ブータンヒマラヤの地形-山間盆地の河岸段丘に関するいくつかの問題-

抄録

Characteristics of fluvial terraces in intermontane basins of the Bhutan Himalaya are investigated from the viewpoint of the geomorphic evolution during the Quaternary.
Main rivers in the Bhutan Himalaya generally flow in remarkably deep valleys with steep gradients from northern to southern areas. Intermontane basins with several fluvial terraces, however, are developed in the middle reaches of these rivers, where gradients of river beds are small. In view of chronological sequence, the wider distribution of the Lower terrace surface (L2 surface) at Wangdiphodrang, central Bhutan, may be caused by the increase in sediment load due to the advance of glaciers in the upper reaches during the Little Ice Age. The characteristics of fluvial deposits of the middle terrace surface (M2 deposits) suggest catastrophic flows which can result from englacial or proglacial lake bursts. The development of fluvial terraces and deposits in the Bhutan Himalaya has been mainly affected not only by climatic changes but also by occasional catastrophic floods like glacial lake outbursts.
The bedding planes of the older fluvial terrace deposits at Wangdiphodrang dip upstream. It suggests the Quaternary uplift of the southern Bhutan Himalaya, that is, the front of the crystalline nappe. Nevertheless the rear side of the nappe front are less eroded. Moreover Quaternary fluvial terraces and deposits in the intermontane basins seem to be rather less developed in the Bhutan Himalaya than in the Nepal, Kumaun and Punjab Himalayas. These show possibilities whether in the Bhutan Himalaya the rate of uplift might be smaller or the initiation of rapid uplift might be rather later than that in the central and western Himalayas.