutonism because this time appears to correspond to late phase intrusions. The three ignimbrites sampled, which fill the Vallé phase valleys, have ZFT ages of 5-7 Ma. ZFT ages of samples in the crest of the Andes are 200-300 Ma indicating very little erosion has occurred since they were uplifted. We propose that uplift of the Northern Peruvian Andes has been occurring since ~18 Ma and that the Rio Pativilca has responded by incising down, therefore ages recorded in this transect record that uplift. Our erosion rate calculations indicate that at ~18 Ma, erosion rates changed from Puna phase rates of 75 m/Myr to Valle phase rates of 150-200 m/Myr. At 5-7 Ma uplift rates increased again from Vallé phase rates to Cañón phase rates of 300-400 m/Myr. We are currently modeling the influence that the evolving topography had on the closure isotherm and the pattern of young cooling ages.