A Study On The Relationship Between Fluid Geochemical Characteristics And Earthquake Activity In The Eastern Part Of The North China Craton

The North China Craton is one of the typical ancient and massive Cratons.Due to the westward subduction of the Pacific plate and the collision of the North China and Yangtze plate that happened during the Late Mesozoic,the magmatism,tectonic activity and lithospheric structure of the eastern and western parts of the North China Craton have been significantly different,which is contrary to the"stable" feature of the Craton.The North China Craton has been widely noted by scholars because of the specific destructive.Current studies show that the western part of the North China Craton retains its stable properties and has an overall basin framework with weak magmatism and tectonic activity.In contrast,large-scale volcanic activity occurred in the eastern part of the North China Craton during the Early Cretaceous,with significant ductile tectonic deformation,crustal thinning and magmatic activity,these phenomena began to weaken in the eastern part after the Cenozoic.In recent years,scientific questions on the timing,extent and mechanism of North China Craton have been a hot topic of research,and exploring the deep processes of surface shallow geological formations and North China Craton destruction based on the correlation between magmatism and shallow geological response is also one of the major scientific questions.The high seismicity of the North China Craton,with moderate to strong earthquakes spread over almost the whole of North China except the interior of the Ordos block,is a natural testing ground for seismic science research.Fluids connect the different circles of the Earth,and are one of the most active material components and media inside the Earth,as well as a window and manifestation of the energy released inside the Earth.Due to its wide distribution and fast response,it can respond to the state and change of the underground condition in a sensitive and objective way.It has been widely used in the study of deep earth dynamics,tectonic activity and seismic activity.The most intuitive and easily observable components of geofluids include natural spring,geothermal gas and soil gas.This paper analyses the origin and characteristics of fluids in different tectonic units of the North China Craton based on the geochemical characteristics of hot spring(123 geothermal water samples and 26 gas samples)and soil gas fluxes(190 Rn and CO2 measurement points)around the typical fault with strong seismic activity in the eastern part of the North China Craton(Tangshan Fault),and discusses the genesis of the differences in fluid origin and characteristics.The spatial distribution of soil gas Rn and CO2 fluxes in the Tangshan Fault Zone and their controlling factors are explored.The relationship between fluid geochemical characteristics and seismic activity within the North China Craton is also discussed.The results of geothermal water show that the water chemistry types within the North China Craton are multiformity,with cations dominated by Na+ and K+,the western region has higher TDS values,and water-rock reactions are the main factor controlling the water chemistry types;most geothermal water belong to partially mature and immature water,but the eastern region tends to be more Mg-angle in triple graph,related to the dilution effect of geothermal water in the eastern region.The source of geothermal water in the eastern and central parts is atmospheric precipitation,with oxygen shift occurring in the high temperature geothermal water in the central Yanggao basin.The eastern region has a more enriched δD than the western region,associated with the presence of giant and thick carbonate sediments in the region.The mean recharge elevation in the east is 1487.6 m and 1906.9 m in the west,with differences in recharge elevation.K-Mg ion temperature scale results show that the mean storage temperature is 38.4℃ in the west and 80.7℃ in the east and central parts,which is in good spatial agreement with sufer heat flow values in the region.The distribution locations of historical large earthquakes within the North China Craton are in good spatial consistency with high storage temperature and the location of hot springs,and large earthquakes within the region mainly occurred in four seismic zones in the North China Craton.Results for hot spring gases indicate that CO2 concentrations in samples from the eastern and central parts of the North China Craton range from 0.01～7.91%,several orders of magnitude higher than those in air(0.03%),and that N2 content is characteristic of a medium-low temperature hydrothermal system and comes from the atmosphere.The central part samples near the Ordos Basin are characteristic of crustal gas,while the eastern samples are typical of subduction zone gases.He-Ne isotope results show that the mantle source contribution of helium isotopes in hot spring gases ranges from 0～31.38%.The mantle source contribution decreases from east to west and correlates well with lithospheric thickness.The results of carbon isotope show that there are more carbonate and mantle-derived carbon in the eastern part,while carbon from sedimentary sources in the western part.The results of soil gas fluxes from the Tangshan Fault indicate that the Rn fluxes of soil gas in and around the Tangshan Fault zone range from 0.01 to 409.31 mBq/(m2·s),with most sampling sites having Rn fluxes in the range of 1.00～100.00 mBq/(m2·s)and a mean flux value of 40.58 mBq/(m2·s);the CO2 fluxes of soil gas range from 8.92～1377.51 g/(m2·d),with CO2 fluxes ranging from 10.00 to 200.00 g/(m2·d)at most sampling sites and a mean flux value of 98.21 g/(m2·d),which is higher than the CO2 fluxes of most other faults worldwide.The high flux and concentration of soil gas Rn in Tangshan area are highly consistent with the spatial distribution of seismic activity(low b value),which are distributed in the areas where large earthquake have occurred in history and are controlled by the spatial distribution of fault zones.It shows that the permeability and regional stress state of fault zones are the main factors controlling the concentration and flux of Rn,and are less affected by the conditions measured.This paper discusses the geochemical characteristics and sources of geothermal water within the North China Craton,the variance in fluids from different tectonic units,and the geochemical characteristics and controlling factors of soil gas fluxes in a typical strong seismically active fault zone in the eastern part of the North China Craton.While providing data support and theoretical guidance for fluid geochemistry within the scope of North China Craton,it is believed that the destruction of North China Craton has an impact on shallow geofluid.Deep fluid and high heat flux are closely related to seismic activity.