Leg 116

Distal Bengal Fan

Three sites (Site 717 to Site 719) were drilled during Leg 116 to investigate the geological phenomena associated with the collision of India with Eurasia, 3,000 km to the north in the Himalayan Mountains. Continental collision began in the Eocene with "soft" collision, possibly between continental India and an island arc seaward of Asia, causing rapid terrigenous sedimentation on the incipient Bengal Fan. The "hard" continent-continent collision began later, resulting in the first main uplift of the Himalayas. Increased resistance to shortening across the Himalayas and continued seafloor spreading at the Southeast Indian Ridge places the Indian-Australian Plate under severe compressive stress and the oceanic crust and overlying sediments are deformed into east-west trending, long-wavelength undulations. Locally, deformation is occurring on high-angle faults, 5 to 20 km apart, that form a series of fault blocks; movement along these blocks has significantly affected local sedimentation. Growth of the Bengal Fan has continued to the present, producing the worldÍs largest submarine fan, nearly 3,000,000 km2.

Leg 116 operations documented that the history of the Bengal Fan extends back to at least 17 Ma, pushing the onset of the first main phase of Himalayan uplift to early Miocene time or perhaps even earlier. The fan is at least 1.3 km thick some 2,500 km from the Ganges delta. Both Himalayan uplift and sea-level fluctuation have markedly influenced the relative rate of fan growth and the type of sediment deposited, the most obvious change being from the silty turbidite sections of Miocene and Pleistocene age, which accumulated at a rate of 350 m/m.y. in the Pleistocene, to the mainly mud turbidite Pliocene sections separated by pelagic clays that accumulated at a rate of 70 m/m.y. A hiatus of nearly 1 m.y.-duration, documented in the sedimentary record between the Pliocene and late Pleistocene, may be related to a change in deep-ocean circulation or in terrigenous-sediment supply during that time. The Ganges-Brahmaputra drainage basin is the primary source of sediments for the fan, accompanied by significant contributions from the continental margins of the western Bay of Bengal and a lesser contribution from local seamounts.

The process of turbidite deposition appears to have continued steadily ever since the onset of faulting and thus motion on the faults has been gradual and fairly constant, although the rate may have increased slightly with time. Intraplate deformation began to affect this part of the central Indian Ocean at ~7 Ma and, since there has been about 350 m of uplift across the fault, the average rate of motion has been 50 m/m.y. Leg 116 also documented dramatic evidence of vigorous hydrothermal circulation, also attributed to this intraplate deformation.