HR: 08:45h
AN: T11G-04    [Abstracts]
TI: Orographic barriers, high-resolution TRMM rainfall, and relief variations along the eastern Andes
AU: * Bookhagen, B
EM: bodo@stanford.edu
AF: Geological and Environmental Sciences, Stanford University, Braun Hall 118, Stanford, CA 94305, United States
AU: Strecker, M R
EM: strecker@geo.uni-potsdam.de
AF: Institut für Geowissenschaften, Universität Potsdam, Karl-Liebknecht-Str. 24, Haus 27, Potsdam-Golm, 14476, Germany
AB: The complex interplay between erosion and tectonics shapes landscapes on various time and length scales. Thus, determining the relative importance of tectonic and climate-driven processes is key for our understanding of the evolution of tectonically active mountain belts. Each mountain belt may exhibit an individual and complex response to these processes that may be characteristic for differently sized sectors of the orogenic system. Here, we focus on small (10¹km) to medium (10²-10³km) scale relations along the eastern flank of the meridionally oriented South American Andes. The central Andes and the major part of the northern Andes are subject to strong rainfall and erosion on their eastern side. There, rainfall has a strong gradient with wet, northern and central parts and presently drier areas south of the Andes bend at ~17°S. While lithology and geologic units vary along strike, the general deformation regime of the northern and central eastern Andes is similar with east vergent thrust-fault systems. We use high-resolution Tropical Rainfall Measurement Mission (TRMM) and SRTM topographic data to characterize elevation, relief, and hillslope angle of peak rainfall at orographic barriers constituting the eastern flank of the orogen. Over a distance of more than 3500km along the eastern flank of the Andes facing the Amazon Basin, we find that peak rainfall (>3.5m/yr) occurs at a mean elevation of 1.3±0.17km, a mean 3- km-relief of 0.95±0.08km, and areas characterized by moderate mean hillslope angles of 18.3±1.7°. We find that topographic relief is the best first-order rainfall predictor. South of the eastward-convex bend of the Andes, rainfall amounts decrease and there exists no distinct, high rainfall peak. The reduction in rainfall is accompanied by a reduction of relief involving several hundred meters at the mountain front as rivers exiting the eastern Andes tend to form large sedimentary fan systems that reduce the distance between minimum and maximum elevation. For ~500km south of the bend, the mean hillslopes of the eastern Andes maintain moderate values before they decrease significantly. We relate this reduction to a change in deformation style from east to more west vergent fault systems at ~24°S due to the compressional reactivation of inherited Cretaceous extensional fault systems. This, in turn, results in slow, gently westward-rising hills with steep western sides. The lack of a pronounced high-relief zone on the eastern, windward side does not create a focused rainfall peak that exceeds the necessary threshold amounts for many erosive processes. In summary, there are two key results of our work that will help understand the shaping of mountain belts through erosive and tectonic processes: First, high rainfall peaks with amounts above 3.5m/yr in the northern and central eastern Andes exceed the triggering mechanisms for many erosive hillslope processes. There, hillslopes are moderately steep despite varying lithology and deformation-rate changes. Second, a minimum 3-km-relief of ~1km is needed to create a significant rainfall peak. This relief threshold is very constant for 3500km along strike of the orogen and suggests relief to be one of the controlling factors for peak rainfall formation in this environment. The lack of high-relief barriers on the eastern Andes in southern Bolivia and NW Argentina prevents the formation of a rainfall peak that in turn would lead to an area with focused erosion.
DE: 1625 Geomorphology and weathering (0790, 1824, 1825, 1826, 1886)
DE: 1824 Geomorphology: general (1625)
DE: 8175 Tectonics and landscape evolution
DE: 8177 Tectonics and climatic interactions
SC: Tectonophysics [T]
MN: 2007 Fall Meeting