THERMOCHRONOLOGIC RECORDS OF CANYON INCISION AND WESTERN ALTIPLANO DEFORMATION, SOUTHERN PERU

The Cotahuasi-Ocoña River in southern Peru has incised a canyon over 3 km deep into the surrounding Altiplano surface in response to Cenozoic uplift that produced the only major barrier to atmospheric circulation in the Southern Hemisphere. Understanding how tectonics and climate have interacted throughout the evolution of this major orogen requires a detailed history of surface uplift, but often paleoelevation techniques are hampered by large uncertainties and potentially systematic errors. Deciphering surface uplift through the record of canyon incision offers a promising alternative.

We present apatite (U-Th)/He data that record a transient cooling response to canyon incision. Although there is abundant evidence in the region that uplift and incision started by c. 30 Ma, our data suggest that a major pulse of over 2.4 km of incision started at c. 9 Ma in the middle reaches of the canyon. Volcanic flows near the bottom of the present canyon floor dated with ⁴⁰Ar/³⁹Ar geochronology constrain the end of significant incision to 2.3 Ma.

Spatial patterns of uplift can offer additional clues about geodynamic mechanisms of uplift and potential orogen-wide implications. Despite the prevalence of normal and thrust faults that deform Oligocene-age sediments throughout the western margin of the Altiplano, there is little evidence in southern Peru for significant movement on these structures associated with the latest period of uplift. Many of the faults mapped in the field do not deform c. 14 to 16 Ma volcanic layers or sediments, and a valley-bottom transect of zircon (U-Th)/He data shows no distinct jumps that would be associated with significant movement on a single structure. This evidence suggests that the latest stages of uplift were not exclusively related to plateau-bounding fault systems. Furthermore, we see evidence that at least some of the recent uplift extended out to coastal regions. These data suggest that the latest period of canyon incision was associated with either block uplift of the western margin and coastal regions, or very long-wavelength warping into a monocline. Such deformation could have been produced by mid- to lower-crustal flow of material, or by lithospheric delamination.