The metamorphism of the carbonate rocks of the SE Zanskar Tibetan zone
has been studied by `'illite crystallinity'' and calcite-dolomite
thermometry. The epizonal Zangla unit overlies the anchizonal Chumik
unit. This discontinuous inverse zonation demonstrates a late to
post-metamorphic thrust of the first unit over the second.
The studied area underwent a complex tectonic history: - The tectonic
units were stacked from the NE to the SW, generating recumbent folds, NE
dipping thrusts and the regional metamorphism. The compressive movements
were active under lower temperature conditions, resulting in late
thrusts that disturbed the metamorphic zonation. The discontinuous
inverse metamorphic zonation dates from this phase. - A NE vergent
backfolding phase occurred at lower temperature conditions. It caused
the uplift of more metamorphic levels. - A late extensional phase is
revealed by the presence of NE dipping low angle normal faults, and a
major high angle fault, the Sarchu fault.
The low angle normal faults locally run along earlier thrusts (composite
tectonic contacts). Their throw has been sufficient to reset a normal
stratigraphic superposition (young layers overlying old ones), but
insufficient to erase the inverse metamorphic relationship. However, the
combined action of backfolding and normal faulting can locally lessen,
or even cancel, the inverse metamorphic superposition. After deduction
of the normal fault translation, the vertical component of the original
thrust displacement through stratigraphy is 400 m, which is a value far
too low to explain the temperature difference between the two units. The
horizontal component of displacement is therefore far more important
than the vertical one. The regional distribution of metamorphism within
the Zangla unit points out to an anchizonal front and an epizonal inner
part. This fact is in agreement with nappe tectonics.