南海中北部陆缘横波速度结构及其构造意义

摘要

The joint exploration of P-wave and S-wave can obtain amount of information on lithology and geophysical properties, which effectively improve the composition constraint of the deep crustal structure. Based on the optimum P-wave model, the shear waves recorded on the radial components of profile OBS2006-3 have been modeled by 2-D ray-tracing. The S-wave velocities in sedimentary layer 1 and sedimentary layer 2 are of 0. 7～0. 9 km/s and 1. 6～1. 7 km/s,respectively. The Vp/Vs ratios in these sediments vary from 2. 64～3. 16 to 2. 41. The relatively high Vp/Vs ratios arc probably related to unconsolidated sediments. In the Mcsozoic sedimentary layer, the S-wave velocity increases gradually from 2. 6 km/s at the top to 3. 0 km/s at the bottom, and the Vμ/V. Ralio is of 1. 74 ～ 1. 80. The Vp/Vs ratio of 1. 74 ～ 1. 82 in the crust is considered to he the continental crust. The S-wavc velocity and Vp/Vs are 4. 0 ～ 4. 2 km/s and 1. 73～1. 78 of the High Velocity Layer (HVL) in the lower crust, which suggest that the HVL has a mafic composition that may originate from underplating of the igneous rocks after the cessation of the seafloor spreading.%纵横波联合勘探可以得到更多关于岩石圈层岩性、物性等介质属性方面的信息,有效提高地壳物质组成的约束性.在纵波速度结构模型的基础上,通过射线追踪和走时拟合对OBS2006-3地震剖面径向分量的转换震相进行了横波速度结构模拟.结果表明:沉积层1、沉积层2的横波速度分别为0.7～0.9 km/s和1.6～1.7 km/s,波速比由2.64～3.16变化到2.41左右,较高的波速比表明沉积层固结程度不高；中生代沉积层横波速度由浅部的2.6 km/s增大到3.0 km/s,波速比为1.74～1.80;地壳中波速比为1.74～1.82,表明该剖面地壳为陆壳性质；下地壳高速层的横波速度为4.0～4.2 km/s,纵横波速比为1.73～1.78,揭示高速层的物质组成为铁镁质的基性岩,推断其与海底扩张停止之后的岩浆底侵作用有关.