Youngest marine fossil evidence in Tibet for disappearance of the Tethyan Ocean
Abstract:The Tethyan Ocean that once covered parts of southern Tibet was eliminated when the Indian and Eurasian continents collided along the Yarlung Tsangpo suture zone. The timing of the last marine sedimentary rocks thus places a constraint on this first-order tectonic event. Although several sedimentary successions have been reported from southern Tibet their documentation is commonly incomplete or important questions remain regarding stratigraphic continuity and/or the reliability of fossil identifications.
We critically assess existing data in order to correlate and compare between sections in three zones: one to the north of the suture on the Asian margin and two to the south on the northern margin of India. We examine the planktonic foraminiferal and nannofossil biostratatigraphy of the youngest sections, which occur in the Tibetan Himalayan succession and lie around 100 km south of the suture. This also includes a previously unreported section 70 km east of Gamba. We also consider the ages of radiolarians from cherts in mélange zones immediately south of the suture zone in Ladakh, NW India and Tibet. We discuss the implications of our results for understanding the timing of continent-continent collision. In doing this we take into account the effects of crustal loading, eustatic sea level variation, orogenesis associated with the on-going convergence between India and Asia together with the likely migration rate of any fore-deep in front of a colliding continental mass.
Our results demonstrate that a marine seaway remained in existence south of the Yarlung Tsangpo suture zone until at least Priabonian (38-33 Ma) time. Importantly, as all sections are truncated by erosion or faulting we note that this remains a maximum age estimate for the last marine sedimentation in this area.