Abstract
The Arun and Tista Rivers, which flow across the Himalayas, are commonly known as antecedent valleys that overcame the rapid uplift of the Higher Himalayan ranges. To clarify whether the idea of antecedent rivers is acceptable, we investigated the geomorphology of the Himalayas between eastern Nepal and Bhutan Himalayas. The southern part of Tibetan Plateau, extending across the Himalayas as tectonically un-deformed glaciated terrain named as ‘Tibetan Corridor,’ does not suggest the regional uplift of the Higher Himalayas. The 8,000-m class mountains of Everest, Makalu, and Kanchenjunga are isolated residual peaks on the glaciated terrain composed of mountain peaks of 4,000–6,000 m high. The Tibetan glaciers commonly beheaded by Himalayan glaciers along the great watershed of the Himalayas suggest the expansion of Himalayan river drainage by glaciation. For the narrow upstream regions of the Arun and Tista Rivers with less precipitation behind the range, it is hard to collect enough water for the power of down-cutting their channels against the uplifting Himalayas. The fission track ages of the Higher Himalayan Crystalline Nappe suggest that the Himalayas attained their present altitude by 11–10 Ma, and the Arun and Tista Rivers formed deep gorges across the Himalayas by headward erosion.