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

In this paper we present the current geological knowledge and the results of new geological and structural investigations in the Cho Oyu-Sagarmatha-Makalu region (Eastern Nepal and Southern Tibet). The tectonic setting of the middle and upper part of the Higher Himalayan Crystallines (HHC) and Tibetan Sedimentary Sequence is characterized by the presence of pervasive compressive tectonics with south-verging folds and shear zones overprinted by extensional tectonics. In the middle and upper part of the HHC two systems of folds (F2a and F2b) have been recognized, a ecting the S1 high- grade schistosity causing kilometer-scale upright antiforms and synforms. The limbs of these upright folds are a ected by F3 collapse folds, top-to-SE extensional shear zones and extensional crenulation cleavages linked to extensional tectonics. The uppermost portion of the HHC and the lower part of the Tibetan Sedimentary Sequence is a ected 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 micaschists 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 ductile/brittle deformation and thin levels of cataclasites and brings the slightly metamorphosed Ordovician limestones into contact with the North Col Formation. 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 ®rst stage (M1) is eclogitic as documented by granulitized eclogites collected at the top of the Main Central Thrust Zone 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 high T (750±7708C), and was followed by recrystallization in the amphibolite facies of low pressure and high T (M3). The ®rst event has also been recorded in the overlying Barun Gneiss, where M1 was followed by decompression under increasing T, the M2 event, producing the dominant mineral assemblage (garnet-sillimanite-biotite), and then by strong decompression under high T, with growth of andalusite, cordierite and green spinel. Also, changes in phase compatibilities suggest an increase in metamorphic temperature (T) coupled with a decrease in metamorphic pressure (P) in some of the thrust sheets of the MCT Zone. A telescoped metamorphic zonation ranging from the sillimanite to the staurolite 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 whole ductile shear zone, particularly in metapelites both below and above the Makalu leucogranite and in micaschists of the staurolite zone