Geology of the Higher Himalayan Crystallines in Khumbu Himal (Eastern Nepal)

In this paper we present the current geological knowledge and the results o f new geological and structural investigations in the Cho Oyu-Sagarmatha-Ma kalu region (Eastern Nepal and Southern Tibet). The tectonic setting of the middle and upper part of the Higher Himalayan C rystallines (HHC) and Tibetan Sedimentary Sequence is characterized by the presence of pervasive compressive tectonics with south-verging folds and sh ear zones overprinted by extensional tectonics. In the middle and upper part of the HHC two systems of folds (F2a and F2b) have been recognized, affecting the S1 high-grade schistosity causing kilom eter-scale upright antiforms and synforms. The limbs of these upright folds are affected by F3 collapse folds, top-to-SE extensional shear zones and e xtensional crenulation cleavages linked to extensional tectonics. The uppermost portion of the HHC and the lower part of the Tibetan Sediment ary Sequence is affected by two major extensional fault zones with a top-to NE direction of movement. The lower ductile extensional shear zone brings into contact the North Col Formation with the high grade gneisses and micas chists of the HHC. It is regarded as the main feature of the South Tibetan Detachment System. The upper low-angle fault zone is characterized by ducti le/brittle deformation and thin levels of cataclasites and brings the sligh tly metamorphosed Ordovician limestones into contact with the North Col For mation. Extensional tectonics continued with the formation of E-W trending high angle normal faults. Three metamorphic stages of Himalayan age are recognized in the HHC of the Sagarmatha-Makalu region. The first stage (M1) is eclogitic as documented b y granulitized eclogites collected at the top of the Main Central Thrust Zo ne in the Kharta region of Southern Tibet. The second event recorded in the Kharta eclogites (M2) was granulitic, with medium P (0.55-0.65 GPa) and hi gh T (750-770 degrees C), and was followed by recrystallization in the amph ibolite facies of low pressure and high T (M3). The first event has also be en recorded in the overlying Barun Gneiss, where M1 was followed by decompr ession under increasing T, the M2 event, producing the dominant mineral ass emblage (garnet-sillimanite-biotite), and then by strong decompression unde r high T, with growth of andalusite, cordierite and green spinel, Also, cha nges in phase compatibilities suggest an increase in metamorphic temperatur e (T) coupled with a decrease in metamorphic pressure (P) in some of the th rust sheets of the MCT Zone. A telescoped metamorphic zonation ranging from the sillimanite to the staur olite and biotite zones is characteristic of the ductile extensional shear zone which is the lower part of the STDS in the Sagarmatha region. Evidence for decompression under increasing temperature, anatexis and leucogranite emplacement accompanying extension in the HHC was found throughout the whol e ductile shear zone, particularly in metapelites both below and above the Makalu leucogranite and in micaschists of the staurolite zone. (C) 1999 Els evier Science Ltd. All rights reserved.