4.2.2 Phase D2: The SW-directed Nyimaling - Tsarap Nappe
The Tethys Himalaya in Zanskar represents the front of the Nyimaling Tsarap nappe. In the studied area, the complete transition from ductile to brittle deformation associated with the southwestward thrusting of the Tethys Himalaya can be observed from the base to the top of the nappe. Ductile deformation is restricted to the transition zone between the HHCS and the TH and is only preserved in the upper part of this zone, where the structures related to this deformation have not been overprinted by the strong penetrative deformation related to extensional movements along the ZSZ.
The southwestward directed D2 deformation is marked in the Tethys Himalaya by three types of structures: low-angle decollements, high-angle reverse faults and folds. Open decollement folds within the calcareous sedimentary series of the Carboniferous Lipak Formation and the Triassic Lilang Group are certainly the most obvious expression of D2 (Fig 4.2 and Fig 4.3). These superficial F2 folds with their north-south striking axes are especially spectacular within the Lilang group where they have an amplitude and wavelength ranging from ten to several hundred metres and are easily distinguishable on satellite views. These folds display long upper limbs dipping gently towards the east and short under limbs dipping steeply to the west or sometimes to the east. Their axial surfaces plunge moderately to steeply towards the east, which indicates a vergence towards the west. These folds are tectonically detached from the underlying rigid Panjal Traps along local decollement faults at the interface between these two units. Decollement faults however also occur within the different formation of the Lilang Group, giving especially to this unit a typical ramp-and flat geometry.
Decollement faults are also commonly observed within the carbonaceous Lipak Formation but are of an order of magnitude smaller than the folds in the Lilang group. The folds in Lipak are tectonically detached from the underlying rigid Devonian Muth quartzites (See Fig 4.3) and do also have a westward vergence. The N-S orientation of the F2 fold axes in the studied area is different from the general trend of the fold axes in the rest of Zanskar, where they are systematically oriented NW-SE (Herren, 1987). We interpret this feature as a local N-S reorientation of initially NW-SE oriented fold axes during the Phase D3.
All the other formations which are not carbonaceous are only very weakly deformed by folding and rather form thrust slices, sliding over each other along local decollements. These low-angle decollements are discreet structures as they follow single stratigraphic horizons and thus do not perturb the normal stratigraphic succession. Two stratigraphic horizons were especially efficient as decollement planes. The first horizon is formed by the argilites of the Surichun member forming the top of the Cambrian Kurgiakh Formation. In the Upper Lahul region, Vannay (1993) reported a major thrust plane following the Surichun Member, which is responsible in the Baralacha La area for a tectonic transport of at least 15 kilometres towards the SW. The second horizon along which layer-parallel decollement occurred preferentially is, for obvious reasons, the gypseous member of the Lipak Formation. Thus, although the normal stratigraphic succession between the Carboniferous Lipak and Permian Po Formations is nearly always preserved, this gypseous horizon certainly accommodated much of the compressive tectonics at the front of the Nyimaling Tsarap nappe. Local decollements also formed along stratigraphic boundaries between or within other sedimentary formations but are of lesser importance and spatial extent.
The third type of structures related to D2 are high-angle reverse faults which, by cutting through the sedimentary formations, disrupt the normal stratigraphic succession and cause the tectonic repetition of certain formations. Such structures are rare in the studied area and a clear tectonic repetition was only observed along one thrust plane in the Tanze syncline, where the Lipak Formation is overlain by a repetition of the Ordovician Thaple, Devonian Muth and Carboniferous Lipak Formations. More to the north, the same thrust surface superposes the Panjal Traps over the Permian Kuling Formation (Fig 4.3).