Tectonic Implications of U-Pb Zircon Ages of the Himalayan Orogenic Belt in Nepal

Abstract

Metasedimentary rocks of the Greater Himalaya are traditionally viewed as Indian shield basement that has been thrust southward onto Lesser Himalayan sedimentary rocks during the Cenozoic collision of India and Eurasia. Ages determined from radioactive decay of uranium to lead in zircon grains from Nepal suggest that Greater Himalayan protoliths were shed from the northern end of the East African orogen during the late Proterozoic pan-African orogenic event. These rocks were accreted onto northern Gondwana and intruded by crustal melts during Cambrian-Ordovician time. Our data suggest that the Main Central thrust may have a large amount of pre-Tertiary displacement, that structural restorations placing Greater Himalayan rocks below Lesser Himalayan rocks at the onset of Cenozoic orogenesis are flawed, and that some metamorphism of Greater Himalayan rocks may have occurred during early Paleozoic time.

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

“…Both of our samples show strong similarities to previously published spectra for the TH and GH zones (Fig. 3) [7]. However, there are several important di¡erences.…”

“…3) or dilution by Paleozoic zircon, as our data show a large population of zircons older than 1.0 Ga (60% of sample). Thirdly, previous samples from the LH contain only zircons older than 1.6 Ga [7,13], whereas nearly 75% of the zircons from our LH sample are younger than this and range as young as 525 Ma. Thus, characterization of the LH as containing only s 1.6 Ga zircons is a result of analyzing suites of zircons from older stratigraphic units.…”

“…The 'crystalline axis' model [5,6] portrays the TH and LH as depositional basins separated by a basement high represented by the GH. More recently, DeCelles et al [7] proposed a third, 'accreted terrane', model in which the GH represents the basement of an exotic terrane that accreted to the northern Indian margin during the Late Cambrian^Early Ordovician, and that the TH sedimentary succession is an overlying cover sequence. Each of these models predicts a unique set of stratigraphic, biogeographic, and geochronologic patterns for the geologic record of the Himalayan region.…”

“…Here we use a combined stratigraphic and geochronologic approach to test these models and create an integrated model of the paleogeographical relationship of the three lithotectonic zones, prior to Cenozoic thrusting associated with accretion of India to Asia. The accreted terrane model [7] is based on published detrital zircon age spectra of the GH and TH that show a wide range of Precambrian to Late Cambrian zircons, in contrast to LH bedrock samples that contain zircons exclusively older than 1.6 Gyr. These data, combined with a similar distribution of Nd model ages for sedimentary rocks, suggested that the LH is, en masse, characterized by older detrital materials than the two northern lithotectonic zones [7,8].…”

“…The accreted terrane model [7] is based on published detrital zircon age spectra of the GH and TH that show a wide range of Precambrian to Late Cambrian zircons, in contrast to LH bedrock samples that contain zircons exclusively older than 1.6 Gyr. These data, combined with a similar distribution of Nd model ages for sedimentary rocks, suggested that the LH is, en masse, characterized by older detrital materials than the two northern lithotectonic zones [7,8]. Samples of Cenozoic syn-orogenic molasse and modern stream sediment from the LH have been interpreted based on this idea, i.e., younger 5001 400 Ma grains as having been derived solely from the GH or TH [9,10].…”