Structural and geochronological evidence for the leading edge of the Greater Himalayan Crystalline complex in the central Nepal Himalaya

The Himalaya is commonly described as a three layer-two fault stack. Namely, a high-grade crystalline core featuring an inverted metamorphic field gradient, the Greater Himalayan Crystalline complex (GHC), is separated from units above and below by shear zones. The Lesser Himalayan Sequence (LHS) underlies the GHC below the Main Central thrust, and the Tethyan Himalayan Sequence (THS) overlies it along the South Tibet detachment. However, the southern Main Central thrust hanging wall consists of a lower unit dominated by a right-way-up metamorphic sequence of biotite ± garnet schists (Bhimphedi Group) and an upper unit with only anchizone metamorphism (Pulchauki Group). The Bhimphedi Group is commonly equated to the GHC, while the Pulchauki Group is well correlated to the THS. However, no shear zone separates these units. We present new structural and geochronological data along the boundary between the GHC and the Bhimphedi Group in the Kathmandu region. These data reveal an ~ 200 m thick Early–Middle Miocene top-north-northeast shear zone that we term the Galchi shear zone. We correlate this shear zone to the South Tibet detachment on the basis of lithological, metamorphic, structural, and chronological criteria. The Galchi shear zone merges with the Main Central thrust to the south, bounding the leading edge of the GHC. This result, combined with recent work in the western Himalaya, suggests that the locally preserved leading edge of the GHC is sub-parallel to the arc of the orogen and the southern Main Central thrust hanging wall is dominated by THS rocks. This orogenic architecture rules out wedge extrusion and channel flow-focused denudation kinematic models for the Himalayan orogen, but is accommodated by tectonic wedging kinematic models, including channel tunneling models with modified timing.

Research Highlights► A ~200 m thick top-north shear zone, the Galchi shear zone, separates the Greater Himalayan Crystalline complex and the Bhimphedi Group in the central Himalaya. ► The Galchi shear zone appears to merge with the Main Central thrust to the south. ► Lithologic distribution, metamorphic field gradients, and shear kinematics indicate that the Galchi shear zone represents the southern extension of the South Tibet detachment. ► The emplacement of the Himalayan crystalline core was accomplished via tectonic wedging kinematics.