Bainang Terrane, Yarlung-Tsangpo suture, southern Tibet (Xizang, China): a record of intra-Neotethyan subduction-accretion processes preserved on the roof of the world

Journal of the Geological Society, May 2004 by Ziabrev, Sergey V, Aitchison, Jonathan C, Abrajevitch, Alexandra V, Et al

The Dazhuqu terrane comprises a series of ophiolitic bodies traceable along the Yarlung-Tsangpo suture zone with major outcrops in the Xigaze and Luobusa areas (Aitchison et al. 2004). Near Zedong and Luobusa ophiolitic rocks are faulted against the Zedong terranc or thrust northwards over lowermost Miocene conglomerates developed along the southern margin of the Lhasa terrane (Aitchison et al. 2002). In the Xigaze district, the ophiolite is thrust northwards over the Xigaze terrane (Burg 1983; Wang et al. 1987). The southern margin of the terrane lies, in most areas, at the Miocene north-directed Renbu-Zedong thrust (Yin et al. 1994, 1999), which places Indian terrane rocks over the ophiolite. In the Bainang district, where there is an Sshaped sigmoidal bend in the Yarlung-Tsangpo suture zone, earlier contacts can be observed at south-directed thrusts that are locally truncated by strike-slip faults (Girardeau et al. 1985a; Ratschbacher et al. 1994).

Several ophiolitic massifs, in the Xigaze area, form a nearly continuous belt over 150 km long and up to 25 km wide. Ophiolitic sections are mostly north-facing with the sequence repeated across dextral strike-slip faults. Although tectonically disrupted and heavily attenuated, sections locally display a complete ophiolitic sequence from fresh Cr-diopside-rich harzburgites to marine sedimentary cover on mafic volcanic rocks (Nicolas et al. 1981; Girardeau et al. 1984, 1985a, b). Radiolarian biostratigraphy constrains the timing of eruption of ophiolitic basalt to the late Barremian-early Aptian (Zyabrev et al. 1999; Ziabrev et al. 2003). Aitchison et al. (2000) interpreted the Dazhuqu terrane ophiolite as having originated in an intraoceanic suprasubduction zone setting and this is supported by detailed mineralogical and petrochemical studies in the Xigaze area (Hebert et al. 2000, 2001).

The Bainang terrane, on the southern side of the suture zone, was interpreted by Aitchison et al. (2000) as a subduction complex, and is the subject of this paper. It contains units previously referred to as infra-ophiolitic thrust sheets of radiolaritcs (Burg & Chen 1984) or Upper Jurassic-Lower Cretaceous red radiolaritcs (Girardeau et al. 1984). The terrane is bounded to the north by ophiolitic rocks of the Dazhuqu terrane and to the south by the Indian terrane. Good exposures exist near Donglha, Xialu and Bainang (Fig. 1). In most sections studied, the terrane is chert dominated and is characterized by a northfacing tectonic pile of oceanic lithologies repeated by a series of south-verging imbricated slices. Radiolarians reported from siliceous rocks near Xialu range in age from the Mid-Jurassic to midCretaceous (Aptian) (Wu 1993; Matsuoka et al. 2001, 2002).

Passive margin rocks of the Indian terrane or Tethyan (Tibetan) Himalaya lie south of the suture. Thick Permian to Cretaceous continental rise deposits (Liu & Einsele 1994) merge southward into a continuous Ordovician to Eocene shelf sedimentary succession of marine carbonate, sandstone, siltstone and shale (Bureau of Geology and Mineral Resources of Xizang Autonomous Region 1993; Jadoul et al. 1998). Ordovician-Early Permian epicontinental deposition in shallow seas linked to Tethys terminated with rifting that evolved into detachment of Peri-Gondwana microcontinents and, ultimately, the opening of Tethys. The Mesozoic sequence records increased tectonic subsidence in the Carnian-Norian, followed by building of carbonate platforms. Drowning along the entire length of the carbonate platform occurred in the early Callovian with deposition of oolitic ironstone superseded by Late jurassic deposition of black shales (Gaetani & Garzanti 1991; Jadoul et al. 1998). The development of the passive continental margin facing the Tethyan domain was punctuated by a series of rifting episodes related to Gondwana disintegration and associated with intraplate volcanism (Gaetani & Garzanti 1991). In correlative sections of the western Himalaya, the Zanskar shelf merges northward with Mesozoic slope-rise deep-sea deposits of the Lamayuru Complex and its distal equivalent, the Karamba Complex (Danclian & Robcrtson 1997; Robertson & Sharp 1998).