地学雑誌
Online ISSN : 1884-0884
Print ISSN : 0022-135X
ISSN-L : 0022-135X
ヒマラヤ・チベット山塊の隆起・削剥過程と地形発達
池田 安隆
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1997 年 106 巻 2 号 p. 168-183

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Present-day denudation of the Himalaya-Tibetan plateau occurs mainly at its southernmargin (High Himalayan front) by headward erosion of small tributaries of Ganges, Brahmaputra, and Indus. At this erosion front, large unroofing more than severalkilometers, which may exceed annealing depths of fission-tracks in apatite and zircon, couldoccur in a short period of time. This would result in dramatically accelerated erosion rates, as have been deduced from cooling age determinations of rocks from near the erosion frontand have often been misinterpreted. Observations of present-day river load of Ganges andBrahmaputra and the sediment volume of the Bengal deep -sea fan and the Ganga basin haveindicated that denudation rate of the Himalayas has been fairly uniform (1.0±0.3mm/yr) since Early Miocene time up to the present. This suggests that the topographic relief of theplateau's front has essentially been constant during the past 17-20Myr. Simple evolutionarymodels of the Himalaya-Tibetan plateau, considering both denudation and crustal thickeningdue to plate convergence, suggest that the plateau had grown as high as the present height (-5km) only 15-30Myr after the onset of collision, without reaching the dynamicequilibrium between denudation and uplift. This implies that the height of the plateau iscontrolled by crustal strength, which is likely to be equal to the gravity-induced stress (-1.4kbar) due to -5km relief. Plate convergence since Early Miocene time has been accommodatednot by crustal thickening but by expansion of plateau's width andsubsequently bylateral (east-ward) escape of the Tibetan plateau.

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