Date of Award
Master of Science (MS)
The Peruvian Andes have attained modern elevations of ~4000 m. Many peaks along the drainage divide are in excess of 5500 m with ~2-3 km of vertical relief, and canyons cut into the western edge have up to 3.5 km relief. The Rio Pativilica Canyon is cut into a folded Mesozoic marine sequence intruded by granitic rocks of the Coastal Batholith, which are overlain by Tertiary volcanics: this canyon has relief in excess of ~3 km. Based on geomorphological evidence, two episodes of valley incision cut the Puna erosional surface: 1) The Vallé stage, which is characterized by broad valleys with ~2000-2500 m of incision; and 2) the younger Cañón stage, characterized by steep canyons with ~2000-2500 m of incision. The thermochronological record of rocks in the Canyon was evaluated to understand the timing of incision. Zircon fission-track (ZFT) and zircon U+Th/He (ZHe) ages from the canyon are all nearly ~35 Ma except at the very top of the canyon when ages drop to ~25 Ma. These zircon cooling ages most likely represent a late episode of cooling following the last phase of plutonism in the Coastal Batholith.
Helium and fission track ages on apatite allow a detailed understanding of the timing of canyon incision. Here we integrate apatite fission-track (AFT) and apatite U+Th/He (AHe) results with known geomorphology. The Vallé stage surfaces are filled locally by 2000+ m of a Mio-Pliocene ignimbrite. This ignimbrite has ZFT and ZHe ages of ~5 Ma, so this phase of incision must predate these ignimbrite ages. In a transect from the coast across the piedmont and up the modern Canyon, 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 age pairs allow estimates of erosion rates, and these data suggest substantial increases in erosion rates at ~15 Ma and again at ~5-6 Ma.
Incision of the canyon is probably caused by either increase in erosion rates on a previously uplifted block, or progressive uplift of the block and concurrent incision. Based on available evidence, we propose that uplift of the Peruvian Andes has occurred since ~15 Ma and that the Rio Pativilca has responded by incising down as uplift has progressed. Increases in erosion rates are a result of increased uplift rates and occur at the start of the Vallé stage at ~15 Ma when erosion rates increased from ~75 m/Myr to 150-200 m/Myr. Erosion rates increase again at the start of the Cañón stage at ~5-6 Ma when rates increased to 300-400 m/Myr. These data suggest that uplift of this part of the Andes is slightly younger than uplift of the central Andes (i.e. Altiplano and Puna) farther to the south, and therefore these results support he idea of northward growth of the orogenic belt.
Montario, Matthew James, "Thermochronological evidence for Neogene incision of the Rio Pativilca Canyon, northern Peru" (2006). Geology Theses and Dissertations. 60.