CONSTRAINTS ON THE DEPOSITIONAL AGES OF LESSER HIMALAYAN ROCKS IN CENTRAL NEPAL AND THEIR STRUCTURAL IMPLICATIONS

Abstract

The lack of good exposures and paucity of datable horizons in central Nepal has hindered the ability of geologists to piece together a relatively cohesive and straightforward stratigraphic succession within the Lesser Himalaya. U-Pb isotopic analyses of detrital zircons from the Modi Khola valley indicates maximum depositional ages of ~1875 Ma for the Kuncha Formation, ~1800 Ma for the Fagfog Formation, and ~ 1780 Ma for the Kushma Formation. The intrusive 1831 ± 17 Ma Ulleri augen gneiss provides a minimum depositional age bound for the Kuncha. Combined, these data suggest the Kuncha Formation is the oldest member of the Lesser Himalayan series in central Nepal. Additionally, 13C data suggest the Malekhu Formation of the Lakharpata Group was deposited before ca. 1250 Ma. A field mapping comparison based on the redefined stratigraphy indicates the Ramgarh thrust is located >10 km farther south than previously mapped, potentially reducing regional shortening estimates.