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 original disposition of terrancs within the YarlungTsangpo suture zone has been greatly disrupted and former relations between terrancs are not well constrained. Therefore, reconstruction of the tectonic evolution of the area is difficult. Most early models invoked the existence of a single Andean-type convergent plate margin along the northern side of Neotcthys but analogy with the modern western Pacific and SE Asia suggests that reality may have been considerably more complex. Development of the Xigaze terrane is interpreted as having been related to evolution of the magmatic arc along the southern edge of the Lhasa terrane (Einsele el al. 1994; Durr 1996). The Xigaze terrane was formerly regarded as being floored by the Dazhuqu ophiolite (e.g. Burg & Chen 1984; Girardeau et al. 1984; Einsele el al. 1994; Durr 1996), but these two units arc ubiquitously in tectonic contact and there is no a priori reason why they should have been genetically related (Aitchison et al. 2000). The cooccurrence and remarkably consistent north-south distribution of the broadly coeval Zedong (magmatic arc), Dazhuqu (forearc ophiolite) and Bainang (subduction complex) terrancs led to their interpretation as evidence for a south-facing intra-oceanic subduction system that lay within the Neotethys (Aitchison et al. 2000) and the existence of more than one convergent margin. As more details and constraints on the evolution of terrancs within the Yarlung-Tsangpo suture zone become available the complexity and sophistication of models for this zone increase.

Methods

Preliminary examination of the Bainang terrane at several sections revealed a structural style and lithologies reminiscent of subduction complexes. The most complete section occurs near Bainang and this was selected for detailed study because of excellent exposures. This area was mapped in detail (1:25000) to discern map-scale structures and obtain a solid basis for structural and biostratigraphic data and interpretations. Special attention was paid to the nature of contacts between different lithologies. Depositional contacts are locally preserved and these were used to reconstruct an oceanic plate stratigraphy. Radiolarian biostratigraphy was applied as a key method to constrain ages and as a means of cross-checking the reconstructed lithostratigraphic succession established during field-mapping. all prospective lithologies were extensively sampled with sedimentological features and details of mesoscopic structural patterns documented. Individual traverses were sampled systematically along continuous profiles to clarify structure and trace possible age progressions. Radiolarians were picked from dilute HF and/ or HCl acid residues, and imaged using a Hitachi SEM. Identification of taxa and age assignment for Middle jurassic to middle Cretaceous radiolarian assemblages arc based on recent taxonomic study and biostratigraphic zonation of Tethyan radiolarians (Jud 1994; O'Dogherty 1994; Baumgartner et al. 1995). For Lower Jurassic and Triassic assemblages other zonal schemes (Pessagno & Whalen 1982; Kishida & Hisada 1985; Yell 1987; Carter et al. 1988; Hori 1990; Carter 1993) were applied. Over 130 radiolarian-bascd ages were acquired in the course of this study.