THRUST PACKAGES OF 1.68 Ga INDIAN SUPRA-CRUSTAL ROCKS IN THE MIOCENE SIWALIK BELT,CENTRAL NEPAL HIMALAYAS

The Siwalik Belt is a frontal fold|thrust belt of the Himalayas and composed of thick sequence of foreland basin sediments derived from the Himalayas during the last 15 to 17 million years.From this Miocene belt in the central Nepal,we discovered exotic thrust packages of the Middle Proterozoic rocks,which has been regarded as the Siwalik Group or post|collisional sediments correlatable with the Subathu or Murree Formation in India.The thrust belt,called the Bagmati Belt,is narrowly distributed in the Siwalik Belt,22km to the north of the Main Frontal Thrust (MFT or HFF) that is an active fault and considered to be the deformation front of the Himalayan orogen.The Main Boundary Thrust (MBT) which separates the Siwalik Belt from the Lesser Himalayan Belt runs 7km to the north of the thrust packages.Within the belt,tectonic slices of 400m to 1km in thickness are repeated three to five times due to thrusts,sandwiching a thin slice of the Siwalik beds.The thrust package consists of the pre|Siwalik sedimetary rocks and sills of dolerite,and named as the Bagmati Group.The thickness is only about 800m due to tectonic repition by thrust,although the group has been considered to be a continuous sequence attaining 2200m in thickness.We divided the Bagmati Group into three formations,each of which shows an upward|coarsening and thickening sequence of 200 to 350m in thickness.All sequence is composed of red|brown orthoquartzite,pink quartzite,micaceous shale and thin sandstone interbed and rhythmite,mottled hematite and hematitic pisolite.We interpret that the Bagmati Group was deposited in shallow lacustrine and desert environments.