中国大陆下扬子及邻区地壳结构基本特征：深地震测深研究综述

Basic features of crustal structure in the Lower Yangtze and its neighboring area of Chinese Mainland：Review of Deep Seismic Sounding research

摘要:

系统回顾了20世纪70年代以来在中国大陆下扬子及其邻区开展的深地震测深工程，总结了相关宽角反射/折射地震资料的波组特征及地壳结构的基本特征。下扬子及邻区深地震测深资料普遍具有较清晰的初至震波Pg、上地壳底界面反射波P1、中地壳底界面反射波P3、莫霍界面反射波Pm及上地幔顶部首波Pn，震相连续，易追踪对比。受浅表沉积盖层或基岩等的影响，Pg波常出现局部延迟滞后或走时超前现象。尤其是大别造山带个别炮点的反射能量较弱或同相轴扭曲、波形紊乱等，均与地壳内界面和莫霍面的深度突变或破碎有关。该区域地壳结构大致分为上、中、下3层，但视资料情况中地壳和下地壳又可进一步划分为2个亚层。华北地台和扬子地台地壳厚度30～36km，莫霍界面形态变化较缓，存在局部隆起，下地壳平均速度 6.7±0.3km/s。但大别造山带下方地壳厚度32～41km，莫霍界面下凹且出现4～7km垂向错断，下地壳平均速度 6.8±0.2km/s。

Abstract:

The Deep Seismic Sounding(DSS)projects carried out from the 1970s of the last century in the lower Yangtze region and its neighboring area were reviewed. Then the basic wave group features of those wide angle reflection/refraction record sections，and that of the crustal structure are summarized. It shows that there were totally five clear wave groups on the record sections，which include the first arrival Pg，the reflection P1 from the bottom interface of the upper crust，the reflection P3 from the bottom interface of the middle crust，the strong reflection Pm from the Moho boundary，and the refraction Pn from uppermost mantle. Generally，these phases are easily consistently traced and compared，despite that some first arrivals are delayed or arrive earlier than normal due to the shallow sedimentary cover or bedrocks. Especially，in the Dabie Mountain region the seismic events of a few shots gathered always have weak reflection energy，coaxial twists，or disorganized waveform. This could be attributed to the variations of reflection depth，the disrupt of Moho，and the discontinuity of the reflection boundary within the crust. The regional crustal structure is composed by the upper，middle，and lower crusts，among which the middle and lower layers can be further divided into two weak reflection ones respectively because of the clear seismic evens on the record sections. The crustal thickness of the North China and Yangtze platform are 30～36km，and the Moho exhibits a flat geometry despite some local uplifts. For these two tectonic areas，the average compressional wave velocity in lower crust is 6.7±0.3km/s. Nevertheless beneath the Dabie Mountain area the crustal thickness is 32～41km，the Moho bends down sharply and takes an abrupt 4～7km offset in the vertical direction. The average compressional wave velocity in lower crust beneath the Dabie Mountain area is 6.8±0.2km/s.