GEOLOGICAL TRANSECT ACROSS THE NORTHWESTERN HIMALAYA IN EASTERN LADAKH AND LAHUL (A MODEL FOR THE CONTINENTAL COLLISION OF INDIA AND ASIA)

The detailed geological mapping and structural study of a complete tra nsect across the northwestern Himalaya allow to describe the tectonic evolution of the north Indian continental margin during the Tethys oce an opening and the Himalayan Orogeny. The Late Paleozoic Tethys riftin g is associated with several tectonomagmatic events. In Upper Lahul an d SE Zanskar, this extensional phase is recorded by Lower Carboniferou s synsedimentary transtensional faults, a Lower Permian stratigraphic unconformity, a Lower Permian granitic intrusion and middle Permian ba saltic extrusions (Panjal Traps). In eastern Ladakh, a Permian listric normal fault is also related to this phase. The scarcity of synsedime ntary faults and the gradual increase of the Permian syn-rift sediment thickness towards the NE suggest a flexural type margin. The collisio n of India and Asia is characterized by a succession of contrasting or ogenic phases. South of the Suture Zone, the initiation of the SW verg ent Nyimaling-Tsarap Nappe corresponds to an early phase of continenta l underthrusting. To the S, in Lahul, an opposite underthrusting withi n the Indian plate is recorded by the NE vergent Tandi Syncline. This structure is associated with the newly defined Shikar Beh Nappe, now p artly eroded, which is responsible for the high grade (amphibolite fac ies) regional metamorphism of South Lahul. The main thrusting of the N yimaling-Tsarap Nappe followed the formation of the Shikar Beh Nappe. The Nyimaling-Tsarap Nappe developed by ductile shear of the upper par t of the subducted Indian continental margin and is responsible for th e progressive regional metamorphism of SE Zanskar, reaching amphibolit e facies below the frontal part of the nappe, near Sarchu. In Upper La hul, the frontal parts of the Nyimaling-Tsarap and Shikar Beh nappes a re separated by a zone of low grade metamorphic rocks (pumpellyite-act inolite facies to lower greenschist facies). At high structural level, the Nyimaling-Tsarap Nappe is characterized by imbricate structures, which grade into a large ductile shear zone with depth. The related cr ustal shortening is about 87 km. The root zone and the frontal part of this nappe have been subsequently affected by two zones of dextral tr anspression and underthrusting: the Nyimaling Shear Zone and the Sarch u Shear Zone. These shear zones are interpreted as consequences of the counterclockwise rotation of the continental underthrusting direction of India relative to Asia, which occurred some 45 and 36 Ma ago, acco rding to plate tectonic models. Later, a phase of NE vergent ''backfol ding'' developed on these two zones of dextral transpression, creating isoclinal folds in SE Zanskar and more open folds in the Nyimaling Do me and in the Indus Molasse sediments. During a late stage of the Hima layan Orogeny, the frontal part of the Nyimaling-Tsarap Nappe underwen t an extension of about 15 km. This phase is represented by two types of structures, responsible for the tectonic unroofing of the amphiboli te facies rocks of the Sarchu area: the Sarchu high angle Normal Fault , cutting a first set of low angle normal faults, which have been crea ted by reactivation of older thrust planes related to the Nyimaling-Ts arap Nappe.