THE HIMALAYA-BENGAL FAN DENUDATION ACCUMULATION SYSTEM DURING THE PAST 20 MA

Mass balances for both denudation in the Himalayas and sediment accumu lation in the Subhimalayan basins, including the Bengal deep-sea fan b ut 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 p ast 20 million years. Coarsening and increased sediment accumulation r ates in the foreland basin and in the Bengal foredeep indicate accentu ated 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 t he 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. A verage denudation during the past 20 m.y., as derived from geothermoba rometric 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 so lute loads of the modem Himalayan rivers indicate a mean chemical denu dation rate of 17 m/m.y. The distinct decrease in sediment accumulatio n 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 si gnificant decline in tectonic activity. Pre-20 Ma sediments in the Sub himalayan basins were derived mainly from the southern margin of the T ibet plateau or from sources outside the study area.