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THE NICOBAR SUBMARINE FAN AND RELATIONSHIP WITH THE BENGAL FAN: PRELIMINARY RESULTS FROM IODP EXPEDITION 362, INDIAN OCEAN

Abstract : The Bengal-Nicobar Fan has been long-studied to investigate possible links between Himalayan tectonics and the Asian monsoons. Despite the many DSDP, ODP, and IODP expeditions in the region, the Nicobar Fan is largely undersampled, even though it contains key information on the tectonostratigraphic evolution of the eastern Indian Ocean. In contrast to the Bengal Fan that records Holocene sediment gravityflows (SGF), the Nicobar Fan, lying east of the Ninetyeast Ridge, is inactive because subduction of the ridge starting in the Late Pleistocene blocked the sediment supply from the north. IODP Expedition 362 (AugustOctober 2016) drilled two boreholes within the sedimentary cover of the Indian Ocean Plate offshore from the north Sumatra subduction zone to investigate the role of input materials in the seismogenesis of megaearthquakes. Here, the 1-to-5 km-thick sedimentary succession comprises a basal pelagic layer overlain by sediments of the Nicobar submarine fan. Drill sites are located at 3°N 91°E, ~ 250 km southwest of the subduction zone, on the eastern flank of the Ninetyeast Ridge where the input section is ~1.5 km thick. Sites U1480 and U1481 were drilled, cored and logged to a maximum depth of 1500 m below seafloor (mbsf), and reached the 60-70 Ma igneous oceanic crust of the Indian plate. The recovered sediments represent a nearly continuous Late Cretaceous to Recent deep-marine sedimentary section that consists of silicilastic sediments deposited from various SGFs (including turbidity currents and debris flows), interpreted as Nicobar Fan, underlain by a diversity of abyssal-plain environment sediments containing essentially hemipelagic, pelagic, tuffaceous and igneous lithologies overlying ocean crust. The Nicobar Fan represents > 90% of the input section on the drill sites and is characterized by a succession of muddy to sandy SGF deposits, including abundant plant-fragment-rich debrites, with rare interbeds of calcareous mud. Sediment accumulation rates reached 100-400 m/Ma in the late Miocene to Pliocene (25-1250 mbsf), but were considerably reduced since ~1.6 Ma (5-20 m/Ma). Underlying the Nicobar Fan, an Oligocene-Miocene unit composed of siliciclastic mud and rare sandy SGF deposits (turbidites as deep as ~1500 m in Site U1481) may record the early stages of the fan in the abyssal plain. Sediment accumulation rates are low (2-15 m/Ma) and the amount of siliciclastic material tends to decrease with depth. Below this, the Late Cretaceous to Oligocene pre-fan unit comprises a suite of pelagic sediments (tuffaceous mudstones, chalk, calcareous mudstones) characterized by a slow (1-5 m/Ma) and erratic (many hiatuses) sedimentation, interbedded with several magmatic intrusions and extrusions. Expedition 362 demonstrates that the Nicobar Fan was active between ~1.6 and ~9 Ma, and possibly since ~30 Ma. The observed mineralogical assemblage of the SGF deposits is consistent with a provenance from Himalayan rivers and the succession is interpreted to represent different stages of fan development from initiation to abandonment. Expedition 362 results will enable direct comparison between the Nicobar Fan and records of sedimentation elsewhere in the Bengal and Indus Fan and provide a holistic view of the Indian Ocean fan system history.
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Submitted on : Tuesday, February 27, 2018 - 9:56:20 AM
Last modification on : Tuesday, June 30, 2020 - 8:26:02 AM

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Hugo Pouderoux, Kevin Pickering, Kitty Milliken, Steffen Kutterolf, Et Al.. THE NICOBAR SUBMARINE FAN AND RELATIONSHIP WITH THE BENGAL FAN: PRELIMINARY RESULTS FROM IODP EXPEDITION 362, INDIAN OCEAN. International Meeting of Sedimentology 2017, Oct 2017, Toulouse, France. pp.719. ⟨insu-01718063⟩

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