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Evolution of the Himalaya since Miocene time: isotopic and sedimentological evidence from the Bengal Fan

¹ CRPG-CNRS, BP 20, 54501 Vandoeuvre les Nancy, France
² LGE-CNRS, Université St Jérôme, CO 431, 13397 Marseille, France

We report Sr, Nd, O, and H isotopic data and clay mineral abundances for turbidite sediments recovered in ODP Leg 116 cores from the Bengal Fan at 1°S. The samples studied cover the period between c. 17 Ma and the present. We also present new and compiled data on the isotopic compositions of potential source regions for the Bengal Fan sediments. _Nd(0) values in the Bengal Fan sediments (all samples) define a narrow range about – 16.0. ⁸⁷Sr/⁸⁶Sr values (all samples) are also in a narrow range near 0.741. ¹⁸O values in quartz separates define a narrow range at +12.8±0.5. Coarse biotite-chlorite separates give ¹⁸O = 3.6–5.6. Combined ¹⁸O values of quartz and biotites indicate a metamorphic source. Clay mineral abundances define two clay facies: an illite-chlorite-rich assemblage (IC) and a smectite-kaolinite-rich assemblage (SK). ¹⁸O in the IC clay fractions is 11.5–15, while SK clays are 18.2–22.6. The narrow range of isotopic values throughout the deposition history implies that the source of the Bengal Fan sediments has not changed since the early Miocene, despite changes in sedimentation rate, sedimentary facies, tectonic history and climactic regime. The difference between ¹⁸O in the IC and SK clay fractions represents different alteration histories of the same source material. The SK clays appear to have been altered at low T in the Indo-Gangetic Plain, while the IC clays and coarse fractions preserve metamorphic signatures. The narrow range of the Sr values, despite wide variation in Rb/Sr ratio, also argues for a source that underwent isotopic homogenization shortly before erosion and deposition of the sediment. The source that meets these criteria is the High Himalayan Crystalline series (HHC) or a close analogue, although subordinate contributions (probably <20%) from the Lesser Himalaya (LH) and Tibetan Sedimentary Series (TSS) are possible.

A model in which the HHC are exposed to erosion since the early Miocene on the south flank of the orogen by thrusting along the MCT, while the TSS is simultaneously removed by northward-directed normal faulting satisfies the constraints above. The results of this study require that the Himalaya have been a significant topographic feature since at least the early Miocene. Independent evidence supports this contention. Variations in the sedimentation style in the Bengal Fan since that time appear to represent a combination of factors, including tectonic activity and the coupled effects of climate and sea-level changes.

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