Research On Tectonic Characteristics Of The Middle Section Of Changle-Zhao’an Fault Belt In Fujian Coastal Area

The Changle-Zhao’an fault belt is located along the coast of Fujian province,the southeastern Margin of the Eurasian plate.It is affected by the Indian Ocean plate and the Pacific plate or the Philippine plate.The crustal activity and deformation are strong,and it is a relatively active medium-strong earthquake-prone area on the southeast coast of China.However,the understanding of the deformation characteristics of the fault is still vague.This paper selects the middle section of Changle-Zhao’an fault belts（Putian-Quanzhou section）as the research object,investigated the structural traces of the middle section of the fault belts in the wild,observed a large number of tectonics cleavage belt’s occurrence,collected samples related to the fracture and tested,then discussed properties and scale of the middle section of the fault,tectonic deformation characteristics,metamorphism-deformation period,neotectonics,tectonic stress field and its tectonic evolution.Changle-Zhao’an fault belt was formed in the early stage of Yanshan movement,and its overall perfor Mance thrust faults with left strike-slip faults.The basic dike swarm,and the"dark particle inclusions"present in the nearby granite indicated that in mesozoic the fault belt may cut through the earth’s crust,initiated a crustal-Mantle Magma mixing event.The Magmatic activity of late Mesozoic in the area was also controled by Changle-Zhao’an fault belt.According to the zircon U-Pb dating results of Magma rock samples,there were at least two phases of Magmatism in Late Mesozoic:the late Jurassic^earlier early Cretaceous,the later early Cretaceous^earlier late Cretaceous,produced from different background of the extrusion and extension.Rocks in the middle section of the fault belt showed different degrees of metamorphic deformation,which were supposed to be closely related to the formation and development of fault.In addition to developed ductile shear belts in some areas,the fault belt’s tectonic deformation feature were dominated by dense tectonics cleavage belts of tens of kilometers,and there were two dominant trends,NE and NW.Among them,the NE-trending cleavage belts sometimes show ductile-brittle superimposed composite cleavage zones,mainly in the stratum of the Nanyuan Group in the West.The deep ductile cleavage belts may be raised to shallow through a strong thrust action in the early stage,then formed by superposition at the later stage:the late Jurassic^earlier Early Cretaceous ductile cleavage,the later late Cretaceous^Paleogene brittle cleavage,represented two stages extrusion tectonic background.The formation of ductile cleavages may also be affected by magmatic activity.The density of the cleavage zone in the eastern later granitic rocks was obviously weakened,and the late brittle cleavage belt may only develop,or it may be related to the rock fabric.The NW cleavage belts exhibit brittleness and may be mainly formed in the later Late Cretaceous^Paleogene.Changle-zhao’an fault belt had three stages tectonic stress field in late Mesozoic:NW-SE extrusion in the late Jurassic^earlier early Cretaceous;the later early Cretaceous^earlier late Cretaceous,the tectonic stress field changed from extrusion to extension,and the stress direction was NW-SE;the later late Cretaceous-Paleogene,the tectonic stress field resumed NW-SE compression,while they might be superimposed NE-SW compressive stress.The tectonic stress field in the neotectonic period was still dominated by NW-SE extrusion,the activity of fault belt was mostly characterized by the inherited activities of the old fault or cleavage belts,and intermittent distribution;since the Holocene,the activity intensity had been greatly weakened.In recent years,earthquakes had occurred mostly in the fault belt of the sea area,and there were also distributions at the intersection of NE and NW fault belt in the land.In particular,Quanzhou section has conditions for earthquakes.In this paper,the formation and evolution of Changle-Zhao’an fault belt was roughly divided into four stages,namely the formation period（T₃^J₂）,the strong extrusion period（J₃^K₁¹）,the conversion extension stage（K₁^2K₂¹）and the relative stability stage（K₂^2Q）.