Comprehensive geological and geophysical study in middle southern region of China
中国中南地区综合地质地球物理研究

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.