Northward younging zircon fission-track ages from 13 to 2 ma in the eastern extension of the Kathmandu nappe and underlying lesser Himalayan sediments distributed to the south of Mt. Everest

This study is concerned with the tectono-thermal history of the Kathmandu nappe and the underlying Lesser Himalayan sediments (LHS) that are distributed in eastern Nepal. We carried out zircon fission-track(ZFT) dating and obtained 16 ZFT ages from the eastern extension of the Kathmandu nappe, the Higher Himalayan Crystal-line, Kuncha nappe, and the Main Central Thrust (MCT) zone. The ZFT ages of the frontal part of the Kathmandu nappe range from 13.0 ±0.8 Ma to 10.7 ±0.7 Ma and exhibit a northward-younging tendency. These Middle Miocene ZFT ages indicate that the frontal part of the Kathmandu nappe remained at a temperature above 240^° C until the termination of its southward emplacement at 12–11 Ma. The ZFT ages of the LHS range from 11.1 ±0.9 Ma in the southern part of the Okhaldhunga Window to 2.4 ±0.3 Ma of the augen gneiss in the northern margin and also exhibit a northward-younging age distribution. The ZFT ages show the northward-young-ing linear distribution pattern (−0.16 Ma/km) along the across-strikesection from the frontal part of the Kathmandu nappe to the root zone, without a significant age gap. This distribution pattern indicates that the Kathmandu nappe, the underlying MCT zone, and the Kuncha nappe cooled from the frontal zone to the root zone as a thermally united geologic body at a temperature below 240^° C. An older ZFT age (456.3 ±24.3 Ma), which was partially reset at the axial part of the Midland anticlinorium in the central part of the Okhaldhunga Window, was explained by downward heating from the “hot” Kathmandu nappe. The above evidence supported a model that southward emplacement of the hot Kathmandu nappe resulted in a thermal imprint on the upper part of the LHS; however, the lower part did not reach 240^° C.