NEOGENE DENUDATION OF THE EASTERN HIMALAYAS: 40AR/39AR DETRITAL GEOCHRONOLOGY AND PETROFACIES EVALUATION OF PLIOCENE-PLEISTOCENE DUPI TILA FORMATION OF THE BENGAL BASIN

The unroofing history of the Himalayan mountain chain is in part recorded in sediments deposited in subsiding foreland basins to the south. The eastern Himalayan clastic wedges are preserved at the Assam-Bengal depositional system toward the eastern syntaxial area of the Himalayas. Sedimentation history suggest progradation of the depocenter toward south-southwest due to advancing Himalayas as it has been consuming the remnant ocean basin trapped between the Indian craton and Burma.

Mio-Pliocene siliciclastic sediments (~4.5 km thick) are draped over by ~2.5-km-molassic Plio-Pleistocene Dupi Tila sediments which are composed of light brown to pink, coarse- to very fine-grained, sandstone, siltstone, silty clay, mudstone and shale with some conglomerate beds. Sandstone petrographic data reveals a dominance of quartzolithic composition with abundant sedimentary and metamorphic lithic fragments. Abundance of kyanite and sillimanite together with epidote, chloritoid, and garnet (mostly almandine) contents reflect unroofing of deeper crustal levels, including green schist, amphibolite and granulite facies source rocks.

⁴⁰Ar/³⁹Ar geochronology reveals dominance of muscovites with early to-late Miocene peaks with subordinate Paleogene to Cretaceous modes. These cooling ages in general resemble those of the Miocene muscovite ages of the Assam-Bengal system. The conspicuous age modes are consistent with two Miocene tectonic events in the Higher Himalayas. Similar ⁴⁰Ar/³⁹Ar and fission-track dates are also obtained from the ODP (Leg 116) drilled sediments of the Bengal fan. The lack of abundance of Cretaceous muscovites distinguishes these Plio-Pleistocene sediments from the Paleogene units, which suggest either (1) changes in dispersal drainage network and/or (2) exposure of different rocks through orogenic unroofing during the Neogene. This study and fission-track study on the ODP samples from the distal Bengal fan suggest that the mean cooling ages are mostly between 3 and 10 million years older than the depositional ages, which implies high uplift and denudation of the source terranes in the eastern Himalayas.