Hydrogeochemical Characteristics Of Springs In Nantinghe Fault Zone In Yunnan Province,China

Affected by magmatism and deep faults activities,the southwestern Yunnan was intense seismic activity and rich in geothermal resources.The Nantinghe fault zone(NTHF),which is located in the southwestern Yunnan,is the largest and most distinctive fault zone in the north-east-trending arc structure system.NTHF was one of the most active fault zones in southwestern Yunnan.Geothermal activity is frequent,and there were many hot springs exposed along the fault zone,which was suitable as a monitoring point for underground fluid activity.Based on the data of ion concentration and isotopic composition of hot spring water,this paper analyzed the sources,hydrochemical types,trace element content characteristics,deep thermal storage temperature,circulation depth of hot spring,and water-rock reaction degree.The relationship hydrate ion anomalies and seismic activity had been discussed and a hot spring fluid geochemical model had been established.All the above studies provide scientific background data support for anomaly verification and earthquake situation tracing,and it is of great significance in improving the level of fluid geochemical earthquake short-term prediction.The activities of fault zones may be monitored well by hydrogeochemical methods.From July 2015 to July 2020,concentrations of dissolved species including the major,trace elements and stable isotopes in 194 water samples collected from 12 thermal springs of NTHF were used to characterise their hydrogeochemical feature and to reveal the relationships between water chemistry and regional tectonic activity.And Xingfu hot spring was taken as a continuous sampling point from February 2019 and water samples were collected every three days.There were 169 water samples until now.The results show that:(1)The stable isotope data(?D,?18O)suggested that the water of hot springs were fed by circulation of meteoric waters,where the east and west branch fault recharge elevation ranges were 1.6~2.1 km and 1.0~1.9 km;(2)There were four main types of water chemistry in the study area: HCO3-Na?HCO3·SO4-Na?SO4-Ca?HCO3-Ca.The east branch fault zone was HCO3-Na type water.There were HCO3·CO4-Na and HCO3-Na in the northeastern segment of the west branch,HCO3-Na water in the middle section,and HCO3-Ca water in the southwestern segment;(3)Si O2 in the region was 11.1~101.0 mg/L,and aqueous geothermometry suggests that thermal storage temperature was 42.7~137.8°C.This data,along with an estimated temperature,gave an estimated circulation depth for the spring water of 0.6~2.5 km.Maolan and Xiaodingxi in the northeastern segment of the west branch were partially balanced water,while the eastern branch and other hot spring were immature water with low salinity.Comprehensive enrichment factors and contents of trace elements reflected that the water-rock reaction degree was weak in the internal circulation of the fault;(4)The dissolved Cl-and SO42-in Xingfu hot spring were obviously abnormal to the seismic precursor of near-field earthquake.Five days before the Yongde MS4.4 earthquake,the Cl-content increased by about40.6% compared with the background value and the epicentral distance is 77 km.one day before Lincang ML3.3 earthquake,the SO42-content increased by about 27.2%compared with the background value and the epicentral distance is 13 km;(5)Based on the above data and comprehensive data,the geochemical model of hot spring fluid in NTHF has been established.It shows that effective short-term and imminent precursory anomalies for the seismic activity in the NTHF can be provided by monitoring the circulation state and water rock reaction of hot spring in the fault zone.In a word,it is a great significance to judge and recognize the short-term and impending precursory anomalies of earthquakes with the study of the hydrogeochemical characteristics of the hot spring in NTHF.