Isotopic constraints on the age and provenance of the Lesser and Greater Himalayan sequences, Nepalese Himalaya

The Main Central thrust of the central Himalaya juxtaposes the Lesser Himalayan sequence (footwall) with the Greater Himalayan sequence (hanging wall), both of which are regarded as components of the northern passive margin of India. Considerable uncertainty surrounds their respective ages and sedimentary derivation. U-Pb and Sm-Nd isotopic studies of samples from the Langtang area, central Nepal, demonstrate important distinctions between the two rock packages. The Greater Himalayan sequence had a sedimentary provenance that included a major source of 0.8-1.0 Ga zircons, implying a Late Proterozoic age. The source of the Precambrian part of the Lesser Himalayan sequence contained 1.87-2.60 Ga zircons, and its depositional age is probably Middle Proterozoic. Nd isotopic characteristics of the two sequences are different: at 21 Ma, the Greater Himalayan rocks had ε_Nd values between -14.6 and -18.5 while their Lesser Himalayan counterparts had values between -21.4 and -25.9, giving rise to major differences in their model ages. The Lesser and Greater Himalayan sequences at Langtang are interpreted as correlative to the lower and upper parts of the Vindhyan Supergroup, currently exposed south of the Gangetic plain in northern India. On the basis of available age data, the exposed hanging wall of the Main Central thrust in central and eastern Nepal contains no Archean-Early Proterozoic basement. If the fault does root into the basement at depth, it must do so farther north than recently published cross sections indicate. This implies a total displacement on the fault that is substantially greater than the 140-210 km required by tectonic overlap in the eastern Himalayas.