TECTONIC AND METAMORPHIC EVOLUTION

OF THE CENTRAL HIMALAYAN DOMAIN

IN SOUTHEAST ZANSKAR

by Pierre Dèzes

5. METAMORPHISM

Wenn ich zehn Jahr jünger wäre, würde ich sehr versucht sein, eine Reise nach Indien zu machen, nicht um etwas Neues zu entdecken, sondern um das Entdeckte nach meiner Art anzusehen. J.W. von Goethe, Brief an Knebel, 1787

5.1 Introduction

The studied area of Zanskar is located at the boundary between two major Himalayan domains of contrasting metamorphic grade: the High Himalayan Crystalline Sequence in the southwest and the Tethys Himalaya in the northeast. These two domains are separated by a major extensional shear zone, the Zanskar Shear Zone. This ~1 kilometre wide tectonic structure dips ~20° to the NE and runs parallel to the NW-SE trending Kurgiakh valley. It is within this narrow zone that one can observe the very rapid, though progressive, decrease in metamorphic grade between the TH and the underlying HHCS.

The footwall of the ZSZ is formed by rocks belonging to the top of the HHCS. These rocks are of high metamorphic grade and form the jagged relief of the Great Himalayan Range with its numerous glaciers and high summits often reaching over 6000 meters. In the studied area, the very top of the HHCS is characterized by the presence of a leucogranitic intrusion complex, forming a ~1 kilometre thick belt of leucogranitic plutons directly below the lower boundary of the ZSZ.

Above the leucogranitic plutons, the Zanskar Shear Zone marks the transition between the HHCS and TH. From base to top, the metapelites within the ZSZ show a reduced, but complete, succession of kyanite to biotite zone mineral assemblages, indicating a rapid upward decrease in metamorphic conditions.

The hanging wall of the Zanskar Shear Zone is formed by sedimentary series belonging to the base of the Tethys Himalaya (Karsha or Phe Formations). These sedimentary series are generally only weakly metamorphosed and form a much smoother topography.

The metamorphic evolution of the Zanskar region was already the object of several studies. Most of these studies where essentially concentrated on the high-grade metamorphic rocks of the High Himalayan Crystalline Sequence (Choudhuri, 1987; Dèzes et al. 1999; Epard et al., 1995; Fuchs, 1987; Gapais et al. 1992; Gilbert, 1986; Guntli, 1993; Herren, 1987; Jain and Manickavasagam, 1993: Kündig, 1988, 1989; Patel et al., 1993; Pognante, 1992, 1993; Pognante and Lombardo, 1989; Pognante et al., 1987, 1990; Routh, 1993; Searle and Rex, 1989; Searle et al. 1992; Spring, 1993; Steck et al., 1993; Vannay, 1993; Vannay and Steck, 1995). Fewer studies dealt with the low-grade metamorphism in the Tethys Himalaya (Fuchs and Linner, 1995; Garzanti and Brignoli, 1989; Spring, 1993; Spring et al., 1993; Steck et al., 1993, 1998).

Three successive Himalayan metamorphic events can be individualized in the studied region, The two first events are associated with crustal thickening and are of prograde regional metamorphic type. The last metamorphic event is associated with isothermal exhumation and is retrograde:

The first prograde regional metamorphism (M1) is only observed in the southwestern part of the studied area where it reaches greenschist facies conditions. This metamorphism is most probably related to the crustal thickening associated with the early NE vergent Shikar Beh nappe (D1).

The second prograde regional metamorphism (M2) is observed over the entire studied area and is characterized by a complete Barrovian metamorphic zonation ranging from anchizone in the Tethys Himalaya to sillimanite + k-feldspar zone in the lowermost part of the HHCS. This zonation which also corresponds to the gradual transition from lower greenschist to lower granulite facies, is marked by the progressive apparition of all the Barrovian index minerals. As these minerals are generally well distinguishable, even with the naked eye, a good estimation of the spatial distribution of the metamorphic mineral zone boundaries can already be established on the field. The increasing metamorphic grade is also well evidenced by the progressive transformation of the Tethyan Himalaya siltitic and arkosic sediments into the HHCS slates, schists, gneisses and migmatites. This progressive transformation of the low-grade metasediments into high-grade metamorphic rocks occurs over a surprisingly short distance which coincides with the ZSZ. The metamorphic event M2 is related to crustal thickening and underthrusting associated with the SW vergent Nyimaling-Tsarap nappe during D2 and D3.

The last metamorphic event (M3) is only clearly observed within the medium- to high-grade rocks of the High Himalayan Crystalline Sequence. M3 is marked by the apparition of low-pressure, high- to medium-temperature metamorphic minerals. These minerals grew as a consequence of retrograde re-equilibration of the high-grade M2 mineral assemblages. M3 is characterized by isothermal decompression associated with the rapid tectonic exhumation of the HHCS from below the TH during D4.