ネパールヒマラヤのスラストテクトニクス : フィッション・トラック年代と山脈上昇過程 Thrust Tectonics and Uplift Process of the Nepal Himalaya Revealed from Fission-Track Ages
The Himalaya is a fold-and-thrust belt in the northern margin of the Indian continental plate, and a product of the collision between India and Asia since the Eocene. The Himalayan orogen consists of several tectonostratigraphic units bounded by diachronous thrusts younging from north to south; namely, the Kangmar Thrust in southern Tibet, the Main Central Thrust, the Main Boundary Thrust, and the Main Frontal Thrust. These thrust faults are splays off from a major mid-crustal decollement named the Main Detachment Fault or the Main Himalayan Thrust. In he Nepal Himalaya, there is also an out-of-sequence thrust, which cuts across the Main Central Thrust, within the Lesser Himalaya.<BR>Abundant geological and geomorphological evidence has suggested that the Himalayan uplift began in the Tethys (or Tibetan) Himalaya, which is situated to the north of the present crest line of the Himalaya (the Higher Himalaya); the uplift then shifted southward, and was finally accelerated in the Higher Himalaya since the Pliocene. Such a migration of the uplift through time is attributed to the southward younging of the activity of above thrusts.<BR>Nine zircon fission-track ages from the crystalline rocks of the Higher Himalaya and the Main Central Thrust zone in the Kali Gandaki and Modi Khola valleys, central Nepal, show a linear increase from 1.2Ma to 2.3Ma with increasing elevation. An average denudation rate of 0.9mm/y during this period is thus calculated. Assuming a closing temperature of 260°C for fission-tracks in zircon and a paleo-geothermal gradient of 35°C/ km, averagedenudation rates since the Late Pliocene are over 3mm/y. The denudation rates increase as approach the present, and rearch >6mm/y after 1.2Ma. The trend of linear increase of fission-track ages with increasing elevation is not disturbed by the Main Central Thrust; therefore suggesting the cessation of Main Central Thrust activity in the Late Pli ocene and a flexural uplifting since then, not a differential movement along the Main C entral Thrust. Three zircon fission-track ages from granites of the Kathmadu Nappe located south of the out-of-sequence thrust are about 9Ma, and yield an average denudation rate of 0.8mm/y for the period 9-0Ma.<BR>The rapid uplift of the Higher Himalaya in central Nepal since the Late Pliocene may have been due to elastic doming of the Lesser Himalayan rocks beneath the Higher and Lesser Himalayas. The doming has resulted from the fault-bend folding of the rocks along the ramp of the out-of-sequence thrust at depth caused by the northward slippage of the Indian crust along the Main Detachment Fault.