THE MAKING OF MOUNT EVEREST: CHANNEL FLOW, AND LOW-ANGLE NORMAL FAULTS IN THE COMPRESSIONAL HIMALAYAN OROGEN (Invited Presentation)

Mount Everest (8850m) has two north-dipping low-angle normal faults cutting the massif, the upper Qomolangma detachment (QD) placing Ordovician sedimentary rocks above Everest series upper greenschist – amphibolite facies rocks, and the lower Lhotse detachment (LD) placing Everest Series above sillimanite gneisses, migmatites and leucogranites. The two faults merge into one large ductile shear zone (South Tibetan Detachment zone) to the north along the Rongbuk valley in Tibet. Pressure-temperature constraints and structural restoration of the STD shows that the fault acted as a passive roof fault during southward extrusion of the footwall. U-Pb dating of leucogranites in Everest (21-20 Ma), Nuptse (23.6 ± 0.7 Ma) and along the Rongbuk valley (granites as young as 15.6 – 15.4 Ma) show that ductile extrusion of >100 km occurred during the Early Miocene with brittle faulting followed during exhumation at <15.4 Ma. The channel flow model was conceived as a result of detailed field mapping, fabric analyses, thermobarometry and geochronology. Channel flow - the ductile extrusion of a partially melted layer of mid-crustal rocks – was operating along 2000 km of the Himalaya during the Early Miocene. At ca 10 Ma the GHS locked, and was jacked up as deformation propagated southward to the Lesser Himalaya. Uplift of the GHS was achieved through underplating of Lesser Himalayan thrust sheets beneath.