TERTIARY HIMALAYAN STRUCTURES AND METAMORPHISM IN THE KULU VALLEY (MANDI-KHOKSAR TRANSECT OF THE WESTERN HIMALAYA) - SHIKAR-BEH-NAPPE AND CRYSTALLINE NAPPE

The Crystalline Nappe of the High Himalayan Crystalline has been exami ned along the Kulu Valley and its vicinity (Mandi-Khoksar transect). T his nappe was believed to have undergone deformation related only to i ts transport towards the SW essentially during the ''Main Central Thru st event''. New data has led to the conclusion that during the Himalay an orogeny, two distinctive phases, related to two opposite transport directions, characterize the evolution of this part of the chain, befo re the creation of the late NE-vergent backfolding. The first phase co rresponds to an early NE-vergent folding and thrusting, creating the T andi Syncline and the NE-oriented Shikar Beh Nappe stack, with a displ acement amplitude of about 50 km. Two schistosities, together with a s trong stretching lineation are developed at a deep tectonic level unde r amphibolite facies conditions (kyanite-staurolite-garnet-two mica sc hists). At a higher tectonic level and in the southern part of the sec tion (Tandy Syncline and southern Kulu Valley between Kulu and Mandi) one or two schistosities are developed in the greenschist facies grade rocks (garnet-biotite and biotite schists). These structures and the associated Barrovian type metamorphism are all related to the NE-vergi ng Shikar Beh Nappe. The creation of the NE-verging Shikar Beh Nappe m ay be explained by the reactivation of a SW dipping listric normal fau lt of the N Indian flexural passive margin, during the early stages of the Himalayan orogeny. In the second phase, the still hot metamorphic rocks of the Shikar Beh Nappe were folded and thrust towards the SW ( mainly along the MBT and the MCT with a displacement in excess of 100 km) onto the cold, low-grade metamorphic rocks of the Larji-Kulu-Rampu r Window or, near Mandi, on the non-metamorphic sandstones of the Gang es Molasse (Siwaliks). Sense of shear criteria and a strong NE-SW stre tching-lineation indicate that the Crystalline Nappe has been overthru sted towards the SW. Thermometry on synkinematically crystallised garn et-biotite and garnet-hornblende pairs reveals the lower amphibolite f acies temperature conditions related to the Crystalline Nappe formatio n. From the muscovite and biotite Rb-Sr cooling ages, the Shikar Beh N appe emplacement occurred before 32 Ma and the southwestward thrusting of the Crystalline Nappe began before 21 Ma. Our model involving two opposite directions of thrusting goes against the conventional idea of only one main SW-oriented transport direction in the High Himalayan C rystalline Nappes.