THE CONTACT BETWEEN THE HIGHER HIMALAYA CRYSTALLINES AND THE TIBETAN SEDIMENTARY SERIES - MIOCENE LARGE-SCALE DEXTRAL SHEARING

Space and time evolution of the synmetamorphic structures across the metamorphic pile have been studied in several areas along the Himalayan belt (east and central Nepal, Garhwal, Zanskar). From one area to the other the evolution is very similar: (1) At the base of the pile, in the Main Central Thrust (MCT) shear zone, the stretching lineation, penetrative and regularly oriented N0-degree-E to N30-degrees-E, indicates the MCT transport direction, very constant all along the belt, from the Eohimalayan main metamorphic development up to the late-metamorphic movements. (2) At the top of the pile, at the contact between the crystalline unit and its sedimentary cover, gravity-driven structures are confirmed (north-vergent folds, ductile normal faulting). However, there are numerous local indications of late Miocene (syn-to late emplacement of the leucogranitic plutons) dextral shearing. (3) In between, across the medium part of the pile, the stretching lineation shows a conspicuous progressive regional clockwise rotation, clearly indicated by the strain trajectories mapped in Nepal and Garhwal. Points 1 and 2 show that the crystalline-sedimentary boundary, despite its apparent structural and metamorphic continuity, is not only a normal fault but also an important dextral shear zone, which has acted since the upper Miocene as the main southern limit of the eastward extruding Tibetan block.