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