UNROOFING OF THE HIMALAYAS - A VIEW FROM APATITE FISSION-TRACK ANALYSIS OF BENGAL FAN SEDIMENTS

Apatite fusion-track analysis of EarlY Miocene to Quaternary distal Bengal Fan sediments from ODP Leg 116 provides a unique perspective on denudation processes associated with the Tertiary Himalayan orogeny. These sediments yield pooled apatite fission-track ages that are 0- to 10-m.y. older than depositional ages. Mean track lengths range from 13.2 to 14.8 mum. These young ages and long track lengths indicate that rocks in the upper crust of Bengal Fan source areas cooled rapidly (greater-than-or-equal-to 15-degrees-C/m.y.), implying denudation rates of several hundreds of meters per million years. These observations support the hypothesis that nearly continuous unroofing of a large source area, similar to the dimensions of the present Himalayan and southern Tibetan Plateau deformation front, and an efficient transport system, similar to the present Himalaya-Ganges-Brahmaputra system, supplied sediment to the Bengal Fan since the Early Miocene.