Analysis of critical radiogenic isotope ratios (Nd and Sr) for bulk samples of metasedimentary formations with clastic protoliths, combined with U-Pb dating of detrital zircons recovered from these formations, have yieided a powerful tool for
discriminating between the major lithotectonic units of the Himalayan orogen, thus defining the major tectonic boundaries that divide them in regions where such structures are often obscured by polyphase, high-strain deformation.
In the Sutlej Area, northwest Himalaya, detrital zircon age populations suggest that cratonic India has supplied both a Neoproterozoic source and a Palaeoproterozoic-Late Archaean source that have contributed detritus to the pre-Himalayan depositional basins of the High Himalayan Crystalline Series (HHCS), whereas only the latter source has contributed to the protolith of the northern exposures of the Lesser Himalayan Series (Inner LHS). Whole-rock Nd and Sr isotope systematics confirm the isotopic distinction between the HHCS and the Inner LHS and equate the protoliths of the HHCS and the Outer LHS, thus defining the Main Central Thrust (between the HHCS and the Inner LHS) and the Jutogh Thrust (between the Inner and Outer LHS). Analyses of the enigmatic Haimanta Group broadly suggest correlation with the HHCS, although stratigraphically younger samples appear to have been derived from the older, Palaeoproterozoic-Late Archaean source.
To assess the validity of this technique along strike of the Himalayan orogen, an equivalent study was undertaken in Bhutan (eastern Himalaya). Both zircon and bulk rock isotopic analyses confirm the isotopic distinction between the HHCS and much of the LHS. Although the LHS can be subdivided on isotopic grounds, these subdivisions do not lie in simple Inner/Outer zones as in the Sutlej section. Moreover, a substantial Mesoproterozoic zircon population, unobserved in the western Himalaya, is found within the HHCS. These results provide critical constraints on the evolving palaeogeographic environment within which the Proterozoic and Palaeozoic formations that constitute the core of the Himalayan orogen, were deposited
