Mesozoic Structural Characteristics In Tuliemaodu Area, Inner Mongolia

The tuliemaodu area of Inner Mongolia located between the north China block and Siberia plate. It is the suture site of North China plate and Erlian-Xilinhot block which isolated from North China plate. The study area is on the Paleozoic accretionary complex zone and its'tectonic site should be the eastward extension of the Paleo-Asian Ocean suture. Because the interaction of Siberian plate, North China plate and Paleo-Asian Ocean plate, the study area experienced the closure of the Paleo-Asian Ocean, and the plates put together with each other. Then the pacific plate subducted under the Eurasia in Mesozoic and a strong tectonic activity and superposition activity start up. After the series of process, the study area became a special composite orogenic belt.By the background information and the experience field work, the author had grasped basically the tectonic features and the corresponding mechanism in Mesozoic of the study area. From the Late Paleozoic to Mesozoic, this area had exercised the followed tectonic activity:1. In Late PaleozoicIn Late Paleozoic, the study still grow the wide Hercynian trough between the Siberian platform and the North China Platform. After the Hercynian polycyclic tectonic movement and migration from land to ocean, this trough gradually closed from west to east from The end of early Permian. so Hercynian cycle is the main one the Tianshan-Inner Mongolia-Larix geosyncline's'cycles. the Middle Permian strata of the study area mainly experienced transformation of the late Hercynian cycle.The late Hercynian cycle'exemplification in the study area is NW' compression. As a result, this compression lead to the middle permian strata' folds and faults in the north-east trending. As the effect of fold vertical lifting, the triassic strata lost.2. Late JurassicThe performance of Indo-tectonic in the district's strata record lost because the missing of the triassic strata. Contrast to the regional informations, Indo-China movement only transformed inheritly the Late Hercynian tectonic framework in this area.The Yanshan cycle is a important movement which cast structural framework in this area. It's mainly performance is strong folds, faults and volcanic activity. In study area, in addition to NE late Paleozoic strata were exposed to the outside strip intermittent, the Yanshan tectonic activity of the Mesozoic continental margin activity belt is the dominance in this area. In terms of the characteristics of tectonic activity, the Mesozoic is the strong period of tectonic-magma activity. It is not only measured the main structural framework in the study area and controlled Mesozoic basin-range direction and distribution of geological bodies, but also established a modern spatial distribution of the mountain valley. According to the Yanshan cycle stratigraphy, magmatic activity and tectonic movement, etc, the main movement which transformed the Late Jurassic strata is the third act of middle Yanshan cycle and the forth act of the late Yanshan cycle. The act IV of Yanshan tectonic cycle lead to the angular unconformity between the late Jurassic and early Cretaceous strata in the study area.â‘ the actâ…¢of middle Yanshan cycleTectonic stress field of the middle Yanshan cycle continued and inherited basically Hercynian cycle of stress direction. It's feature is NW extrusion. And this made the NE fold in Late Jurassic and the accompanying NE left-thrust fault, NW dextral normal fault and near EW right-lateral strike-slip fault. Because the Pan-continental break from Late Jurassic to Early Cretaceous and Siberia plate drifted south due to ridge expansion, the sea of Okhotsk demised. After Siberia and North China plate, Heilongjiang plate continent- continent collision, the North-South continent got together once again. At the meanwhile, due to the NW subduction of the Pacific plate, the Intracontinental high strain was formed and NW extrusion is the main stress direction.â‘¡the actâ…£of Late Yanshan cycleTo the act IV of Late Yanshan cycle, due to the direction changing of the Pacific plate motion, the feature of the regional stress field had changed along from NW compression of the act III of the Yanshan cycle to NE compression of the actâ…£of Late Yanshan cycle. The result of the stress field is that the original NE NW SN fault moved again. With the changing of mechanical properties and direction of movement, the faults in the study area had the features of multi-period activities and composite mechanical properties. The folding in this period was not obvious. This is because pre-existing faults are widely developed, easy to lead to the formation of stress release and difficult to fold. With the changing of stress direction, the origin NE NNE faults whose property were left-thrust transformed into normal faults then made large-scale mid-acidic dykes to intrusive into them. That is the original reason about the dykes what we see today. Yet the original NW faults showed clearly the nature of the extrusion. It specifically expressed that the tectonic lenses and schistosity belts had grow within the fracture zone and near the fault plane. And they showed the features of composite mechanical properties.3. Early Cretaceous tectonicIn this period, a lot of tectonic events happened as the crust-lithospheric thinning in Central Asian Orogenic Belt and the south regions of northeast Asia and left-lateral strike-slip fault movement caused by northern extension of the Tan-Lu fault and the eastern margin of proliferation. All these made the stress in this area shows the obvious characteristics of the extrusion from NE to SN. And these features were showed by the act V of the Yanshan tectonic cycle.The original faults in this area moved again by effect of the role of the SN compressive stress field. The specific performances were obviously that NE NNE faults left-slip and NW faults right-slip and near EW faults showed the feature of extrusion while near SN ones showed the feature of tension. The specific forms are that besides complex superposition of the nature of the faults, the dykes who intrusive earlier into the faults along NE NNE directions took place in broken transformation and quartz veins in later period intrusive into the faults along the three directions mentioned above.