THE MESOZOIC-CENOZOIC EVOLUTION OF THE TECTONIC AND CLIMATIC PATTERNS, NW CHINA

The formation of the arid northwest China (ANW) is significant to the understanding of the present tectonic and environmental patterns of China. Based on the integrated study of the tectonic and environmental events in northwest China, we found four-phases of tectonic movement, including Indosinian, Early Yanshanian, Late Yanshanian and Himalayan, occurred in this region during Mesozoic and Cenozoic, which had played important role in the formation of ANW. Generally, it was dry and hot during the period from Triassic to Eocene but has been dry and cool since Oligocene in this region. The closure of the Paleo-Asian ocean and the subsequent collision of Qiangtang, Lhasa and India with Asia made northwest China gradually moving away from the ocean and resulted in the dropping of moisture. The climate has changed through three transitional phases, i.e., the wet and hot Late Triassic-Early Jurassic including 5 wet and hot climatic events, the dry and hot Late Jurassic-Late Cretaceous, including 5 dry and hot climatic events, and the dry and cool Oligocene-Quarternary, including 5 dry and cool climate events. We also found that the tectonic and the climate patterns were different in the north and south of this region. Taking the Paleo-Tienshan and Paleo-Qilian Mts. as a boundray, strong tectonic movement and dry climate occurred in the north, but relatively steady and wet climate in the south during the period of Late Triassic-Jurassic. However, in Cretaceous, as the tectonic movement was intensified in the west but remained steady in the east, and climate was dry in the south but wet in the north of NW China. The formation of ANW is the consequence of both the global change and the local tectonic movement. The local tectonic movement happened usually prior to the global change. The occurrence of the dipolar ice-sheet, the uplift of the Tibetan Plateau and the retreating of the Paratethys from the Tarim jointly caused the formation of ANW since Oligocene.