南海北部陆坡块体搬运沉积体系的地震响应与成因机制

块体搬运沉积体系是一种重力作用下形成的深水沉积体系,在全球大陆边缘沉积体系中扮演着重要角色.块体搬运沉积作用不仅造成危害极大的深水地质灾害,而且与海洋天然气水合物和深水油气的形成过程与富集有密切联系.根据新获得的海底地形地貌资料和地震资料,揭示了块体搬运沉积体系识别特征,明确了其分布范围.这些块体搬运沉积体系具有明显的滑坡形态和沉积构造;头部表现张性断裂构造,内部有旋转断块和流动构造,趾部表现为挤压和逆冲构造.块体搬运沉积体系表现出不同的成因类型.通过白云凹陷天然气水合物的分布模式和白云块体搬运沉积体系有限元数值模拟研究,认为白云块体搬运沉积体系与天然气水合物分解密切相关,进一步给出6种水合物分解模型情况下海底稳定性的安全因子,随着水合物分解量的增加,斜坡的稳定性逐渐减弱,当水合物的分解达到一定数量时,就会发育海底块体搬运沉积体系.

Seismic characteristics and triggering mechanism analysis of mass transport deposits in the northern continental slope of the South China Sea

Mass transport deposits (MTDs) are one kind of deepwater sedimentary system driven by the gravity flow action, and play an important role in the deep-water sedimentation in the global continental margins. MTDs are not only one of the most serious deep-water geo-hazards, but also have close relationships with the formation and accumulation of marine gas hydrates and deepwater hydrocarbon. Based on the topographic and high resolution seismic data in the hydrocarbon exploration region, we reveal the discerned characteristics and distribution of the MTDs in the northern slope of the South China Sea. The MTDs are characterized by apparent geomorphology and sedimentary structures. The head of MTDs has well developed extensional fault; the middle of MTDs has slide and turnover block; in the toe region of MTDs occurred compressive structures and thrust faults. Several mechanisms can induce the development of MTDs. However, Baiyun MTDs were inferred to be dominated by the dissociation of gas hydrate after we have analyzed the model of gas hydrate reservoirs and numerical modeling by using elastic-plastic finite element method for the MTDs in the Baiyun sag. We calculated 6 sets of safety factors with 6 different dissociation levels of gas hydrate, and found that with the increasing dissociation of gas hydrate, the stability of submarine slope decreases.