The Himalaya-Bengal fan denudation accumulation system during the past 20 Ma

Mass balances for both denudation in the Himalayas and sediment accumulation in the Subhimalayan basins, including the Bengal deep-sea fan but excluding the Indus fan, yield 7.1 x 10(6) km(3) and 7.4 x 10(6) km(3) (s +/- 20%, rock of 2.75 g/cm(3) density), respectively, for the past 20 million years. Coarsening and increased sediment accumulation rates in the foreland basin and in the Bengal foredeep indicate accentuated tectonic activity and unroofing in the Himalayas since that time. The sediment volume includes greater than or equal to 1 x 10(6) km(3) of Neogene Bengal fan sediment that was lost via the Nicobar fan to the Sunda accretionary wedge. In addition, the Indian peninsular rivers contributed about c. 0.6 x 10(6) km(3) of solid load to the basins. Average denudation during the past 20 m.y., as derived from geothermobarometric data and restored cross sections, occurred most rapidly along the High Himalayan crystalline chain (vertical unroofing c. 1000 m/m.y.; northward lateral retreat of southern Himalayan slope, exposed to monsoonal rain, less than or equal to 3.5 km/m.y.) and much slower in the Tethyan sedimentary zone to the north (average 150 m/m.y.). The solute loads of the modem Himalayan rivers indicate a mean chemical denudation rate of 17 m/m.y. The distinct decrease in sediment accumulation on the outer Bengal fan between about 7 and 1 Ma (in contrast to the Indus fan) is probably caused by exogenic factors rather than by a significant decline in tectonic activity. Pre-20 Ma sediments in the Subhimalayan basins were derived mainly from the southern margin of the Tibet plateau or from sources outside the study area.