Tectonometamorphic discontinuities within the Greater Himalayan Sequence in Western Nepal (Central Himalaya):Insights on the exhumation of crystalline rocks.

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.