PALEO-ELA DEPRESSION AND REGIONAL ELA GRADIENTS IN THE CORDILLERA HUAYHUASH, PERÚ

The Cordillera Huayhuash, Perú (~10°15’ S) provides an opportunity to improve our understanding of spatial gradients of tropical equilibrium line altitude (ELA) and ELA depression during the Last Glacial Maximum (LGM). The high peaks of this range are currently glaciated, and past glaciations include the Holocene, late Glacial, LGM, and a probable older stage. We present results from a study to map glacial features, determine the glacial chronology, and reconstruct paleo-ELAs throughout the range. Cosmogenic ages (10Be) of quartz-bearing rocks on moraine ridges are used to determine the extent of the LGM and other major stages of glaciation in this little known area. Field work focused on the Jahuacocha valley, draining the western side of the cordillera, and the Carhuacocha and Mitucocha valleys, both major drainages of the eastern side of the range. Glacial features were mapped using hand-held global positioning system units, as well as aerial photographs, and Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) satellite data. We use ASTER-derived digital elevation models, 3 arc second (~90 m) shuttle radar topography mission (SRTM) digital elevation data for South America, and digitized 1:100,000 topographic maps to reconstruct ELAs. Initial cosmogenic dates from Jahuacocha moraines suggest that the glaciers of the late Glacial were nearly as great as at any time during the preserved geologic record. The elevation of maximum glacial extent is similar in these three major drainages, suggesting a smaller-than-expected variation in ELA depression, if glacier maxima were contemporaneous. Valley hypsometry is variable throughout the range, and probably was an important control on glacier development. In some cases, low altitude tributaries contribute a substantial ice volume to the main valleys, which may be an important key to constraining paleo snowline altitude variations. From the mapped moraine positions and valley shapes we reconstruct probable glacier area and ELA for each stage. Field and satellite based glacier reconstructions in the Jahuacocha, Carhuacocha, and Mitucocha valleys are a major step in developing an improved understanding of the spatial gradients in paleo glacier extent, ELA, and ELA depression at a critical tropical location.