Mantle degassing along strike-slip faults in the Southeastern Korean Peninsula

Abstract

On September 12, 2016, a ML 5.8 earthquake hit Gyeongju in the southeastern part of the Korean Peninsula (SeKP), although the area is known to be far from the boundary of the active plate. A number of strike-slip faults are observed in heavily populated city areas (e.g., Busan, Ulsan, Pohang, and Gyeongju). However, dissolved gases related to the active faults have rarely been studied despite many groundwater wells and hot springs in the area. Here we report new results of gas compositions and isotope values of helium and carbon dioxide (CO2) in fault-related fluids in the region. Based on gas geochemistry, the majority of gas samples are abundant in CO2 (up to 99.91 vol.%). Measured 3He/4He ratios range from 0.07 to 5.66 Ra, showing that the mantle contribution is up to 71%. The range of carbon isotope compositions (δ13C) of CO2 is from −8.25 to −24.92‰, showing mantle-derived CO2 is observed coherently where high 3He/4He ratios appear. The weakening of faults seems to be related to enhanced pressures of fluids containing mantle-derived helium and CO2 despite the ductile lower crust underneath the region. Thus, we suggest that the SeKP strike-slip faults penetrate into the mantle through ductile shearing.

Mantle-Derived Helium Emission near the Pohang EGS Site, South Korea: Implications for Active Fault Distribution

“…Based on the N 2 -Ar-He ternary diagram ( Figure 2), nonreactive gases (N 2 , Ar, and He) in the Heunghae and Yeonil areas display a two-component mixing relationship between the mantle and atmospheric end members. Lee et al [9] have shown that fault gases in the southeastern Korean peninsula are continental gases rather than subduction zone gases. Dissolved gases in groundwater from the Sinkwang area are atmospheric with low He/Ar ratios (0.003 to 0.001).…”

“…Dissolved gases in groundwater from the Sinkwang area are atmospheric with low He/Ar ratios (0.003 to 0.001). Lee et al [9] addressed that some gas samples which are severely contaminated by air in southeastern Korea are due to shallow well depths (130 to 296 m). However, in the Heunghae and Yeonil areas, it is unlikely that well depths (Table 1) and contribution from deeply derived gases are relevant.…”

“…It is also possible to quantify the contribution of each source [8]. Mantle-derived helium and CO 2 degassing through faults in the southeastern Korean peninsula has recently been reported [9].…”

Geological degassing enhances microbial metabolism in the continental subsurface

“…This process, also termed mantle degassing, is the transition of volatiles from the mantle (supercritical) to the subcritical zone of the upper crust fueled by lower pressure of volatiles near the surface compared to the mantle 20 . Modern Earth has few areas with active mantle degassing, which are usually restricted to terrestrial volcanoes or hydrothermal vents in oceans 21,22,23,24 . At hydrothermal vents, chemolithoautotrophs initiate the microbial trophic network and proliferate at high rates leading to high microbial cell numbers 1 .…”

Seismic Scatter Associated with Earthquakes that Ended a Quiescent Period in Southeastern Korea

“…In addition to these, several faults were newly included as active faults associated with the M 5.8 earthquake (Lee et al 2018(Lee et al , 2019. Several cultural heritage sites, including the capital of the longest dynasty in ancient Korea, as well as many industrial facilities, including nuclear power plants, are located in the vicinity of these faults.…”