The main geological features of the Himalayan orogen are described in the region between the Khimti Khola and the Dudh Khosi valleys (eastern-central Nepal). The study on the basis of geological mapping, structural and petrographic analysis still in progress. In the investigated area, the Greater Himalayan Sequence (GHS) may be divided in two different domains basing on lithological, structural and metamorphic criteria. The lower domain (GHS-L) mainly consists of a metasedimentary sequence recording an increase in metamorphic grade upward, passing
from the staurolite zone to the sillimanite zone and, locally, to anatexis. The upper domain (GHS-U, corresponding to the Higher Himalayan Crystallines of much literature) is characterized by high-grade metamorphic rocks, often anatectic, recording a progressive decrease in peak-pressure structurally upward. The tectonic juxtaposition of the highgrade, anatectic rocks of the GHS-U over the low-grade metasedimentary rocks of the Lesser Himalaya Sequence is accommodated by the Main Central Thrust Zone, roughly centred on the GHS-L. This major boundary
zone identifies a several km-thick shear zone characterized by a top-tosouth ductile shearing, responsible for the pervasive development of mylonitic foliations, mylonites, mesoscopic shear zones and for the tectonic juxtaposition of slices of different lithologies (e.g. phyllonite, augen-gneiss, two-micas paragneiss and micaschist, slate, quartzite, marble and Ca-silicate granofels).
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