Paleocene–Eocene record of ophiolite obduction and initial India‐Asia collision, south central Tibet

Abstract

Uppermost Cretaceous to Eocene marine sedimentary sequences occur both to the south and north of the Yarlung Zangbo suture in south central Tibet. They consist of Indian‐margin strata of the northern Tethyan Himalaya and Asian‐margin strata of the Gangdese forearc. Both assemblages are characterized by major changes in depositional environment and sedimentary provenance at ∼65 Ma and an appearance of detrital chromium‐rich spinel of ophiolite affinity (TiO2 generally <0.1 wt%) during the Paleocene. Ophiolitic melange exposed along the suture could have provided a source for detrital spinel. The melange occurs in the hanging wall of a north dipping, south directed mylonitic shear zone which includes a tectonic sliver of mafic schist. Amphibole from the schist yields 40Ar/39Ar ages of ∼63 Ma, which we attribute to cooling during slip along the shear zone and southward obduction of the melange. Melange obduction was coeval with the development of an angular unconformity within the Gangdese forearc basin to the north (between late Maastrichtian time and ∼62 Ma). Upper Paleocene to middle Eocene sandstones in the northern Tethyan Himalaya yield 200–120 Ma U‐Pb detrital zircon ages and 190–170 Ma 40Ar/39Ar detrital mica ages. These detrital grains were most likely sourced from regions north of the Yarlung Zangbo suture, suggesting that onset of India‐Asia collision in south central Tibet is middle Eocene or older in age. Collectively, our results support previous suggestions that oceanic rocks were obducted onto the northern margin of India during latest Cretaceous–earliest Tertiary time. Coeval changes in Gangdese forearc sedimentation raise the possibility that this obduction event marks onset of tectonic interaction between India and Asia at ∼65 Ma. Alternatively, in concert with the conventional view of Eocene collision initiation, the obducted oceanic rocks may be of intraoceanic origin, while coeval changes in Gangdese forearc sedimentation may be a consequence of an increase in the rate of ocean‐continent convergence following the demise of the intraoceanic subduction zone.

Shoshonites in Southern Tibet Record Late Jurassic Rifting of a Tethyan Intraoceanic Island Arc

“…Although little of the intraoceanic system is preserved, its collision is clearly recorded along the suture zone in detrital sediments. Coarsegrained clastic sediments derived from the northern India and supra-subduction zone terranes accumulated within the collision zone and obduction is heralded by the first arrival ophiolitic detritus shed into latest Paleocene marine sediments on the northernmost margin of India (Ding et al 2005;Zhu et al 2005). Ophiolite emplacement was also coincident with the development of widespread melange along the northern margin of India (Liu 2001).…”

Constraints on the early uplift history of the Tibetan Plateau

“…We also have directly dated an Early Eocene (zone of Buryella clinata-Thursocyrtis ampla) radiolarian chert in Saga County in the northern Tethyan Himalaya (SI Fig. 12 in SI Appendix), where previous studies have only inferred a mid-Paleogene age on the basis of Paleocene biostratigraphy and stratigraphic relationships (43,45). Thus, by using the disappearance of marine facies as a measure of early surface uplift, we conclude that the emergence of the Himalaya occurred postEocene at the earliest, possibly even more recently.…”

Late Cretaceous (ca. 90Ma) adakitic intrusive rocks in the Kelu area, Gangdese Belt (southern Tibet): Slab melting and implications for Cu–Au mineralization

“…1a) (Yin and Harrison, 2000). The Yarlung-Tsangpo suture marks the final collision between India and Asia in the Early Tertiary ($65-55 Ma) (Patzelt et al, 1996;Ding et al, 2005;Najman, 2006;Mo et al, 2007;Wu et al, 2007;Chung et al, 2009;Chen et al, 2010). A remarkable tectonic feature in the southern Lhasa Block is the occurrence of the NS-striking normal faulting or graben systems, possibly due to east-west extension during the Miocene (18-13 Ma) (Williams et al, 2001;Nomade et al, 2004) (Fig.…”