Geochemical Features Of Fluid In Major Fault Zones Along The Eastern Boundary Of The Sichuan-yunnan Rhombic Block

The convergence of the Indian and Eurasian plates has forged the Qinghai-Tibet Plateau,a region with a distinct geological composition.The Sichuan-Yunnan rhomboidal block,at the plateau’s southeastern edge,is critical for material escape,China’s north-south seismic belt,and tectonic activity.Its eastern boundary encompasses the Xianshuihe,Anninghe,Zemuhe,and Xiaojiang fault zones,known for intense late Quaternary deformation and seismic activity.With over 350 hot springs,this area offers an ideal setting to investigate fluid geochemistry,genesis mechanisms,and short-term earthquake fluid predictions related to boundary faults.Previous research primarily focused on individual fault zone fluid geochemistry within the Sichuan-Yunnan rhomboidal block’s eastern boundary fault system,such as hot spring ions,gases,and soil gases.However,the geochemical attributes of this large-scale fault system and its connection to stress loading and regional seismic activity under the eastward expansion of the Qinghai-Tibet Plateau remain elusive.Thus,exploring this issue and uncovering the link between fluid migration-release processes,seismic activity,and stress loading are of paramount importance.This study analyzes 31 soil gas Rn and CO₂flux measurement profiles deployed along the eastern boundary fault system of the Sichuan-Yunnan rhombic block,revealing significant differences in Rn and CO₂fluxes among different fault zones.Concurrently,the helium isotope correction value（Rc/Ra）calculation uncovers prominent mantle degassing characteristics in the Kangding-Moxi region.Based on the three-endmember mixing model of helium,we calculate the three-endmember contribution rates of helium（atmospheric He _A,mantle He _M,crustal He _C）in the spring overflow gas along the eastern boundary of the Sichuan-Yunnan rhombic block.The results indicate strong mantle degassing in the Xianshuihe fault zone,while the He in the Anninghe-Xiaojiang fault zone mainly originates from the crust.Additionally,the excellent correspondence between the S-wave low-velocity zone and the high ³He/⁴He ratio in the intersection area of the Xianshuihe,Longmenshan,and Anninghe fault zones may reflect early mantle melting or asthenospheric mantle upwelling in this region.The analysis ofδ¹³C values of CO₂in the spring overflow gas in the eastern boundary fault system of the Sichuan-Yunnan rhombic block suggests that CO₂may come from multiple sources,with crust-derived carbon being dominant.Using a He-CO₂ ternary mixing model,the contribution rates of CO₂from the mantle（M）,carbonate（CAR）,and organic matter（ORG）are found to be different in the studied fault system.In this system,the CO₂contribution from carbonate rocks and organic sediments exceeds 90%,while the mantle carbon ratio is relatively low.Due to the convergence of the Indian and Eurasian plates and the blocking effect of the Yangtze Craton,the Kangding-Moxi section of the Xianshuihe Fault Zone becomes a high-pressure stress focal area.Considering the ³He/⁴He values,high strain rates,and rapid surface uplift,it is inferred that mantle-scale dynamic processes are present in this region.The geothermal architecture of the southeastern Tibetan Plateau margin delineates that the Xianshuihe Fault Zone is predominantly governed by mantle-derived heat flow,prominently concentrated in the Kangding region.In contrast,the Anninghe-Xiaojiang Fault Zone is primarily regulated by crustal heat flow.Investigations on the association between helium isotopes and regional stress fields exhibit that compressional stress is more conducive than extensional stress to enhance fault penetration,escalate permeability,and promote the release of deep volatile substances such as helium and carbon dioxide.Mantle fluid degassing in the Kangding-Moxi area implies the potential involvement of mantle-derived CO₂in the earthquake nucleation mechanism.As deep mantle fluids migrate towards the surface along fault zones,overpressured fluids may modify regional crustal structures,permeability,and rock fracturing,resulting in earthquakes and expediting radon gas emanation from crustal rocks.This research unveils the fluid geochemical features of the fault system at the eastern boundary of the Sichuan-Yunnan rhomboid block,offering crucial insights into fluid migration and release processes,mantle-crust interactions,geothermal resource distribution patterns,and the correlation between earthquake nucleation mechanisms and mantle fluids in the area.