4.2 Main Structural Elements in the NW Himalaya
From south to north, the Himalayan belt can be divided in the following main structural elements.
The Main Frontal Thrust (MFT): It is along this still active structure that the Sub-Himalaya is thrust towards the SW over the quaternary fluvial deposits of the Indian plains.
The Main Boundary Thrust (MBT): This structure separates the metapsammitic schists and phyllites of the Lesser Himalaya (hanging wall) from the conglomerates and sandstones of the Sub-Himalaya (footwall). The SW-directed movements associated with this structure are characterised by brittle deformation (cataclastites).
The Main Central Thrust (MCT): This structure is one of the most important tectonic elements associated with the Himalayan orogen; it separates the high-grade metamorphic rocks of the High Himalayan Crystalline Sequence (hanging wall) from the weakly metamorphosed series of the Lesser Himalaya (footwall). Deformation along this structure was mainly ductile. The MCT can be traced along the entire frontal zone of the Himalayan belt and appears in tectonic windows as the Kishtwar Window (KW) or the Larji-Kulu-Rampur Window (LKRW) as well as in klippes as the Simla Klippe.
The Crystalline Nappe: This SW-directed nappe is formed by the High Himalayan Crystalline Sequence and was exhumed by thrust faulting along the MCT over the Lesser Himalaya. This nappe is also sometimes referred to as «slab», «wedge» or «sheet». Internal deformation within this nappe is responsible for a large amount of crustal thickening. The Kalath Fold (Thöni, 1977; Epard et al., 1995) is a major SW vergent fold associated with the thrusting of the Crystalline Nappe on the Main Central Thrust towards the SW.
The Shikar Beh Nappe: The existence of an early phase of NE vergent nappe stacking within the HHCS was proposed by Steck et al., 1993 and represents an exceptional feature in the Himalaya, where compression structures are classically characterised by SW-directed thrusting and folding. The Tandi syncline represents one of the most striking tectonic structures associated with this deformation phase. Vannay (1993) demonstrated that this syncline formed during a phase of NE-directed folding associated with the Shikar Beh Nappe. No thrust zone associated with this NE vergent nappe was observed along the Leh - Rohtang La transect but more to the East, along the Tso-Morari - Spiti transect. Steck et al., 1998 suggest that the Lagudarsi Thrust represents the northeastern front of the Shikar-Beh Nappe.
The South Tibetan Detachment System (STDS), also called North Himalayan Shear Zone (NHSZ), represents a major system of north-dipping structural detachments at the boundary between the High Himalayan Crystalline Sequence and the Tethys Himalaya. This structure was first identified by Caby et al. (1983) and Burg (1984). A detailed analyses of the STDS was made by Burchfiel et al. (1992). Deformation along this structure was accommodated either by dextral strike-slip or by extensional shearing. Unlike the MCT, the STDS is not a continuous structure along the entire Himalayan belt. Various strands belonging to this structure have been identified and one of them is the Zanskar Shear Zone (ZSZ). The ZSZ is a ~150 km long extensional structure that caused a structural detachment between the HHCS and the TH. Most of the deformation along the ZSZ was accommodated by low-angle ductile normal shearing, but high-angle normal faults as the Sarchu Fault (Spring, 1993) or the Dutung-Thaktote Normal Faults (Steck et al. 1998) are also associated with the ZSZ. Another strand of the STDS identified in the NW Himalaya is the Chandra Dextral Shear Zone (CDSZ). According to Vannay and Steck (1995), the CDSZ did not cause a structural detachment between the HHCS and the TH; deformation along this structure was essentially accommodated by ductile dextral strike-slip and is responsible for an E-W reorientation of linear structures. Although the CDSZ and the ZSZ both belong to the STDS, the relationship between these two structures is not clearly understood. It was proposed by Vannay (1993) that they form an en echelon structure.
The Sarchu Dextral Shear Zone (SDSZ): This shear zone corresponds to a N-S couloir of ductile deformation associated with the dextral underthrusting of the High Himalayan Crystalline sequence (or Kenlung Serai Unit) below the Nyimaling Tsarap nappe (Steck et al. 1993). This structure is responsible for a N-S reorientation of linear structures and can be followed from the Nyimaling region in the north to the Main Central Thrust in the south.
