In-sequence shearing within the Greater Himalayan Sequence in Central Himalaya: deformation and metamoprhism by crustal accretion from the Indian plate

The Greater Himalayan Sequence (GHS) is the main metamorphic unit of the Himalayas, stretching 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 (Godin et al., 2006). Several shear zones have been recognized within the GHS, usually regarded as out of sequence thrusts. In the GHS in Central Himalaya we identified a tectonic and metamorphic discontinuity, above the MCT, with a top-to-the SW sense of shear (Higher Himalayan Discontinuity: HHD) (Carosi et al., 2010; Montomoli et al., 2014). 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 HHD zones 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 main tectonic feature running for several hundreds kilometers, documented at the regional scale and dividing the GHS in two different portions. The occurrence of even more structurally higher contractional shear zone in the GHS (above the HHD): the Kalopani shear zone in the Kali Gandaki valley, points out to an even more complex deformation pattern within the metamorphic core. Rcent U-Th-Pb in situ monazite ages of the Kalopani s.z. point to a ~ 36-38 Ma age of the exumation of the higher portion of the GHS. The most popular models for the exhumation of the GHS (i.e extrusion and channel flow), based on the MCT and STD contemporaneous activities, are not able to explain the occurrence of the HHD and of other older in-sequence shear zones. Any model of the tectonic and metamorphic evolution of the GHS should account for the occurrence of the tectonic and metamorphic discontinuities within the GHS and its consequences on the metamorphic paths and on the assembly of Himalayan belt.