Zanskar : Tectonic evolution

4.2.3 Phase D3: Ductile underthrusting of the HHCS below the TH

The southwestward propagation of the deformation D2 in the Nyimaling - Tsarap Nappe also affected the basal series of the Tethys Himalaya which will ultimately form the High Himalayan Crystalline Sequence. In a first stage, these, mainly proterozoic, basal series were still welded to the Tethys Himalaya, but as deformation proceeded, and as the Tethys Himalaya formed an increasingly thick stack of rocks, the lower part of the Tethys Himalaya became ductile enough to accommodate the compressive deformation through ductile shearing. From that time on, the compressive forces were essentially concentrated along a ductile shear zone at the base of the Nyimaling - Tsarap Nappe. Movements along this thrust zone contributed to the further underthrusting and metamorphisation of the basal proterozoic series, leading to the ultimate metamorphic differentiation between the Tethys Himalaya and the High Himalayan Crystalline Sequence.

In the studied area, D3 is marked by an old N-S lineation, a strong N-S elongation of the pebbles in the Ordovician Thaple conglomerates and the N-S reorientation of D2 NW-SE fold axes. These features are essentially preserved at the base of the Tethys Himalaya and to a lesser extent at the very top of the Zanskar Shear Zone where subsequent deformation associated with D4 was not strong enough to erase completely the D3 structures.

P3 folds affecting quartz ribbons and mafic horizons as well as an S3 crenulation cleavage in metapelitic rocks are preserved at the top of the ZSZ. These D3 structures show interference structures with D4. The overprint of D4 on D3 increasing gradually through the ZSZ, almost none of the S3 structures subsist at the base of the Zanskar Shear Zone.

Towards the bottom of the Zanskar Shear Zone, D3 is only preserved as inclusion trails within synmetamorphic rotated garnet porphyroblasts (fig. 4.4). These snowball garnets systematically show a top to the S-SW sense of shear throughout the ZSZ, which indicates that this structure acted as a ductile thrust zone during D3 before being reactivated as a ductile extensional shear zone during D4. This superposition of extensional shearing over an earlier thrust zone at the top of the HHCS was also recognized along other segments of the ZSZ by Patel et al. (1993).

At a larger scale, D3 is marked by a zone of dextral strike-slip that can be followed from the Nyimaling region in the north down to the MCT in the south (Fig 4.1). This N-S trending structure encompasses the Nyimaling and Sarchu dextral shear zones (Steck et al. 1993) and shifts the western domains to the north with respect to the domains east of this zone of dextral strike-slip. This zone also corresponds to the westernmost end of the Nyimaling dome and the amphibolite grade rocks of the HHCS around the Kishtwar window as well as to the eastern termination of the crystalline domes of the HHCS in Zanskar.

Fig 4.4: D3 syntectonic garnet porphyroblast in a metapelitic rock from the Kyanite zone (Giambul valley). Shear sense creteria deduced from the helicitic inclusion trails indicate top-to-the-SW rotation of more than 180°. Shear sense criteria in the matrix indicate an opposite top-to-the-NE movement associated to D4.

The progressive rotation in the thrusting of the Nyimaling-Tsarap Nappe from a top-to-the-SW direction during D2 to a top-to-the-south direction during D3 is most probably related to the counterclockwise rotation of the Indian continent relative to Asia between 45 Ma and 36 Ma as documented by plate tectonic reconstitution (Patriat and Achache, 1984; Dewey et al. 1988; Scotese et al. 1988).