The Baralacha La Thrust System: The region of the Baralacha-La is characterised by a great number of SW vergent thrusts and folds. These low-angle decollements follow the Surichun Member of the Kurgiakh Formation and disturb only weakly the normal stratigraphic succession. Vannay and Steck (1995) conclude that the Baralacha La Thrust System is characteristic of a shallow structural level. These authors interpret these thrusts as representing an imbricate structure, that developed at the front of the SW vergent Nyimaling-Tsarap Nappe. The Parang-La Thrust at the front of the Mata Nappe represents an equivalent of the Baralacha La-Thrust along the Tso-Morari- Spiti transect (Steck et al. 1998). These two thrust systems are linked together by an en echelon structure.
The Nyimaling-Tsarap Nappe: This name was proposed by Steck et al. (1993) for the whole thrust pile of sedimentary rocks situated between the Indus Suture Zone to the north and the Baralacha-La to the south. The internal structure of this SW vergent nappe corresponds to an imbricate structure as defined by De Margerie and Heim (1888) and updated by Steck et al. (1993, 1998). A progressive change in the style of deformation was evidenced from the northeastern part of the Nyimaling-Tsarap nappe towards the southwest as higher structural levels are exposed. Along the Indus Suture Zone in Ladakh, the root zone of the Nyimaling-Tsarap nappe is exposed as a consequence of the uplift of the Nyimaling crystalline dome. In this region, it was observed that the southwestward-directed deformations at the base of the nappe are essentially accommodated by ductile shearing. From the root zone towards the southwest, and as increasingly higher structural levels of the Nyimaling-Tsarap nappe are exposed, the deformation becomes more and more brittle. This is marked by the progressive development of a ramp-and-flat tectonic style towards the frontal part of the Nyimaling-Tsarap nappe. The upper part of the Tethys Himalaya is thus sliced into several overlapping structural units bounded by a succession of low-angle northeast dipping ramps. These brittle detachments are most likely anchored in a zone of ductile deformation towards the base of the Nyimaling-Tsarap nappe. The amount of deformation and translation becomes less important within the structural units forming the front of the nappe (Baralacha La Thrust). A recent study (Steck et al. 1998) revealed that more to the East, in the Tso-Morari area, several superposed recumbent nappes can even be evidenced within the Tethys Himalaya. According to Steck et al. (1998), the Nyimaling-Tsarap nappe represents the equivalent of the uppermost nappe (the Mata nappe) in this nappe stack.
The HHCS Domes: the High Himalayan Crystalline Sequence is characterised by the presence of several dome structures. The Bhazun Dome, the Cishoti Dome and the Barnaj Body are such structures found north of the Kishtwar window and described by Kündig (1989); the Haptal Dome south of Padum was described by Herren (1987). In SW Zanskar, the HHCS also forms a dome we propose to call the Gianbul Dome.
The Nyimaling Crystalline Dome: In NE Ladakh, the Tethys Himalayan Sedimentary series are limited to the north by a dome of metamorphic rocks and Cambro-Ordovician granites similar to the High Himalayan Crystalline Sequence. The rocks from the Nyimaling Crystalline Dome recorded the ductile deformation associated with the southwestward tectonic transport of the Nyimaling-Tsarap Nappe. The dome structure corresponds to a NE vergent backfold and was acquired later on, during an episode of dextral transpression (Stutz, 1988; Stutz and Steck, 1989; Steck et al. 1993). More to the east, in the Tso-Morari region, the equivalent of the Nyimaling Crystalline Dome is structurally underlain by the Tetraogal Nappe and the Tso-Morari Nappe (Steck et al., 1998).
The Indus Suture Zone (ISZ): This structure marks the limit between the Indian Plate and the Asian plate. It is along the Indus Suture zone that the Indian plate was subducted below Asia. Remains of oceanic crust and island arcs (Dras volcanics), mixed with flysch and molasse deposits, can be found within the ISZ as well as in the Spontang Klippe.