Age of Initiation of the India-Asia Collision


    I have been involved in field work within the Himalayas attempting to directly constrain the age of initiation of the India-Asia collision.   The collision of India with Asia is perhaps the most profound tectonic event to have occurred in past 100 Ma. It is responsible for the uplift of the Himalayas and Tibetan Plateau and has been argued to have been responsible for geological, geochemical, and climatological consequences of global extent. Yet, the age of initiation of this collision remains poorly constrained. The literature is replete with estimates that range from the Late Cretaceous (>65 Ma) to latest Eocene (<40 Ma) with little consensus in between.

     Rowley (1996) reviews the available stratigraphic evidence from the Himalayan region, and concludes that only in the western Zanskar-Hazara region is the age well constrained as starting in the Late Ypresian (~<52 Ma). In the most eastern sections of Tertiary rocks thus far recognized within the Tethyan Himalayas north and east of Everest (Mount Qomolangma) normal, shallow shelf-type carbonates extend into the Lutetian, without evidence of a change in sedimentation to the top of the section, so the start of collision must be still younger- See Zhu et al. 2005- as this is now known to be incorrect with the youngest shelf carbonates in the Zhepure Shan now known to be no younger than zone P8 (~50 Ma) - or late Ypresian in age. Along-strike of the Indus Yarlung Zangbo suture thick submarine delta-fan complexes derived from erosion of the Himalayan-Tibet system provide independent estimates that agree with a diachronous collision initiating in the late Ypresian in the west and progressing into and perhaps through the Lutetian in the east. The stratigraphic and magmatic history along the north side of the suture are compatible with such a diachronous history. This diachroneity has important implications for estimates of the accommodation of strain within this orogenic system. We now know that if diachroneity exists it must be older than late Ypresian, with evidence from ages of metamorphism in Tso Morari and adjacent areas implying initiation there at perhaps 53 Ma,  Hence the conclusions of this paper regarding diachroneity are no longer correct.

    A follow up paper on this focussed on the Zhepure Mountain sequence using backstripping techniques to analyze the subsidence history in this section. This paper reviews the stratigraphic section preserved on Zhepure Mountain, on the north flank of Everest (Mount Qomolongma) and its implied subsidence history. Zhepure Mountain lies about 65 km south of the Indus-Yarlung Zangbo suture and contains the most complete and youngest passive margin shelf sediments in the Tethyan Himalayas. On the basis of the subsidence history of the preserved section there is no evidence of acceleration of the subsidence up to the youngest rocks. Therefore collision-related loading and accelerated subsidence must post-date the youngest sediments preserved, which date from the early Lutetian. Hence accelerated subsidence at Zhepure Mountain must post-date about 45.8 Ma. In the Zanskar and Hazara region to the west, the initiation of collision is stratigraphically well constrained as starting in the Late Ypresian (~<52 Ma) implying a significant component of diachroneity to the initiation of this collision. Our work in the Zhepure Shan (=Mountain) demonstrates that the age of the uppermost shelf carbonates reported by WIllems et al. was incorrect. The uppermost deepening upward carbonates are late Ypresian in age and conformably and gradationally overlain by ophiolite-detritus and volcanic arc derived clastics of late Ypresian age (See immediately below).

    Our most recent work documented by Zhu et al. (2005) specifically examined the Zhepure Shan sections. From the Abstract: We document the stratigraphy and provenance of the lower Tertiary terrigenous sections in the Zhepure Shan region of the Tethyan Himalaya, southern Tibet, using petrographic and geochemical whole-rock and single-grain techniques. The Cretaceous–early Tertiary shelf deposits of shallow marine carbonates and siliciclastics of the former Indian passive margin near the western end of the Zhepure Shan are conformably overlain by lower Tertiary clastic rocks. Sandstones in the Jidula Formation (Paleocene) mostly contain monocrystalline quartz grains of cratonic origin. In contrast, significant amounts of immature framework grains with a distinct ophiolitic and volcanic arc influence are present in the Youxia (Early Eocene) and Shenkeza (post–Early Eocene) formations. Major, trace, and rare earth element concentrations in both sandstones and shales complement the petrographic data and indicate that the source of the Jidula Formation consisted primarily of quartzose basement rocks, probably of Indian continental origin, whereas the sediments of the Youxia Formation were mainly derived from the uplifted Gangdese arc-trench system associated with the obduction of the Asian subduction complex. The compositions of Cr-rich spinels in the Youxia and Shenkeza sandstones resemble those from fore-arc peridotites and were most likely derived from the arc and ophiolite rocks along the developing Yarlung-Zangbo suture to the north. No spinels have been observed in the Jidula sandstones. Therefore, the early Tertiary detrital clastics in the Zhepure Shan record a marked change in provenance and sediment character and specifically at the time of deposition of the Youxia Formation, which contains a zone P-8 foram assemblage. This change indicates that the onset of India-Asia collision and the first development of the foreland basin immediately south of the India-Asia suture zone occurred at 50.6 ± 0.2 Ma in the both the western (Zanskar) and eastern (this study) Tethyan Himalaya.

Rowley (1996), Rowley (1998), Zhu et al. (2005)

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