The Crystalline Nappe of the High Himalayan Crystalline has been
examined along the Kulu Valley and its vicinity (Mandi-Khoksar
transect). This nappe was believed to have undergone deformation related
only to its transport towards the SW essentially during the `'Main
Central Thrust event''. New data has led to the conclusion that during
the Himalayan orogeny, two distinctive phases, related to two opposite
transport directions, characterize the evolution of this part of the
chain, before the creation of the late NE-vergent backfolding.
The first phase corresponds to an early NE-vergent folding and
thrusting, creating the Tandi Syncline and the NE-oriented Shikar Beh
Nappe stack, with a displacement amplitude of about 50 km. Two
schistosities, together with a strong stretching lineation are developed
at a deep tectonic level under amphibolite facies conditions
(kyanite-staurolite-garnet-two mica schists). At a higher tectonic level
and in the southern part of the section (Tandy Syncline and southern
Kulu Valley between Kulu and Mandi) one or two schistosities are
developed in the greenschist facies grade rocks (garnet-biotite and
biotite schists). These structures and the associated Barrovian type
metamorphism are all related to the NE-verging Shikar Beh Nappe. The
creation of the NE-verging Shikar Beh Nappe may be explained by the
reactivation of a SW dipping listric normal fault of the N Indian
flexural passive margin, during the early stages of the Himalayan
orogeny.
In the second phase, the still hot metamorphic rocks of the Shikar Beh
Nappe were folded and thrust towards the SW (mainly along the MBT and
the MCT with a displacement in excess of 100 km) onto the cold,
low-grade metamorphic rocks of the Larji-Kulu-Rampur Window or, near
Mandi, on the non-metamorphic sandstones of the Ganges Molasse
(Siwaliks). Sense of shear criteria and a strong NE-SW
stretching-lineation indicate that the Crystalline Nappe has been
overthrusted towards the SW. Thermometry on synkinematically
crystallised garnet-biotite and garnet-hornblende pairs reveals the
lower amphibolite facies temperature conditions related to the
Crystalline Nappe formation.
From the muscovite and biotite Rb-Sr cooling ages, the Shikar Beh Nappe
emplacement occurred before 32 Ma and the southwestward thrusting of the
Crystalline Nappe began before 21 Ma. Our model involving two opposite
directions of thrusting goes against the conventional idea of only one
main SW-oriented transport direction in the High Himalayan Crystalline
Nappes.