中国中南地区综合地质地球物理研究

利用重、磁和天然地震资料对中南地区进行了综合地质地球物理研究.根据研究区的重、磁异常的分布特征,可将其分为4个重力异常区、3个航磁异常区.根据重力资料反演计算得到的研究区的地壳厚度在295～41 km之间,总的趋势为西厚东薄,地壳厚度与地形起伏基本上呈镜像关系.根据磁力资料计算得到的研究区居里界面在12～40 km之间变化.地震层析成像结果表明研究区内的速度分布总体上体现了纵横交错的断块特征.在纵向尺度上,江汉－洞庭盆地以及周缘造山带的上地壳结构变化不大,中地壳和下地壳则普遍受到现今构造活动的改造,以致岩石的结构发生了一定的变化.它们主要表现为低速区域的扩大,尤其是在地壳下部尤为突出,这与断陷盆地的拉张以及造山带岩石层的底侵和拆沉作用密切相关.通过对研究区地球物理场的分析计算,在研究区共提取主要断裂带34条.根据岩石层板块大地构造理论,依据岩石层结构、地壳结构和结晶基底等深部结构的不同,将研究区中板内不同构造单元——块体作为一级构造单元,块体之间的深大断裂带作为块体的边界——块体结合带,据此原则在研究区中划分出两个一级构造单元,五个二级构造单元.

Comprehensive geological and geophysical study in middle southern region of China

We use gravity,magnetic,and earthquake data to carry out a comprehensive geological and geophysical study in the middle southern region of China.According to the distribution of gravity and magnetic anomalies,four gravity anomalous zones and three magnetic anomalous zones are identified.According to calculations of gravity data,the crustal thickness ranges between 29.5 and 41 km with a general trend of being thick in the west and thin in the east.There is a mirror image relationship between crust thickness and terrain relief.According to magnetic data,the depth of Curie interface ranges between 12 and 40 km in the study area.The crust of study region is divided into four layers by velocity interfaces at 3,13,23km and 36km,and below the crust two velocity interfaces at 71km and 120km are adopted in seismic tomographic study.Tomographic imaging result indicates that velocity distribution generally reflects the characteristics of crossed fault blocks.Longitudinally,the upper crust structure in Jianghan-Dongting basins and their marginal orogenic belts does not change much,the middle and lower crust were generally transformed by modern tectonic activities,leading to certain changes in lithospheric structure.Their main presentation is the enlargement of low velocity areas,it is particularly so in the lower part of the crust.This is closely related to the tension of depressive basins and lithospheric intrusion in orogenic belt.Analysis and calculations of geophysical fields reveal 34 principal fault zones in the study area.According to geotectonic theory of lithospheric plate,as well as the variations of deep-seated structures of lithosphere,crustal structure and crystalline basement,the different tectonic units-massives in the study area can be taken as primary tectonic units.The deep grand fault zones between them can be taken as boundary contact zones.In light of this principle,two primary tectonic units and five secondary tectonic units are identified.