雷琼拗陷地壳结构特征

Crustal structure features of the Leiqiong depression in Hainan Province

根据1984-1985年在雷琼地区完成、长约210km的白延-江洪深地震测深资料，应用有限差分法、时间项法反演和二维非均匀动力学射线追踪等方法，得到白延-江洪地壳剖面的二维非均匀构造图像．结果显示：琼北及雷州半岛地壳厚约25～26km，地壳平均速度在琼州海峡南段琼北凸起约6．16km·s^-1，海峡北段雷琼拗陷区约5．70km·s^-1；地壳速度偏低，下地壳尤为明显，仅约6．3～6．5km·s^-1；结晶基底显示为厚约2～4km、速度5．5～6．0km·s^-1的速度梯度带，Moho界面上、下界面速度分别是6．5km·s^-1和8．05km·s^-1的一级速度间断面，没有发现明显的壳幔过渡带．白延-江洪剖面位于雷琼拗陷更新世玄武岩覆盖区，推测地壳厚度减薄以及低速度构造主要反映了该区域自新生代以来一直延续至第四系晚期的多期火山岩浆喷发，热地幔对地壳底部的底浸、拆层与地壳仍处于相对温热状态相关．

Seismic data obtained from an about 210 km long Baiyan-Jianghong deep seismic sounding profile, which was completed in Leiqiong area in 1984 and 1985, were processed by means of finite difference tomography, time term inversion and 2-D dynamic ray tracing methods. And 2-D inhomogeneous crustal structure images along Baiyan-Jianghong seismic line were determined. The results show that the crustal thickness in northem Hainan province and Leizhou Peninsula is about 25 - 26 km. In northem Hainan island embossment of the southern part of Qiongzhou Straits, the average crustal velocity is about 6.16 km·s^-1 and 5.70 km·s^-1 in the Leiqiong depression of northern segment of Qiongzhou Straits. The crustal velocity is particular low and especially obvious in lower crust where it is only about 6.3 - 6.5 km·s^-1 ; The crystalline basement presents a velocity gradient belt, the thickness of which is about 2 - 4 km and the velocity is approximately 5.5 -6.0 km·s^-1 . Moho, the velocities of which on its two sides are 6.5 km·s^-1 and 8.05 km·s^-1 respectively, shows one-order velocity discontinuity, and there is no obvious crust-mantle transitional zone to be found. Because the Baiyan-Jianghong seismic line lies in the Leiqiong depression area with basalt covering of the Pleistocene, it is suggested that the thinner crust and lower Velocity in this area mainly reflect the multierupting of volcanic magma from the Cenozoic till the late Quaternary as well as the underplating and delaminating of hot mantle to the crustal bottom which are interrelated with the crust be still in comparatively hot state.