Synthrusting metamorphism, cooling, and erosion of the Himalayan Kathmandu Complex, Nepal

Abstract

In this paper we tackle some of the outstanding problems of the Himalaya, in particular the external zone in the Kathmandu Complex, using an integrated approach involving field mapping, microstructure, thermobarometry, and geochronology. The result is a new model showing the evolution of one major Main Central Thrust: therefore we refute suggestions that the Kathmandu Complex is a klippe or separate thrust sheet. Compared to the Main Central Thrust sheet in the High Himalaya, the Kathmandu Complex shows differences in deformational and metamorphic features and timing of metamorphism that are consistent with its position some 100 km south of the High Himalaya, fairly near the leading edge. Unless there was substantial volume loss between the time of peak metamorphism and the beginning of thrusting then our geobarometry results indicate that the Main Central Thrust wedge was ∼40 km thick on the northern side of the Kathmandu Complex and <20 km thick on the southern margin. Initiation of the Main Central Thrust occurred at ∼22 Ma, possibly during the closing stages of peak amphibolite facies metamorphism; slip at elevated temperature (500°–600°C) continued until ∼14 Ma. This is a little longer than has previously been proposed. In marked contrast to the famous inverted metamorphism on the Main Central Thrust in the High Himalaya, the metamorphic zonal scheme in the Kathmandu Complex is right way up with the exception of a thin zone of greenschist facies thrust related dynamically metamorphosed rocks at the base. These mylonites postdate the high‐grade regional amphibolite metamorphism and give an illusion of inverted metamorphism. A likely reason for the contrast is that the Main Central Thrust cut up section toward the foreland and therefore at Kathmandu, carries high levels in the metamorphic structure. Our model involves reactivation of the Main Central Thrust at 7–8 Ma as inferred from published monazite and mica ages, but because the Kathmandu rocks show no evidence for high‐temperature reactivation at this time, we presume that the late reactivation involved only the internal High Himalaya zone while the Main Central Thrust was inactive in the external Kathmandu zone. We attempt to quantify rates of cooling, exhumation and thrusting during time period 22 Ma to the present.