Chronology of tectonic events in the crystalline core of the Himalaya, langtang National Park, central Nepal

Abstract

The Main Central Thrust (MCT) is an important intracontinental subduction zone that accommodated a significant proportion of shortening between India and Asia during Tertiary Himalayan orogenesis. Argon 40/argon 39 geochronology indicates at least two distinct periods of thrust movement on the MCT in the Langtang National Park region of central Nepal. Ductile deformation and associated mylonitization characterizing the earlier event are constrained by 40Ar/39Ar dating of muscovites to have occurred sometime before 5.8 Ma. A later period of brittle deformation, resulting in the juxtaposition of rocks of different lithology within the MCT zone, occurred at approximately 2.3 Ma, based on 40Ar/39Ar dating of neoblastic muscovites from the brittle fault zones. The hanging wall of the MCT contains amphibolite to upper amphibolite grade gneisses and small leucogranite bodies assigned to the Greater Himalayan sequence. Argon 40/argon 39 cooling ages of muscovite and biotite from the gneisses range from 4.6 to 9.7 Ma. These dates contrast with previously obtained 16–21 Ma U‐Pb monazite and zircon ages for metasedimentary rocks from the same structural levels [Parrish et al., 1992], indicating relatively slow cooling over the early to middle Miocene interval for much of the MCT hanging wall. However, one 19.3 Ma biotite 40Ar/39Ar cooling age for a sample from the uppermost portion of the hanging wall is only slightly younger than U‐Pb monazite ages for nearby anatexites, possibly suggesting rapid cooling of the uppermost Greater Himalayan sequence by tectonic denudation associated with the structurally higher South Tibetan detachment system. The general consistency of 40Ar/39Ar ages throughout the 11‐km‐thick Greater Himalayan sequence suggests rapid cooling in late Miocene time, probably due to an increase in erosion rate related to ramping on the structurally lower Main Boundary Thrust. An alternative possibility would be massive hydrothermal resetting in late Miocene time of the entire sequence.