Emplacement of hot Lesser Himalayan nappes from 15 to 10 Ma in the Jumla–Surkhet region, western Nepal, and their thermal imprint on the underlying Early Miocene fluvial Dumri Formation

Abstract

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.

Rift‐related origin of the Paleoproterozoic Kuncha Formation, and cooling history of the Kuncha nappe and Taplejung granites, eastern Nepal Lesser Himalaya: a multichronological approach

“…In addition, we report the similar FT age of about 5 Ma for detrital zircon and 2.3 Ma for apatite from the overlying kyanite-sillimanite gneiss of the HHC. On the basis of these thermochronological data from eastern Nepal and new data from western Nepal (Sakai et al 2011(Sakai et al , 2013, we discuss the origin and the thermal history of Kuncha and overlying metamorphic nappes, and propose a lateral cooling model of the nappes.…”

“…On the basis of these thermochronological data from eastern Nepal and new data from western Nepal (Sakai et al . , ), we discuss the origin and the thermal history of Kuncha and overlying metamorphic nappes, and propose a lateral cooling model of the nappes.…”

Detrital geochronology of the Gondwana sequence of the Lesser Himalaya of the Nepal Himalaya

“…Low temperature thermochronology from the metamorphic core shows the cooling age of white mica from 16 to 12 Ma (Larson et al, 2017). Likewise, multichronological study of the Kucha Formation and weakly metamorphosed Dumri Formation suggests that the exhumation and emplacement of Greater Himalaya were after 15 Ma (Sakai et al, 2013). On the other hand, another school of thought argue older exhumation of the Greater Himalaya with discoveries of discontinuities of different generations within the metamorphic core (Carosi, Montomoli, & Iaccarino, 2018).…”

“…Likewise, low‐temperature thermochronology shows that the Greater Himalaya was exhumed after 15 Ma (Larson et al, ), which is consistent with the fission track and U–Pb detrital zircon analysis from the Middle Siwaliks (Bernet et al, ). In addition, Sakai, Iwano, Danhara, Hirata, and Takigami () illustrated that the youngest unit of the sedimentary basin the Miocene Dumri Formation must have been deposited before the exhumation and emplacement of the Greater Himalaya.…”

Zircon multichronology: Fission‐track, U‐Pb, and combined fission‐track–U‐Pb studies

“…Sakai et al . (2013a,b) present applications of zircon multichronology using a combination of inherited U–Pb ages and reset FT dates to show that a hot metamorphic nappe covering the lesser Himalayas cooled laterally; the authors provide a new hypothesis for the extrusion of metamorphic rocks in an active nappe. Finally, Iwano et al .…”