| The～1600km long Altyn Tagh Fault (ATF) bounds the Tarim plate with rigid lithosphere to the northwest and the Tibetan plateau with relatively weaker lithosphere to the southeast, and is the most important and the largest strike-slip fault inside the plateau. Its evolution during the Cenozoic not only has been all along the research focus for the geologists, but also provokes hot debates throughout the geoscience world. In this thesis, I use the sedimentary and structural records within the Qaidam Basin, the largest sedimentary basin inside the Tibetan plateau and located at the southern side of the ATF, as the proxies to study the Cenozoic tectonic process of this huge strike-slip fault. The conclusions are summarized as follows:(1) The Cenozic sequences of the northwestern Qaidam Basin are clearly divided into 3 units. The lower unit consists of the Lulehe and the Xiaganchaigou formations. During their depositing period (before ca.36Ma), the northwestern Qaidam Basin was a broad lake that may be across the present Altyn Shan and link to the southeastern Tarim Basin. The middle unit includes the Shangganchaigou and the Xiayoushashan formations, which are composed of fluvial-alluvial coarse-grained sediments with upward coarsening rhythm and provenance from the Altyn Shan, indicating the Altyn Shan continually uplifted since then and culminated at the end of the deposition of the Xiayoushashan formation (ca.15Ma). The upper unit consists of the Shangyoushashan, the Shizigou and the Qigequan formations, unconformably covers the underlying strata, and obviously overlaps towards the Altyn Shan, implying the range and altitude of the Altyn Shan both reduced at the time. Although the provenance of the upper unit was also the Altyn Shan, it differs greatly from that of the middle unit, suggesting a tectonic alteration occurring between the two units. However, inside the Qaidam Basin, the sedimentary environment had been all along lacustrine during the whole Cenozoic, and the sediments were fine-grained mudstone and siltstone, indicating the uplift of the Altyn Shan since ca.36Ma did not affect there and was restricted along the northwestern Qaidam Basin.(2) The EW-trending fault system in the northwestern Qaidam Basin and the NW-trending fault system inside the basin are completely different, and do not form curvilinear thrusting controlled by the left-slip on the ATF. The EW-trending faults distribute in a narrow belt (ca.30km wide) along the southern side of the ATF, and are in en-echelon arrangement, indicating that they were controlled by sinistral shear. These faults generally dip north, cut downward to the basement and mostly upward to the top of the Xiayoushashan formation. They thrusted imbricately to the south, tilting southward the basement of the northwestern Qaidam Basin. They started to be active at ca.36Ma, strongly activated at the end of the Xiayoushashan formation (ca. 14.9Ma), but became weak since then. The NW-trending faults distribute more expansively, and controlled the formation of anticlinal belts inside the Qaidam Basin. They dip NE or SW, and have two-layer structures:in the deep, they are steep-dipping basement-involved faults with wide anticlines in the hanging walls, while in the shallow layer, they are thin-skinned backthrust faults, forming close anticlines in the surface. They formed under sinistral transpressional settings, and started to develop at the beginning of the Shizgou formation (ca.8.2Ma), but strongly activated at the end of Qigequan formation (ca.1.534～0.277Ma). So they are two different fault systems forming at different periods, in different areas and under different controlling factors, and do not form curvilinear thrusting. The transition between them may mark an important tectonic event in the Qaidam Basin as well as the northern Tibetan plateau.(3) Coupling analysis between the Qaidam Basin and the Altyn Tagh Fault system proves that the Altyn Shan started to uplift as early as ca.36Ma, while large-scal left-slip motion on the ATF commenced since ca.15Ma. This suggests a two-stage model for the Cenozoic evolution of the ATF. During the early stage between ca.36～15Ma, the ATF was mearly a basal shear zone confined in the mid-lower crust, creating obviously surface uplift and EW-trending faults in the northern margin of the Tibetan plateau, which formed the rudiment of the Altyn Shan. During the late stage since ca.15Ma, the ATF eventually broke the whole crust, initiating large-scale left-slip movement and altering gradually the early Altyn Shan to the present framework. The diamond-shaped main body of the present Altyn Shan was part of the Qilian segment of the early Altyn Shan which was later cut and displaced by both the ATF and the North Altyn fault. Left-slip movement on the ATF transfers the N-S compressional stress from the south to NE-SW, leading to the prominent uplift of NW-trending Qilian Shan and Eastern Kunlun Shan. This suggests that there should be two types of structures in the northern Tibetan plateau during the Cenozoic: the early EW-trending structures forming during ca.36-15Ma, and the late NW-trending ones forming since 15Ma. The former was greatly altered by the latter and preserved only in some local regions, for instance in the Qiadam Basin, the EW-trending faults in its northwestern part belong to the early structures, while those NW-trending faults inside it are late structures.
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