中国东部海区及其邻域岩石层结构与地球动力学特征研究

Lithosphere structure and geodynamics characteristics of China eastern seas and adjacent region

中国东部海区及其邻域受到来自欧亚、太平洋和菲律宾海板块运动的共同作用,其是认识中国边缘海形成演化的天然实验室.本文通过分析中国东部海区及其邻域的地球物理场特征,采用重力反演方法计算了该地区的莫霍面深度,根据地震层析成像的反演结果分析了研究区的地壳和岩石层速度结构,并从地热、断裂带、天然地震分布和应力场特征等方面对该区域的地球动力学特征做了进一步的分析.通过分析、反演计算可以得出:中国东部海区的陆架区的重、磁场特征与中国大陆重、磁场特征分布趋势与走向相同,是大陆地球物理场向海域的延伸.东海陆架盆地区和中国大陆东部的地壳结构和走向趋势相同,是大陆地壳向海域的延伸.东海东部及其边缘海域、海槽为过渡性质地壳,由于地壳性质的转变,说明冲绳海槽应为东海大陆架的自然终结.地震层析成像反演结果为研究岩石层地幔的结构组成以及形成演化的深部动力因素提供了依据,32+km深度处的速度图像基本上反映出莫霍界面正下方的速度分布特征及莫霍面的起伏形态,其与重力反演计算的结果相符合.70km深度上的速度图像反映出东部海区深部结构南北分块的特征,海区表现负速度扰动,推断已部分进入软流圈.110km深度处的速度图像表现为大范围的低速异常,只有扬子块体有正异常显示,进一步验证华南华北块体减薄.中国东部海区及其邻域地球动力学特征主要为:冲绳海槽高热流异常区显著,热流分布明显和沟弧盆体系的构造特征相吻合,呈现NE向条带状分布;断裂体系显示出"东西成带"的待征;冲绳海槽和琉球海沟区域具有地震活动的成带性,地震震中的分布密度远大于远离板块边界的大陆内部;其现代应力场从北到南由NE向逐渐转为NEE至SE向,总体呈向东发散的扇形分布.

The China eastern seas which is influenced by Eurasian plate, Pacific plate and Philippine Sea plate is a large natural laboratory to study on the evolution of marginal sea. The research on Moho distribution and lithosphere structure in China eastern seas and adjacent regions based on the tomography results from seismic body wave data, the geodynamics characters results from geothermy, fracture zone and stress and inversion results of gravity data in research area have been carried out. According to analysis, we consider that the gravity-magnetic character of China eastern seas have the same tendency as the continent, We also inverse the depth of Moho used Bouguer gravity anomaly and figure out that Okinawa Trough is likely to be the end of continental shelf of the China eastern seas. The tomography results provide new evidences for the mantle structure and the force of formation and evolution. The velocity map of 32＋ km depth coincides to the inversion of Moho and generally depicts the up and downs of Moho velocity. In the 70km depth, that the seas＇ velocity is low, as a result, it shows the seas, rather than continent, get to asthenosphere. In 110km depth, only the subduction zone shows positive velocity disturbance. Northern and Southern of China blocks are negative velocity disturbance. We speculate that the lithosphere in Northern and Southern of China may get thinner. To the geodynamics characteristics, geothermy distribution shows belted feature. The approximate east-west fault zones shows belted feature in the shallow. The epicenter distribution in China eastern seas also shows belted feature, and in Okinawa Trough and Ryukyu Island Arc it becomes intensive than other areas. The stress character shows fan- shaped and totally has NE, NEE and SE orientation.