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

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[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

A transpressive crustal-scale dextral shear zone is documented in the Variscan Basement of northeastern Sardinia. It indicates the presence of a shear deformation parallel to the belt overprinting previous D1 structures related to nappe stacking and top-to-the-S and -SW thrusting. The L2 stretching lineation points to an orogen-parallel stretching and to a general change in the tectonic transport from D1 to D2. Phase D1 developed during initial frontal collision, whereas the D2 deformation was characterized by dextral shearing during the increasing curvature of the Ibero-Armorican arc. Transpressional deformation developed in a regime of decreasing pressure. It caused telescoping of the Barrovian isograds and the exhumation of the low- to medium-grade metamorphic rocks. In this sector of the Variscan belt, exhumation is due to continuing compression with an increasing component of horizontal displacement. The overall change of the shortening direction in a large sector of an orogenic belt, with the occurrence of increasing orogen-parallel displacement, may be regarded as a general mechanism affecting the exhumation of rocks and preventing the thickened collisional crust from undergoing a generalized gravitational collapse.