Tectonostratigraphy, deformation, and metamorphism of the Himalayan mid-crust exposed in the Likhu Khola region, east-central Nepal

Detailed field study of the Likhu Khola region in east-central Nepal has provided the basis for new lithological, metamorphic, and structural interpretations of the region. Metamorphic mineral assemblages define an inverted metamorphic field gradient from middle greenschist facies rocks (micaceous phyllitic schist) in the structurally lowest levels observed to upper amphibolite facies migmatitic rocks (sillimanite migmatite) in the structurally highest levels observed in this study. Quartz textures and c -axis orientations indicate that deformational temperatures also generally increase upward in the structural section, ranging from ∼490 °C to >650 °C. These temperatures are compatible with observed metamorphic mineral assemblages at lower structural levels, indicating that deformation and metamorphism may have been contemporaneous. All rocks in the mapped area are pervasively deformed and typically record a top-to-the-south sense of shear; no discrete large-scale thrust or normal-sense structures were observed. The deformational temperatures recorded, however, are similar to those observed in the immediate hanging wall of the Main Central thrust, the base of the exhumed metamorphic core. The new observations from the Likhu Khola region are compatible with nearby studies that highlight structural, metamorphic, and temporal discontinuities within the exhumed Himalayan mid-crust.