The core of the Greater Himalayan Sequence in the Mugu-Karnali area (Western Nepal) is affected by a thick shear zone with development of nearly 4 km of mylonites (Mangri shear zone). It is a contractional shear zone showing a top-to-the-SW and WSW sense of shear. The shear zone developed during the decompression, in the sillimanite stability field, of rocks that previously underwent relatively high-pressure metamorphism deformed under the kyanite stability field. P-T conditions indicate that the footwall experienced higher pressure (1.0-0.9 GPa) than the hanging wall (0.7 GPa) and similar temperatures (675°-700 °C). U-Pb in-situ dating of monazites indicate a continuous activity of the shear zone between 25 and 18 Ma. Samples from the lower part of the Greater Himalayan Sequence underwent similar ductile shearing at ~ 17-13 Ma. These ages and the associated P-T-t paths revealed that peak metamorphic conditions were reached ~ 5-7 Ma later in the footwall of the shear zone with respect to the hanging-wall pointing to a diachroneity in the metamorphism triggered by the shear zone itself. Mangri Shear Zone, with the other recently documented tectonic and metamorphic discontinuities within the Greater Himalayan Sequence, point to the occurrence of a regional tectonic feature, the High Himalayan Discontinuity, running for more than 500 km along the strike of the Central Himalayas. It was responsible of the exhumation of the upper part of the Greater Himalayan Sequence starting from 28 Ma, well before the activation of the Main Central Thrust and the South Tibetan Detachment. Our data point out that exhumation of the Greater Himalayan Sequence was partitioned in space and time and different slices were exhumed in different times, starting from the older in the upper part to the younger in the lower one

Kyanite-bearing migmatitic paragneiss of the lower Greater Himalayan Sequence (GHS) in the Kali Gandaki transect (Central Himalaya) was investigated. In spite of the intense shearing, it was still possible to obtain many fundamental information for understanding the processes active during orogenesis. Using a multidis- ciplinary approach, including careful meso- and microstructural observations, pseudosection modelling (with PERPLE_X), trace element thermobarometry and in situ monazite U–Th–Pb geochronology, we constrained the pressure–temperature–time–deformation path of the studied rock, located in a structural key position. The migmatitic gneiss has experienced protracted prograde metamorphism after the India–Asia colli- sion (50–55 Ma) from ~43 Ma to 28 Ma. During the late phase (36–28 Ma) of this metamorphism, the gneiss underwent high-pressure melting at “near peak” conditions (710–720 °C/1.0–1.1 GPa) leading to kyanite- bearing leucosome formation. In the time span of 25–18 Ma, the rock experienced decompression and cooling associated with pervasive shearing reaching P–T conditions of 650–670 °C and 0.7–0.8 GPa, near the sillimanite–kyanite transition. This time span is somewhat older than previously reported for this event in the study area. During this stage, additional, but very little melt was produced. Taking the migmatitic gneiss as representative of the GHS, these data demonstrate that this unit underwent crustal melting at about 1 GPa in the Eocene–Early Oligocene, well before the widely accepted Miocene decompressional melting related to its extrusion. In general, kyanite-bearing migmatite, as reported here, could be linked to the production of the high-Ca granitic melts found along the Himalayan belt

The Greater Himalayan Sequence (GHS) is one of the major tectonic units of the Himalaya\ud running for more than 2400 km along-strike. It has been considered as a coherent tectonic unit\ud bound by the South Tibetan Detachment (STD) and the Main Central Thrust (MCT). However,\ud thrusts within it have been recognized in several places and have been mainly interpreted as\ud out-of-sequence thrusts being active after the main phase of exhumation of the crystalline unit\ud after the MCT activated. Recent integrated studies allow the recognition of several ductile shear\ud zones in the core of the GHS, with top-to-the-SW-sense of shear (Higher Himalayan Discontinuity\ud (HHD)). U–Th–Pb in situ monazite ages provide ages older than the MCT. Data on pressure and\ud temperature evolution testify that these shear zones affected the tectonometamorphic evolution of\ud the belt and different pressure and temperature conditions were recorded in the hanging wall and\ud footwall of the HHD. The correlation of the WNW–ESE-trending HHD with other discontinuities\ud recognized in the GHS led to the proposal that it is a tectonic feature running for several hundred\ud kilometres, documented at the regional scale dividing the GHS in two different portions

In the Kali Gandaki valley (central Nepal), a ductile, high-temperature, contractional shear zone with a top-to-the-SW sense of shear, known as Kalopani Shear zone (KSZ), is located within the uppermost part of the Greater Himalayan Sequence (GHS). We mapped and investigated this shear zone in in detail, in order to unravel its age and role in the evolution of the GHS. Pseudosection modeling and inverse geothermobarometry reveal that rocks involved in the KSZ experienced pressure-temperature conditions between 0.6-0.85 GPa and 600-660°C. U-Th-Pb in-situ LA-ICP-MS and SHRIMP dating on monazite point to retrograde metamorphism related to the KSZ starting from ~ 41-30 Ma. The kinematics of the KSZ and associated erosion and/or tectonics, caused the Middle-Late Eocene exhumation of the GHS in the hanging wall of the KSZ zone at least nine million years before the activities of the High Himalayan Discontinuity, the Main Central Thrust, and the South Tibetan Detachment. Structural data, metamorphic conditions and geochronology from the KSZ, compared to those of other major tectonic discontinuities active within the GHS in the Kali Gandaki valley, indicate that shear deformation and exhumation were not synchronous but migrated downward and southward at different lower levels within the GHS. These processes caused the exhumation of the hanging-wall rocks of the activated shear zones. The main consequence of this tectonic is that exhumation was driven by an insequence shearing mechanism progressively involving new slices of the Indian crust and not solely by the coupled activity of Main Central Thrust and South Tibetan Detachment.

The largest crystalline unit representing the mid-crust in the Himalayan belt is the Greater Himalayan Sequence (GHS) which stretches all over the 2400 km of length of the belt. The GHS, recognised since the first geological explorations of the Himalayas, has been considered for a long time as a coherent tectonic unit, exhumed by the contemporaneous shearing along the Main Central Thrust and the South Tibetan Detachment System in the time span ~ 25-17 Ma. A multidisciplinary approach, integrating geological mapping, structural analysis, petrology and geochronology allowed to better constraints on its internal architecture characterised by several levels of tectonic-metamorphic discontinuities on the regional scale with a top-to-the-S/SW sense of shear and active since ~ 40 Ma. The GHS is consequently divided in three main tectonic units exhumed progressively from the upper part to the lower one by ductile shear zones, later involving the Lesser Himalayan Sequence. Above the Main Central Thrust a cryptic tectono-metamorphic discontinuity (Higher Himalayan Discontinuity; HHD) has been recognized and mapped in Central-Eastern Himalaya. The mapping of the HHD has been allowed by the use of a multidisciplinary approach involving structural analysis, geochronology and petrography. A new map of Western Nepal is presented. In this framework the popular models of exhumation of the GHS mainly based on the contemporaneous activity of the two bounding shear zones (Main Central Thrust and the South Tibetan Detachment) and considering the GHS as a coherent tectonic unit, should be reconsidered. An in-sequence shearing tectonic model, from the deeper to the upper structural levels, further affected by out-of-sequence-thrusts, is more appropriate to explain the deformation, metamorphism and exhumation of the mid-crust in the Himalayan belt. Geological mapping of the Himalayan belt is very far away to be exhaustively completed. Anyway during the last 20, and particularly during the last few years, it has been notably improved due to a new multidisciplinary approach.