Abstract
The Dongsha Island (DS) is located in the mid-northern South China Sea continental margin. The waters around it are underlain by the Chaoshan Depression, a relict Mesozoic sedimentary basin, blanketed by thin Cenozoic sediments but populated with numerous submarine hills with yet less-known nature. A large hill, H110, 300 m high, 10 km wide, appearing in the southeast to the Dongsha Island, is crossed by an ocean bottom seismic and multiple channel seismic surveying lines. The first arrival tomography, using ocean bottom seismic data, showed two obvious phenomena below it: (1) a low-velocity (3.3 to 4 km/s) zone, with size of 20 × 3 km2, centering at ~4.5 km depth and (2) an underlying high-velocity (5.5 to 6.3 km/s) zone of comparable size at ~7 km depth. MCS profiles show much-fragmented Cenozoic sequences, covering a wide chaotic reflection zone within the Mesozoic strata below hill H110. The low-velocity zone corresponds to the chaotic reflection zone and can be interpreted as of highly-fractured and fluid-rich Mesozoic layers. Samples dredged from H110 comprised of illite-bearing authigenic carbonate nodules and rich, deep-water organisms are indicative of hydrocarbon seepage from deep source. Therefore, H110 can be inferred as a mud volcano. The high-velocity zone is interpreted as of magma intrusion, considering that young magmatism was found enhanced over the southern CSD. Furthermore, the origin of H110 can be speculated as thermodynamically driven, i.e., magma from the depths intrudes into the thick Mesozoic strata and promotes petroleum generation, thus, driving mud volcanism. Mud volcanism at H110 and the occurrence of a low-velocity zone below it likely indicates the existence of Mesozoic hydrocarbon reservoir, which is in favor of the petroleum exploration.
Highlights
A mud volcano is formed by eruption of mud, gas, and fluid, often containing hydrocarbon, water, and so on; it is an indicator of petroleum leakage from deep [1]
Given the thick Mesozoic layer around, the chaotic reflection zone below H110 is interpreted as comprising of highly fractured Mesozoic strata (Figure 9b)
A mud volcano is always indicative of hydrocarbon leakage [8], often with methane hydrate forming, when in deep sea
Summary
Radiated Pulse) sub-bottom profiles, and abundant authigenic carbonate and deep-water authigenicin carbonate deep-water organisms dredged samples from the seabed [12]. Remain unknown, due to lack of deeper thewaters waterssoutheast southeastofofthe theDS, DS,there thereare arealso alsoseveral severalsubmarine submarinehills hills(Figure (Figure). Remain unknown, due to lack of deeper thewaters waterssoutheast southeastofofthe theDS, DS,there thereare arealso alsoseveral severalsubmarine submarinehills hills(Figure (Figure2) Whether they are volcano or mud volcano is unknown, due to less investigation, but Whether they are volcano or mud volcano is unknown, due to less investigation, but importantfor for exploration exploration of topographically large one,one, H110,. Hill MV8 were previously recognized mud volcanoes [12]
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