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

Bainang terrane

A NE-SW-oriented, 35 km long tectonic lens of Bainang terrane rocks is preserved at a well-developed bend in the trace of the Yarlung-Tsangpo suture zone located east of the township of Bainang. Exposure pinches out tectonically near Bainang in the west and eastwards towards Dazhuqu. The terrane is bounded to the NW by the Dazhuqu terrane. The contact is a south-directed thrust that dips 60-70� NW and places an ophiolitic assemblage in the hanging wall over the Bainang terrane. To the south, the Bainang terranc is juxtaposed, along another moderately to steeply (45-85�) dipping south-directed thrust, over a footwall of Indian terrane lithologies. The overall geological structure within the terrane is that of an imbricate thrust stack containing numerous north-facing and chiefly south-verging tectonic slices. Slices incorporate oceanic pelagic and hemipelagic lithologies such as chert, siliceous, calcareous and tuffaceous mudstone, limestone, and siliceous and calcareous shale. Individual thrust slices are thin (5-60 m) and pinch out over short distances. Units shown on the geological map and cross-sections (Fig. 2) represent distinctive packages of these slices in which similar lithologies arc tectonically stacked. Some packages are internally deformed by folds on various scales and shearing is widespread. Deformation intensity increases progressively from NW to SE across strike. The abundance of widely spaced layer-parallel shear zones increases until an anastomosing cleavage peaks with development of a strong foliation in the SE. An east-west-striking sinistral strike-slip fault locally truncates the southern margin of the Bainang terrane where it offsets 1 km^sup 3^ size fragments of the Dazhuqu terranc by at least 20 km. Four NW-SE-striking faults diagonally crosscut the western part of the mapped area with sinistral offsets of 250-500 m and are synthetic to the sinistral strike-slip fault along the southern flank of the terrane. Late strike-slip faulting is interpreted as related to collisional deformation (Ratschbacher et al. 1994).

Stratigraphy and structure

Original lithostratigraphic sections within the terrane are heavily disrupted but this is compensated for by exceptional exposure. Sufficient original contacts between lithologies can be locally observed such that the original stratigraphie succession is determinable with reasonable confidence. Much of the terrane is structurally disrupted but block-in-matrix style melange is rare. Detailed field investigation permits the recognition of five mappable lithotectonic units in the Bainang area. Their discrimination is based on the proportions of characteristic lithologies, and structural style (Table 1). From north to south, the units are the Bangga, Zongxia, Maniga, Yalongmai and Renchingang units. Structural variation permits recognition of the Sakabu and Tsashibu subunits within the Maniga unit and the Chiangdui and Baigang subunits in the Yalongmai unit. all names are taken from local villages situated close to outcrops of each unit. In general, units trend NE-SW. Overlap between consecutive units is somewhat discordant and individual units wedge out along strike (Fig. 3). The two northern units have similar stratigraphie records that differ noticeably from the other three. Accordingly, units are combined into the northern and southern tracts.