Uplift Mechanism of the Kathmandu Nappe, Central Nepal: Geochronological Approach

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  • 中央ネパール，カトマンドゥ・ナップの上昇機構―放射年代学的アプローチ―
  • チュウオウ ネパール カトマンドゥ ナップ ノ ジョウショウ キコウ ホウシャ ネンダイガクテキ アプローチ
  • Geochronological Approach
  • -放射年代学的アプローチ-

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Abstract

The Himalayan orogen represents a fold-and-thrust belt characterized by a series of foreland-propagating thrust systems with an out-of-sequence thrust (OST). Geological and geomorphological evidence implies that the Himalayan upheaval started from the north and shifted gradually southward with time. The uplift seems to have been accelerated in the Higher Himalaya since the Pliocene. To investigate the role of OST in recent uplifts of the Higher Himalaya and to estimate the denudation rate of the latter, we carried out fission track (FT) dating on zircon and apatite of high-grade pelitic and granitic gneisses from both sides of an inferred OST north of the Kathmandu nappe and along a traverse across Gosainkund Lekh in the northern root zone of the Kathmandu nappe.<BR>Nine zircon FT ages of sites in the Gosainkund Lekh area range from 1.5 Ma (at 1525 m) to 2.8 Ma (at 5045 m). Ages of four samples from the northern slope of the area increase linearly with elevation, and yield an average denudation rate of 2.4 mm/y. On the other hand, six samples from the southern slope yield identical ages (2.5-2.8 Ma) regardless of their sample elevations (1150-5045 m). The zircon ages of the area suggest that rocks on the southern slope passed evenly the depth of closure temperature of zircon at around 2.6 Ma. Following this, the northern part of the southern slope (the highest part of Gosainkund Lekh) and the northern slope were uplifted more rapidly than the southern slope.<BR>Eight zircon FT ages and seven apatite FT ages from rocks around the Sheopuri zone range from 4.4 Ma to 6.9 Ma and 4.3 Ma to 5.5 Ma, respectively. These ages vary irrespective of their sample elevations (1200-2400 m). Furthermore, zircons and apatites from both sides of the OST (Trisuli-Likhu fault) yield almost the same FT ages, suggesting that the OST has been inactive since the latest Miocene. It is noteworthy that zircon FT ages from the sample elevations between 1200 m and 1800 m in the present area together with zircon FT ages (ca. 9 Ma) reported from early Paleozoic granites in the southern part of the Kathmandu nappe suggest a general trend of younging towards the north between 9 Ma and 1.5 Ma. On the basis of such a trend, two possible uplift mechanisms of the Kathmandu nappe and its northern root zone in the Higher Himalaya are presented. Both mechanisms might be related to crustal ramp due to northward wedging of Indian mid-crust beneath the Himalaya.

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