THE INFLUENCE OF ALLOGENIC PROCESSES ON THE BENGAL FAN SEDIMENT ARCHIVE (Invited Presentation)

Sediment repositories are influenced by both autogenic and allogenic processes in their hinterlands. This study uses detrital geochronology and thermochronology to assess the influences of tectonics and climate on the Bengal Fan sediment archive which was cored during IODP Expedition 354.

The Bengal Fan is fed by the Ganges and Brahmaputra rivers. The main drivers affecting the Bengal Fan sediment archive are Eastern Himalayan tectonics, the Asian monsoon, and for the recent history, Northern Hemisphere Glaciation which affects the quantity of sand that is delivered to the deep basin during interglacial versus glacial periods.

IODP Exp 354 collected Bengal Fan sediment to the base of the fan, which at the location drilled, is late Oligocene in age. Geochronological data (detrital zircon U-Pb ages) and thermochronological data (detrital rutile and apatite U-Pb, mica Ar-Ar, zircon fission track ages) were obtained from samples throughout the cored interval. Of particular interest in this study is the youngest part of the record. The oldest sample with short lag times (<1 Ma), indicative of very rapid exhumation, has a depositional age of ~3.5 Ma. The youngest sample with a long lag time (>6 Ma), has a depositional age of ~5 Ma (Najman et al, GSAB, 2019). This increase in exhumation rates reflects rapid exhumation of the eastern Himalayan syntaxis.

The increase in hinterland exhumation that we record in the Bengal Fan is broadly matched by an increase in Trans-Himalayan detritus since Late Pliocene, as demonstrated from our zircon U-Pb data (Blum et al., Scientific Reports, 2018). This increase cannot easily be explained by syntaxial exhumation, since the Trans-Himalayan source region lies upstream of the syntaxis, although it may reflect exhumation of a Trans-Himalayan “lid” of the syntaxis. Alternatively, it may reflect the onset of Northern Hemisphere Glaciation when the higher altitudes of the Transhimalayan region could have been disproportionately affected by greater glacial erosion. This high proportion of Transhimalayan material continues in Pleistocene samples, but declines again in the modern Brahmaputra sample. This decline may reflect the change from glacial to interglacial conditions, at which time glacial erosion of the Transhimalayan high ground played a less dominant role in sediment generation again.