HR: 11:05h
AN: T32C-04    [Abstracts]
TI: Long-term denudation rates in the central Himalayas: What can we learn from detrital zircon fission-track dating and specific-stream power modeling?
AU: * Bookhagen, B
EM: bodo@crustal.ucsb.edu
AF: Institute for Crustal Studies, University of California, Santa Barbara 1140 Girvetz Hall, Santa Barbara, CA 93106-1100 United States
AU: Burbank, D W
EM: burbak@crustal.ucsb.edu
AF: Institute for Crustal Studies, University of California, Santa Barbara 1140 Girvetz Hall, Santa Barbara, CA 93106-1100 United States
AU: Brewer, I D
EM: Ian.Brewer@stos.co.nz
AF: Shell Todd Oil Services Ltd, 167 Devon St West, New Plymouth, 4620 New Zealand
AU: Garver, J I
EM: garverj@union.edu
AF: Department of Geology, Olin Building, Union College 807 Union ST, Schenectady, NY 12308-2311 United States
AB: The central Himalaya encompasses areas characterized by high rates of rock uplift and erosion. It is also heavily influenced by the Indian summer monsoon, which modulates the sediment-transport capacity of Himalayan rivers. In order to constrain long-term denudation rates, we analyzed detrital zircon fission-track samples from 6 major river catchments that span 800 km along the southern flank of the central Himalaya. The age distribution of each sample (~70 zircons counted) reflects spatial variations in catchment denudation at million year time scales. Five catchments have similar grain-age distributions with a primary population of ages centered at 5±1 Ma. Several of these catchments display a secondary population of ages ranging from 12-15 Ma. However, the biggest catchment (Karnali) in a central position is characterized by a dominant population of older grain ages centered at ~10 Ma. Here, we seek to explain these puzzling results by using stream power modeling based on high-resolution space-borne precipitation (TRMM) and topography (SRTM). We use precipitation to predict discharges and thus derive a more realistic stream-power model, with higher values in the eastern catchments and lower values in the more arid, northern regions that are draining parts of the Tibetan Plateau. The precipitation distribution shows two distinctive orogen-parallel bands of high rainfall values: (1) at the orographic barriers of ~1 km elevation in frontal part of the Lesser Himalaya; and (2) around 2-3 km of elevation on the southern flank of the Greater Himalaya. This more northerly band is discontinuous along the range, and is nearly absent in the Karnali catchment, thus creating drier conditions and lower specific discharges. We hypothesize that regions of high specific stream power will coincide spatially with the dominant sources of eroded sediment. Given that cooling ages depend on both altitude and latitudinal position in a laterally advecting orogen, spatial differences in where high stream power occurs may explain differences in detrital ages. In a SRTM-based analysis, we use specific stream power above a threshold value to define rapidly eroding parts of each catchment. The hypsometry (elevation vs. area distribution) of these regions shows that all but the Karnali catchment have prominent area-probability distribution peaks at ~1.5 km elevation and several have a secondary peak at ~3-5 km. In contrast, high stream power in the Karnali occurs nearly uniformly between 1 and ~4 km elevation. Such a distribution is predicted to yield a broader, older population of cooling ages, similar to that displayed by the fission-track data. In the other catchments, primary sediment yield from a narrow range of lower altitudes should produce the observed younger ages with a principal source near the toe of the Greater Himalaya.
DE: 1040 Radiogenic isotope geochemistry
DE: 1625 Geomorphology and weathering (0790, 1824, 1825, 1826, 1886)
DE: 8102 Continental contractional orogenic belts and inversion tectonics
DE: 8122 Dynamics: gravity and tectonics
SC: Tectonophysics [T]
MN: Fall Meeting 2005