In order to decipher the origin and tectonothermal history of the Kuncha nappe, we undertook a geological investigation in the Taplejung window in eastern Nepal, and carried out multichronological analyses of zircon, apatite, and mica of the Kuncha Formation and Taplejung granites. Three granite bodies that intrude into the Kuncha Formation show fission-track (FT) ages of 6.2 to 4.8 Ma for zircon and 2.9 to 2.

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Newly discovered peloidal limestone from the summit of Mount Qomolangma (Mount Everest) contains skeletal fragments of trilobites, ostracods and crinoids. They are small pebble-sized debris interbedded in micritic bedded limestone of the Qomolangma Formation, and are interpreted to have been derived from a bank margin and redeposited in peri-platform environments. An exposure of the Qomolangma detachment at the base of the first step (8520 m), on the northern slope of Mount Qomolangma was also found. Non-metamorphosed, strongly fractured Ordovician limestone is separated from underlying metamorphosed Yellow Band by a sharp fault with a breccia zone. The 40 Ar-39 Ar ages of muscovite from the Yellow Band show two-phase metamorphic events of approximately 33.3 and 24.5 Ma. The older age represents the peak of a Barrovian-type Eo-Himalayan metamorphic event and the younger age records a decompressional high-temperature Neo-Himalayan metamorphic event. A muscovite whole-rock 87 Rb-86 Sr isochron of the Yellow Band yielded 40.06 ± 0.81 Ma, which suggests a Pre-Himalayan metamorphism, probably caused by tectonic stacking of the Tibetan Tethys sediments in the leading margin of the Indian subcontinent. Zircon and apatite grains, separated from the Yellow Band, gave pooled fission-track ages of 14.4 ± 0.9 and 14.4 ± 1.4 Ma, respectively. These new chronologic data indicate rapid cooling of the hanging wall of the Qomolangma detachment from approximately 350°C to 130°C during a short period .

A multichronological study of the weakly metamorphosed Early Miocene fluvial Dumri Formation and the overlying Kuncha and Lesser Himalayan Crystalline nappes has helped to clarify the timing and heat source of the metamorphism, as well as the history of emplacement and cooling of the nappes. U–Pb ages of detrital zircons from the Dumri Formation show peaks at ca 1 Ga and 500–600 Ma, and fission‐track dating indicates a thermal imprint on the formation at 11–10 Ma. In the Naudanda Quartzite, overlying the Kuncha Formation, reset fission‐track detrital zircon ages are 9.5 Ma, but the U–Pb ages are older than 1.7 Ga. The differences in the U–Pb ages for detrital zircons from the Dumri and Kuncha Formations indicate that the Kuncha nappe was never exposed at the surface during the Early Miocene when the Dumri Formation was deposited. Two‐mica garnet schists of the Main Central Thrust (MCT) zone have fission‐track detrital zircon ages of 7.8 Ma and 40Ar–39Ar muscovite ages of 19 Ma, though the zircons have U–Pb ages older than 600 Ma. Our interpretation is that the heat for the Dumri Formation metamorphism came from the overlying nappes. The timing of the thermal imprint on the Dumri Formation indicates that the metamorphic nappe reached its present position before ca 10 Ma. The Parajul Khola granite in the frontal zone of the Kuncha nappe yields U–Pb zircon ages of 1.89 Ga, and it underwent an Early Miocene thermal event, cooling below 240°C at 14.7 Ma and below 100°C at 10.3 Ma. These thermochronological data suggest that the frontal part of the Kuncha nappe was exposed at the surface around 15–14 Ma and cooled immediately, but inner parts of the nappe were still hot during its emplacement.

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Volcanic rocks recovered from the Japan Sea during ODP Legs 127 and 128 were analyzed by 40 Ar-39 Ar whole-rock stepwise-heating experiments. All three experiments on samples from Site 795 in the Japan Basin revealed disturbed age spectra, but they are consistent with crystallization ages of 15 to 25 Ma for the samples. At Site 797 in the Yamato Basin, three of the five samples showed plateau ages of 18-19 Ma. At Site 794 in the northern Yamato Basin, three of the five samples revealed concordant age spectra of 20-21 Ma. The radiometric age results are consistent with the estimated ages for the oldest sediments at Site 797 based on the biostratigraphy, but are significantly older than those of the oldest sediments at Site 794. However, the radiometric ages are concordant with previously inferred ages for the formation of the Japan Sea floor based on radiometric age data from dredged igneous rocks from the Japan Sea. The present results indicate that formation of the Japan Sea floor started at least 19-20 Ma ago and give more precise age constraints.