40AR/39AR DATING OF DETRITAL MUSCOVITES FROM MODERN RIVERS OF NEPAL

The ⁴⁰Ar/³⁹Ar ages of detrital muscovites have been obtained on both single grains and bulk splits from nine samples of sands modern rivers in central and western Nepal; one from the lower reaches of the Karnali and eight from the Narayani and its tributaries the Trisuli, and Dordi.

The distribution of the single grain ages range from quite narrow (total range 4 to 8 Ma, n=199) to relatively wide (90% of grains in the range 4 to 25 Ma; n=210). Although the correlation is not strong the coarser material tends to give the youngest ages. Ages from the downstream samples of the Karnali and Narayani are distinctly different The mode and mean of the 210 grains analyzed in the Karnali sample are 18 and 16 Ma, respectively. The corresponding data for the Narayani sample are ~9.5 and 10.5 Ma.

None of the age distributions of the single grain data contain a significant “zero-age” component. This contrasts with previously published data for sandstones from the Tertiary Siwalik Group in central Nepal and the distal Bengal fan. The lack of very young material in the modern sands of central and western Nepal suggests that this portion of the Himalayan orogen has not experienced an episode of extremely rapid exhumation in the past few million years. This is consistent with the notion that portions of the mountain range alternate between being the sites of rapid uplift and erosion (>12 mm/year); during the past few million years it appears that only the syntaxes of the Himalaya have undergone such rapid erosion but the data from the Bengal fan and the Siwaliks indicates that these regions must not have been the only regions which experienced such and episode. However, these data argue that no region in central Nepal has experienced sustained rates of erosion greater than ~6 mm/year during the past two to three million years.