Paleomagnetism has played an important role in quantifying the Mesozoic evolution of “Proto-Tibet”. We present here our recent paleomagnetic data from five Middle-Upper Jurassic sedimentary sequences of the eastern North Qiangtang Terrane at Yanshiping. The new paleomagnetic results from 99 sites, 1,702 samples and reveal paleopoles at 79.1°N/306.9°E (dp=3.9°, dm=6.3°) for Quemo Co Fm, 68.9°N/313.8°E (dp=2.1°, dm=3.7°) for Buqu Fm, 66.1°N/332.1°E (dp=2.7°, dm=4.6°) for Xiali Fm, 72.4°N/318.6°E (dp=3.9°, dm=6.7°) for Suowa Fm, and 76.9°N/301.1°E (dp=7.9°, dm=13.2°) for Xueshan Fm, respectively. These results indicate that Yanshiping experienced latitudinal changes from ~24.5° N to ~22.0º N over the time interval 171.2 - <157.5 Ma, accompanied by clockwise (CW) rotations of ~19.8±9.4º between ~171.2 and 161.7 Ma and counterclockwise (CCW) rotations of ~15.4±13.4º between ~161.7 and <157.2 Ma. We attribute the change in rotation sense at approximately ~161.7 Ma to the initial collision of the Lhasa and Qiangtang terranes. Using this and other paleomagnetic data from the Lhasa, Qiangtang and Tarim terranes, as well as other geological evidence, such as tectonism-related sedimentary sequences, volcanism, and HP metamorphism, we propose a new conceptual evolution model for the Mesozoic QT and Tethyan Oceans, including 3 intra-continental collisions (South-North Qiangtang, Qiangtang-Songpan-Ganzi and Lhasa-Qiangtang) and post collisional extensions.
YAN Maodu
The Simao Basin is located in the south of the Yunnan province and the southeast of the Qinghai-Tibet plateau. It is classified as the Sanjiang tectonic domain belongs to the eastern part of Tethyan tectonic domain. The thick and continuous Early Cenozoic strata preserved in the basin is thought to be an ideal achieve to reconstruct the history of tectonic evolution in this area as well as on the southeastern plateau. The most complete Early Cenozoic strata in the Simao Basin are located in Xiaojinggu Town, Jinggu County, which mainly includes the sedimentary strata of the Mengyejing Formation, the Denghei Formation and the Mengla Formation. Previously, the chronological study of sedimentary strata in the Simao Basin is mainly concentrated in the Mengyejing Formation with potassium salt. However, scholars still have significant controversy about the deposition time of this group at this stage. Further, a complete sedimentary profile containing the middle and lower part of the Mengyejing Formation could not obtain due to vegetation cover and village construction. Through the systematic thermal demagnetization analysis of the 361.86-meter-thick borehole that encompasses the entire Mengyejing Formation, a Paleocene-Cretaceous high-resolution magnetic chronology results were obtained in this area initially.
YAN Maodu
The Qujing Basin is located in the eastern part of Yunnan Province, is a long and narrow rift basin with north-south trend in shape. The Basin preserves thick and continuous Cenozoic sediments, which can be divided into Xiaotun Formation, Caijiachong Formation and Ciying Formation from bottom to top. These thick Cenozoic sediments deposited are ideal achieves used to explore the history of local deformation process affected by the collision of the Indian-Eurasian plate as well as the evolution of the Indian monsoon in the Cenozoic. Previously, the macrochronological framework of these stratum was mainly defined by biological fossils, but high-resolution chronology with precise chronological control has not been carried out, thus limiting the understanding of tectonic evolution and climate and environmental changes since the Eocene in Yunnan. Based on the paleomagnetic test performed on the 300-meters thick boreholes drilled in the Qujing Basin as well as the U-Pb age (35.49 ± 0.78 Ma) results of volcanic tuff zircon collected from the top of the Caijiachong Formation, we then present the preliminary results of a precise chronological controlled high-resolution magnetic chronology record.
