4.3 Discussion

 The close juxtaposition of two domains, the High Himalayan Crystalline Sequence and the Tethys Himalaya, showing very different styles of deformation made it often difficult to correlate precisely a deformation phase observed in one domain with its equivalent in the other domain.

We have nevertheless subdivided the tectonic evolution of SE Zanskar into six phases of deformation. This subdivision was made on the basis of structural arguments but also on the basis of metamorphic arguments which will be presented in the next chapter.

Both the HHCS and the TH originally formed a single continuous succession of sedimentary rocks ranging from the Proterozoic to the Eocene. The now observable metamorphic difference between these now distinct domains is the result of the tectonic thickening of the sedimentary series during the collision between India and Eurasia, and thus, of the burrial and metamorphisation of the mainly Proterozoic sediments at the base of the Nyimaling-Tsarap Nappe formed by the whole stack of Tethyan Himalaya sedimentary rocks. The tectonic uncoupling of the Proterozoic series from the younger series is the result of the formation of a ductile thrust, reactivated as an extensional shear zone, at the base of this nappe.

Structural observation basically reveal that the studied area underwent three clearly distinct tectonic episodes. The two first episodes are marked by compressive structures (folds, thrust planes, nappes) showing opposite sense of movement (D1, NE vergent and D2+D3, SW vergent). The third episode is marked by extensional structures (Shear Zone D4, doming D5, normal faults D6). While these three major episodes clearly succeeded one another through time, deformation phases belonging to a same tectonic episode might have occurred contemporaneously at different structural levels.

The Zanskar Shear Zone, which is marked by a top-to-the NE sense of shear, did not form in a system globally in extension but, paradoxically, this "extensional" structure formed while the Himalaya was still globally in compression. At large scale D4 does indeed correspond to the extrusion of the HHCS as a SW vergent nappe, thrust over the Lesser Himalaya along the Main Central Thrust. At the top of the HHCS, the Zanskar Shear Zone formed contemporaneously with the MCT, as a result of differential movements between the SW vergent ductile crystalline Nappe and the more rigid Tethyan Himalaya series. In other words, the southwestward extrusion of the HHCS as a large scale nappe during D4 was accomodated at its base by a thrust plane (the MCT) and at its top by an "extensional" shear zone (the ZSZ).

The superposition of high-angle ultramylonites over ductile deformation structures within the Zanskar shear zone is most probably the consequence of a single protracted episode of deformation. Continuous deformation along an extensional shear zone does indeed bring lower crustal rocks to the surface, resulting in a gradual transition from the ductile to the brittle domain within the ZSZ.

It should also be noted that, while ductile shearing occurred at depth, brittle normal faults must have started to form in the upper structural levels represented by the Tethys Himalaya. This evolution of a shear zone from a ductile to a brittle part of the crust was first proposed by Ramsay (1980), although in a basement-cover context. Such relation between discrete brittle faults in a high tectonic level and a ductile shear zone in a structural deeper level was also proposed by Steck et al. (1993) for the Nyimaling-Tsarap nappe during the compressive phase.

An interesting feature is the zone of dextral shearing associated with D3 that borders the western end of the HHCS in Zanskar, and which we have tentatively correlated with the Sarchu and Nyimaling Dextral Shear Zones. When looking at the geological map of the NW Himalaya, one can see that the domains lying west of a north-south running line connecting Leh and Kulu are shifted to the north over an horizontal distance of about 50 kilometres with respect of their equivalent lying east of this line. The eastern Zanskar dextral shear zone, together with the Sarchu and Nyimaling Dextral Shear Zones, might thus belong to a major zone of dextral strike-slip, stretching from the Indus Suture Zone to the Main Central Thrust. An equivalent structure, but of senestral strike slip, could also be inferred from the geological map for the region of Warwan at the western extremity of the Zanskar HHCS. The delimitation of the Zanskar High Himalayan Crystalline Sequence by two zones of strike slip would explain why this particular region indents the Tethys Himalaya more than the adjacent areas, and could also explain, why the ZSZ is restricted to the Zanskar region.