Search

Since 1999, independently derived geophysical and geological models have been published arguing for an intra-oceanic subduction system along essentially the entire width of the India–Eurasia collision belt. This idea conflicts with earlier proposals, where in the eastern part of the convergence zone Neotethyan mid-ocean ridge-generated lithosphere directly north of the Indian craton was consumed beneath Eurasia in Tibet in an Andean-type plate configuration. New palaeomagnetic data are reported from three Barremian–Aptian (∼ 120 Ma) sequences of chert, siliceous mudstones and volcaniclastic rocks. These rocks form the uppermost part of the Dazhuqu supra-subduction zone ophiolite terrane, which crop out along substantial portions of the India–Eurasia (= Yarlung Tsangpo) suture zone in southern Tibet. The declination data provide little regionally-useful tectonic information; they are dominated by local rotations, presumably related to the Dazhuqu terrane's initial obduction onto the India plate in the Palaeocene and subsequent movement(s) as India later collided and indented into Eurasia. The inclination data are, however, useful because they yield consistent sub-equatorial formation sites (2.7 °S ± 3.0°, 7.9 °N ± 2.7°, 1.4 °N ± 5.7°), which correspond with the location of the Neotethyan intra-oceanic subduction system inferred from the seismic tomographic data. At the time these Dazhuqu terrane rocks formed, the intra-Tethyan subduction zone would have been about 2500 km south of Eurasia.

The Bainang terrane, an intra-oceanic island arc 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 (≪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.

The histories of individual conglomeratic units along the Yarlung-Tsangpo (River) suture zone in southern Tibet reflect significant phases in the Mesozoic to Cenozoic tectonic evolution of this area. Several temporally distinct conglomerate units are recognized along the suture, and their detailed examination permits analysis of the collision between India and Asia. Upper Jurassic to Lower Cretaceous conglomerates crop out within the Sangri Group along the southern Lhasa terrane. They are dominated by limestone and andesitic volcanic cobbles derived entirely from the Lhasa terrane. These rocks have experienced amphibolite facies metamorphism, and exhibit a strong penetrative regional foliation. Thick successions of the Palaeocene Liuqu Conglomerate crop out within the suture from Xigaze to Lhaze. They contain detritus sourced from intra-oceanic terranes associated with the suture zone, as well as clasts of Indian affinity, while Lhasa and Xigaze terrane-derived material is notably absent. These conglomerates record an early suture zone event prior to India-Asia collision. Uppermost Oligocene to Lower Miocene 'Gangrinboche facies' conglomerates crop out on the southern edge of the Lhasa terrane along the length of the suture. Several correlative units within this facies exhibit broadly similar stratigraphic histories. A basal depositional contact upon an eroded Lhasa terrane surface is ubiquitous with initial clast derivation from the north. Up-section, the first arrival of coarse-grained, suture-zone and India-derived clasts, is abrupt. These southerly derived clasts predominate by the top of most sections. An a really restricted succession of gently dipping Late Neogene ultramafic breccias unconformably overlies folded Liuqu Conglomerate near Quanggong. Other Neogene sediments are extensive west of Mount Kailas. Deposition of coarse clastic sediments is presently continuing along the length of the Yarlung Tsangpo. Discrimination and detailed investigation of each of these units will improve our understanding of the evolution of the India-Asia collision.

Well-preserved, abundant radiolarians provide high-precision biostratigraphic age constraints on the timing of the eruption of ophiolitic basalts exposed along the Yarlung–Tsangpo suture zone in southern Tibet. Dazhuqu terrane ophiolites were generated in an intra-oceanic supra-subduction zone setting within a relatively short (

Ophiolitic rocks distributed along the Yarlung Tsangpo suture zone in southern Tibet are the few remaining fragmentary remnants of many thousands of kilometres of the ocean space that formerly existed between India and Eurasia. Portions of mid-Jurassic and mid-Cretaceous intra-oceanic island arcs can be recognized amongst those rocks that have been studied in detail. Complete suprasubduction zone ophiolite successions are preserved in the Dazhuqu terrane, which crops out both east and west of Xigaze. Radiolarians in inter-pillow cherts and immediately overlying sedimentary rocks indicate a Barremian ophiolite generation event. Palaeomagnetic data show that this ophiolite formed at equatorial latitudes south of the Lhasa terrane before its south-directed emplacement onto the northern margin of India. Highly refractory ultramafic rocks in the Luobusa ophiolite appear to be of Mid-Jurassic age and are potentially related to intra-oceanic island arc remnants in the nearby Zedong terrane. Ophiolitic massifs along the suture in western Tibet are thrust southwards onto northern India and record Late Jurassic ocean-floor development. Miocene north-directed back-thrusting associated with India-Asia collision has further complicated interpretation of regional geology. The ophiolitic rocks of the Yarlung Tsangpo suture zone provide evidence for the former existence of multiple oceanic island arc segments within Neotethys and suggest that consumption of the oceanic space between India and Asia was more complicated than has been predicted by existing models

Lower Miocene conglomerates crop out along the length of the Yarlung Tsangpo suture zone on the southern margin of the Lhasa terrane. These conglomerates, known by various local names, are correlated herein as the Gangrinboche conglomerates. All units exhibit broadly similar stratigraphic histories and a basal depositional contact upon an eroded surface of rocks of the Lhasa terrane is ubiquitous. At most localities the tops of sections are either removed by erosion or truncated by north-directed thrusts. These conglomeratic molasse units developed in response to the India/Asia collision and record aspects of its development. In all units initial clast derivation was from the Lhasa terrane on the northern margin of the Yarlung Tsangpo suture zone. Up-section the first appearance of clasts derived from terranes within the suture zone and the northern margin of India, all of which lie to the south of any outcrops of Gangrinboche conglomerates, is observed. Although these units were previously thought to be Eocene, analysis of fossil and structural constraints indicates Early Miocene deposition. As development of the Gangrinboche conglomerates records a significant phase in the evolution of the India–Asia collision understanding of their age and stratigraphic evolution has wider implications for regional tectonic models.

Extensive field investigations along the Yarlung–Zangbo suture zone in southern Tibet reveal the presence of now fragmented remnants of a south-facing intra-oceanic subduction system. This system developed within Tethys during the Cretaceous. The associated arc, forearc ophiolite, and subduction complex were emplaced onto the leading edge of India at the end of the Cretaceous. Rapid sedimentation in oblique-slip basins and disruption of water-saturated sediments into melange was widespread and concomitant with ophiolite emplacement. We describe the tectonic entities that developed during this previously unrecognized phase of Tethys–Tibet evolution and present a new model for the evolution of this portion of Tibet.

Detailed investigations along the Yarlung Tsangpo suture zone, Tibet result in the following conclusions. (1) Arc, forearc, and subduction complex elements of at least one intra-oceanic island arc which once lay within Tethys and accreted to India prior to its collision with Asia are preserved within the suture. (2) Collision-related tectonic melange is widespread. (3) Temporallydistinct conglomeratic molasse units were developed; they are related to different collision events. (4) Improved radiolarian fossil data place constraints on timing of the development of intra-oceanic terranes and melange formation. (5) A simple unconformable contact occurs between Upper Oligocene–Lower Miocene conglomerates and the southern Lhasa terrane; the socalled south-directed "Gangdese thrust" does not exist. These new results suggest the former existence of an intra-oceanic subduction system within Tethys and indicate that considerable revision of existing models for the evolution of the Yarlung Tsangpo suture zone is required.