DEFORMATION WITHIN THE GREATER HIMALAYAN SEQUENCE IN CENTRAL HIMALAYA: IN-SEQUENCE SHEARING AND METAMORPHISM BY CRUSTAL ACCRETION FROM THE INDIAN PLATE

The Greater Himalayan Sequence (GHS) is the main metamorphic unit of the Himalayas, stretched for over 2400 km, bounded to the South by the Main Central Thrust (MCT) and to the North by the South Tibetan Detachment (STD) whose contemporanous activity controlled its exhumation between 23 and 17 Ma. Several shear zones and/or faults have been recognized within the GHS, usually regarded as out of sequence thrusts. Recent investigations in the GHS in Central Himalaya allowed to identify a tectonic and metamorphic discontinuity, above the MCT, with a top-to-the SW sense of shear (High Himalayan Discontinuity: HHD). U-(Th)-Pb in situ monazite ages provide temporal constraint of initiation of the HHD at 27-25 Ma, older than the Main Central Thrust, and continuing up to 17 Ma. Data on the P and T evolution testify that this shear zone affected the tectono-metamorphic evolution of the belt and different P and T conditions have been recorded in the hanging-wall and footwall of the HHD. The correlation of the HHD with several other discontinuities recognized in the GHS led to propose that it is a tectonic feature running for several hundreds kilometers, documented at the regional scale and dividing the GHS in two different portions.

In Western and Central Nepal the occurrence of even more structurally higher contractional shear zone in the GHS (above the HHD): the Tyar shear zone (Mugu-Karnali valley) and the Kalopani shear zone ( Kali Gandaki valley), points out to a more complex deformation pattern within the metamorphic core.

These recent findings suggest that GHS is build up by the progressive accretion of Indian crustal slices since the Eocene-Oligocene. The GHS is made up by several crustal slices showing younging deformation and metamorphism from the upper one to the lower one.