Norian conodonts of the South Qiangtang Terrane, North Tibet, and their palaeogeographic implications

Abstract

Prior to their extinction at the Triassic-Jurassic boundary, conodonts were relatively diverse and abundant during Norian time. The great faunal turnovers at the base of the Norian and the base of the Alaunian (Middle Norian) confirm that the end-Triassic mass extinction was protracted through Late Triassic time rather than a single event. Norian conodonts also display obvious geographic distribution. Thus, investigation of Norian conodonts is not only of biostratigraphic significance, but it will elucidate the end Triassic mass extinction event as well as Late Triassic palaeobiogeography. Lacian (Early Norian) and Alaunian faunas appear to have spread extensively through Tibetan terranes of the Eastern Tethys, yet evidence of a continuous Lacian to Alaunian conodont succession as well as faunal turnover at the base of the Alaunian is sparse. Our biostratigraphic investigation of carbonate deposits of the Mopanshan area of the South Qiangtang Terrane has yielded abundant Norian conodonts. The Lacian Ancyrogondolella uniformis-Norigondolella hallstattensis Fauna and the Alaunian Mockina postera-Norigondolella steinbergensis Fauna are well-represented in the study region. The Norian conodont succession of the Eastern Tethys may be correlative with that of the Western Tethys and Eastern Pacific. The obvious geographic distribution provides evidence that the Indus-YarluZangbo Suture Zone (IYZSZ) separated Eastern Tethys into northern Tethys and southern Tethys realms. Norian conodonts are less abundant and diverse in the southern Tethys realm relative to the northern (or peri-Gondwana) Tethys. The Himalaya Terrane in the south belonged to southern Tethys whereas the Lhasa and the South Qiangtang terranes were part of the northern Tethys. Moreover, the absence of Norigondolella steinbergensis from the southern Tethys seemed to differentiate this realm from the northern Tethys.