【摘要】 利用中国地震台网和ISC台站1980~2004年的地震数据,反演了南海东北部及其邻近地区的Pn波速度结构和各向异性.上地幔顶部的速度变化揭示出区域地质构造的深部特征:华南地区速度较高并且变化平缓,具有构造稳定地区的岩石层地幔特征;华南沿海尤其是滨海断裂带附近出现低速异常,表明该断裂可能穿过壳幔边界深达上地幔顶部.南海北部至台湾海峡较高的速度与华南地区类似,反映出大陆边缘和陆架地区的岩石层地幔性质;西沙海槽附近较高的速度不仅反映了华南大陆向南的延伸,而且与海槽裂谷拉张引起的地幔上拱有关,整个南海北部没有发现大规模地幔热流的活动痕迹.相比之下,南海东部次海盆的上地幔顶部存在明显的低速异常,对应于海底扩张中心的地幔上涌区,表明岩石层地幔强烈减薄甚至缺失;台湾东部-吕宋-菲律宾北部的低速异常与地震、火山活动以及岩浆作用紧密相关,揭示了西太平洋岛弧俯冲带的活动特征;南海东北部的洋-陆边界清晰,南海东部和菲律宾海西部较高的速度代表了海洋岩石层地幔的性质.Pn波各向异性反映出区域性构造应力状态及岩石层地幔的变形痕迹:华南地区的各向异性较小,说明这一构造稳定地区的岩石层地幔变形程度较弱;南海北部的快波方向与地壳浅表层构造的伸展方向一致,主要反映了中、新生代以来的大陆边缘张裂和剪切作用对岩石层地幔结构的影响;琉球-台湾-吕宋岛弧两侧各向异性十分强烈,平行于海沟的快波方向表明菲律宾海板块和欧亚大陆的相互作用导致俯冲板块前缘的岩石层地幔强烈变形;台湾东南海域快波方向的变化可能与欧亚大陆和菲律宾海板块俯冲机制的转换以及岩石层被撕裂有关.更多还原

【Abstract】 Using seismic data from Chinese and ISC stations between 1980～2004,we inverted Pn wave velocity and anisotropy of the northeastern South China Sea and adjacent region.The velocity variations at uppermost mantle reflect the features of regional geology and tectonics.The southeastern China region has fast velocities correspondent to the lithospheric mantle in tectonically stable regions.Slow velocities appear near the Binhai fault zone along the coast of southeastern China,which indicates a possible penetration of the fault zone into the uppermost mantle.Similar to southeastern China,fast velocities throughout the northern South China Sea and Taiwan Strait reveal a property of the continental lithospheric mantle,while fast velocities in the Xisha Trough reflect a southward extension of continental margin and a mantle upwarp produced by rifting.There is no evidence showing large-scale mantle heat flows in the northern South China Sea.However,pronounced slow velocities are observed in the eastern sub-basin of the South China Sea,which is thought to be an extinct mantle upwelling center with high heat flows,indicating a thinning or removal of lithospheric mantle.Along the eastern Taiwan,Luzon and northern Philippine,slow velocities are related to seismic and volcanic activity and magmatism in the arc zone,while fast velocities in the eastern South China Sea and western Philippine Sea reflect the oceanic lithospheric mantle.Pn velocity anisotropy also reveals the stress state in regional tectonics and the history of deformation of lithospheric mantle.Southeastern China has small anisotropy in response to less deformation there.Clear anisotropy is observed in the northern South China Sea,with fast directions of anisotropy aligned with shallow structures,which reflect the evidence of the Mesozoic-Cenozoic rifting and shear deformation in the lithospheric mantle.Strong anisotropy is also observed along the Ryukyu-Taiwan-Luzon arc zone and the fast directions parallel to those trenches indicate intense deformation in the lithospheric mantle of the leading edge of the Philippine Sea plate and Eurasian continent.The change of fast directions of anisotropy near eastern Taiwan is probably caused by a conversion of collision mechanisms between the Eurasian continent and the Philippine Sea plate,and the tear of the lithosphere mantle.更多还原