Oblique-slip basin development during island are collision along the Yarlung-Tsangpo suture zone, Tibet

Rapidly deposited coarse clastic rocks of the Liuqu Conglomerate record a phase of sedimentary basin development along the Yarlung-Tsangpo suture zone (YTSZ) in Tibet sometime between Late Cretaceous-earliest Eocene. Sediments are dominated by conglomerates and deposition occurred in both terrestrial and sub-aqueous settings. Facies changes are commonly abrupt with rapid changes in clast types, grain size and depositional styles. Proximal deposits are locally offset relative to their original source terranes. These coarse clastic sedimentary rocks are interpreted to have been deposited in oblique-slip basins that developed between the leading (northern) edge of the Indian margin and an Early Cretaceous island arc that lay within Tethys. Sediments were derived from both areas and the conglomerates record the history of collision between these terranes. A total absence of clasts derived from terranes to the north of the Yarlung-Tsangpo suture suggests that basins in which the coarse clastic units accumulated developed prior to the main collision between India and Asia. Liuqu Conglomerate, crops out over a 150 km strike length along, and restricted to, the YTSZ near Xigaze. These sedimentary rocks have not been the subject of any previous detailed investigation and hold a key to understanding important phases in the tectonic evolution of the suture. Liuqu Conglomerate varies from one area of outcrop to another but is essentially a coherent mappable unit. Coarse-grained mineralogically and texturally immature sediments indicate deposition proximal to source. Fossils from lacustrine sediments (Tao, 1988) indicate that deposition occurred sometime between latest Cretaceous to earliest Eocene. Formation of the unit thus postdates Early Cretaceous intra-oceanic arc activity (Aitchison et al, 2000) and significantly predates the final India-Asia convergence. Lithofacies present in the conglomerate are grouped into associations that are interpreted in terms of major depositional environments. Facies Association 1-alluvial fan, Facies Association 2-braided river, Facies Association 3-distal braided river, Facies Association 4-lower shoreface, Facies Association 5-standing body of water below wave base. Liuqu Conglomerate was deposited upon, and contains clasts of, Early Cretaceous intra-oceanic arc-related terranes and the Indian passive margin. These conglomerates are the oldest sedimentary rocks in Tibet in which a mixture of clasts derived from these various terranes is present. As clast petrography indicates the close proximity of source terranes, which must have developed in strikingly different tectonic settings any depositional model needs to explain their juxtaposition. Our preferred model for accumulation of the Liuqu Conglomerate is within a valley, or series of valleys, that most probably developed along the collisional axis between an Early Cretaceous intra-oceanic arc to the north and the Indian continent, which lay south of the accreting arc.