HR: 12:05h
AN: T12D-08    [Abstracts]
TI: Sensitivity of Denudation Rates to Latitudinal and Orographic Variations in Climate, Central Andes, Bolivia.
AU: * Insel, N
EM: nadinsel@umich.edu
AF: University of Michigan Department of Geological Sciences, 1100 North University Avenue, Ann Arbor, MI 48109, United States
AU: Ehlers, T A
EM: tehlers@umich.edu
AF: University of Michigan Department of Geological Sciences, 1100 North University Avenue, Ann Arbor, MI 48109, United States
AU: Schaller, M
EM: mirjam@umich.edu
AF: University of Michigan Department of Geological Sciences, 1100 North University Avenue, Ann Arbor, MI 48109, United States
AB: Continental denudation rates are commonly thought to be sensitive to both tectonics and climate, but the importance of each effect is difficult to constrain. In this study we present 17 new cosmogenic ¹⁰Be data to quantify medium-term, basin-averaged denudation rates from the Andean fold and thrust belt, Bolivia. Sampling occurred across two different transects in both the northern (~-15°S) wet and southern (~- 19°S) dry Subandes. These data are compared with (1) modern latitudinal variations in precipitation, (2) orographically controlled E-W climate gradients, and (3) differences in morphological parameters. In addition, results are used to (4) verify temporal variations in denudation rates by comparing millennial-scale (cosmogenic nuclide-derived) and decadal-scale (sediment yield-derived) rates.
Cosmogenic nuclide-derived denudation rates are as follows: In the southern transect denudation rates vary by two orders of magnitude and range between 0.04 to 2.20 mm/yr over timescales between 0.4-19 kyr. The highest values and largest variability in denudation rates are observed in the smallest catchments with drainage basin areas <60 km². In the northern transect denudation rates range between 0.13 and 0.95 mm/yr over timescales between 0.5-6.8 kyr. Here, the highest values are observed in small basins located along the frontal range of the Andean mountain chain. Interestingly, the mean value for the cosmogenic nuclide-derived denudation rate is 0.57 +/- 0.07 mm/yr for the dry south and 0.35 +/- 0.04 mm/yr for the wet north. In contrast, modern denudation rates calculated from suspended sediment flux data suggest that average rates in the south (0.3 mm/yr) are 3-4 times slower than in the north (1.3 mm/yr).
Previous results exhibit strong spatial and temporal variations in denudation rates and lead to the following implications: (1) Cosmogenic nuclide-derived denudation rates are poorly to uncorrelated with drainage basin characteristics such as relief or slope. (2) An obvious relationship is observed between denudation rates for catchment sizes >1000 km² and precipitation. (3) Denudation rates in small catchments (<200 km²) do not show a statistical relationship with climate. Climatic effects seem to be superimposed by tectonic influences (e.g. proximity to active faults). (4) Temporal variations in denudation rates might be related to an anthropogenic increase in sediment yield in areas with intense farming. Another hypothesis is that climate changed during the integrated time interval and cosmogenic nuclide-derived denudation rates inherited a past climate signal.
DE: 1130 Geomorphological geochronology
DE: 1150 Cosmogenic-nuclide exposure dating (4918)
DE: 8177 Tectonics and climatic interactions
DE: 9360 South America
SC: Tectonophysics [T]
MN: 2007 Fall Meeting