The eastern edge of the Gurla Mandhata core complex: an orogen-perpendicular strain boundary in NW Nepal Himalaya

This study defines the nature of the eastern boundary of the Gurla Mandhata core complex and assesses its significance in the broader context of along-strike termination of metamorphic core complexes. Integrated field mapping and monazite U–Th/Pb petrochronology constrain the magmatic and tectonometamorphic evolution of migmatitic rocks, leucogranitic dykes and sills, and the Chuwa Granite exposed along the eastern margin of the Gurla Mandhata core complex (NW Nepal Himalaya). The dykes and sills are interpreted as feeders to the Chuwa Granite, emplaced during crustal shortening and southward extrusion at ∼20 – 17 Ma. Migmatites record retrograde metamorphism from ∼18 to 14 Ma, and migmatization persisted until ∼14 Ma, coeval with onset of orogen-parallel extension in the Gurla Mandhata core complex. The Gurla Mandhata core complex and its eastern edge share a thermal pulse typical of southward extrusion of the Himalayan metamorphic core from ∼22 Ma to 15 Ma. However, shear-sense differences from ∼15 Ma to 13 Ma suggest the existence of a strain partitioning boundary at the eastern edge of the core complex, coinciding with the northward trace of the Indian basement Great Boundary Fault, which is interpreted to have reactivated and influenced the location of the core complex’ eastern termination.