STRUCTURAL AND U-TH-PB ZIRCON GEOCHRONOLOGICAL STUDIES INDICATE THAT THE CENTRAL HIMALAYA DEVELOPED VIA TECTONIC WEDGING

Channel flow models for the Himalayan development feature a climatic shift leading to focused erosion. This erosion exhumes the crystalline core to the surface between two faults. However, the two bounding faults merge towards the foreland in the western Himalaya. Therefore the crystalline core was emplaced at depth there, befitting a “tectonic wedging” kinematic evolution. Structural mapping and zircon geochronology are used to test two hypotheses: (1) the Himalaya developed through tectonic wedging, or (2) the Himalaya evolved via multiple mechanisms, with an along-strike progression from tectonic wedging in the west to channel flow / erosional exhumation in the center and east. Results from the northern margin of the Kathmandu Nappe (central Himalaya) reveal a merger of the two faults towards the foreland. The upper bounding fault here features (A) a ~300 m thick shear zone coincident with the right-way-up kyanite isograd; (B) paragneisses and schists, with minor leucogranitic lenses within and below the shear zone; and (C) dominantly top-north sense of shear recorded by multiple shear criteria. The bounding fault merger is not exposed, but the bounding fault zones outcrop separated by <300 m just north of the merger. 36 U-Th-Pb ion microprobe spot analyses were acquired from 24 zircon grains from a leucogranitic lense deformed by top-north shear bands. Grains have complex cores with some ≤ 30 μm prismatic rims. Grain cores yield variable, pre-Oligocene ages and low U/Th ratios (~50 to 250). Rim analyses (from 8 zircons) have latest Oligocene – earliest Miocene ²³⁸U/²⁰⁶Pb ages and variable U/Th ratios (~50 to 1700). Core data are interpreted as inherited/mixed ages; rim data reveal zircon that grew during crystallization of the leucogranite. These results, together with published Ar-Ar thermochronology, indicate that the top-north upper fault was active here in the Early Miocene. This timing of motion is consistent with such chronology elsewhere in the range. Preservation of the leading edge of the Himalayan crystalline core requires that the southern segments of the upper bounding fault did not breach the surface during motion. A tectonic wedging kinematic evolution satisfies key observations for regions separated by ~1000 km. The frontal tip of the crystalline core can be inferred along the strike of the orogen.