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

[1] A high-temperature shear zone, Toijem shear zone, with a top-to-the-SW sense of shear affects the core of the Higher Himalayan Crystallines (HHC) in western Nepal. 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. PT conditions indicate that the footwall experienced higher pressure (∼9 kbar) than the hanging wall (∼7 kbar) and similar temperatures (675°-700°C). Monazite growth constrains the initial activity of the shear zone at 25.8 ± 0.3 Ma, before the onset of the Main Central Thrust zone, whereas the late intrusion of a crosscutting granitic dike at 17 ± 0.2 Ma limits its final activity. Monazites in kyanite-bearing gneisses from the footwall record prograde metamorphism in the HHC from ∼43 to 33 Ma. The new data confirm that exhumation of the HHC started earlier in western Nepal than in other portions of the belt and before the activity of both the South Tibetan Detachment System (STDS) and Main Central Thrust (MCT) zones. As a consequence, western Nepal represents a key area where the channel-flow-driven mechanism of exhumation, supposed to be active from Bhutan to central-eastern Nepal, does terminate. In this area, exhumation of crystalline units occurred by foreland propagation of ductile and, subsequently, brittle deformation. Citation: Carosi, R., C. Montomoli, D. Rubatto, and D. Visonà (2010), Late Oligocene high-temperature shear zones in the core of the Higher Himalayan Crystallines (Lower Dolpo, western Nepal), Tectonics, 29, TC4029,

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

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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.

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Anatectic melt inclusions (nanogranites and nanotonalites) have been found in garnet\ud of kyanite-gneiss at the bottom of the Greater Himalayan Sequence (GHS) along the Kali\ud Gandaki valley, central Nepal, c. 1 km structurally above the Main Central Thrust (MCT). In\ud situ U–Th–Pb dating of monazite included in garnets, in the same structural positions as melt\ud inclusions, allowed us to constrain partial melting starting at c. 41–36 Ma. Eocene partial\ud melting occurred during prograde metamorphism in the kyanite stability field (Eo-Himalayan\ud event). Sillimanite-bearing mylonitic foliation wraps around garnets showing a top-to-the-SW\ud sense of shear linked to the MCT ductile activity and to the exhumation of the GHS. These findings\ud highlight the occurrence of an older melting event in the GHS during prograde metamorphism in\ud the kyanite stability field before the more diffuse Miocene melting event.\ud The growth of prograde garnet and kyanite at 41–6 Ma in the MCT zone, affecting the bottom of\ud the GHS, suggests that inverted metamorphism in the MCT zone and folded isograds in the GHS\ud should be carefully proved with the aid of geochronology, because not all Barrovian minerals grew\ud during the same time span and they grew in different tectonic settings

The most popular models regarding the exhumation of the Greater Himalayan Sequence (GHS), such as extrusion, channel flow, critical taper and wedge extrusion, require prolonged activity of the two bounding shear zones and faults, the Main Central Thrust (MCT) and the South Tibetan Detachment (STD). We present the crystallization age of an undeformed leucogranite that intrudes both the GHS and the Tethyan Himalaya Sequence (THS). Zircon and monazite U‐Pb ages in the leucogranite give ages between 23 and 25 Ma constraining, at that time, the end of shearing along the STD. Our results limit the contemporaneous activity of the MCT and STD to a short period of time (~1–2 Ma) and thus argue against exhumation models requiring prolonged contemporaneous activity of the MCT and STD.