GEOLOGY OF THE NORTHERN ARUN TECTONIC WINDOW

The Arun Tectonic Window (ATW) and its inverted metamorphic zonation were first described by Bordet (1961) and Hagen (1969) in their regional surveys of the eastern Nepal Himalaya. The ATW is centred on the Arun antiform (“ trans\|anticlinal de l’Arun”, Bordet, 1961), a major late structure, c. 100km long, which strikes north to north\|northeast, transversely to the E—W tectonic trend of the eastern Himalaya from the lower Arun valley to southern Tibet. From south to north, i.e. from the core of the window upwards in the nappe pile, the tectonic units exposed in the ATW are:(1) The Lesser Himalayan Tumlingtar Unit (Nawakot nappes of Hagen,1969), a thick sequence of greenschist\|facies Upper Precambrian metasediments, bounded to the north by a thrust zone (Main Central Thrust 1 of Maruo & Kizaki, 1983; Main Central Thrust Zone of Meyer & Hiltner, 1993). (2) The Lesser Himalayan Crystalline nappe (LHC), comprised of staurolite to kyanite grade micaschists and granitic orthogneiss (Kathmandu Nappes of Hagen,1969), lying on top of the low\|grade metasediments. (3) The Higher Himalayan Crystalline nappe (Tibetan Slab of Bordet, 1977), bounded on both side of the ATW by thrust sheets defining a major syn\|metamorphic thrust (Main Central Thrust of Bordet,1961; Main Central Thrust 2 of Maruo & Kizaki, 1983).In this contribution some results of geological investigations in the hitherto unrecognized northern part of the ATW (Kharta region of the Arun—Phung Chu valley and Ama Drime—Nyonno Ri range), are presented. The Kharta region is 30km east of the Everest—Makalu massif and sits in the western limb of the Arun antiform, whereas the Ama Drime—Nyonno Ri Range, to the east of Kharta, is right in the core of the Arun antiform. Here the ATW exposes a section of deep tectonic levels of the Lesser Himalayan Crystalline nappe and MCT zone which elsewhere in the Nepal Himalaya are concealed below the overlying Higher Himalayan Crystalline nappe.