Meso-Cenozoic Thermal Regime And Lithospheric Thinning In The Qin Shui Basin,CNCC

The North China Craton has been in a stable state since it was cratonized 1.8 billion years ago.The existing evidences of Geophysics,geochemistry and petrology indicate that the eastern part of the North China Craton has been destroyed about 125 million years ago,and the western part of the North China Craton has been thinned.During Mesozoic-Cenozoic,the thick and cold lithosphere of Paleozoic was gradually replaced by the thin and hot asthenosphere or oceanic mantle of Cenozoic.Based on the study of the present geothermal field,geothermal flow,thermophysical parameters,thermal evolution history,thickness of thermal lithosphere and thermal flow structure of Qinshui Basin in the central part of North China Craton,it can reflect the evolution information of deep lithosphere,and provide the constraints and evidence support for the dynamic mechanism of lithosphere evolution and thinning in the central and western part of North China Craton.Based on apatite fission track data,aft data and easy%ro method,the Meso Cenozoic thermal history of the south,North and central Qinshui Basin is restored,and the thermal history of the south,North and central Qinshui Basin is compared.On the basis of the above research,combined with the formation data of drilling,the model of crust stratification structure of Geophysics and the basin thermal physical parameters,the deep temperature and the thickness of thermal lithosphere are calculated under the constraints of one-dimensional heat conduction equation and dry basalt solid phase line.The results show that the thickness of the seismic lithosphere in Qinshui Basin is generally thin in the north and south,thick in the middle,thin in the East and thick in the West.The thickness of the seismic lithosphere is 36-43km,with an average thickness of about 41km.The thickness distribution of the seismic lithosphere in Qinshui Basin is characterized by thinning from north to South and from west to East,with an overall variation of 85-130km The average thickness is about 120km,and the thickness of the lithosphere on the north side of the basin is relatively large,about140km.The overall thickness of the Cenozoic fault basin on the west side of the basin is relatively small,between 70-110km.The test results of thermal physical properties of sedimentary rocks in Qinshui Basin show that with the formation changing from old to new,the test results of rocks with different lithology show that the thermal conductivity decreases gradually and the heat generation rate increases gradually.Nowadays,the distribution of geothermal gradient is mainly controlled by the fluctuation of Moho surface.On the whole,the value of geothermal gradient is higher in North and south,and lower in the middle.The geothermal gradient in Qinshui Basin is generally distributed between 2.09?4.76?/100m,with an average geothermal gradient of?2.82±0.103??/100M;the average geothermal gradient in the south is 3.53?/100M,the average geothermal gradient in the middle is 2.24?/100M,and the north is flat The average ground temperature gradient is 2.7?/100M.At present,the distribution of large geothermal flow in the basin is mainly controlled by Moho surface and internal faults of the basin.The overall distribution trend is higher in the south,followed by that in the north and the middle.The geothermal flow is between 44.8-101.8mw/m2,with an average value of 65.22mw/m²The evolution path of the thermal history in the south,North and central Qinshui Basin is basically similar.Since the end of the Mesozoic Jurassic,the value of the thermal flow has been increasing.The value of the thermal flow in the Qinshui Basin reached the peak of 85?107mw/m²in the late Jurassic?Early Cretaceous.At this time,the tectonic activity of the corresponding North China plate is the most intense,causing the upwelling of the asthenosphere,resulting in the thinning of the lithosphere,and the overall heat flow ratio of the crust and mantle of the basin is less than 1 It represents a typical structural model of"cold crust and hot mantle".After the early Cretaceous,the geothermal flow value decreased rapidly,from 85-107mw/m²in general to about70-80mw/m²in Paleogene.Since the recent record,the geothermal flow value decreased slowly.Today,the geothermal flow value in Qinshui Basin is between44.8-101.8mw/m²,with an average value of 65.22mw/m².Some of the higher values are related to the development of faults in Qinshui Basin.The thickness of the early Cretaceous thermal lithosphere in the basin is 40-55km,showing a trend of thin in the South and thick in the north.The thickness in the south is 40.1km,in the middle is53km,and in the north is 54.5km.The thickness of the thermal lithosphere in the basin is 93.9-128.5km.The overall distribution trend is thin in the South and thick in the north.The heat flow ratio of the crust and mantle in the basin is greater than 1,which represents the structural model of"hot crust and cold mantle".The present thermal rheological structure of Qinshui Basin is the rheological structure of"strong crust weak mantle".Under the background of dry mantle and compression,the strength of lithosphere is distributed in the range of 2.446?2.55×10¹²N/m,under the background of tension,the strength of lithosphere is distributed in the range of1.338?1.962×10¹²N/m,and the elastic thickness of lithosphere is distributed in the range of 18.23?18.71km.Generally speaking,the formation of lithospheric thermal structure in Qinshui Basin in the central part of North China plate is mainly caused by the subduction of Pacific plate and upwelling of asthenosphere or mantle plume.Secondly,the early Cretaceous tectonic thermal event in Qinshui Basin has a good coupling relationship with the destructon of North China Craton.