Extensional and compressional faults in the Everest-Lhotse massif, Khumbu Himalaya, Nepal

Two large-scale north-dipping, low-angle normal faults cut the Everest mass if at the top of the High Himalayan slab in Nepal. The upper fault the Qomo langma Detachment, follows the north slope of Everest from above the 'Yello w Band' at c. 8500 m on the Southwest Face down to the Rongbuk glacier. On the south side of Everest this fault places unmetamorphosed Ordovician muds tones and limestones above biotite-grade marbles, calc-silicates and greens chists (Everest series pelites). The lower normal fault, the Lhotse Detachm ent, places greenschist grade calc-silicates and pelites above sillimanite- K-feldspargrade gneisses (quartz+biotite+ garnet+ K-feldspar+ plagioclase sillimanite +/- cordierite assemblages) formed at temperatures above 650 d egrees C. Abundant sheet intrusions, sills and dykes of leucogranite contai ning tourmaline+muscovite +/- biotite +/- garnet are restricted to the foot wall of the Lhotse Detachment. The mid-upper greenschist grade Everest peli tes, some 2000 m thick on the SW face of Everest. form a northward tapering wedge bounded by normal faults below and above. A major compressional faul t, the Khumbu Thrust, bounds the base of a c. 3-6 km thick sheet which cons ists of a series of Bat-lying leucogranite sills or sheets extending more t han 25 km south of Everest. The leucogranite peaks of Ama Dablam, Kangteiga and Tamserku are all part of the same sheet, which, prior to erosion, was probably originally continuous. The main granite emplacement mechanism was by syn-tectonic magma injection by hydraulic fracture propagation during si mple shear, along a series of large sills from their source region at depth to the north. Granite emplacement occurred during upper crustal extension along the top of the High Himalayan slab, although final motion on the Lhot se Detachment post-dated granite emplacement. Both the normal faults at the top of the slab and the Khumbu thrust at the base of the leucogranite shee ts were mechanically linked, resulting in the southward extrusion of rocks formed at c. 12-30km depth within the High Himalayan slab.