Determination of erosion rates along actively growing mountain ranges at the NE margin of Tibet using cosmogenic 10Be
Zusammenfassung der Projektergebnisse
This study provides the first quantitative constraints on the rate of erosion of actively growing mountain ranges at the NE edge of the Tibetan Plateau – a key area of intracontinental mountain building and lateral plateau growth. Owing to systematic along-strike variations in mean slope and local relief, the fault-bounded mountain ranges in this region offer the opportunity to investigate how denudation depends on these topographic indices. We determined catchment-wide denudation rates along two of the actively growing mountain ranges (Yumu Shan and Longshou Shan) from 10Be concentrations in stream sediment. Both ranges expose (1) low-grade metamorphic Palaeozoic rocks in their centre, and (2) unmetamorphosed Cretaceous red beds near the tips of the range-bounding thrust faults. Field observations document debris flows, shallow landsliding, and surface wash as dominant mechanisms of mass transport on the hillslopes. Denudation rates in the Palaeozoic metasediments range from ~100 to ~300 mm ka-1 and correlate linearly with mean slope angle and local relief. In contrast, Cretaceous sediments exhibit higher and more variable denudation rates although slope and relief values are lower in these catchments. The higher denudation rates in the Cretaceous red beds result from their lower mechanical strength as compared to the resistant Palaeozoic rocks. This lithologic control on denudation demonstrates that uniform catchment lithologies and rock strength are essential for deciphering the effect of topographic indices on the rate of denudation. In the Yumu Shan wind gaps, thrust fault scarps at the mountain front, and hanging palaeocatchments indicate that this range continues to grow laterally and vertically. Field investigations, fault scarp profiles, and 10Be exposure dating constrained the rate of rock uplift in the eastern and central part of this range at ~500 and ~800 mm ka-1, respectively. As the uplift rate of the central Yumu Shan exceeds catchment-wide 10Be denudation rates, which range from ~180 to ~280 mm ka-1, the mountain range continues to rise and has not yet reached a topographic steady-state. This is, to our best knowledge, the first study that has demonstrated a pre-steady state topography for a mountain range by quantifying rock uplift and denudation rates on similar timescales with cosmogenic 10Be. Denudation rates obtained for larger catchments in the up to 5500 m high Qilian Shan support the inference that the parameters controlling denudation change through time. During an early growth stage of mountain ranges local relief and the mean hillslope angle increase and there is a linear relation between erosion rate and these two parameters. However, at a later stage slopes approach a threshold value and denudation rates become independent of slope. At this stage the denudation is presumably governed by the long-term rock uplift rate. The stochastic temporal and spatial distribution of landslides through time causes a considerable variability of the observed denudation rates. A key question to be answered in the future is if and how the rock uplift rate can be quantified from 10Be concentrations in river sediment.
Projektbezogene Publikationen (Auswahl)
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(2007). Denudation of actively growing mountain ranges in the foreland of NE Tibet inferred from in-situ produced cosmogenic Be-10, Eos Transactions AGU, 88(52), Fall Meeting AGU 2007
Palumbo L., Hetzel R., Tao M., Li X.
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(2007). Slip rates of active thrusts and rates of river incision in the Qilian Shan (NE-Tibet), Eos Transactions AGU, 88(52), Fall Meeting AGU 2007
Hetzel R., Palumbo L., Tao M.
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(2008). In-situ cosmogenic 10Be denudation rate variations along active tectonic ranges at the NE edge of the Tibetan Plateau. Geophysical Research Abstracts, vol. 10, EGU 2008
Palumbo L., Hetzel R., Tao M., Li X.
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(2009). Deciphering the rate of mountain growth during topographic presteady state: an example from the NE margin of the Tibetan Plateau. Tectonics, 28, TC4017
Palumbo, L., Hetzel, R., Tao, M., Li, X., Guo, J.
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(2009). Spatial variations in catchment-averaged denudation rates from normal fault footwalls. Geology, 37, 1139-1142
Densmore, A.L., Hetzel, R., Ivy-Ochs, S., Krugh, W.C., Dawers, N., Kubik, P.W.
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(2009). Uplift and denudation rates of an actively growing mountain range inferred from in-situ produced cosmogenic 10Be: the Yumu Shan (NE Tibetan Plateau), Geophysical Research Abstracts, vol. 11, EGU 2009
Palumbo L., Hetzel R., Tao M., Li X., Guo J.
