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Communication Dans Un Congrès Année : 2018

Neogene erosion regime and climate of the Himalayan basin from the Bengal fan turbidite record

Résumé

In Himalaya, the monsoon precipitations and tectonic exert a primary control on erosion. Tectonic is generating uplift and maintain high elevation of the range and earthquakes fragilise the mountain front. Monsoon intensity controls the extension of the glacier cover, landslide activity, river incision, vegetation cover, and exports of sediments towards the floodplain. The monsoon also exerts a primary control on the chemical erosion as weathering rates are clearly dependent upon river discharge. In the floodplain, sediment export also is tightly controlled by climate. The seasonality and intensity of the monsoon allow to reach discharge high enough to ensure the efficiency of river transport to the delta (Lupker et al. 2011). This efficient transport also acts as a limiting factor for weathering as it reduces residence time in the floodplain. The comparison between the Ganga with a large floodplain and the Brahmaputra with a narrow floodplain and comparatively lower residence time confirms that Himalayan weathering is limited by transport. IODP Expedition 354 drilled the Bengal fan (France-Lanord et al. 2016) and allows to assemble a comprehensive record of Himalayan erosion over the Neogene and Quaternary. Sediments are predominantly composed of turbidites generated from the Ganga-Brahmaputra shelf. Turbiditic deposition being discontinuous, the Expedition 354 drilled an E-W transect in order to cover as much as possible the displacement of the depocenter. Seven sites were drilled along a 320 km E-W transect at 8°N allowing the restitution of an almost complete record of Himalayan erosion at the scale of the Neogene. The deepest Site U1451 on the Eastern end of the transect reach the base of the fan. This marks the onset of fan deposition on this flank of the Ninetyeast ridge around 20 Ma. Turbiditic sediments show mineralogical, geochemical and isotopic characteristics which reveal a close analogy with modern Ganga-Brahmaputra sediments. Internal variability reveals that turbidites are not permanently a mixing of both rivers. Rather, there are evidences during the Quaternary that Brahmaputra was for some periods the only source of the turbidites. Major and trace element geochemistry show relatively stable compositions throughout the Neogene and Quaternary. They reveal a very weak regime of chemical weathering with no significant variation through time. Concentrations in mobile elements such as Na and K relative to Al are significantly higher than in modern sediments and suggest that weathering or soil erosion is amplified in the modern time. The record of detrital carbonate deposition shows variations with higher concentrations during the Miocene then falling concentrations during the Plio-Pleistocene. This suggests either a change in the exposition of Tethyan series in Himalaya or significantly lower chemical weathering during Miocene. This record contrasts with the ODP Leg 116 record at 1°S in the distal fan, which from 7 to 1 Ma shows an apparent high degree of weathering associated to lower accumulation rates. The record at 8°N indicates that these variations do not reflect changes in the continental source basin but rather reflect a change in the turbiditic export. Overall, the record of erosion at 8°N reveals that a relatively stable erosion regime dominated by physical erosion was established since Miocene. The modern period appears one with higher intensity of chemical erosion. These observations suggest that erosion sustained by a highly seasonal climate able to ensure rapid transport of sediments such as the monsoon was already active 20 Ma ago.
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hal-02328950 , version 1 (23-10-2019)

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  • HAL Id : hal-02328950 , version 1

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Christian France-Lanord, Volkhard Spiess, Albert Galy, Kohki Yoshida. Neogene erosion regime and climate of the Himalayan basin from the Bengal fan turbidite record. 9th International Conference on Asian Marine Geology, Oct 2018, Shanghai, China. ⟨hal-02328950⟩
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