Characterization of mud volcanoes in the northern Zhongjiannan Basin, western South China Sea

Mud volcanoes are important to understand hydrocarbon research and exploration, crustal movement and neotectonic activity, geological engineering, and climate change. They are widely distributed along the Alps-Tethys suture zone, the Pacific belt, and continental slopes. Previous detailed studies on mud volcanoes concentrated mostly on onshore areas. Recent developments in geophysical technology have opened vast deepwater areas for research on mud volcanoes. Based on multibeam bathymetry, multichannel seismic reflection, gravity, and magnetics data, mud volcanoes in the northern Zhongjiannan Basin have been studied comprehensively and systematically. Abundant mud volcanoes were found in the whole Northern Depression of the basin, not only in the Zhongjian Valley but also in the Rizhao Valley, with different sizes, morphologies, and eruption intensities. Five types of mud volcanoes are identified in the study area, which include the following: deep source high-energy large-size mud volcanoes, shallow source low-energy small-size mud volcanoes, budding mud volcanoes (mud diapir), mud volcanoes related to canyon and bottom current channels, and mud volcanoes related to pockmarks and gas chimneys. At the developmental scale, the distribution and type of mud volcanoes (diapirs) in the Zhongjiannan Basin are obviously different than those in the Yinggehai Basin, which is famous for the development of a large number of large mud diapirs in the North-western South China Sea. The region's tectonic and sedimentary environment is the main controlling factor of the development of mud volcanoes. Three sets of Tertiary thick mudstones in the Zhongjiannan Basin are the material basis for the formation of mud volcanoes. Overpressured fluid, generated from the combined factors of undercompaction, hydrocarbon generation, hydrothermal pressurization, and tectonic compression, is the driving force of the eruption of mud volcanoes. Furthermore, earthquakes that occurred in the surrounding areas of the Zhongjiannan Basin are the triggering mechanism of mud volcano eruptions.