拉萨地体内低速异常成因的探讨——壳内局部熔融的地震层析证据  

Interpretation of the Low Velocity Anomalies in the Lhasa Terrain: Tomographic Evidence for Partial Melting in the Crust

作　　者：薛光琦[1] 王有学[2] 宿和平[3] 

机构地区：[1]中国地质科学院矿产资源研究所,北京100037 [2]桂林工学院资源与环境工程系,广西桂林541004 [3]中国地质科学院地质研究所,北京100037

出　　处：《地球学报》2005年第5期411-415,共5页Acta Geoscientica Sinica

基　　金：国土资源重点基础研究专项(编号:20001010)资助。

摘　　要：青藏高原南部天然地震体波的层析反演图像描绘出拉萨地体内在15～90 km深度内低速异常的分布特征.在研究速度结构的基础上,根据熔融程度与纵波波速呈倒数衰减的特性,论证了拉萨地体中低速体的出现与壳内的局部熔融有关.壳内局部熔融则来源于板块俯冲、地壳增厚产生的热而非幔源侵入所造成.同时,也发现了在拉萨以东中下地壳低速体有向东发展的趋势.大面积部分熔融层分布的深度范围约在15～35 km处.另外,据藏南地区的SKS波分裂时间延迟很小的特点,推测藏南地幔中可能不存在各向异性,也就是没有足够的热源使各向异性矿物的品格重新定位.这从另一方面佐证了部分熔融仅存在于地壳内而不是地幔中.The low velocity distribution pattern at the depths of 15～90 km in Lhasa terrain is well imaged through tomographic analysis of seismic body waves acquired in southern Tibet. Based on a study of the velocity structure and the approximate Poisson＇s ratio obtained, it is shown that the low velocity body in Lhasa terrain is correlated with partial melting in the crust. The partial melting is caused by the heat derived from subduction and crust shortening rather than by the mantle material intrusion. According to σvalue distribution, the authors hold that the low velocity body in mid-low crust east of Lhasa tends to extend eastward. The partial melting is restricted to a range of 15～35 km in depth. From the tiny time delay in SKS splitting in the area, it is inferred that the partial melting may not exist in the mantle due to the lack of enough heating for the reorientation of anisotropic minerals. This adds to the evidence for the presence of partial melting only in the crust instead of in the mantle in this area.

关 键 词：青藏高原 地震层析 速度 局部熔融 异常成因 低速体 拉萨 体内 证据 地壳增厚 

分 类 号：P315.72[天文地球—地震学] P618.130.8[天文地球—固体地球物理学]