Abstract
A holistic view of the Bengal–Nicobar Fan system requires sampling the full sedimentary section of the Nicobar Fan, which was achieved for the first time by International Ocean Discovery Program (IODP) Expedition 362 west of North Sumatra. We identified a distinct rise in sediment accumulation rate (SAR) beginning ∼9.5 Ma and reaching 250–350 m/Myr in the 9.5–2 Ma interval, which equal or far exceed rates on the Bengal Fan at similar latitudes. This marked rise in SAR and a constant Himalayan-derived provenance necessitates a major restructuring of sediment routing in the Bengal–Nicobar submarine fan. This coincides with the inversion of the Eastern Himalayan Shillong Plateau and encroachment of the west-propagating Indo–Burmese wedge, which reduced continental accommodation space and increased sediment supply directly to the fan. Our results challenge a commonly held view that changes in sediment flux seen in the Bengal–Nicobar submarine fan were caused by discrete tectonic or climatic events acting on the Himalayan–Tibetan Plateau. Instead, an interplay of tectonic and climatic processes caused the fan system to develop by punctuated changes rather than gradual progradation.
Highlights
Our new integrated Nicobar–Bengal Fan sediment records show a net increase in flux to the eastern Indian Ocean at 9.5–9 Ma, representing the onset of a new sedimentary regime in the east Indian Ocean
Detrital zircon ages (Fig. 2) and petrology from the Nicobar Fan sediments show the sand provenance remained unchanged throughout the middle Miocene to present
Comparison of the zircon age distributions (Fig. 3) show sediment gravity-flow (SGF) deposits exposed on the Andaman– Nicobar Islands are closely similar to Nicobar Fan sediments and that both have affinities with Himalayan-derived units, the TransHimalaya and arc-derived input from erosion of the Indo–Burman Ranges that were expanding westwards during the Pliocene
Summary
The Bengal–Nicobar Fan, Indian Ocean (Fig. 1), has the greatest area and length of any submarine fan, and has long been studied to investigate possible links between Himalayan tectonics and the Asian monsoon (e.g., An et al, 2001; Bowles et al, 1978; Clift et al, 2008; Curray, 2014; Curray and Moore, 1974; France-Lanord et al, 2016; Schwenk and Spiess, 2009; Moore et al, 1974). The stratigraphic results from this expedition (Dugan et al, 2017) are integrated here with results from previous sites on the Bengal–Nicobar Fan and Ninetyeast Ridge (NER) of the Deep Sea Drilling Program (DSDP Leg 22, von der Borch et al, 1974), Ocean Drilling Program (ODP Leg 116, Cochran et al, 1989; Leg 121, Peirce et al, 1989) and IODP (Expedition 353, Clemens et al, 2016; Expedition 354, France-Lanord et al, 2016). We present the first stratigraphic data from the Nicobar Fan and reappraise published chronostratigraphic data from Bengal Fan and NER drillsites into a unified modern timescale to facilitate accurate comparison of depositional records across the whole system. Comparing sediment accumulation rates (SARs) between these sites gives a new and integrative understanding of the timing of fan growth and distribution of fan deposits
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