Deformation Of Wulian-Rizhao Shear Zone And Regional Tectonic Significance,Shangdong Province

Extensional crustal detachment is the most important tectonic response of lithosphere thinning Late Mesozoic extensional structures.The Late Mesozoic detachment of crust resulted in extensive lithosphere thinning.The lithosphere in a vast area of eastern North China was thinned and destructed.The extensional tectonics of the North China carton in the Late Mesozoic was not a unique event,but apart of the regional lithosphere thinning.Wulian-Rizhao ductile shear zone is located in the east margin of North China plate.It is a huge shear zone between Jiaolai basin in northwest and the the Sulu orogenic belt in the south.The shear zone is composed of mylonite,but partly can be retained the combination which is from the mylonitic gneiss,mylonite,brecciated mylonite,cracked mylonite to fault gouge.The microscopic deformation phenomena and quartz c-axis fabric preferred displayed that the turgor convex recrystallization of K-feldspar,the polycrystalline of quartz and the turgor convex recrystallization of quartz,intragranular cracks in feldspar.The ductile shear zone has experienced a deformation and metamorphic processe from low-amphibolite facies,high greenschist facies to the low greenschist facies.Analysis the chronology for the shear zone and near the two sets of granitic rocks,we got the zircon U-Pb ages with 767Ma and 126Ma,which represent the ages of magmatic crystallization.Combined with the tectonic-magmatism relations and the regional geological data analysis,that the former was from Jiaonan orogenic belt of the crystalline basement,while the latter belonged to the same period of magmatic activity with shearing.Combined with the sedimentary strata and chronology of Jiaolai sedimentary basin and the age of Fangzi Rock,we initial limited the activity time of ductile shear zone is no later than about 140Ma,ended at about 122Ma.Wulian-Rizhao ductile shear zone was formed due to regional extension and lithosphere thinning in North China Craton during Late Mesozoic.The tectono-magmatic evolution of Wulian-Rizhao ductile shear zone can be subdivided into three stages:In the first stage(140Ma?130Ma),the detachment fault and ductile shear zone were formed by the regional extension.Lithosphere was thinning and uplift.During the the second stage(130Ma?122Ma),with the continued thinning of the lithosphere,a large number of igneous rocks and extrusive rock intruded.Intrusive rocks were stock,extrusive rock were many dykes,apophysal injection,syntectonic magmatic intrusionwere intruded into the shear zone.The third stage(122Ma?100Ma),lithospheric thinning was continuing,but the speed is slowing down.Shear zone has risen to the level of the upper crust and the detachment has been stopped.The magma intrusion and eruption is still happening.