Different Burial-Cooling History of Triassic Strata between the Western Weibei Uplift and the Northwestern Weihe Basin in Northwest China

Analysis of tectonothermal history of the Yanchang Formation in the western Weibei Uplift and in the northwestern Weihe Basin can reconstruct the cooling history of the southwest most remained Upper Triassic source rock of the North China Plate. Apatite fission-track(AFT) and(U-ThSm)/He(AHe) analysis were used to recover the cooling and uplift history of the Upper Triassic here. Ten sandstones from the Middle–Upper Triassic strata yield AFT ages between 179.8 ± 7.4 and 127.6 ± 8.1 Ma. AHe ages of two sandstones have the value of 37.7 ± 2.3–131.1 ± 8.1 and 45.7 ± 2.8–83.5 ± 5.2 Ma. Time-temperature modeling results showed that tectonothermal history of the Yanchang Formation was initially different in time-space relationships but then became almost identical through time followed by different cooling rate. Modeling results of the Triassic strata in the Qianyang area and the Yaojiagou area revealed three different uplift-cooling stages commencing in the Late Jurassic at ~165 Ma and in Early Cretaceous at ~110 Ma, respectively, both followed by first similar cooling histories to the Early Miocene at ~20 – 23 Ma and then different since the Late Miocene. Uplift-cooling rate since the Late Miocene at ~8 Ma were different between the Western Weibei Uplift and the Northwestern Weihe Basin. The timing, cooling-uplift rates of the Yaojiagou area, which was mainly controlled by movements related to the Liupanshan Mountains, the Qinling Orogens and the Weibei Uplift, had the earliest onset of uplift-cooling for the Upper Triassic series compared to other regions within the Weibei Uplift. Cooling paths for the Upper Triassic series became uniform regionally in the Early Cretaceous marking a key time for the tectonothermal evolutionary history of Upper Triassic series in the southwestern North China Plate.

Characteristics and evolution of faults in the north-central Yin’e Basin and the effects on the coal-seam in the Cretaceous strata

Research on the characteristics of faults and their evolutionary history since the Cretaceous in the Suhongtu-Dagu depressions can provide a theoretical basis for geological evaluation of the coal seams in the Suhongtu Formation in the northern-central region of the Yin’e Basin.Using 3-D seismic-logging inversion techniques,seismic stratigraphic calibration,stratigraphic sequence delineation,and thickness calculations on the Suhongtu-Dagu depressions were carried out to clarify the planar and profile distributions of the faults,as well as the evolutionary history of these faults and the tectonic history of the depressions.The results of this study revealed that the distribution of the faults in the Suhongtu-Dagu depressions in the northern part of the Yin’e Basin varies with region,and the fault system was multi-period,orthotropic,north-east-trending,and north-north-east-trending,with a certain degree of inheritance in terms of the geological setting.Three types of faults were identified:Y-shaped fractures,reverse Y-shaped fractures,and parallel fractures,which can be classified as Paleozoic-Cenozoic continuous syncline faults and intra-depression faults from the top of the Permian to the Upper Cretaceous series and inter-stratigraphic adjustment faults within the Cretaceous System,respectively.The evolution of these faults can be divided into three phases:the controlling faults were the faults that existed before the Early Cretaceous and had been active since then;synclinal faults that formed during the Early Cretaceous;and modified faults that formed since the Early Cretaceous.The development and modification of the coal seams in the Cretaceous Suhongtu Formation in the Hari,Kuanzihu,and Babei sags were strongly controlled and influenced by a multi-phase complex fault system.