Deep structure and orogenic mechanism of the Kunlun Mountains

Abstract:According to the information obtained by geological and geophysical work along INDEPTH- Ⅳ profile, the authors formulated a new model for the Kunlun Mountains orogeny based on detailed comprehensively studies. Some main viewpoints heldby the authors are as follows: (1)In the Early Permian, the subduction of Songpan - Ganzi Ocean toward the Kunlun- Qaidam block resulted in the formation of continental margin arc and extension area of arc. With the continuous collision and squeezing of Songpan - Ganzi and Kunlun-Qaidam blocks, there occurred different tectonic evolutions. (2)Old crystalline basement of Kunlun region was lifted up to form mountains and subjected to strong erosion during the face-to-face squeezing of the land masses, and hence the old crystalline basement and the plutonic rocks were exposed on the surface due to denudation. Kunlun terrane did not overthrust northward to the Qaidam. South Kunlun block thrust over the North Kunlun block, with the breaking depth reaching10 km. (3)The crustal thickening of Kunlun occurred mainly in the lower crust (6.2-6.7 km/s), mainly mafic lithosphere. (4)As the back- arc extension zone of the Kunlun Mountains, the Qaidam Basin was uplifted and experienced subsidence with the Kunlun orogenic uplift, and Cenozoic sedimentary thickness was 12- 14 km resulting from two sorts of actions (deposition and compression). Crystalline basement underwent rifting and formed a new rift, with the width of rift being about 12 km and the depthbeing about 4 km. The conductive zone implied that the rift was connected with the deep thermal fluids through the fracture. (5)The Moho depth of the Qaidam Basin is 52 km, that of the Kunlun Mountains is 65-70 km, and the Moho level lies near Golmud (with a distance of 185 km). (6)The flysch layer thickness of Songpan-Ganzi terrane is 10-14 km. Shallow layers were thickened by Tertiary Fenghuoshan thrusting. The underlying uniform velocity layer (with high conductivity display) is 6.2-6.3 km/s, which is a unique phenomenon of the terrane. The thickness of the accumulation of residual oceanic crust was 15 km, the shallow layer was thickened through Tertiary volcanic nappe system, and 15 km of thick rock bed squeezed into middle crust. (7)The model that Asian lithospheric mantle subducted into the Qaidam mantle lithosphere was denied. The authors hold that the Indian continental lithosphere mantle was separated into two layers, stretching northward along the bottom of the plateau and was subducted over the lithosphere mantle of Qaidam. That is a new way of length adjustment caused by the squeezing process of the north and south of the plateau.