Characteristics of mantle degassing and deep-seated geological structures in different typical fault zones of China

Abstract

n this paper a comprehensive tracing study is conducted on mantle degassing and deep-seated geological structures in different types of fault zones in the continent of China based on the helium isotope data, coupled with some indices such as CO 2 / 3 He, CH 4 / 3 He and 40 Ar/ 36 Ar, and geological tectonics data. There are four representative types of fault zones: (1) Lithospheric fault zones in the extensional tectonic environment are characterized by a small Earth’s crust thickness, a lower CH 4 / 3 He-high R and lower CO 2 / 3 He-high R system, the strongest mantle de-gassing, and the dominance of mantle fluid, as is represented by the Tancheng-Lujiang fault zone. (2) The lithospheric fault zones or the subduction zone in the strongly compresso-tectonic envi-ronment, for instance, the Bangonghu-Nujiang fault zone, are characterized by a huge thick Earth’s crust, with the R/Ra values within the range of 0.43―1.13, and weak mantle degassing with mantle-source helium accounting for 5%―14% of the total. (3) The deep-seated fault zones at the basinal margins of an orogenic belt are characterized by R values being on order of magnitude of 10 -7 , and the CH4/3He values, 10 9 ―10 10 , CO 2 / 3 He values, 10 6 ―10 8 ; as well as much weak mantle degassing. (4) The crustal fault zones in the orogenic belt, such as the Yaojie fault zone (F 19 ), possess a high CH 4 / 3 He-low R (10 -8 ) and high CO 2 / 3 He-low R system, with no obvi-ous sign of mantle degassing. Studies have shown that the deep-seated huge fault zones are the major channel ways for mantle degassing, the main factors controlling the intensity of mantle degassing are fault depth, tectonic environment and crust thickness; the intensity of mantle de-gassing can reflect the depth and the status of deep-seated tectonic environment of fault, while the geochemical tracing studies of gases can open up a new research approach; upwelling ac-tivity of hydrothermal fluids from the deep interior of the Earth may be one of the driving forces for the formation and evolution of the huge deep fault zones. Piedmont fault zones are the locations where deep-seated tectonic activity and crust/mantle structure are transformed, which are of great significance in understanding the mechanisms of formation of orogenic belts and basins.