The Structure Features And Forming Mechanism Of The Huaibei Coalfield

Resulted from the long-termed and complicated tectonic evolution,the Huaibei coalfield is characterized by its complicated fault structure.It is part of the southeastern margin of the North China,one of the major hosting coal belts in North China.The Huaibei coalfield lies on the leading edge and outskirts of the Xu-Su arc-shaped thrust belt.Based on a systematic analysis of the geological data in the study area and in combination with the field survey,we discussed the structural features,tectonic evolution and forming mechanism of the Huaibei coalfield.The results show that:1.The Huaibei coalfield could be subdivided into the northern and the southern units along the Subei fault.The northern unit is featured by thrust faults and Jura-type folds distributed as an arc-shape belt protruding westward,whose tectonic lines are characterized by SN-NEE strikes.The southern unit has got tectonic lines that strike NNW and NNE,and dominated by normal faults and open short-axis folds.The synclines are gentle and cored by Carboniferous-Permian strata,with abundant but deep-buried coal reservoirs.The secondary subdivision of the southern and northern structural units is in line with the faults developed along the leading edge of the nappe,and structural analyses have been conducted within these units.The northern unit is part of the main body of the Xu-Su thrust.The hanging wall of the thrust comprises the Xiaoxian anticline and area to its east,while the footwall lies to the west of the anticline;as for the southern unit,the hanging wall in the east of the Xisipo fault composes the southeastern end of the Xu-Su arc-shaped structure,while the footwall lies to the west of the fault.2.The structural analysis of the mining area of the Huaibei Coalfield(Zhahe mining area,Suixiao mining area,surface nappe structure area,Suxian mining area,Linhuan mining area and Guoyang mining area)indicates that the structure of the mining area formed by 2 or 3 episodes of structural movements.It could be clarified that the Huaibei coalfield had experienced three major tectonic events since the deposition of the coal-bearing strata during Carboniferous-Permian,namely the S-N compression during Indo-China period which formed the E-W fractures,the early Yanshanian NWW-SEE compression that formed the Xu-Su arc-shaped structure belt,and the late Yanshanian NNE-SSW compression that triggered the voluminous NNE-SSW normal faults within the coalfield.3.This paper presents the analyses of the Xu-Su arc-shaped structure both on the plane and along the section.The fault structure is mainly composed of the over-thrusts including imbricate faults,step reversed fault and the fault-related fold.The klippe is a special form of stepped reverse faults.By the measurement of structural elements such as conjugate joints and using the stereographic projection method to solve the paleo-stress field,the maximum principal stress axis of the Xu-Su arc-shape structure was near EW at the earlier stage and changed to the NNE-SSW at later stage.4.The balanced geological profile and structural recovery shows that the shortening rate of Lutuan-Heifengling is 20%and the rate of shortening of Dangshan-Shuoli is 14%.The thrusting of the Xu-Su arc-shaped structure belongs to the pre-expansion mode.The thrust distance of the front fault,main nappe and the trailing edge were 10 km,30 km and 100 km respectively.The trailing edge may have been transported from the west side of the Tanlu Fault to its current position.