| In the coastal area of southeast Fujian province,China,a series of Cenozoic basins,such as Zhangzhou basin,Fuzhou basin and adjacent Chaoshan basin,as well as massively distributed NWtrending faults,constituded a very remarkable geological landscape.Geographically,they make up the front edge of southeast convex,jagged,arc-shape terrestrial block of south China block.These coastal basins,which adjoin the Cenozoic marginal seas of east China and also face with one of the most active orogenic belts(the Taiwan Orogenic Belt)on Earth,most likely recorded the influence of the vigorous trench-arc-basin system on the nearby terrestrial area.The unique geographic and tectonic position of Zhangzhou basin,which is evenly located between Fuzhou basin and Chaoshan basin and also directly across from the Taiwan Orogenic Belt,makes it the most typical and valuable research object.Therefor,taking the Cenozoic tectonic evolution pattern of Zhangzhou Basin in Fujian as an example,the formation mechanism of NWtrending faults on the continental crust of the coastal marginal zone of southeastern China and the zigzag coastal basin are explored.The tectonic effects of modern trench-arc-basin system on the adjacent land blocks can provide examples for a deeper understanding of the continental margin dynamics and ocean-land interaction processes.Based on a combination of comprehensive analysis of a large number of field structural deformation data and geophysical data,as well as numerical simulation,through the study of the four aspects of the Zhangzhou Basin’s geometry,kinematics,chronology and dynamic characteristics,several insights of the basin’s structural evolution pattern and its geodynamic mechanism have been acquired.(1)The Zhangzhou Basin is a fan-shaped extensional basin formed under the constraints of two sets of faults(NE-trending and NW-trending),these two sets of faults are characterized by normal faulting and strike-slip faulting separately.By comprehensively analyzing the spatial differences in the structural pattern around the basin,the structural deformation characteristics of major faults,the integrity and continuity of structural geomorphologic features,and the distribution of Quaternary sediments,it is proposed that the range of the basin is from the arc-shape ridge of northern Yanxi Town,south to Damao Mountain,west to Tianbao Mountain,and east to a wide flat terrain area discontinueslly bounded by Yanxi Town-Chenxiang Town-Guokeng Town-Baiyun Mountain and other places.(2)The Zhangzhou Basin is a Quarternary extensional basin,the main feature is the Quarternary deposit directly overlain the Mesozoic granite.The geometry and structural pattern of the Zhangzhou Basin are mainly controlled by two-stage structural deformation of the NW-trending fault.The early stage was dominated by NW-trending normal faults,resulting in regular spatial changes in the structural configuration around the basin from the coast to the inland: the east side of the basin showed a series of estuary bays formed by strong fault depressions,and the western high mountains resulted in sevaral linear ridges by strong uplift.During the late stage,the strike-slip faults dominated,and three transition extension zones controlled by NW-trending strike-slip faults were formed on the north and east sides of the basin.These NW-trending,sinistral strike-slip faults superimposed and reformed the NE-trending faults formed during the Mesozoic.The transitional stretching in the three transitional extension zones showed a gradually increasing trend from north to south,which was a result of the differential displacement between the nearly parallel NW-trending faults.It’s the reason that caused passive extension of the fan-shaped basin and intermittent boundaries on the east side.(3)During the Cenozoic period,the Zhangzhou Basin experienced a transition from the Mesozoic NE-trending tectonic regime to the NW-trending tectonic regime.Marked by the intrusion of the early Cenozoic mafic dykes in Haimen Island,strong NEE-SWW(NE-SW)extensional effects formed a large number of NW-trending normal faults and high angle joints in the study area.These normal faults constituted the spatially differentiated structural patterns of the graben type and the horst type in the east and west of the basin,respectively,and the mechanical properties represent the horizontal extension of the eastern coastal area of the basin and the horizontal compression of the inland area on the west side,respectively.Hence,the tectonic stress field of the continental margin zone has spatial changes from the sea to the land.(4)The spatial variation of the tectonic pattern in the periphery of Zhangzhou Basin is the result of an unevenly distributed structural stress field.The study area can be spatially divided into three blocks with Shajian,Zhangzhou and the east of Longhai.The fault slip vector results show that three different maximum principal stress states are reflected in these three blocks,respectively.For example,the maximum principal stress in the Shajian area is NW-SE;the Zhangzhou area is dominated by the near-vertical maximum principal stress;the area east of Longhai is characterized by the combination of vertical maximum principal stress and NE-SW maximum principal stress.The maximum principal stress orientation was obtained based on a large number of joint dominant orientation statistics,and the results showed that the basin and its tectonic line to the east were affected by NW-trending strike-slip faults,and nearly 20° counterclockwise rotation occurred relative to the west.(5)In the late Cenozoic,the Zhangzhou Basin had experienced both normal and strike-slip faulting.The analysis of modern seismic activity and focal mechanism solutions shows that the coastal areas of Fujian and the western side of the Taiwan orogenic belt are under different tectonic stress state.These NW-trending strike-slip faults formed in the late Cenozoic period may still be working at present and continue to control the tectonic evolution of the coastal continental margin.The latest normal faulting is the formation of a set of NE-trending secondary structures under the transtension of NW-trending strike-slip faults,with the en echelon ridge in Xiamen-Haicang area being the most typical.(6)The two-stage tectonic evolution of the Zhangzhou Basin was affected by the arc-shape curvature of the continental margin under ocean-continent interaction and the lateral extrusion of the back-arc oceanic crust.The numerical simulation results show that the continental margin can undergo arc-shape flexure under the convergence caused by the subduction.With the flexure of continental margin and lateral extrusion of back-arc oceanic crust and its internal magma island arc being the main feature,which lead the continental crust and subduction zone undergone cooperative bending deformation with relatively convex arc tops.During the retreat of the subducting slab,an arc-shaped strain band and a radial strain band can be formed,in which the arc-shaped strain band will jump toward the ocean,indicating that the subduction zone’s transition to ocean is not the result of the migration of the original subduction zone with the plate,but a new weak zone;radial strain bands have the characteristic of equidistant distribution,which can cause disturbance of the shape of the continental margin,and eventually form fault structures with equidistant distribution inside the continental crust.To sum up,this paper takes the structural evolution of Zhangzhou Basin as an example,combined with regional geological evolution,and put forward the inherited role of the Late Mesozoic northeast structural pattern in the evolution of the basin,and discussed the main controlling factors of the basin formation in the Cenozoic period.Considering the influence of the subduction of the western Pacific plate,the finite element numerical simulation was used to verify the arc-shape bending of the continental margin and the lateral extrusion of the back-arc oceanic crust.The twostage tectonic evolution of the Zhangzhou Basin has been influenced by the ocean-land interaction since the late Mesozoic,and its geodynamic mechanism can be summarized as the response of the distant effect of the Western Pacific subduction zone on the continental crust. |
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