摘要
龙旂热液区(49°39′E)是超慢速扩张西南印度洋中脊(SWIR)上发现的首个活动热液喷口,也是我国开展海底多金属硫化物资源勘探和走向深海大洋的重点研究区域。前期三维层析成像研究成功地揭示了该区的深部结构特征,但仅仅是提供了静态信息,相比之下,地震波各向异性研究是揭示其深部动力学机制的有效手段之一。本文简要介绍了龙旂热液区开展的主动源和被动源海底地震仪(OBS)探测实验,并初步分析了前期三维层析成像的走时残差数据,发现其与方位角之间存在显著的余弦函数关系,表明该区速度结构存在各向异性,但其各向异性来源尚不明确。探索各向异性来源将有望揭示该区的热液循环机制,为认识超慢速扩张SWIR热液区形成的动力学演化过程提供科学依据。因此,本文拟在前期研究基础上,利用主动源和被动源OBS地震数据,通过纵波方位各向异性和横波分裂联合分析的方法,重点研究地壳和地幔各向异性特征,并结合三维速度模型与区域地质背景资料,对获得的快波方向和快、慢波到时差参数进行深入分析,阐明地壳内裂隙分布、应力场变化和地幔流动特征,从而揭示龙旂热液区的热液循环机制、岩石圈形变和深部动力学过程等科学问题。
Dragon Flag Hydrothermal Field(DFHF, 49°39′E), located on the ultraslow-spreading Southwest Indian Ridge(SWIR), was the first active hydrothermal vent discovered in this area; and it is an essential location for seafloor exploration of polymetallic sulfide resources and deep ocean studying for China. A three-dimensional tomographic study successfully revealed the deep structure characteristics of the DFHF, but it only provided the static velocity information. For comparison, seismic anisotropic study is an effective method for investigating the dynamic mechanism of the DFHF. In this paper, we briefly introduce the oceanic bottom seismometer(OBS) surveys of both active and passive sources. Our preliminary analysis shows a cosine relationship between travel-time residuals and azimuths based on the velocity structure obtained from previous three-dimensional tomography, indicating that there is an anisotropy on the velocity structure at the DFHF; but, the source of the anisotropy is unclear until now. Seismic anisotropic study at the DFHF will contribute greatly to hydrothermal circulation mechanism and dynamic evolution process of hydrothermal field on the SWIR. Hence, we intend to conduct seismic anisotropic study by combining azimuthal seismic anisotropy and shear-wave splitting techniques based on the OBS data generated by the active(airguns) and passive(earthquakes) sources. Combined with three-dimensional velocity model and regional geological background data, anisotropic parameters of fast-wave direction and travel-time difference between fast and slow waves are used to depict the distribution of crustal cracks, stress variation and mantle flow. In addition, hydrothermal circulation mechanism, lithospheric deformation and deep dynamics process are further revealed.