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

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, eNd(0) values in the Bengal Fan sediments (all samples) define a narrow range about 16.0.87Sr/S6Sr values (all samples) are also in a narrow range near 0.741. g180 values in quartz separates define a narrow range at + 12.8 + 0.5%0. Coarse biotite-chlorite separates give g180 = 3.6-5.6%0. Combined g180 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). glSO in the IC clay fractions is 11.5-15%o, while SK clays are 18.2-22.6%o. 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 g~80 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. The Bengal Fan is the world's largest submarine fan, extending some 3000km south into the Indian Ocean from Bangla Desh. The Bengal Fan has developed largely in response to the collision of India with Asia and the uplift of the Himalaya in Tertiary time (Gansser 1966; Curray & Moore 1971). Curray & Munasinghe (1989) have divided the Bengal Fan into precollisional and postcollisional sediments, based on the presence of an Early Eocene unconformity. The Fan reaches a maximum thickness of over 22 km and the mass of the postcollision sediments is estimated at 2.9 x 1016 tons (Curray 1991). The Bengal Fan is fed by the GangesBrahmaputra river system, which currently drains both the north and south slopes of the Himalaya. The great relief, monsoon climate and extensive glaciation of the Himalayan region combine to produce some of the world's highest erosion rates. The Ganges-Brahmaputra system provides 12% of the river particulate flux to the world oceans, while providing less than 3% of the water flux (Miiliman & Meade 1983). Chemical erosion is also important; the chemical denudation rates are two to three times higher than the world average in the GangesBrahmaputra watershed (Sarin et al. 1989). The Bengal Fan provides an unparalleled record of Himalayan erosion, which in turn is related to the collision history of India and Asia, and the uplift history of the Himalaya. Erosional processes are also related to climate, thus the Bengal Fan may record information about climate change in the regions that supplied it with sediment. The Bengal Fan has been sampled by underwater drilling, permitting a reconstruction of the erosion history of the Himalaya in Neogene time. We wish to address several questions: What were the sources of sediment to the Bengal Fan, and have these sources varied in time? How From TRELOAR, P.J. & SEA~LE, M.P. (eds), Himalayan Tectonics Geological Society Special Publication, No. 74, pp. 603-621. 603 604 C. FRANCE-LANORD ET AL. can we use the sedimentological record to constrain the erosion history of the source regions? What is the relative importance of physical versus chemical erosion in the denudation of the Himalaya? What role does climate change play in the sedimentation history of the Bengal Fan, and by inference, erosion in the Himalaya? In this paper, we present sedimentological and Nd, Sr, O and H isotopic data on samples recovered from the Bengal Fan by the Ocean Drilling Program during Leg 116. These samples yield a record of sedimentation in the Bengal Fan beginning in Early Miocene time. These data, along with sedimentological data from DSDP Leg 22 and new and compiled data on some potential source regions are used to address the questions listed above. The sedimentological record of the Bengal Fan is an integrated record, sampling the Himalayas across the wide area of the Ganges-Brahmaputra watershed (1.48 million km 2, Milliman & Meade 1983). Thus the Bengal Fan record can yield important constraints on processes in the Himalayan source region unavailable for terrestrial studies. The stratigraphic resolution of the sedimentary record also permits a detailed comparison in time of sedimentary events and possible forcing factors, such as tectonic events or climate change. Geology of Leg 116 Sites Ocean Drilling Project (ODP) sites 717,718 and 719, drilled during Leg 116 are located in the distal portion of the Bengal Fan (Fig. 1). The holes are approximately 8 km apart. Hole 717C penetrated 828 mbsf (metres below sea floor) bottoming in late Miocene sediments, while Hole 718C penetrated 935 mbsf, bottoming in early Miocene sediments. Hole 719B penetrated 466 mbsf, bottoming in late Miocene sediments. The principal lithologies encountered in all holes were silt and mud turbidites, with lesser but variable amounts of biogenic mud turbidites (Shipboard Scientific Party 1989). Correlations between the holes are based primarily on lithostratigraphy and nannofossil biostratigraphy (Stow et al. 1990; Gartner 1990). A number of potential problems exist with the assignment of biostratigraphic ages to these sediments. Most of the sequence appears to have been deposited near or below the CCD, thus the state of preservation of calcareous fossil material is often poor. Second, it is probable that most of the fossils in the turbidites of the Bengal Fan have been redeposited, and thus fossil-bearing horizons would actually be younger than indicated. However, the succession of highest and lowest Fig. 1. Location of ODP Leg 116 Sites and DSDP Site 218 in the Bengal Basin. occurrences of fauna is both self-consistent and closely comparable with that observed in contemporaneous pelagic sediments, suggesting that redeposition has not seriously disturbed the biostratigraphic record (Gartner 1990). Stow et al. (1990) have presented a detailed summary of the sedimentology of Sites 717,718 and 719. Lower to upper Miocene sediments are characterized by silt and mud turbidites (Fig. 2). The latest Miocene, Pliocene, and early Pleistocene are characterized by mud turbidites, with interbedded silt turbidites, biogenic turbidites and pelagic clays. In the middle-upper Pleistocene section, sandy and siltto mud turbidites dominate, except for the last few metres of Holocene pelagic oozes. Variations in sedimentation rates derived from biostratigraphy are consistent with the lithologic variations. OverMIOCENE EVOLUTION, BENGAL FAN 605