HR: 0800h
AN: T31A-0283    [Abstracts]
TI: Quaternary Geologic History of the Rio Tambo, Southern Peru: Repeated Mass-Wasting Events in Western Cordillera Drainages
AU: * Davis, M W
EM: dfarber2@ucsc.edu
AF: University of California, Santa Cruz, Dept. of Earth Sciences, Santa Cruz, CA 95060, United States
AU: Hall, S R
EM: shall@pmc.ucsc.edu
AF: University of California, Santa Cruz, Dept. of Earth Sciences, Santa Cruz, CA 95060, United States
AU: Farber, D L
EM: dfarber@ucsc.edu
AF: University of California, Santa Cruz, Dept. of Earth Sciences, Santa Cruz, CA 95060, United States
AU: Farber, D L
EM: dfarber@ucsc.edu
AF: Lawrence Livermore National Laboratory, 4000 East Ave., Livermore, CA 94550, United States
AU: Audin, L
EM: laurence.audin@ird.fr
AF: IRD-LMTG-UNIVERSITE DE TOULOUSE-CNRS-OMP, Lima, Peru, TOULOUSE, 001, France
AU: Finkel, R
EM: finkel1@llnl.gov
AF: Lawrence Livermore National Laboratory, 4000 East Ave., Livermore, CA 94550, United States
AB: The amount and timing of river incision along the western margin of the Altiplano can provide insight into the climatic, volcanic, and tectonic history of this region. While many studies have focused on the Tertiary geologic history of the western margin of the Altiplano, the Quaternary geologic history of this margin remains largely unstudied. The Pacific draining Rio Tambo river valley in the forearc of southern Peru is one of the large drainages of the western margin. While much of this river is presently cutting bedrock, many locations within the main and tributary channels contain lacustrian, landslide, and volcanic ash deposits of Quaternary age (?). While geomorphologic features such as active and abandoned alluvial fans, fluvial terraces (strath and fill), and pediment surfaces exist along multiple segments of this drainage current geologic maps of the area lack the spatial resolution to effectively use these deposits for constructing the geomorphic evolution of the area. Thus, we have mapped the Quaternary (?) features through field measurements using a theodolite, as well as from remotely sensed data including SRTM DEMs, stereo-pair aerial photos, and ASTER images. Using cosmogenic 10Be to date the surfaces, we are able to place bounds on the age of terrace, lacustrian, and landslide deposits. Although fluvial incision is an important erosive process, which has resulted in this high-relief landscape, we find extensive evidence that mass-wasting is here extremely effective at facilitating the rapid movement of large amounts of material. At numerous times throughout the Quaternary, huge debris flows have dammed the steep fluvial valleys producing vast lakes and thick (up to ~100m ) lacustrian deposits upstream of the landslide deposits. At present, the landslide and lacustrian deposits have been re-incised, in many cases, to bedrock, and only stranded lake and landslide deposits remain high on valley walls. Thus, a large amount of material has been transported out of this drainage basin through first mass-wasting followed by river incision. As the Rio Tambo is located in the hyperarid climatic zone and thus receives very little annual precipitation, landsliding might result from over steepened valley walls associated with nick-point migration or possibly as a result of groundwater sapping. Further, we observe this type of rapid landscape modification in many of the bedrock-rivers in both northern and southern Peru and indeed throughout the western margin of the Peruvian Andes. While previous authors have described the late Cenozoic-present as a period of canyon cutting throughout the western margin, we suggest that mass-wasting and re-incision is the dominant process facilitating the degradation of the high topography of this portion of the the Andes.
DE: 8104 Continental margins: convergent
DE: 8107 Continental neotectonics (8002)
DE: 8175 Tectonics and landscape evolution
DE: 8177 Tectonics and climatic interactions
SC: Tectonophysics [T]
MN: 2007 Fall Meeting