2.5 Purposes of the present work

This work fits in with the geological studies that were undertaken since 1979 by the University of Lausanne in the northwestern Himalaya. A complete section (Steck et al., 1993a and b) from the Indus-Yarlung Suture Zone in Ladakh to the Main Central Thrust in Himachal Pradesh was the result of several geological traverses and PhD works by this team (Stutz, 1988; Spring 1993 and Vannay, 1993). More to the west, the Ladakh, Suru, Zanskar and Kishtwar regions were the subject of several PhD works undertaken since 1978 by researchers from the University of Zurich (Honegger, 1982; Herren, 1987; Kündig, 1988, Stäubli, 1988, Guntli, 1993). There is, however, a lateral gap between the areas covered by these two research groups. This study fills this gap for the Zanskar region and in particular between the area covered by the PhD Work of Spring (1993) in the Sarchu region and the region studied by Herren (1987) along the Doda River in Zanskar.

Given the outstanding quality of the outcrops in Zanskar, this area represents one of the key localities to study the influence of extension tectonics along the Zanskar Shear Zone on the tectonic, magmatic and metamorphic evolution of the Greater Himalayan Domain.

A large part of this study was devoted to fieldwork. During the six months that lasted our three geological expeditions, we could establish a geological map of the studied region, using essentially Spot satellite images as topographic background. The mapped zone covers an area of about one thousand square kilometres at an altitude between 3500 and 6000 meters.

Structural observations were essentially focused on the transition zone between the HHCS and the TH in order to study the tectonic evolution of the Zanskar Shear Zone during the Himalayan orogen.

Petrographic observations were also an important aspect of fieldwork. A large amount of information on the metamorphic and magmatic events associated with the tectonic evolution of this area could be gathered from macroscopic observations.

The stratigraphic analyses of the sedimentary formations of the Tethys Himalaya is also part of the present work, although we did not spend much time on this aspect of the Himalayan geology since quite a lot of excellent work has already been done on this subject by previous researchers.

The second part of this study was dedicated to microscopic observations and laboratory work. A total of 252 thin sections were examined. The study of metamorphic mineral assemblages and microtextural porphyroblast-matrix relations under the microscope contributed greatly to the interpretation of the tectonic and metamorphic history of Zanskar. This optical study was completed by several analytical methods to obtain quantitative data. These methods include U/Pb and Ar/Ar geochronology, microprobe analyses, cationic and net transfer thermobarometry, isotopic thermometry and illite crystallinity.

The main purpose of the present work is to provide quantitative constraints on the age, displacement and slip rates along the Zanskar Shear Zone, on the bases of geochronological and geothermobarometrical data. Although we believe to have answered these questions in a rather satisfying manner, several other questions emerged in the course of this study:

• What role did the Zanskar Shear Zone play in the metamorphic and structural evolution of this part of the Greater Himalayan Domain?
• Why are the sedimentary rocks of the HHCS metamorphosed while their equivalents in the Tethys Himalaya are only weakly altered?
• Why is the Barrovian metamorphic zonation in the transition zone so condensed, and was this metamorphic event overprinted by a later metamorphism?
• How are the brittle normal faults in the Tethys Himalaya (e.g. the Sarchu fault) related to the Zanskar Shear Zone?
• Is the Zanskar Shear Zone correlated in time with other tectonic structures in the Himalaya?
• How are the leucogranites related to the Zanskar Shear Zone, and was the metamorphic gradient within the migmatitic zone sufficient to produce these granitic melts?
• How does a synorogenic extensional structure as the Zanskar Shear Zone form within a globally compressive system, such as the Himalaya?
• Are there Pre-Himalayan structures preserved in the Central Himalayan Domain, what are they and to what event can they be attributed?
• Why do most of the Paleozoic formations thin out and disappear towards the northwestern part of Zanskar?
• How does the geology of southeastern Zanskar correlate with adjacent regions?

We will deal with these questions throughout this work, but this monograph is not written such as to treat each of them individually. We rather tried to structure this monograph such as to tell a coherent, and if possible, linear "story". In this introductory chapter we have given a general overview of the geodynamic settings leading to the formation of the Himalaya, as well as a description of the geologic framework of the NW Himalaya. In the following chapter, we describe the sedimentary formations that were deposited on the northern margin of the Indian continent, for they make up most of the studied area. In chapter 4, we will outline the structural geology of the northwestern Himalaya and more specifically, we will see what deformation phases are observed in southeast Zanskar. Chapter 5 describes the metamorphic history of the studied area, which is a direct consequence of its structural evolution. In chapter 6 we will see how magmatic rocks formed from the sedimentary protolith, as a result of combined high-grade metamorphism and rapid tectonic uplift. In Chapter 7, we will use the data obtained from the metamorphic and magmatic rocks to constrain the age and amount of shear along the Zanskar Shear Zone. In Chapter 8, our datas and observations are confronted to several models proposed to explain synorogenic extension. Finally, in chapter 9, we will make a short recapitulation of the main points covered by this work.