SOIL RADON AND FAULT ACTIVITY ANALYSIS OF THE HUANGCUN FAULT IN THE OUTHWESTERN LUSHI BASIN HENAN PROVINCE

Measurement and analysis of the distribution of radon concentrations in soil air are an effective method to study the manifestations of geodynamic activity in the upper crust. Studying radon emanation is possible to reveal positions/activity of faults in the area,as well as to identify the recent movement intensity of fault blocks. The purpose of this paper is to determine a possible connection between anomalous radon values and the Huangcun fault at Qiaojiayao,where is currently exposing geothermal water in a limited area,with the temperature up to 34℃. Estimation of soil-radon activity is first carried out for faults in southeastern Lushi Basin,Henan Province. Four profile radon surveys are conducted at Huangcun fault that is overlain by Quaternary sediment in the southwest of the Lushi Basin. Radon concentrations are measured at 74 sites in an area about 1.2km2(6002000m) from July to August in 2013. The background and anomaly threshold values calculated are 7.12 kBqm-3 and 12.36 kBqm-3,respectively. Abnormal radon values,with peak signals of about 2～3 times higher than the background level,lead to the detection of a fractured zone,denoted as F2. The spatial variation of soil gas radon values are correlated with the activated Huangcun faults in the southwest of the Lushi Basin. The lower 222Rn activity is recorded on the main fault plane,while relatively higher 222Rn values is sampled a few tens of meters on the both sides of the fault,as a consequence of the dilution effect played by the soil(CO2)efflux. Radon concentrations highlight a decreasing gradient from south to north in the area. On moving further away from the main fault plane toward north,soil-gas activity decreases and becomes more stable. Higher soil 222Rn values are detected on the upthrown side of the Huangcun fault than on its downthrown side. This could be due to the greater structural instability of the downthrown side of the fault,and hence to its greater fracturing,which would result in a higher degree of radon dispersion and/or air dilution. The Qiaojiayao geothermal anomaly is located in the transition zone of radon concentrations between middle values and background values. Radon values illustrate a close relation with the activity of the Huangcun fault. The active fault zones and the associated fractures act as paths for radon migration. The data analysis suggests that the fault could be activated and transformed from a compress-shear to a normal fault. Such a fault is very likely extending down deeply,forming large conduits through which gases and heat move upwards. It may constitute the most important structural element concerning the formation of geothermal water near north of the fault at the Qiaojiayao village. The initial result shows that,the basement tectonic has multiple phrases of activities,and that the compressive-twisting faults in active region can be converted into regional tensional ones,controlling migration of hydrothermal fluids.