Tectonic divisions of the Qinghai-Tibet Plateau and structural characteristics of deformation on their boundaries

Abstract：The determination of macroscopic structural characteristics has great significance for the study of the uplift of the Qinghai-Tibet Plateau and its “dynamic model construction”. The Qinghai-Tibet Plateau represents a gigantic “convergence-intracontinental subduction type” lithospheric block formed by amalgamation of six terranes of three plates (or old land), i.e. the North Kunlun-Altyn Tagh-Qilian terrane of the Tarim-Sino-Korean plate, the South Kunlun terrane and Hoh Xil-Bayan Har terrane of the South China-Southeastern Asian plate and the Qiangtang terrane, Gangdise terrane and Himalayan terrane of Gondwanaland, through multiple breakups, convergences and intracontinental subductions. It is separated from the Indus lithospheric block, Tarim-Alxa-Ordos lithospheric block and Yangtze lithospheric block by the south Qinghai-Tibet Plateau marginal junction zone, north Qinghai-Tibet Plateau marginal junction zone and east Qinghai-Tibet Plateau marginal junction zone respectively. According to the present-day dynamic characteristics, this gigantic lithospheric block (first-order tectonic unit) may be subdivided into four second-order tectonic units, namely, the Himalayan block, northern Tibet block, southern Qinghai block and Kunlun-Altyn Tagh-Qilian block, which are bounded by the Yarlung Zangbo junction zone, Xijir Ulan-Jinsha River junction zone and Central Kunlun junction zone successively. The four blocks may be further divided into several fault-bounded third-order tectonic units (terranes).The various tectonic units making up the gigantic Qinghai-Tibet lithospheric block are situated in a unifying geodynamic system. This geodynamic system is generally manifested as follows：the Himalayan block and Kunlun-Altyn Tagh-Qilian block on the southern and northern sides of the Qinghai-Tibet lithospheric block are asymmetrically thrust toward and stacked on the cold and rigid Indus lithospheric block and Tarim-Alxa-Ordos lithospheric block respectively in the tectonic setting that the Indian plate is continuously and strongly subducted beneath the Eurasian plate and the hot Qinghai-Tibet lithospheric block with ductile rheological properties moves en bloc in a NNE direction. Beneath the northern Tibet block and southern Qinghai block in the interior of the Qinghai-Tibet Plateau, large numbers of low-velocity bodies at depth upwell and propagate horizontally from west to east. In this tectonic setting, the former is superimposed by nearly N-S compression, forming a tectonic framework marked mainly by N-S-trending downfaulted zones and NW- and NE-trending conjugate strike-slip motions； whereas for the latter, except for the Songpan-Garzê terrane that shows south-vergent thrust stacking, the Hoh Xil-Bayan Har terrane is mainly manifested by sinistral strike-slip motion leading to successive eastward extrusion, so that the whole southern Qinghai block experiences thrust propagation toward the Yangtze block and spreading toward the Sanjiang tectonic zone. Therefore, with increasing rate of uplift with time, the plateau also propagates gradually toward the rigid blocks at its peripheries, i.e. the area of the plateau expands progressively. So the plateau boundaries have the nature of propagation. On the basis of the propagation mechanism, two types of dynamic boundary may be distinguished：strike-slip type propagation boundary and thrust type propagation boundary. The typical propagation boundary is located in the Altyn Tagh Mountains on the northern margin of the Qinghai-Tibet Plateau and the Sanjiang area on the eastern margin of the plateau. The dynamic boundary on the southern margin of the plateau belongs to the typical thrust type propagation boundary, while the dynamic boundary in the Qilian and Longmen mountains has both the nature of thrust propagation and the nature of strike-slip propagation.