Distribution of the gas vents at the seabed of the Mid-Okinawa Trough and their controlling factors
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摘要:
海底气体排放是海洋环境中碳物质从岩石圈进入到水圈中的重要地质过程，理解甲烷在该过程中的迁移方式和表征是定量评价海底甲烷排放在全球碳循环中环境效应的重要基础。本次研究使用2013—2016年采集的多波束回声测深以及二维多道地震数据，展示了与海底气体排放有关的地球物理特征，在水体、浅部地层中以及海底界面处分别识别出了束状羽状流、柱状裂隙气体疏导通道和下伏有碳酸盐岩的海床凸起，它们被解释为气体排放的地质表征，这些构成要素在空间上的叠置关系呈现了冲绳海槽中部气体排放特征，研究选取典型实例刻画了这一地质过程并总结了模型。经过分析断层与气体排放地质表征的空间位置关系，提出冲绳海槽研究区内海底气体排放的分布受到了盆地构造活动的控制，冲绳海槽发生的斜向裂谷作用导致了研究区张扭断层的形成，以拉张为主的断层为富甲烷流体提供了垂向运移通道，致使气体排放沿正断层分布。研究表明海底气体排放可以广泛发育在以拉张应力为主的地质环境中。
Abstract:Gas venting at the seabed is recognized as a key geological process for carbon transfer from the lithosphere to hydrosphere. Understanding how methane is transported and the geological expressions in this process is necessary for quantatively evaluating its climatic impact on global carbon cycle. In this study, multi-beam echo-sounder (MBES) and multi-channel seismic (MCS) data acquired between 2013 and 2016 from the Mid-Okinawa Trough are used to show the geophysical features associated with gas vents. We identified the bundle-shaped gas flares in water column, the migration pathways consisting of cylindrical cluster of fractures in the shallow subsurface and the seabed domes underlain by carbonate-cemented sediments. They are regarded hereby as the geological indicators of gas vents, indicating how gases are vented in the Mid-Okinawa Trough. An example has been selected for description to represent the gas vents in the study area. We proposed that the distribution of gas vents in the study area is controlled by basin-scale tectonic activities after analyzing their spatial relationship with the faults. The oblique rifting in the Mid-Okinawa Trough resulted in the formation of the transtensional faults. The faults that formed in the tensile regime provide vertical conduits for gas-rich pore fluids to migrate, which caused that the gas vents were elongated along the normal faults. This study suggests that gas vents on the seabed can occur extensively in the extension-dominated tectonic regime.
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Key words:
- gas vent /
- tectonic control /
- gas flare /
- Okinawa Trough
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