HR: 09:15h
AN: T21E-06    [Abstracts]
TI: Early Himalayan Erosion: Constraints to Models of Crustal Deformation, Changes in Ocean Geochemistry and Global Climate
AU: * Najman, Y
EM: y.najman@lancs.ac.uk
AF: Dept Env Sci, Lancaster Univ, Lancaster, LA1 4YQ, United Kingdom
AU: Bickle, M
EM: mb72@esc.cam.ac.uk
AF: Dept Earth Sci, Cambridge Univ, Cambridge, CB2 3EQ, United Kingdom
AU: BouDagher-Fadel, M
EM: m.fadel@ucl.ac.uk
AF: Dept Earth Sci, UCL-Birkbeck, London, WC1E 7HX,
AU: Carter, A
EM: a.carter@ucl.ac.uk
AF: Dept Earth Sci, UCL-Birkbeck, London, WC1E 7HX,
AU: Garzanti, E
EM: eduardo.garzanti@unimib.it
AF: Dip Geologiche, University Milan-Bicocca, Milan, 20126, Italy
AU: Paul, M
EM: mpaul@crpg.cnrs-nancy.fr
AF: CRPG-CNRS, BP20, Vandoeuvre-Les-Nancy, 54501, France, Metropolitan
AU: Wijbrans, J
EM: wijj@geo.vu.nl
AF: Dept Isotope Geology, Vrije Univ, Amsterdam, 1081, Netherlands
AU: Willett, E
EM: ed.willett@bowleven.com
AF: Cairn Energy, 50 Lothian Rd, Edinburgh, EH3 9BY, United Kingdom
AU: Oliver, G
EM: gjho@st-andrews.ac.uk
AF: School of Geosciences, St Andrews Univ, St Andrews, KY16 9AL, United Kingdom
AU: Parrish, R
EM: rrp@nigl.nerc.ac.uk
AF: NIGL, BGS, Nottingham, NG12 9GG, United Kingdom
AU: Akhter, S
EM: shakhter@univdhaka.edu
AF: Dept Geology, Univ of Dhaka, Dhaka, 1000, Bangladesh
AU: Allen, R
EM: r.allen1@lancs.ac.uk
AF: Dept Env Sci, Lancaster Univ, Lancaster, LA1 4YQ, United Kingdom
AU: Ando, S
EM: sergio.ando@unimib.it
AF: Dip Geologiche, University Milan-Bicocca, Milan, 20126, Italy
AU: Chisty, E
EM: emdad.chisty@cairn-energy.plc.uk
AF: Cairn Energy, 50 Lothian Rd, Edinburgh, EH3 9BY, United Kingdom
AU: Reisberg, L
EM: reisberg@crpg.cnrs-nancy.fr
AF: CRPG-CNRS, BP20, Vandoeuvre-Les-Nancy, 54501, France, Metropolitan
AU: Vezzoli, G
EM: giovanni.vezzoli@unimib.it
AF: Dip Geologiche, University Milan-Bicocca, Milan, 20126, Italy
AB: Dating the onset of Himalayan erosion is critical to understanding crustal deformation, and the proposed link with global climate and ocean geochemistry. The most commonly quoted age of India-Asia collision is ~50 Ma, yet the record of Paleogene Himalayan erosion is scant, either absent or of low age resolution. We identify sediments shed from the rapidly exhuming southern flanks of the eastern-central Himalaya at 38 Ma, in the >1 km thick deltaic Barail Formation of the Bengal Basin, Bangladesh. This formation was previously of disputed provenance and poorly dated. New provenance data from the Barail Formation, (seismic, petrographic, geochemical, and Ar-Ar, U-Pb, ZFT, Sm-Nd and Re-Os isotopic ratios) is consistent with Himalayan, and inconsistent with Indian cratonic or Burman sources. The biostratigraphic and isotopic mineral ages date the Barail Formation as spanning Late Eocene to Early Miocene. Lag time data show that exhumation of the orogen was rapid by 38 Ma. These data 1) reduce the delay between collision, and oldest known record of substantial and rapid erosion of the Indian crust of the central-eastern Himalaya's southern flanks, from >20 Myrs to 12 Myrs, and 2) indicate that thrusting and crustal thickening had generated sufficient relief by 38 Ma to generate efficient erosion. They also explain a previously puzzling discrepancy between the timing of early hinterland metamorphism and exhumation inferred from bedrock and the lack of corresponding evidence in the erosional record. Older sediments derived from the orogen's southern flanks may be found in the future. However, we consider the Barail Formation to represent the onset of significant accelerated erosion from the southern flanks of the central-eastern Himalaya at a regionally applicable scale because 1) earlier input of significant clastic material to the Bengal Basin is inconsistent with the pre-Barail carbonate shelf depositional environment, 2) the Bengal Basin is the only obvious exit point for such detritus, 3) the strata in the Northeast Bengal Basin can be correlated over a large area of the delta in which major Himalayan-derived input initiated sometime around 40 Ma (Lindsay et al., GSAB 1991) and 4) the data are consistent with the exponential increase in accumulation rates at the start of the Oligocene in basins surrounding the India-Asia collision zone (Metivier et al, GJI 1999). Our study 1) supports orogenic models that imply delayed erosion, although the rapid exhumation is earlier than given by models of "channel flow" (Beaumont et al, Nature 2001), 2) supports the contention that Himalayan erosion resulted in increase of the marine 87Sr/86Sr ratio at ~40 Ma and 3) brings into question the proposed link between the onset of Himalayan erosion and the onset of Cenozoic global cooling.
DE: 8102 Continental contractional orogenic belts and inversion tectonics
DE: 8110 Continental tectonics: general (0905)
DE: 8169 Sedimentary basin processes
DE: 9320 Asia
SC: Tectonophysics [T]
MN: 2007 Fall Meeting