2005 Salt Lake City Annual Meeting (October 16–19, 2005)

Paper No. 10
Presentation Time: 10:40 AM


BERNET, Matthias1, VAN DER BEEK, Peter2, HUYGHE, Pascale3, MUGNIER, Jean-Louis2 and PIK, Raphael4, (1)LGCA, Université Joseph Fourier, Maison des Géosciences, 1381 rue de la piscine, Grenoble, 38041, France, (2)Laboratoire de Géodynamique des Chaînes Alpines, Université Joseph Fourier, BP 53, Grenoble, 38041, France, (3)LGCA (Laboratoire de géodynamique des chaines alpines, CNRS, université Joseph Fourier, maison des géosciences, BP 53, Grenoble, AZ 38041, France, (4)CRPG-CNRS, 15, rue N.D. des Pauvres, B.P. 20, Vandoeuvre-Lès-Nancy, 54501, France, matthias.bernet@aya.yale.edu

Dating individual grains with two or more methods is a new trend in detrital thermochronology in recent years. Zircon, a common mineral in clastic sediments, is ideally suited for this approach, because A) it is very robust during weathering and transport, B) it has reasonably high uranium concentrations that allow dating with different methods, and C) the closure temperatures of the different dating methods are relatively high, so that the grains are not easily reset after deposition. The available dating methods are U/Pb dating (crystallization or high-temperature metamorphism), fission-track dating (cooling below ~240°C), and (U-Th)/He dating (cooling below ~160°C). Thermochronology of detrital grains is usually done to determine sediment provenance, as well as exhumation rates and cooling histories of sediment source areas. In our study of detrital zircon from modern river sediment and ancient sandstone samples of the Miocene to Pliocene Siwalik Group in western Nepal, we combine fission-track and U/Pb dating on single zircons. While U/Pb ages help to differentiate between broad source areas, such as the Tethyan Himalayas, Greater Himalayas, or Lesser Himalayas, the fission-track ages reflect the most recent cooling history of these different sources and can be used to estimate long-term exhumation rates. Our results so far indicate that zircons derived from Tethyan Himalayan sources have pre-Himalayan cooling ages (> 100 Ma). Greater and Lesser Himalayan zircons in contrast show a very similar cooling age pattern. Both source regions deliver zircons that either have cooling ages between 15-18 Ma, or they belong to a young group with consistently changing ages, but constant lag times (fission-track age - depositional age). The reoccurring 15-18 Ma age group is an indication of fast cooling and possibly rapid exhumation in the Himalayas around 15-18 Ma. The changing age group is evidence for continuous exhumation at rate of 1 – 1.5 km/m.y.