Sediment Provenance and Depositional History of the Nicobar Fan (Bengal Depositional System) from IODP Expedition 362: Detrital Zircon Geochronology, Apatite Thermochronometry, Sand Petrography and Heavy-Mineral Results
Résumé
IODP Expedition 362 drill sites in the southern Bay of Bengal at 3°N 91°E sampled for the first time a complete stratigraphic section of the Nicobar [submarine] Fan down to the oceanic crust. This part of the Bengal depositional system provides new insights into sediment delivery to the NE Indian Ocean due to erosion, uplift, and denudation of the Himalaya and associated tectonostratigraphic components. The Nicobar Fan comprises various sediment gravity-flow (SGF) deposits, hemipelagites and pelagites. The oldest pre-fan sediments (limestones dated at ~69 Ma) are overlain by volcanic material and slowly accumulated pelagites. Terrigenous input began at ~19 Ma with stacked mud rich SGF deposits (mainly turbidites). From 9.5 Ma, sediment supply and sand content abruptly increased from 1–5 to 10–50 g cm-2 kyr-1. Fan reorganisation occurred at ~2 Ma when a distinctive trench wedge formed as part of the Sunda Subduction Zone. The Nicobar Fan and the Bengal Fan have a comparable sedimentary and stratigraphic architecture. The persistent connection between both fans, shown by geochemical and petrological data and the presence of many submarine channels, ceased at 0.28 Ma when the Nicobar Fan became inactive. Throughout the cored succession of Nicobar Fan SGF deposits, mineralogical variability appears to be controlled by complex and diverse factors, including provenance changes, selective diagenetic dissolution of non-durable minerals (pyroxene, sillimanite, amphibole), and to a lesser extent grain size and hydraulic-sorting processes. U-Pb age spectra of detrital zircons, sand petrography and heavy-mineral analysis suggest that the Nicobar Fan was a major sink for Himalaya-derived material. Apatite fission track (AFT) data shows that source area exhumation, routing and burial were extremely rapid between 3–8 Ma, and that after 3 Ma (possibly around 2 Ma) exhumation rates slowed with a clear lag in time between apatite cooling ages and depositional age. This broadly corresponds to a time of fan re-organisation. The more distant Investigator Fan and the thick accretionary prism of the Sunda subduction zone also include significant amounts of Himalaya-derived material delivered to the easternmost Indian Ocean (Oligocene-late Miocene).