5.3 The prograde regional metamorphism M2

5.3.1 Introduction

The metamorphic grade associated with the second tectonometamorphic event M2 progressively increases from the Tethys Himalaya towards the HHCS. In the studied area, the TH is characterized by slightly downwards increasing low-grade metamorphic conditions (anchizone to epizone). An exception to this rule is observed in the easternmost part of the studied area, where the metasedimentary series at the base of the Tethys Himalaya have reached lower amphibolite facies in the footwall of the Sarchu Fault. Along the Kurgiakh valley, the base of the TH forms the hanging-wall of the ZSZ and is systematically characterized by chlorite zone grade metasediments. The metamorphic grade then increases progressively, but very quickly, across the Zanskar Shear Zone from biotite grade at its top to kyanite grade at its bottom. The ZSZ thus marks the transition between the low-grade metasediments of the TH and the high-grade rocks of the HHCS. This progressive increase in metamorphic grade between the TH and the HHCS across the ZSZ is beautifully illustrated in the studied area along the Gianbul valley (Fig 5.8). This valley offers a natural cross-section through the base of the TH and the HHCS and allows to study optimally the progressive increase in metamorphic grade between these two domains. It is also along the Gianbul valley that we could reach the lowermost structural units of the HHCS, where the highest grade rocks of the studied area are exposed.

The following description of the metamorphic zonation associated with M2 starts with the low-grade metamorphism of the Tethys Himalaya with its lateral and vertical variations. We will then describe the prograde metamorphic zonation between the TH and the HHCS as observed across the ZSZ along the Kurgiakh valley and end this description with the highest grade migmatitic zone of the HHCS exposed in the Gianbul valley.