INVERSE METAMORPHIC ZONATION IN VERY LOW-GRADE TIBETAN ZONE SERIES OFSE ZANSKAR AND ITS TECTONIC CONSEQUENCES (NW INDIA, HIMALAYA)

The metamorphism of the carbonate rocks of the SE Zanskar Tibetan zone has been studied by ''illite crystallinity'' and calcite-dolomite the rmometry. The epizonal Zangla unit overlies the anchizonal Chumik unit . This discontinuous inverse zonation demonstrates a late to post-meta morphic thrust of the first unit over the second. The studied area und erwent a complex tectonic history: - The tectonic units were stacked f rom the NE to the SW, generating recumbent folds, NE dipping thrusts a nd the regional metamorphism. The compressive movements were active un der lower temperature conditions, resulting in late thrusts that distu rbed the metamorphic zonation. The discontinuous inverse metamorphic z onation dates from this phase. - A NE vergent backfolding phase occurr ed at lower temperature conditions. It caused the uplift of more metam orphic levels. - A late extensional phase is revealed by the presence of NE dipping low angle normal faults, and a major high angle fault, t he Sarchu fault. The low angle normal faults locally run along earlier thrusts (composite tectonic contacts). Their throw has been sufficien t to reset a normal stratigraphic superposition (young layers overlyin g old ones), but insufficient to erase the inverse metamorphic relatio nship. However, the combined action of backfolding and normal faulting can locally lessen, or even cancel, the inverse metamorphic superposi tion. After deduction of the normal fault translation, the vertical co mponent of the original thrust displacement through stratigraphy is 40 0 m, which is a value far too low to explain the temperature differenc e between the two units. The horizontal component of displacement is t herefore far more important than the vertical one. The regional distri bution of metamorphism within the Zangla unit points out to an anchizo nal front and an epizonal inner part. This fact is in agreement with n appe tectonics.