Emplacement of hot L esser H imalayan nappes from 15 to 10 M a in the J umla– S urkhet region, western N epal, and their thermal imprint on the underlying E arly M iocene fluvial D umri F ormation

A multichronological study of the weakly metamorphosed E arly M iocene fluvial D umri F ormation and the overlying K uncha and L esser H imalayan C rystalline 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 – P b ages of detrital zircons from the D umri F ormation show peaks at ca 1  G a and 500–600  M a, and fission‐track dating indicates a thermal imprint on the formation at 11–10  M a. In the N audanda Q uartzite, overlying the K uncha F ormation, reset fission‐track detrital zircon ages are 9.5  M a, but the U – P b ages are older than 1.7  G a. The differences in the U – P b ages for detrital zircons from the D umri and K uncha F ormations indicate that the K uncha nappe was never exposed at the surface during the E arly M iocene when the D umri F ormation was deposited. Two‐mica garnet schists of the M ain C entral T hrust ( MCT ) zone have fission‐track detrital zircon ages of 7.8  M a and 40 A r– 39 A r muscovite ages of 19  M a, though the zircons have U – P b ages older than 600  M a. Our interpretation is that the heat for the D umri F ormation metamorphism came from the overlying nappes. The timing of the thermal imprint on the D umri F ormation indicates that the metamorphic nappe reached its present position before ca 10  M a. The P arajul K hola granite in the frontal zone of the K uncha nappe yields U – P b zircon ages of 1.89  G a, and it underwent an E arly M iocene thermal event, cooling below 240° C at 14.7  M a and below 100° C at 10.3  M a. These thermochronological data suggest that the frontal part of the K uncha nappe was exposed at the surface around 15–14  M a and cooled immediately, but inner parts of the nappe were still hot during its emplacement.