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.

Anatectic melt inclusions (nanogranites and nanotonalites) have been found in garnet\ud of kyanite-gneiss at the bottom of the Greater Himalayan Sequence (GHS) along the Kali\ud Gandaki valley, central Nepal, c. 1 km structurally above the Main Central Thrust (MCT). In\ud situ U–Th–Pb dating of monazite included in garnets, in the same structural positions as melt\ud inclusions, allowed us to constrain partial melting starting at c. 41–36 Ma. Eocene partial\ud melting occurred during prograde metamorphism in the kyanite stability field (Eo-Himalayan\ud event). Sillimanite-bearing mylonitic foliation wraps around garnets showing a top-to-the-SW\ud sense of shear linked to the MCT ductile activity and to the exhumation of the GHS. These findings\ud highlight the occurrence of an older melting event in the GHS during prograde metamorphism in\ud the kyanite stability field before the more diffuse Miocene melting event.\ud The growth of prograde garnet and kyanite at 41–6 Ma in the MCT zone, affecting the bottom of\ud the GHS, suggests that inverted metamorphism in the MCT zone and folded isograds in the GHS\ud should be carefully proved with the aid of geochronology, because not all Barrovian minerals grew\ud during the same time span and they grew in different tectonic settings

New single crystal 40Ar/39Ar and apatite fission track ages from the Kanchenjunga region of far east Nepal yield insight into the timing of assembly of the Himalayan midcrust and the mechanisms that controlled its exhumation. The 40Ar/39Ar data are compared with new U(Th)/Pb zircon and monazite intrusive crystallization ages and existing metamorphic monazite ages from across the study area to test for internal consistency and potential excess Ar contributions. This new data set, which significantly enhances the density and spatial coverage available from the region, shows that inferred thrust‐sense discontinuities within the now‐exhumed former midcrustal rocks exposed therein must have ceased motion by ~12 Ma. Furthermore, the spatial distribution of ages across the Kanchenjunga region, older ages (~12–16 Ma) to the south and north and younger ages (~8 Ma) in the middle portion of the transect, is compatible with simulations of tectonic‐enhanced exhumation above a developing duplex system in nearby Bhutan.

The exposed mid-crustal rocks of the Himalayan orogen provide a natural laboratory for constructing the kinematic evolution of the midcrust during a large-scale continental collision. Kinematic models provide testable, geometrically valid, internally consistent, integrated solutions for diverse geological data from deformed regions. We investigated the Tama Kosi region of east-central Nepal with structural, geochemical, and geochrono logical methods to refine a detailed kinematic model for the Miocene Epoch, during which the mid-crust was pervasively deformed, translated southward, and progressively stacked via basal accretion. Geochemical and U-Pb zircon data demonstrate that two similar orthogneiss bodies were derived from different protoliths, one formed through vapor-absent melting at 1940 ± 16 Ma and the other via vapor-present melting at 1863 ± 14 Ma, respectively, indicating that they do not reflect structural repetition. In situ Th-Pb monazite petrochronology from the Mahabharat Range links the orogenic foreland to the exposed mid-crust of the High Himalaya via a coeval, protracted metamorphic growth-crystallization and/or recrystallization record spanning late Eocene or early Oligocene to early Miocene. Differential cooling of white mica, evidenced by 40 Ar/ 39 Ar cooling ages across the studied area, may outline a previously unrecognized out-of-sequence thrust, the occurrence of which is coincident with the location of a sharp break previously recognized from quartz crystallographic fabric deformation temperatures. Together with previous work, these data form the basis for a new, internally consistent kinematic model for rocks of the Tama Kosi region during the Miocene Epoch that tracks the transition from distributed ductile deformation in the mid-crust to deformation along discrete surfaces during their exhumation.

Note: Rock samples were collected in Kathmandu Nepal in summer of 2016 and 2017. The monazite trace elements and ages data were analyzed at the Institute of Geology and Geophysics, Chinese Academy of Sciences by Gautam Prashad Khanal and Jia-Min Wang in 2019. S2.1: Eu N /Eu* N distribution over the 208 Pb/ 232 Th ages. S2.2: HREE distribution over the 208 Pb/ 232 Th ages. S2.3: Yn N /Gd N distribution over the 208 Pb/ 232 Th ages. S2.4: BSE images of the monazites used for grain separate monazite dating. S2.5: BSE images of the monazites in thinsection used for in-situ monazite dating. S2.1: EuN/Eu*N distribution over the 208 Pb/ 232 Th ages. Caption: Plot of Eu N /Eu* N ratio versus Pb 208 /Th 232 age for the analysed monazite grains in the study area.The black bars represent 2 sigma age error, red rectangle represent older cores, and blue circle represent younger rims while green triangle represent insitu grains hosted in Bt, Qz or Kfs. Grey arrows in samples 17KN10 and 17KN09 show feldspar growth trend with decreasing age. S2.2: HREE distribution over the 208 Pb/ 232 Th ages. Caption: Plot of normalised Heavy Rare Earth Elements (HREE N) versus Pb 208 /Th 232 age for the analysed monazite grains in the study area. The black bars represent 2 sigma age error, red rectangle represent older cores, and blue circle represent younger rims while green triangle represent insitu grains hosted in Bt, Qz or Kfs. Grey arrows in samples 17KN10, 17KN09 and 17KN07 show garnet breakdown trend with decreasing age. S2.3: Yn N /Gd N distribution over the 208 Pb/ 232 Th ages. Caption: Plot of Yb N /Gd N ratio versus Pb 208 /Th 232 age for the analysed monazite grains in the study area. The black bars represent 2 sigma age error, red rectangle represent older cores, and blue circle represent younger rims while green triangle represent insitu grains hosted in Bt, Qz or Kfs. Grey arrows in samples 17KN10, 17KN09 and 17Kn07 show garnet breakdown trend with decreasing age. S2.4: BSE images of the monazites grain separates showing laser spot. Caption: Selected BSE images of dated monazite separate grains showing spot locations. The black circles are 24μm and the numbers represent 208 Pb/ 232 Th ages in Ma. S2.5: BSE images of the monazites in thin section showing laser spot. Caption: The thinsection plane polarised image (Upper figure) showing location and textural control of the dated monazites for the sample 17KN09. The red boxes represent the monazite location, red circles laser spot which are 24μm and the red numbers represent 208 Pb/ 232 Th ages in Ma.