More about the missing Tethys in the Yarlung Zangbo suture

The Yarlung-Zangpo suture zone, the youngest in Tibet, marks a site of collision of the Eurasia continent and Indian subcontinent. It is characterized by a discontinuous line of ophiolitic bodies in southern Tibet and beyond. Terranes of different tectonic affinity are scattered along this line and represent remnants of the Tethys, a wide oceanic expanse existed throughout the Mesozoic and eventually closed along the suture in the Eocene. These terranes are targets of an extensive study by the Tibet Research Group, HKU aimed at more comprehensive understanding of the Tethys closure and tectonic evolution of the suture. Here results of structural and biostratigraphic investigation of the Bainang terrane are presented. The Bainang terrane, distinguished in the central part of the suture, comprises rock units previously referred to as infra-ophiolitic thrust sheets of radiolarites or Upper Jurassic-Lower Cretaceous red radiolarites. It is situated between the ophiolitic suite of the Dazhuqu terrane to the north and Indian passive margin series to the south. The terrane is characterized by a pile containing numerous north-facing and chiefly south-verging imbricated slices. These tectonic slices incorporate various lithologies: chert, siliceous and tuffaceous mudstones, limestone, siliceous and calcareous shales. The most complete and well-exposed piece of the terrane near Bainang has been mapped and sampled for radiolarian biostratigraphic study. Five litho-tectonic units are discriminated on the basis of characteristic lithologies and structural styles. From north to south they are: Bangga, Zongxia, Maniga, Yalongmai and Renchingang units. The Bangga unit is mostly composed of red ribbon chert with minor siliceous mudstone and is characterized by scattered shear zones bounding tectonic slices and lenses several tens of meters thick. The Zongxia unit comprises predominant siliceous mudstone with numerous tectonic lenses of red ribbon chert. Conjugate tectonic lenses bounded by shear zones is a dominant structure. In both units obtained radiolarians indicate that chert is Late Triassic to Early Cretaceous and siliceous mudstone accumulated in a short interval from middle to upper Aptian. Random distribution of radiolarian ages along several profiles across the Bangga unit testifies for a rather complex internal structure. The Maniga unit is made up of more diverse lithologies which include: (1) Upper Triassic-Middle Jurassic micritic limestone, calciturbidites, siliceous-calcareous rocks, and varicolored calcareous mudstone; (2) Aalenian-Bajocian ferruginous chert; (3) Bajocian-Oxfordian tuffaceous chert and mudstone, basic tuff and silicified tuff; and (4) Kimmerigian-Tithonian red ribbon chert. Rocks of the unit are intensely sheared with typical phacoidal and S-C fabrics. Intensity of shearing progressively increases southwards (across strike) and eastwards (along strike). To the south, this part of the terrane merges to the Yalongmai unit that includes the same lithologies but with a greater proportion of calcareous mudstone. Rocks are more intensely sheared and structural style is characterized by penetrative foliation with stretching lineation deformed by later sporadically developed crenulation and abundant kink bands. In the southernmost Renchingang unit the predominant lithology is gray calcareous shale with protoliths of meter-thick calciturbidites. Subordinate red radiolarian chert and varicolored calcareous shale compose thin and disrupted but traceable tectonic slices. Structural style is the same as for the previous unit. In general, structure of the Bainang terrane is characterized by slice imbrication with elements of large-scale ramp folds comparable to that of other accretionary wedges. A vertical rather than lateral growth of the wedge is most likely mirrored in the structural patterns of the studied fragment. A long history of sedimentation supposedly in different and separate parts of the Tethys is recorded in the reconstructed stratigraphic columns for different units. The youngest rocks incorporated into the Bainang accretionary wedge are mid-upper Aptian siliceous mudstones, hemipelagic deposits similar to lithologies known from many accretionary wedges on-land. The earliest stage of subduction-accretion is thus inferred to have begun by the end of the Aptian as recorded in the studied fragment of the terrane. Although the Aptian siliceous mudstones of two northerly units possess abundant and sometimes thick tuff layers, the lack of any other clastic rocks shed off a volcanic arc is characteristic for this piece of the accretionary wedge. We suggest this may indicate an intra-oceanic setting for this remnant of the accretionary system. Another feature of the terrane is numerous basic sills that intruded rocks of the three southernmost units prior to shearing. The sills probably originated from pre-Kimmeridgian oceanic intra-plate alkaline basic magmatism on the Tethys floor close to the Indian subcontinent.