Structural, geochronological and geochemical evidence for two distinct thrust sheets in the ‘Main Central thrust zone’, the Main Central thrust and Ramgarh–Munsiari thrust: implications for upper crustal shortening in central Nepal
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Two orogen-scale thrusts structurally underneath Greater Himalayan (GH) rocks characterize the structural architecture of Himalaya in central Nepal. The Main Central thrust (MCT) is at the base of the GH with the Lesser Himalayan (LH) Robang Formation in the footwall, which is the hanging wall of the Ramgarh–Munsiari thrust (RMT). At Kodari-Tatopani and Malekhu, U–Pb detrital zircon age populations from the RMT sheet yield a maximum depositional age of c. 1838 and c. 1871 Ma. U–Pb analyses of igneous zircons from the RMT sheet yield a crystallization age of c. 1750 Ma at both Galchhi and Kodari-Tatopani. The εNd(0) values of pelitic rocks from the RMT sheet at Kodari-Tatopani range from c. −23 to −25; whereas, GH rocks have values from c. −12 to −18. These data indicate that the RMT sheet carries the Palaeoproterozoic LH rock and the MCT carries the GH rock. At Kodari-Tatopani, the thrust previously mapped as the MCT is interpreted to be the RMT. Positively identifying the RMT sheet in all three locations yields a more accurate kinematic evolution and confirms its orogenic-scale presence in central Nepal.