CENOZOIC DEPOSITS IN THE NORTHERN TIANSHAN MOUNTAINS AND ITS IMPLICATIONS FOR NEOTECTONICS AND ENVIRONMENTAL CHANGES
天山北麓晚新生代沉积及其新构造 与古环境指示意义 *

The exceptional topographic relief in High Asia provides an opportunity to quantify the uplift history related to the India-Asia collision,as well as the effects of the high topography on global circulation patterns.The Tianshan Mountains just lie in the actively deforming part of the India-Asia collision zone.The thrust faulting and folding of the Neogene and Quaternary sediments in the piedmont depression zones of the Tianshan Mountains,form linear,roughly east-west anticlines at the northern and southern borders of the orogenic belts,indicating crustal shortening and thickening in the Late Cenozoic time.Such thick Cenozoic terrestrial sequences were shed into foreland basins,holding great potential for understanding the relationships between mountain building,rock denudation,sediment deposition,and climate.To better understand the uplift history of the Tianshan Mountains,we undertook a paleomagnetic study of ca.1200m thick piedmont sediments,from which we collected 1400 specimens in the northern foot of the mountains.Our paleomagnetic investigations of the Late Miocene-Pleistocene terrestrial sequences provide the first detailed and long-term chronological control for the thick molasse deposits in the studied region,thus providing a solid base for discussing the uplift history since the Late Miocene.The magnetostratigraphic results indicate that an episode of uplift of the Tianshan Mountains occurred around ca.7Ma ago marked by the occurrence of the molasse deposits,and the uplift possibly lasted to ca.2.58Ma.The uplift between 7Ma and 2.58Ma in the studied region was mainly in response to the tectonic uplift.However,the prominent Early Pleistocene Xiyu Formation,which consists of conglomerates,is at least partially associated with the mountain glaciations as a response to the onset ice sheet in the Northern Hemisphere since 2.58Ma.Additionally,the existence of the angular unconformity between the Xiyu Formation and its overlying Middle Pleistocene strata should indicate tectonic movement around the end of the Early Pleistocene.In this sense,the accumulation of the Early Pleistocene Xiyu Formation was mostly controlled by both climatic cooling and tectonics.