Abstract

The Main Central Thrust (MCT) is one of the most tectonically significant structures in the Himalayan orogen. Detailed geologic mapping and structural analysis of the MCT in the Langtang National Park region of central Nepal reveals that this segment of the fault zone experienced multiple episodes of south-directed movement, under both brittle and ductile conditions, during the Tertiary period. Early (mid-Miocene) movement resulted in the development of mylonitic fabrics synchronous with amphibolite-facies metamorphism. The mean orientation of the dominant mylonitic foliation is N28°W, 38°NE. An associated mineral/stretching lineation plunges 40° to N40°E. Kinematic indicators suggest hanging-wall movement to the southwest relative to the footwall along the north-dipping fault. It is not possible to constrain the magnitude of high-temperature displacement on the MCT at the longitude of Langtang. Late-stage structures in the MCT zone at Langtang include a series of imbricate, brittle thrust faults that separate different lithostratigraphic units and correspond to metamorphic discontinuities. We interpret this fault system as a duplex structure. Muscovite 40Ar/39Ar cooling ages from the MCT zone range from 8.9-6.9 Ma. Because the nominal closure temperature of Ar diffusion in muscovite (approximately 625 K) is higher than the apparent temperature conditions under which late brittle deformation occurred, we suggest that brittle deformation was a latest Miocene-Pliocene phenomenon. Another major Himalayan fault, the Main Boundary Thrust (MBT), was developing to the south of Langtang at approximately the same time. We speculate that brittle faulting within the MCT zone may have initiated as the MCT zone was transported over a ramp in the MBT.

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