Abstract:
The highest plateau in the world has attracted geologist’s attention for centuries. Significant crustal shortening, which led to the eventual construction of the Cenozoic Tibetan Plateau, began in the Himalaya in the south and the Qilian Shan in the north (over than 1000km) at approximately the Middle Cenozoic. The Paleozoic and Mesozoic tectonic histories in the Himalayan-Tibetan orogeny exerted a strong control on the Cenozoic strain history and strain distribution. Moreover, the Tibetan plateau formed by several terrane slices each of which is about several hundred kilometer wide. Assuming that the lithosphere thickness is also on the 100 km scale, the small width-thickness ratio must have induced a strong influence of mantle dynamics on crustal deformation because of the strong crustal deformation that is located mainly at the continent edges.
The Qiangtang terrane in Central Tibet, with its high-pressure rocks, is a key area to unravel the evolution of the Paleo-Tethys. The presence of widespread Mesozoic subduction mélange and high-pressure rocks in the Qiangtang terranes can have importance consequences for the formation of the Cenozoic Tibetan plateau. Long-lasting and detailed mapping work offer us the possibly to unravel the geological evolution of the Qiangtang terrane and further test existing geodynamic models in order to better understand the evolution of small terranes.
1:50,000 mapping results from the Rongma area in the central Qiangtang terrane show that the Qiangtang metamorphic belt can be separated into a Paleozoic autochthonous basement and an overlying allochthonous thrust stack of subduction mélange that contains high-pressure rocks and Permian sediments. Detrital zircon ages (youngest peak at 591 Ma) and an ~ 470 Ma granite intrusion age constrain the age of the Qiangtang Basement to be between the Late Precambrian to Middle Ordovician. This is in agreement with the observed unconformity between basement and overlying Mid–Late Ordovician strata. Based on detrital age spectra and an Early Ordovician unconformity we prefer that the Qiangtang terrane has Gondwana affinity during the Early Paleozoic.
The occurrence of Late Triassic eclogite and glaucophane-bearing schists in the Central Qiangtang terrane indicates the existence of a suture zone between the North and South Qiangtang terranes before the Late Triassic. Detailed 3D mapping analyses indicate that the high-pressure rocks were exhumed from underneath the south Qiangtang terrane in an extensional setting caused by the pull of the northward subducting slab of the Shuanghu–Tethys. High-pressure rocks, sedimentary mélange and margin sediments were thrusted on top of the ophiolitic mélange that was scraped off the subducting plate. Both units were subsequently thrusted on top of the south Qiantang terrane continental basement. These conclusions are also supported by sediments and magma ages.
Onset of Late Triassic sedimentation marked the end of the amalgamation of both Qiangtang terranes and the beginning of spreading between Qiantang and north Lhasa to the south, leading to the deposition of thick flysch deposits in the Jurassic. Strongly folded Jurassic flysch is unconformably overlain by weakly deformed mid-Cretaceous conglomerate and volcanics. This relationship demonstrates closure of Banggong Hu-Nujiang Ocean during Early Cretaceous. Collision between the Lhasa and Qiangtang terranes led to fast exhumation in the Qiangtang terrane during ~140-90 Ma, with exhumation rates about 0.15-0.3 mm/y. The Qiangtang terrane was above sea level up to the mid-Cretaceous and experienced significant denudation prior to mid-Cretaceous times. This model implies substantial crustal thickening and perhaps plateau formation in central Tibet prior to the Indo-Asian collision.
A new model for the exhumation of (ultra-) high-pressure ((U)HP) rocks was developed, based on the observation of exhumed and obducted (U)HP rocks in the study area. It is suggested that exhumation was caused here by pulling up of the subducted slab in a double divergent subduction setting. The long northward subducting slab is envisaged to have extracted the short southward subducting slab. This model was further investigated with numerical modelling. It is sugested that the proposed mechanism may explain the very fast and recent exhumation of eclogites of the d'Entrecasteaux Island off the coast of Papua New Guinea, as well as the fast opening of the Woodlark Basin.
High-pressure rocks