Long-Lasting Boiling-Wells: Geochemical Windows into the Tectonic Activity of the Maodong Fault (China)

The Maodong Fault (China) was mainly active during the Late Pleistocene. However, in the past century, numerous destructive earthquakes have occurred along the fault zone, indicating its continuing activity. Therefore, refined monitoring of the tectonic activity along the fault is required. Boiling-Wells located in the Maodong Fault Zone were selected for this purpose. The parameters, including the rare earth elements (REE) and gas components, such as CO2, Rn, and Total Volatile Organic Compounds (TVOC), in the wells were analyzed. By combining field observations with the analytical data, we constrained the relationships between the anomalies of the hydrochemical composition and the gas composition in the Boiling-Wells and the Maodong Fault: (1) CO2 and TVOC in the Boiling-Wells originated from Cenozoic magmatism and associated intrusive rocks. High concentrations of Rn are closely linked to tectonic activities of the Maodong Fault. CO2, TVOC, and Rn are all transported to the Boiling-Wells along the Maodong Fault, with CO2 acting as a carrier gas for Rn. (2) REE in the Boiling-Wells was mainly sourced from CO2 fluids that originated from deep-seated Cenozoic magmas and intrusive rocks. The concentrations of the REE and their distribution patterns were controlled by the input of CO2 fluids and by epigenetic processes. (3) The abnormally high contents of Ca2+, HCO3-, Pb2+, and Al3+ in the Boiling-Wells are attributed to the migration of externally-derived (deep) CO2 fluids through the Maodong Fault. (4) The anomalies of the gaseous (Rn, CO2, and TVOC) and hydrochemical components (Ca2+, HCO3-, Pb2+, Al3+, n-ary sumation REE, and REE patterns) in the Boiling-Wells are closely related to the tectonic activity of the Maodong Fault. Therefore, the long-lasting Boiling-Wells provide an excellent geochemical window into the evolution of the Maodong Fault. Our study documents that the contents and variations of specific hydrochemical and gaseous components of Boiling-Wells are well-suited geochemical tracers to identify and characterize the tectonic activity of the Maodong Fault. This method is also applicable for the monitoring of tectonic activities of major faults zones with comparable preconditions worldwide.