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

Abstract: The Bainang terrane, an intra-oceanic island are subduction complex into which Tethyan oceanic rocks were accreted during the Cretaceous, is preserved within the Yarlung-Tsangpo suture zone of Tibet. The lithostratigraphic succession established from field mapping records a long history of sedimentation in different portions of the central Tethyan domain from Late Triassic to mid-Cretaceous time. These rocks are preserved within a south-verging imbricate thrust stack of thin ([much less than]1 km thick) northward younging tectonic slices. Five lithotectonic units were mapped in the terrane and these units are assigned to two distinct tracts. The northern tract, which accumulated on the north side of Neotethys, was probably separated from its southern counterpart by a mid-ocean ridge. Detailed radiolarian biostratigraphy is used to constrain the timing of depositional events within each tract. Oceanic plate stratigraphy of the northern tract records its northward travel and mid-Cretaceous (late Aptian) approach towards a south-facing intra-oceanic subduction zone. Rocks in the southern tract developed closer to the Indian subcontinent and experienced thermotectonic subsidence and Mid-Jurassic basic alkaline intraplate magmatism. They were probably accreted late in the Cretaceous. Variations in structural style across the terrane indicate deformation at different depths and vertical growth of the wedge rather than lateral accretion. The overall tectonostratigraphy of the terrane reflects its development in a remote intra-oceanic setting.

Keywords: Tibet, Mesozoic, accretionary wedges, subduction, accretion, Indus- Yarlung-Zangbo suture zone, radiolarians.

The Yarlung-Tsangpo suture zone in Tibet marks the zone of collision between India and Eurasia. The once vast Tethyan Ocean closed along this suture during Cenozoic continent-continent collision. Much of what lay within this ocean was subducted, smeared out or otherwise destroyed through subduction during convergence between India and Asia or disappeared during the collision. all that remains is now trapped within the few kilometres width of the suture zone. Investigations of terranes within the suture provide insights into the architecture and evolution of the Tethyan Ocean interior.

The evolutionary history of the Neotethys has been inferred from the continuous sedimentary record preserved in the northern Indian passive margin series (Gaetani & Garzanti 1991; Liu & Einscle 1994). However, this interpretation addresses only the sedimentary response to multiple rifting events along the southern Neotethyan margin. The depositional history and architecture of the central Neotethyan domain remains unexplored. Although the movement of India towards Eurasia is well documented from magnetic anomalies on the Indian Ocean floor and other magnetic data (Klootwijk et al. 1992), the sequence of events that accompanied India-Asia convergence is thus far poorly understood especially in the eastern (Tibetan) segment of Neotethys. In spite of an early suggestion (Allegre et al. 1984; Proust et al. 1984) of the possibility of intra-Tethyan subduction in this sector of Tethys, the later and more widely accepted model (Searle et al. 1987) assumes that the entire north-south extent of oceanic lithosphere was subducted along the southern margin of the Lhasa terrane. Summaries of the structure and evolution of this region appear somewhat dissimilar to those for areas further west (Kohistan and Ladakh), where the Spontang and Kohistan-Dras volcanic arcs, associated with intra-oceanic subduction, are well documented (Searle et al. 1987, 1999; Corfield et al. 1999). The simplified account of the eastern part of the Neotethys has recently been reassessed with the recognition of remnants of an intra-oceanic subduction system (Aitchison et al. 2000; McDermid et al. 2002) and seismic tomographic images under the region (Van der Voo et al. 1999).

In this paper, we present the results of detailed geological mapping, and structural and radiolarian biostratigraphic studies of the Bainang terrane. The terrane developed as a subduction complex, on the southern edge of a south-facing intra-oceanic subduction system, which grew above a northward-subducting slab of Neotethyan ocean lithosphere. Facies of the central Neotethyan domain were accreted into this subduction complex. New data elucidate the tectonic setting and evolution of this terrane, and they are interpreted in terms of depositional setting and the mode and timing of subduction-related accretion. We contrast the geological development of the Bainang terrane with that of other terranes in the region and consider this in the broader context of Neotethys evolution.

Regional tectonic framework

Six tectonostratigraphic units (terranes) that developed prior to India-Eurasia collision are recognized within and bounding the Yarlung-Tsangpo suture zone (Fig. 1). From north to south, we briefly outline their nature and key features related to the evolution of the Tethys, following the nomenclature introduced by Aitchison et al. (2000).