Coal Metamorphism Sequence Of The Qinshui Basin And Its Response To The Deep Process Evolution

It was an effective way for the coalification delinking the coupling and response event-time sequences of deep processes to research the hydrocarbon generation, gas accumulation and oil formation of the coal-accumulating basin, which was also the key method to analyze the basin dynamics background. Based on the collected data about the formation, evolution and coal metamorphism of the central North China Craton, combined with the field geological survey, the event-time sequences of the coalification and deep processes of the Qinshui basin were established. After placing the coalification in the interaction framework of different spheres in deep earth, the coupling relationship and response mechanism between coal metamorphism sequence and deep process of the Qinshui basin were studied. The following results were obtained:1. The coalification showed non-uniform characteristics in space and time for Qinshui basin. The late Paleozoic coal of the Qinshui basin was controlled by the geothermal metamorphism up to gas-fat coal stage at the late Triassic, and the vitrinite reflectance (R0) isoline inherited the planar distribution of the middle-lower Triassic strata. To the end of the middle Jurassic, the nearly EW distribution of the Ro isoline in the northern and southern basin was formed, and the Ro isoline in central basin showed abnormal to the late Triassic. To the end of the early Cretaceous, the nearly NS distribution of the R0isoline in the central basin was earliest formed in the late Jurassic, when the present coal metamorphism pattern was finally completed and the coalification terminated.2. There were different asthenospheric systems in different areas of the Qinshui basin. Because of the collision between Yangtze and north China plate, a NNE asthenosphere channel from the southern Ordos block to the northern Shanxi plot upwelled during late Triassic, and at the38°N it was converged with another deep flow belt caused by the collision between Siberia and north China plate. Being influenced by the Pacific plate subduction and collision during Jurassic to Cretaceous, the asthenosphere or mantle convection was connected westwards to the asthenosphere channel shaped in late Triassic at Yangquan and Pingshun, respectively, and the deep process achieved. The volcanic activity showed a typical explosived characters in northern Qinshui basin which was related to the distant effect of the India-Eurasia collision in Cenozoic.3. Coalification showed a spatio-temporal coupling and response to the deep process. Some results from the sequence of the coalification in the deep process of Qinshui basin indicated:i. In Triassic the deep activities exhibited normally, ii. Because of the plate subduction and collision before the middle Jurassic, the asthenosphere upwelled in the northern and southern and parts of the central basin by the end of Triassic. Meanwhile, the deep flow moving westwards superimposed at the northern and southern basin. The metamorphism pattern of the late Paleozoic coal at the northern and southern basin was shaped by the control of deep activities. iii. The metamorphism pattern of the central Qinshui basin completed because of the reactivated deep process in late Jurassic to early Cretaceous, while the deep activities in the northern and southern basin continued to the end of the early Cretaceous. The present coal metamorphism pattern completed and the deep process terminated at that time.4. The modification or thinning of the lithospheric showed non-uniform characteristics in space and time for Qinshui basin. The asthenosphere deeper Qinshui basin upwelled due to the subduction and collision of the Yangtze, Pacific and India-Eurasian plate, meanwhile, the lithospheric thinning was possibly controlled by the thermal or chemical erosion. The lithospheric thinning may be started from the latest Triassic, reached peak value in late Jurassic to early Cretaceous, and early finished in the latest early Cretaceous.