4.2.4 Phase D4: Ductile top to the NE extension along the ZSZ
An apparently paradoxical feature of mountain building processes is the development of major extensional structures within overall compressive systems. Although the significance of these extensional structures has been recognized only recently, such structures have been found in most orogenic belts, where they appear to play a major role in the exhumation of high-grade metamorphic rocks. In several inactive mountain belts like the Norwegian Caledonides (Norton, 1986; Andersen et al., 1991; Fossen, 1992; Wilks and Cuthbert, 1994, Chauvet and Séranne, 1994; ), the Variscan belt of Europe (Malavieille, 1993), or the northwestern American Cordillera (Crittenden et al., 1980; Allmendinger et al., 1983; Dallmeyer et al., 1986; Metzger et al., 1991), such extensional features have been attributed mainly to late orogenic processes, developing only once shortening nearly ceased. However, in active mountain belts like the Andes (Dalmayrac and Molnar, 1981), the Alps (Steck, 1980; Ratschbacher et al., 1989; Steck and Hunziker, 1994), or the Himalaya (Burg, 1983; Burchfiel and Royden, 1984 & 1985; Searle, 1986; Herren, 1987; Copeland et al., 1990; Pêcher et al., 1991; Hubbard et al., 1992; Burchfiel et al., 1992; Hodges et al., 1992 and 1993; Brunel et al. 1994; Burg et al., 1994; Inger, 1994; Scaillet et al., 1995; Edwards et al. 1996; Sorkhabi et al., 1996; Davidson et al., 1997), clear evidence shows that major extensional structures were active contemporaneously with shortening and crustal thickening.
One of the most spectacular examples of syn-orogenic extension can be observed in the Himalaya. For nearly 2000 km along this belt, a system of north-dipping detachments separates the high-grade metamorphic sequence of the High Himalayan Crystalline Sequence from the overlying, weakly metamorphosed sediments of the Tethyan Himalaya. These extensional structures are referred to as the South Tibetan Detachment System (STDS; Burchfiel et al., 1992).
The Zanskar Shear Zone (ZSZ) is a ~ 150 km long segment of the STDS separating the HHCS from the TH in the Zanskar region of the NW Himalaya (Herren, 1987). This structure corresponds to a 1 kilometre thick zone of ductile shear, or mylonitic zone (Passhier and Trouw, 1996) following the transition domain between the Tethys Himalaya and the High Himalayan Crystalline Sequence (Fig 4.1).
The ZSZ records two major phases of deformation. As described above, relict D2+D3 microstructures, essentially expressed by rotated garnets, indicate an early top-to-the-SW sense of shear. These features indicate that the ZSZ most probably acted initially as a major syn-metamorphic thrust, along which the HHCS was underthrust below the TH (see also Patel et al., 1993).
Multi-stage extensional D4 structures are superposed on the compressional D3 fabrics. The earliest D4 phase structures correspond to pervasive C-S fabrics indicating top-to-the-NE extension. The shearing surfaces are parallel to the shear zone boundaries and define the main foliation, which bears a penetrative, down-the-dip mineral lineation. In the studied area, the ZSZ dips constantly 20° to the NE. In the lowermost part of the ZSZ, leucogranitic dikes are progressively reoriented and boudinaged through shearing, to finally become sub-parallel to the foliation (Fig 4.6 and Fig 4.7). The main penetrative foliation is superposed by a steeper, anastomosing foliation that corresponds to a C'-type shear band cleavage (Fig 4.5). This cleavage is oblique to the shear zone boundaries and dips 40-60° to the NE. A conjugate cleavage, dipping gently toward the SW, is less well developed. These shear band cleavages record extension during a later stage of the ductile D4 deformation.
Along with the reorientation of the leucogranitic dikes and the C-S or C'-S fabrics, other shear sense indicators like mantled porphyroblasts, asymmetric P3 microfolds, back-rotated boudins and mica fishes, systematically indicate top-to-the-NE sense of shear associated with D4. These microstructure are associated with the growth of retrograde metamorphic minerals which will be described in Chapter 5.4.
The morphology of the transition zone between the High Himalayan Crystalline sequence and the Tethys Himalaya depends on the type of rocks sheared within the Zanskar Shear Zone. In those regions where metapelitic - metapsammitic rocks were affected by ductile shearing, the transition between the HHCS and the TH is gradual and the ZSZ does not appear as a marked morphologic structure in the topography. This feature is commonly observed from the Gianbul valley to the Gumburanjun. In those regions where ductile shearing affected more competent rocks (i.e. orthogneisses), the transition between the HHCS and the TH is often brutal and the top of the ZSZ is characterised by a slickenside. Such a brutal contact is exposed in the Mune - Reru area, where it forms a marked structure that can even be recognised on satellite images.
Although most of the extensional tectonics associated with D4 were concentrated along the Zanskar Shear Zone (Fig 4.8 and Fig 4.9), the Tethys Himalaya was also affected by this phase. In this domain, D4 is marked by the reactivation of D2 thrusts as low-angle normal faults and backfolding in the carbonaceous horizons (Carboniferous Lipak and Triassic Lilang). Both low-angle folds and backfolding are, however, much less developed in Zanskar than in the adjacent Suru and Sarchu regions, most probably because in these latter regions extensional tectonics were not, or only weakly, accommodated by ductile shearing between the HHCS and the TH.