ACTIVE EXHUMATION OF A HIGH-GRADE ASYMMETRIC DOME IN THE SUTLEJ VALLEY (NW-HIMALAYAS, INDIA)

The crystalline zone of the Himalayan orogen exposed along the Sutlej Valley consists of two high-grade metamorphic cores, which are separated by major shear zones and which have a completely different tectonometamorphic evolution: (i) The upper unit corresponds to the High Himalayan Crystalline Sequence (HHCS), composed mainly of amphibolite facies to migmatitic paragneisses revealing an inverted metamorphic gradient. The HHCS is bounded at its base by the Main Central Thrust, and it is separated from the overlying low-grade sediments of the Tethyan Himalaya by the extensional South Tibetan Detachment System. The HHCS core exhumed to upper crustal levels in the Early Miocene. (ii) A second metamorphic dome, the Lesser Himalayan Crystalline Sequence (LHCS), of Lesser Himalayan affinity mainly consists of amphibolite facies orthogneisses derived from an Early Proterozoic granites and is bounded by the Munsiari Thrust at its base, and by the extensional Karcham Normal Fault (KNF) at its top, cross-cutting the mylonites of the Main Central Thrust.

Structural results indicate that the dominant ductile fabric in the HHCS reflect the exhumation of the HHCS through combined thrusting along the Main Central Thrust and extension along the South Tibetan Detachment System. The LHCS is most likely exhuming by a similar tectonic mechanism but exhumation controlled by combined out-of-sequence thrusting along the Munsiari Thrust and extension at the level of the Karcham Normal Fault is in the field of the brittle-ductile transition zone and brittle cataclastic deformation. In contrast to the Miocene cooling ages of the HHCS, the LHCS rocks of the Sutlej section record a significantly younger cooling and higher cooling rate, as they are characterized by muscovite Ar/Ar ages at 6.6-4.3 Ma, as well as by zircon and apatite FT ages at 2.7-1.7 Ma and 0.9-0.7 Ma. Exhumation of the LHCS, which actually forms a huge asymmetric SW vergent dome is still active. It appears consequently that following the exhumation of the HHCS controlled by the activation of the Main Central Thrust and South Tibetan Detachment System during Early Miocene, a new phase of comparable tectonically-controlled exhumation of high-grade metamorphic rocks is currently occurring in the active, frontal part of the Himalayan orogen.