Integrated tectonostratigraphic analysis of the Himalaya and implications for its tectonic reconstruction

Abstract

The isotope geochronology of isochronously deposited Cambrian strata from different tectonostratigraphic zones of the Himalaya confirms new stratigraphic, sedimentological, and faunal evidence indicating that the Himalaya was a single continental margin prior to collision of India with Asia. Lesser, Greater, and Tethyan Himalaya represent proximal to distal parts of a passive continental margin that has been subsequently deformed during Cenozoic collision of India with Asia. Detrital zircon and neodymium isotopic data presented herein discount the prevailing myth that the Lesser Himalaya has a unique geochronologic and geochemical signature that is broadly applicable to modeling the uplift history of the Himalaya. The conclusion that all pre-Permian Lesser Himalaya strata lack young detrital zircons that are present in the Greater and Tethyan Himalaya underpins previous arguments that the Main Central Thrust forms a fundamental crustal boundary that separates the Indian craton from an accreted terrane to the north. The supposition that Himalayan lithotectonic zones differ in detrital zircon age populations has also been used to reconstruct the unroofing history of the Himalaya during foreland basin development in the Cenozoic. Our data conflict with the underlying assumptions implicit in these studies in that samples of similar depositional age from both the Lesser and Tethyan Himalaya contain detrital zircons with similar age spectra. Similarities between the Kathmandu Complex and the Tethyan Himalaya support stratigraphic continuity between the former and either age-equivalent Greater Himalayan protolith or the Tethyan. Assuming that the complex rooted along the Main Central Thrust, these strata would simply have escaped intense metamorphism during Cenozoic tectonism. Alternatively, the complex may represent a part of the Tethyan Himalaya that was emplaced during an early stage of movement along a south-directed thrust fault located near the present-day structural position of the South Tibetan Fault System.