ヒマラヤの主中央衝上断層のスプレイイング The Spraying of the Main Central Thrust in Nepal and Bhutan Himalayas
Among the well known major thrusts in the Himalayan area, the Main Central Thrust (MCT hereinafter) was geo-structurally studied. The MCT is characterized by the distinct reverse metamorphism, whereby the Himalayan gneiss forming the Greater Himalaya overlies the less metamorphosed Lesser Himalayan sequences.<BR>The author studied the MCT zone along three valleys in western Nepal, eastern Nepal and central Bhutan. As a result it was evident that the MCT does not comprise only a single fault plane, but rather at times sprays into a number of thrusts, <I>i.e</I>. two to four in western Nepal, six in eastern Nepal and more than ten in central Bhutan. It is clear that the thrust fault corresponding to the MCT clearly increases the number of branched-out thrusts from west to east.<BR>Furthermore, according to existing literature (Maruo and Kizaki, 1981; Kano, 1982), rock facies of the foot-wall of the MCT in the Lesser Himalayas moving from west to east show a steady transition in degree of metamorphism from phyllite with oil showings, to phyllite and crystalline schist, and to garnetiferous schist. Also, moving from higher elevation to lower elevation, the transition of metamorphism is from gneiss at high elevation, crystalline schist at mid elevation and metasediments with sedimentary structures at the lower elevation.<BR>This implies that the degree of metamorphism is less in the more deep seated strata, and the deep seated strata is more exposed at the surface in the western Himalayas. Also, in contrast to the fact that Siwalik formation is widely distributed in the western Himalayas, sometimes even showing an overturn structure, that in Bhutan shows very narrow distribution suggesting only minor uplifting and no exposure of deeper portion of thrust zone in the eastern Himalayas.<BR>On the basis of the above findings of the study, the MCT and its equivalents clearly undergo increased spraying from west to east. Number of sprayed thrusts are two in western Nepal where the deep portion of the thrust zone is exposed and reach 10 or more in the eastern Himalaya (Bhutan), where only the shallow portion of the thrust zone is observed due to the slow or the recently started uplifting.<BR>This indicates that the deep portion of the thrust comprises a single plane, while in the shallow portion, there is greater spraying of thrust to form the thrust zone and this phenomenon has been observed along the MCT zone in Himalayas.