Synorogenic extension in the Tethyan Himalaya documented by structural studies and the Kubler index, Lachung La area, NW India

Tectonic observations in the Tethyan Himalaya reveal an important extension al event that succeeds the emplacement of SW-verging nappes. A major thrust , called the Kum Tso Thrust, has been backfolded and reactivated by normal faulting associated with this event. Measurements of the Kubler index, coupled with characterization of clay-siz e paragenesis show the effect of normal faulting on the regional metamorphi c zonation and indicate that important extension zones, like the Sarchu-Lac hung La Normal Fault Zone (SLFZ), exist within the Tethyan Himalaya. Diagen etic limestones from within the SLFZ are characterized by the occurrence of mixed-layered clay phases, kaolinite and an illite with a 001 peak >0.4 De lta degrees2 theta. This zone is bordered by two anchizonal-to-epizonal zon es, where illite peaks become narrower. Further to the NE the successive ap pearance of biotite, chloritoid, garnet and garnet-staurolite-kyanite assem blapes testifies to an increase in metamorphic grade. The cataclastic sampl es from the normal faults contain kaolinite, smectite and a 'broad' illite, indicating that extension occurs under diagenetic conditions.