4.2.5 Phase D5: Doming
The doming episode is only apparent in the High Himalayan Crystalline Sequence, where the D5 phase is associated with an important bulging of this domain. The dome formed by the HHCS, has an approximately elliptical shape, with a NW-SE trending long axe, perpendicular to the general directions of transport of the TH nappes. The dome swells to a maximal width in the middle of its long axe and pinches at both ends, in the Suru region to the NW and in the Sarchu region to the SE. We will refer to the HHCS dome in SW Zanskar as the Gianbul Dome.
Along the Gianbul valley, we could observe that the ZSZ is warped by the doming phase. We thus disagree with the interpretation of Kündig (1988), that ductile extensional shearing along the ZSZ was the consequence of doming.
This warping or folding of the ZSZ, together with the metamorphic isograds, around a dome explains why the metamorphism decreases in the areas south of the dome as in the Chenab valley. Indeed, if the metamorphic isograds were not folded, one would expect a gradual increase in the metamorphic gradient towards the south.
Because of the folding of the ZSZ around the Gianbul Dome (Fig 4.12), it is also expected that the southern side of the dome should be characterized by the presence of the ZSZ but tilted towards the south. On the southern part of the dome, the ZSZ could thus be confused with a NE vergent thrust zone because the ZSZ is here dipping to the SW and the tilted D4 shear sense criteria would still reveal a ductile top to the NE sense of shear. Preliminary data from Steck, Epard and Robyr (oral communication) reveal that, in the Miyar Valley, the southern limit of the Gianbul Dome is characterized by the presence of a ductile, low-angle, SW dipping shear zone (Miyar Thrust). The structural analysis of the Miyar Valley region reveals an early top to the NE sense of shear of the Miyar Thrust superposed by SW dipping high-angle normal faults (similar to the D6 normal faults on the northern side of the dome). Although Steck et al. (oral communication) interpret the Miyar thrust as the ductile sole of the early NE vergent Shikar-Beh Nappe, we rather consider the Miyar thrust and the ZSZ as forming a single continuous folded extensional structure (Fig 4.12).
The consequence of the folding of the ZSZ around the Gianbul Dome, is that the shear zone has to crop out again somewhere south of the Chenab valley and the north of the MCT. Such a structure has not been described so far but the geology of this region is still little known. It is expected that the southernmost reappearance of the ZSZ should be characterised by a set of high-angle brittle normal faults, because the regions south of the Chenab are only affected by greenschist facies.
The cause of doming of the HHCS in Zanskar is unclear. It is indeed very unlikely that the doming of the HHCS is the consequence of tectonic denudation related to extensional movements along the ZSZ. Such a process is often described in zones where exhumation of high-grade rocks is associated with crustal extension as in the Bassin and Range or the Cordilleran metamorphic core complex (Spencer, 1984). In these settings, the removal of a large portion of the overburden through tectonic denudation leads to isostatic rebound of the exhumed domains and to the warping of the low-angle normal shear zone. Although we observe a warping of the ZSZ around the Gianbul Dome, the doming of the HHCS occurred in a context of crustal shortening.
|Phase D4: Ductile extension along the ZSZ||Phase D6:High-angle normal faults|