YAN Maodu
The thick Cenozoic sediments deposited in Yunnan are ideal achieves used to explore the history of local deformation process affected by the collision of the Indian-Eurasian plate as well as the evolution of the Indian monsoon in the Cenozoic. However, due to the lack of precise age control, the early Neogene strata in Yunnan are poorly constrained. The Qujing Basin in the northern part of Yunnan Province preserves thick and continuous Cenozoic sediments, which can be divided into the Xiaotun Formation, the Caijiachong Formation and the Ciying Formation from bottom to top. Through the combination of the field outcrop profile and the borehole core, the research team obtained the stratified stratum of the Xiaotun Formation and the Caijiachong Formation with a total thickness of 251 m in the Qujing Basin. The U-Pb geochronology of the top volcanic tuff layer (35.49 ± 0.78 Ma), Caijiachong mammal fossil group (late Eocene) as well as magnetic stratigraphy collectively reveals that the age at the bottom of the Xiaotun Formation is 46.2 Ma, the top of the Caijiachong Formation should be < 36.2 Ma, and the epoch line of the two groups is 41.2 Ma. However, due to the weak influence of tectonic activities in the late Cenozoic and the small deformation of the formation, the terrain in the middle of the basin is relatively flat, resulting in the inability to obtain the top of the continuous Caijiachong Formation and the upper Ciying Formation samples. A total of 320.1 meter core covering the entire Ciying Formation and the Caijiachong Formation was obtained through the continuous drilling mission carried out in the center of the basin. Among them, the overall lithology of the core of the Ciying Formation (0-216.3 m) is dominated by gray mudstone and siltstone, and several layers of coal seams are intercalated; while the lower Caijiachong Formation (216.3-305.5 m) is grayish and grayish green mudstone. The lithology of the Xiaotun Formation (305.5-320.1 m) is mainly dominated by red mudstone.
YAN Maodu
The most complete Early Cenozoic strata in the Simao Basin are located in Xiaojinggu Town, Jinggu County, which mainly includes the sedimentary strata of the Mengyejing Formation, the Denghei Formation and the Mengla Formation. Due to the tectonic uplifting of the mountain in the late Cenozoic, the syncline structure caused the top of the Mengyejing Formation, the Denghei Formation and the Mengla Formation to be exposed to the surface. However, a complete sedimentary profile containing the middle and lower part of the Mengyejing Formation could not be obtained due to vegetation cover and village construction. The chronological study of sedimentary strata in the Simao Basin is mainly concentrated in the Mengyejing Formation with potassium salt. However, there still has significant controversy about the deposition time of this group at this stage. Recently, a continuous and complete high-resolution sequence (361.86 m in thickness) of the Mengyejing Formation was obtained through the continuous drilling. Among them, the Mengyejing Formation (0-353.3 m) is mainly a set of purple-red muddy silt and mudstone combination, while the underlying Mangang Formation (353.3-361.86 m) is a set of gray-white sandstone.
YAN Maodu
This data set contains a deep drilling paleomagnetic age data near the open sea in the middle reaches of Heihe River. The borehole is located at 99.432 E and 39.463 n with a depth of 550m. The samples of paleomagnetic age were taken at the interval of 10-50 cm. The paleomagnetic test was carried out in the Key Laboratory of Western Ministry of environmental education of Lanzhou University. The primary remanence of the samples was obtained by alternating demagnetization and thermal demagnetization, and the whole formation magnetic formation was obtained by using the primary remanence direction of each sample, and then the sedimentary age of the strata was obtained by comparing with the standard polarity column. The results show that the bottom boundary of the borehole is about 7 Ma and the top boundary is 0 ma.
HU Xiaofei PAN Baotian
This data set contains two shallow drilling data near Heiquan in the middle reaches of Heihe River: 140 meters and 68.2 meters deep respectively. Paleomagnetic age samples were taken at 10-50 cm intervals from the two boreholes, and the magnetostratigraphic sequences of the two boreholes were obtained by testing these samples.
HU Xiaofei PAN Baotian
