Abstract

Early Miocene to Quaternary sedimentary strata drilled from the distal Bengal fan in the central Indian Ocean during Leg 116 of the Ocean Drilling Program are divided into six zones by modal proportions of detrital heavy minerals. We identified the provenance of sediments in each zone on the basis of the mineral assemblage and propose a two-phase uplift of the Higher Himalayas since 17 Ma. The oldest sedimentary strata, ranging from 17.1 to 15.2 Ma, were derived mostly from the sedimentary rocks of the Lower Himalayas and the Tethys Himalayas. The zone above these strata (15.2-10.9 Ma) contains minerals derived from the Higher Himalayas. The main provenance of the zone dated between 10.9 and 7.5 Ma also appears to be the Higher Himalayas. Calcic amphibole, which indicates contribution from the Higher Himalayas, appeared at 15.2 Ma and abruptly increased at 10.9 Ma. The acme of the uplift in the first phase was between 10.9 and 7.5 Ma. From 7.5 to 6.5 Ma, the amount of elements from the Higher Himalayas abruptly decreased. It is proposed that the Higher Himalayas were eroded, and their source area became smaller. Concurrently, pumpellyite disappeared. Because this mineral is currently found only in the limited area of the upper drainage basin of the Indus River, we presume that the drainage system of the Ganges and Indus rivers changed between 7.5 and 6.5 Ma, in association with tectonic movement. The zone dated between 6.5 and 0.9 Ma is composed of two types of sedimentary units, one from the Higher Himalayas and the other from the Indian subcontinent and Sri Lanka. The provenance of the zone younger than 0.9 Ma is thought to be the Higher Himalayas, where a large peridotite body was thrust up over the Tethys Himalayas. The second main phase of uplift of the Higher Himalayas occurred after 0.9 Ma.

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