The Meso-cenozoic Lithospheric Thermal Structure And Its Constraints On The Differential Tectonic Thermal Evolution In The Southwestern Ordos Basin

The thermal structure and evolution of lithosphere is the frontier field of the study of the deep thermal dynamics of the basin,and it is also a hot issue of the study of the North China Craton destruction and the genetic mechanism and tectonic thermal evolution of the Ordos Basin in the west of China.On the basis of systematic collection of present-day geothermal field data from different tectonic units in the study area,this paper has systematically analyzed the Cenozoic thermal evolution and its palaeothermal field characteristics in different tectonic units in the study area and obtained geothermal heat flow parameters during the geological and historical periods using the test and analysis of paleothermal scale parameters such as mirror body reflectivity and apatite fission trace and thermal history simulation methods.On this basis,the thermal structure of the Meso-Cenozoic lithosphere of different tectonic units in the study area was computationally characterized,and the thermal evolution variability of different Meso-Cenozoic tectonic units in the study area and its relationship with the thermal lithosphere thickness variation were discussed and analyzed.The main achievements are as follows:(1)The analysis of paleothermal scale parameters and thermal history simulations of drill core samples from different structural units in the southwestern part of the basin indicate that the Meso-Cenozoic era of the study area underwent three tectonic-thermal evolutionary stages with different geothermal heat flows,(1)The stable rising period of heat flux:gradual increase of heat flow from Triassic to Late Jurassic,from about 60m W/m~2to about 75m W/m~2.(2)Maximum heat flux peak:from Late Jurassic to the end of Early Cretaceous,the heat flux increased rapidly,and reached the maximum value at the end of Early Cretaceous.The heat flux in this period was distributed in 75m W/m~2?103m W/m~2;(3)period of declining geothermal heat flow:since the Late Cretaceous,the heat flow has been decreasing steadily,and the value of heat flow has gradually decreased to 65±m W/m~2.(2)The results of calculation and analysis of thermal lithosphere thickness and structure show that the thermal lithosphere thickness of the Late Triassic(230±Ma)in the southwest basin is distributed over(92.10?112.80)km,and the mantle heat flow ratio of different structural units is greater than 1,which is a typical"hot shell and cold mantle"lithospheric structure,which is reflected in the early Mesozoic deep thermal activity in the study area is relatively weak;the thickness of the thermal lithosphere at the end of Early Cretaceous is obviously thinning,and different tectonic units appear obvious differences,Tianhuan depression area is relatively large(49.50km),Yishan slope and Weibei uplift thickness is thinner,which are40.52km and 38.87km,but the mantle heat flow ratio of different tectonic units are less than 1,which shows the lithospheric thermal structure of"cold crust and hot mantle"type;the thermal lithosphere thickness of different tectonic units from the Late Cretaceous to the present has increased significantly,with the Tianhuan depression thickening to 136.10 km,and the Yishan slope and Weibei uplift thickening to 95.93 km and 93.76 km respectively,but maintaining the characteristics of the Early Cretaceous lithosphere thickness difference thinning period east thinning west thickening,and the mantle heat flow ratio of each tectonic unit is greater than 1,showing that the"Hot shell cold mantle"type lithospheric thermal structure.(3)A comprehensive comparative analysis of the thermal structure and tectonic-thermal evolution of the Cenozoic lithosphere in the southwestern part of the basin and its related regional magmatic hydrothermal activity shows that the tectonic-thermal anomalies such as high heat flow,high ground temperature gradient and significant thinning of the thermal lithosphere thickness in the Early Cretaceous are likely to be closely related to the regional magmatic hydrothermal activity,especially in the Qingyang-Fuxian area in the south-central part of the basin.Since the late Cretaceous,the gradual increase in the thickness of the thermal lithosphere and the decrease in the gradient of paleo-temperature in the study area are probably related to the gradual decrease in thermal activity in the deep part of the regional crust,but due to the fact that the thickness of the thermal lithosphere has inherited the differential distribution state of the Early Cretaceous,the heat flow and temperature gradient in the study area are characterized by an east-high-west distribution.