4.1.2 Chronology of deformation in the NW Himalaya (Fig 4.1)
The following abbreviations are used in this work for the structural analyses: D: deformation phase; S: Foliation (So = bedding; otherwise, S stands for a pervasive schistosity or a crenulation cleavage); L: Mineral or stretching lineation; F: Fold
D1: This phase corresponds to early NE-directed movements associated with the formation of the Shikar-Beh Nappe (Steck et al. 1993). The studied area is only marginally affected by this tectonic event. The Tandi Syncline and an early metamorphism of Barrovian type, restricted to the southern part of the HHCS, formed during this phase.
D2: This is the main compression phase affecting the sedimentary series of the Tethys Himalaya and corresponds to the development of the SW vergent Nyimaling Tsarap Nappe (Steck et al., 1993). Deformation associated with this phase propagated gradually from the Indus Suture Zone towards the SW as the Indian plate was progressively subducted below Asia along the Indus Suture Zone. The Baralacha La Thrust is a major frontal structure associated with this phase.
D3: This phase corresponds to the N to NNW underthrusting below the Tethys Himalaya of the series that will eventually form the High Himalayan Crystalline Sequence. The main structures associated with this phase are the Sarchu and Nyimaling Dextral Shear Zones as well as the Zanskar Shear Zone.
D4: Is a phase of extensional ductile shearing at the top of the Crystalline Nappe (HHCS) and low-angle normal faulting in the Tethys Himalaya. These deformations are associated with the southwestward-directed exhumation of the High Himalayan Crystalline Sequence from below the Tethys Himalaya. Most of the deformation related to D4 was concentrated along the Zanskar Shear Zone. Extensional movements along the ZSZ are supposedly contemporaneous with the activation of thrusting along the MCT.
D5: Corresponds to an episode of doming of the HHCS (Gianbul Dome).
D6: This last phase is marked by high-angle brittle-ductile normal faults around the HHCS domes.
Situated at the contact between the High Himalayan Crystalline and the Tethys Himalaya, the studied area is dominantly characterized by ductile extensional tectonics associated with D4. This deformation phase is superposed on the earlier compressive D1, D2 and D3 phases and is essentially concentrated within a 1 kilometre thick shear zone, the Zanskar Shear Zone. Two other phases, D5 and D6, are also associated with extensional tectonics and, although they appeared later than D4, their very existence is intimately associated with this main extensional phase.