| The sedimentary basins and the orogenic belts are the two basic tectonic units of the continents, which constitute a tectonic system that closely related in both spatial development and mechanisms. As the most immediate and specific geological carrier of the orogenic evolution, the sedimentary basins provide an important window to understand the development of the orogenic belts.Since the Cenozoic, the continental collision between India and Eurasia plates and the continuous northward indentation reactivated the ancient Tianshan orogenic belts. Consequently, the basins on the north and south sides of the Tianshan developed very thick successive Cenozoic sediments simutaneously, which documented the evolution history of the Tianshan Mountains and furtherly can help to deeply explore the big scientific problems, such as the environmental effect, the aridification of the Asia hinterlands and the paratethys that caused by the Tianshan Mountains uplift.However, there has been much arguments over the age of the Cenozoic strata distributed in these basins, which greatly constraints the understanding of the timing,stages, magnitudes and the climatic effects etc. induced by the Tianshan Mountains uplift event. Therefore, this study chose the central and eastern part of the Kuqa depression on the south side of the Tianshan Mountains, carried out the high-resolution magnetostratigraphic works on the Cenozoic sediments, and clarified the controversial magneto stratigraphy. Based on the above, depending on the quantitative and qualitative tools like paleomagnetism, balanced cross-section restoration and structural history, and the indexes such as the growth strata,sedimentation rates and flux, etc, the research conducted the elaborate basin evolution analysis, demonstrated the basin-mountain coupling between the Kuqa depression and south Tianshan Mountains, reestablished the tectonic evolution history of the south Tianshan Mountains since the Cenozoic, and preliminarily discussed the dynamic mechanism of the deformation. The main conclusions and progresses achieved including:1. Our magnetostratigraphic research combined the Pollen and paleontological data shows seven good age constraints on the Kumugeliemu group?Suweiyi?Jidike?Kangcun?Kuqa formation and Xiyu formation(the top is unknown) in the Kuqa depression,with ages of ?42-38?38-36?36-13?13-6.5?6.5-2.6 Ma and <2.6 Ma respectively.2. Paleomagnetic results reveal that about 8.2° clockwise rotation occurred in the Kuqa depression during 42.2-2.6 Ma. Specifically, small-scale clockwise rotations happened during 42.2-38, 38-36 and 36-13 Ma, with the rotations of about 0.8°? 1° and 2.2°,respectively,showing the increasing trend. However,about 4.90 anticlockwise rotations took place during 13-6.5 Ma as a consequence of the dextral Talas-Ferganhna faults. Later the strong India-Eurasia collision influence rotated the Kuqa depression about 9.1° clockwisely.3. The balanced cross-section restorations of the three seismic sections at the south foot of the Tian Shan Mountains indicate that the crustal shortening and deformation is featured by the E-W differentiation and N-S zonation. The crustal shortening interpreted from the three sections gradually decreases in W-E direction,and also shows the same zonation in N-S direction, which means that all the tectonic deformations in Cenozoic initiated at about 36 Ma. And the deformation extended to the basins at the southern part from the mountains at the northern part, forming the piggyback thrusts and the related folds, and the deformation increased periodically,reaching the peak at about 2.6 Ma.4. According to the equal thickness maps of the Cenozoic residual strata in Kuqa depression, the sedimentation fluxs of the Kumugeliemu group-Suweiyi formation,Jidike formation, Kangcun formation and Kuche formation are figured out, with the values of 30.05, 73.85, 328.19 and 1179.89 t·Ma-1/m2 respectively. Combined the previous contemporaneous researches about the climate and provenance, etc. we think that the stepwise increase of the sedimentation fluxs of the strata in the Kuqa depression responded to the periodically uplift of the south Tianshan Mountains.5. We got the sedimentation rates of study area in Kuche depression through the high resolution magneto stratigraphy sequence combined with the previous relevant researches. The sedimentation rate was relatively stable and low between 42-13 Ma with the value of about 5 cm/kyr, and increased continuously since about 13 Ma and reached peak at 6.5 Ma, and obviously declined between 6.5-2.6 Ma. Meanwhile, the sedimentation rates also display the spatial differences. That is, the sedimentation rate of the second-row Ke-Yi tectonic belt was clearly lower than that of the third-row Qiulitage tectonic belt. In the study, the changes of the sedimentation rates were thought as a result of the periodically tectonic uplift of the south Tian Shan started at about 13 Ma and the gradual basin-wards expanding of the fold-thrust belt in Kuqa depression, which caused the increasingly uplift of the earlier depositional areas,reduced the deposition space to sediment, and then constantly decreased the sedimentation rate.6. The evlution history of the crustal shortening, tectonic development revealed by the balanced cross-section restoration technique and the history of sedimentary recorded by sedimentation rates and fluxs are regarded as a response to the four-stages tectonic evolution of the south Tianshan Mountains since Cenozoic, which includes the regional extension subsidence period(65-?36 Ma),compressional shortening and uplift period(?36-13 Ma),intensive period(?13-6.5 Ma) and peak period(?6.5 Ma-present).7. Based on the E-W differences of the crustal shortening and deformation of the south Tianshan Mountains, also combining the tectonic rotation results of the Tarim basin in Cenozoic, this study concluded that the gradual northword migration of the Pamir syntaxis drived by the India-Eurasia collision influence and the rotations of the Tarim basin mobilized by the subsequent wedging between Pamir and Tianshan Mountains are the primary cause of the tectonic deformation and shortening occurred in Tianshan Mountains since the Cenozoic. |
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