Roles of fault structures on the distribution of mantle-derived CO2 in the Bohai Bay basin, NE China

Abstract Based on studies on the contents, origin and filling history of CO2, and the seismic interpretation of the Bohai Sea, the sea area of the Bohai Bay basin, the distribution of mantle-derived CO2 and roles of fault structures have been examined in this study. Mantle-derived CO2 is mainly distributed in the structures within the Bozhong area, the center of the Bohai Sea. Laser Raman spectrum shows that CO2 inclusions are symbiotic with hydrocarbon inclusions with a charging time no earlier than 5.1 Ma. Late-stage (5.1–0 Ma) active basement faults are the major pathways for the migration of mantle-derived CO2. The intensity of the activity influences the CO2 content. Abundant CO2 occurs in the structures with basement faults that have relatively high fault activity rate (FAR) values. Along the basement fault, high CO2 content appears near the segments with high FAR values. Fault combination patterns affect the accumulation layers of mantle-derived CO2. Structures with fault combination patterns that resemble a “flower” or “Y” commonly accumulate CO2 in Cenozoic shallow layers as well. The branch faults that intersect with basement faults facilitate the adjustment of CO2 from deep to shallow layers. The uplifted mantle is the deep tectonic context, and the upwelling mantle magma is the source of the development of mantle-derived CO2. Below the middle crust, mantle-derived CO2 may be vertically transported along the lithosphere-scale strike–slip faults. Within the middle and upper crust, the late-stage active basement faults are the major conduits for mantle-derived CO2.