9. Conclusion

9 Conclusion

In this work on the structural and metamorphic evolution of SE Zanskar, we have tried to give an account of the successive events that led to the present-day close juxtaposition of two contrasted Himalayan domains, the Tethys Himalaya and the High Himalayan Crystalline Sequence. We have seen that these two domains originally formed a continuous series of sedimentary rocks deposited on the northern margin of the Indian continent and range in age from Upper Proterozoic to Upper Triassic. The contrasted metamorphic overprint of these two domains is the result of a series of strictly Himalayan events we will recapitulate.

The studied area is affected, although only marginally, by a first event of crustal thickening associated with the emplacement of the NE-vergent Shikar-Beh Nappe. This event is marked by a weak metamorphism of lower greenschist facies characterised by the growth of tiny biotite flakes on the southern side of the Shingo-La and decreases progressively towards the NE.

SE Zanskar is subsequently affected by a second event of crustal thickening associated with the emplacement of the SW vergent Nyimaling-Tsarap Nappe. Relic shear sense indicators suggest that the ductile thrust plane of this SW vergent nappe is preserved in the studied area. We propose that it is along this structure that the lowermost sedimentary series were underthrusted below the Nyimaling Tsarap Nappe. It is essentially during this event that the sedimentary series acquired their metamorphic overprint.

The burial of the sedimentary series is associated with a metamorphism of Barrovian type showing a complete zonation from chlorite to kyanite zone. Our thermobarometric data confirm our petrographic interpretations that the rocks from the different metamorphic zones equilibrated at significantly different depths along the kyanite geotherm. These data also indicate that crustal thickening associated with the emplacement of the Nyimaling Tsarap Nappe was very important, for the highest grade metamorphic rocks now exhumed were buried at depths we calculated to have been over 40 kilometres.

The latest stage of emplacement of the Nyimaling-Tsarap Nappe is marked by a rotation of the underthrusting direction resulting in a reorientation of the fold axes in a N-S direction.

The subsequent stages of the tectonic-metamorphic history of SE Zanskar are related to the exhumation of the high grade metamorphic rocks. This exhumation is marked by a reversal in the shear sense of the thrust plane at the base of the Nyimaling-Tsarap Nappe. Indeed, we propose that the Zanskar Shear Zone was formed by the reactivation of this thrust plane in an opposite direction. The exhumation of the High grade rocks is responsible for the constriction of the Barrovian metamorphic isograd within the Zanskar Shear Zone and for the present-day juxtaposition of the rocks from the base of the Nyimaling-Tsarap Nappe (the HHCS) and the upper parts of this same nappe (the Tethys Himalaya).

The main issue of this work was the quantification of the amount and age of ductile extension along the Zanskar Shear Zone. On the basis of our thermobarometric results, we have shown that the highest grade rocks present in the ZSZ have equilibrated at a depth of 34±3 km and that these rocks have been brought to the same elevation as rocks that equilibrated at a depth of 22±1 km, implying a vertical displacement of 12±3 km. The coupling of these data with a geometric model of the shear zone allowed us to estimate the net-slip along this structure to be at least 35±9km. However, a series of arguments lead us to assume that the total displacement along the ZSZ might have been up to ~100km. The age and duration of ductile extensional shear was constrained by our geochronological data. The cessation of ductile extensional movements along the ZSZ was estimated at 19.8 ±0.1Ma through Ar/Ar muscovite cooling ages of undeformed leucogranitic dikes cross-cutting the base of this structure. A close estimate of the onset of extension was obtained through U/Pb monazite cooling age of the leucogranites, which yield an age of 22.2±0.2Ma. These data confirm the contemporaneity of extensional shearing along the ZSZ and thrusting along the MCT.

Several models are discussed to tentatively explain what processes could have led to the exhumation of the HHCS. We suggest that the HHCS represents a slice of high-grade rocks uncoupled from the subducting Indian upper-crust. This uncoupling occurred because buoyancy forces hindered further descent of this slice of crustal rocks. Consequently, the main detachment zone is transferred from the thrust plane at the base of the Nyimaling Tsarap Nappe to the Main Central Thrust. This caused the HHCS to be sandwiched between the still subducting Indian crust and a buttress formed by the Tethys Himalaya. Combined simple shear and pure shear, assisted by buoyancy forces led to the upward expulsion of the HHCS as a large-scale nappe limited at its base by the MCT and its top by the ZSZ.

The exhumation of the HHCS is marked by the apparition of a series of metamorphic minerals defining an isothermal decompression path. We suggest that the tertiary leucogranites formed in the migmatitic zone as a consequence of isothermal decompression-induced vapour-absent melting. The intrusion of these leucogranitic melt in the ZSZ might have lubricated this detachment and contributed to increase the amount of net slip along this structure.

The last stages of exhumation are marked by the formation of large domes in the HHCS and by high-angle brittle compensation faults.

This reconstitution of the sequence of events we present to explain the present geology of Zanskar is, of course, greatly influenced by contemporary models and theories on orogenic tectonics and is therefore prone to change as these ideas evolve. Nevertheless, we believe that this work contains several valuable observations on the metamorphism, the magmatism and the tectonic structures, associated with the formation of the Himalaya.