Strain softening mechanisms in a regional scale shear zone: the Main Central Thrust Zone in the Mt. Makalu area (Eastern Nepal)

Deformation in mountain belt at middle/deep crustal levels is quite often heterogeneous and localized in high-strain zone, i.e. ductile shear zones, with the development of mylonites. One of the largest worldwide shear zones is exposed in the Himalaya where the Main Central Thrust Zone (MCTZ), a km thick ductile (to brittle) shear zone, tectonically juxtaposes the medium- to high grade metamorphic rocks of the Greater Himalaya Sequence structurally above the lowto medium- grade metamorphic rocks of the Lesser Himalaya Sequence (Hodges, 2000). In this contribution we investigate the processes active during the MCTZ evolution in the Makalu area (Eastern Nepal) where Precambrian orthogneiss, locally known as Num-orthogneiss, are ductile sheared and structural analysis testifies how a progressive transformation from orthogneiss to micaschist, trough a transitional zone, is present. Quantitative textural evolution, bulk-rock major and trace element chemical changes and mineral phases chemical changes have been characterized studying representative samples from “pristine” orthogneiss, transitional zone and micaschist. Kinematic indicators developed both at the meso and microscales, such as S-C fabric, rotated porphyroclasts and mica fishes point to a top-to the-south sense of shear. Kinematic indicators are progressively much more developed associated with changes of foliation morphology (from a spaced anastomosing schistosity to a continuous) starting from orthogneisses to micaschist. ICP-MS trace elements data confirm the common protolith origin of the studied rocks. Quantitative modal phase variations have been investigated with image analysis techniques (on BSE-SEM images) pointing to a progressive disappearance of feldspars balanced by increasing of micas. This latter process is assisted by progressive increasing of XMg in white micas and of a transition from biotite to phlogopite. Major and trace elements comparison between less deformed rocks (“pristine orthogneiss”) and higher deformed ones (micaschist) have been computed using the Isocon method (Grant, 2005). In summary, microstructural and geochemical analyses led to recognize a non-isochemical process of strain softening related to phase transformation (feldspar replaced by micas) enhanced by fluids infiltration and Mg metasomatism, which progressively favoured deformation localization within the MCTZ.