TECTONO-METAMORPHIC EVOLUTION OF THE GREATER HIMALAYAN SEQUENCE (GHS) IN WESTERN AND CENTRAL NEPAL (CENTRAL HIMALAYA): INSIGHTS ON THE EXHUMATION OF DEEP-SEATED CRUSTAL ROCKS

The Greater Himalayan Sequence (GHS) is the exhumed metamorphic core of the Himalayan Belt. This tectono‐metamorphic unit is bounded by two regional scale ductile‐to brittle shear zones, active since the Miocene: the basal “thrust sense” Main Central Thrust (MCT) and the “normal sense” South Tibetan Detachment System (STDS) at the top. In this contribution structural, metamorphic and geochronological evolution of the GHS is investigated along two geological transects in the Nepalese Himalaya (Central Himalaya), which are: (i) the Mugu Karnali Valley, in Western Nepal, where very few informations are present in the geological literature; and (ii) the Kali Gandaki Valley, in Central Nepal, where the main geological features are much better known. Detailed pressure (P) – temperature (T) – deformation (D) – time (t) paths have been gained along both transects, combining meso‐ and micro‐structural analyses, forward and inverse metamorphic petrology methods (e.g. pseudosections and geothermobarometry) and in situ U‐Th‐Pb monazite geochronology. Field as well as microstructural observations permit to identify P‐T‐t‐D discontinuities within the GHS, in both transects, where high‐temperatures (HT) contractional (top‐to‐the S/SW) shear zone are developed. The findings of P‐T‐t‐D discontinuities testify how the GHS has a much more complex internal architecture, respect to a tectonic unit bounded by two faults. Geochronological data, moreover, points out how these discontinuities are older than the MCT along the same structural transects. Detailed compilation of geological literature supports how these findings seems a regional feature, at least in Central Himalaya. These data, collectively, suggest that exhumation of the GHS didn't occur at the same time everywhere, but it was partitioned in space and time and where different slices were exhumed in different times, starting from the older in the upper part to the younger in the lower one. These slices were tectonically juxtaposed deep in the crust, by the progressive propagations towards the foreland of ductile contractional shear zones.