宁夏及邻区地壳上地幔结构特征分析

Analysis of Crust and Upper Mantle Structure in Ningxia and its Adjacent Area

摘要:

介绍了宁夏及邻区1970年以来多种地球物理探测研究成果，揭示了该地区不同活动构造单元的深部地壳构造背景。分析表明， 活动块体相互作用、地壳深部断裂、地壳低速体、速度结构的差异和岩性的不同、上地幔顶部隆起、莫霍界面较大的起伏以及复杂的壳幔过渡带等构造特征与该区域强震的形成和发生有较为密切的关系。最后讨论了该区域强震发生的机理。

Abstract:

Ningxia and its neighboring area are located in three blocks including the Erdos block，Alashan block and the Qinghai-Xizang block.There are many active faults in this area，such as the Haiyuan fault，the eastern piedmont fault of Helanshan，the Huanghe fault，and the Liupanshan fault.Because of the special stress environment in this area，all earthquakes of magnitude greater than 8 and most of those 5 to 7.9 occurred along the boundaries of the three active tectonics blocks.Researchers began exploration in this district in the 1960s and have used many methods such as wide-angle reflection and refraction seismic exploration，electrical prospecting，shallow seismic exploration，the gravity method，Global Positioning System （GPS），geodetic deformation survey，seismic array，and surveying and prospecting of active faults.Many valuable results have been obtained by using the various types of study methods.This paper summarizes the results of 40 years of such studies in this area by geoscientific researchers.The active block hypothesis states that strong earthquakes occur in such areas that the position wave velocity changes from high to low or the placement of the upper low-velocity body changes.Distribution of the geotemperature field in this area shows that many strong earthquakes occur in the position of high temperature and anomalous temperature gradient.The differential movement of block interaction is very strong.The results of wide-angle reflection and refraction seismic exploration also indicate the occurrence of the upper Moho uplift phenomenon in the strong seismic focus，in which superheated materials likely rise and melt rock as the hot mantle rock ascends in this area.By examining approximately 40 years of research in this area，including the deep seismic sounding profile，high gravity and magnetic anomalies，and inversion of seismic array data，we conclude that the Alashan and Erdos blocks are relatively stable and that the earthquake activity is weak，whereas the earthquake activity of the northeastern margin of the Qingzang-Tibet Plateau and Yinchuan Basin is stronger.The result of the probe of the deep seismic sounding profile shows that the upper Moho in the Yinchuan Basin is an uplift that resembles a concave bowl from east to west.Geothermal data support this theory.A low-velocity body exists in the crust of Yinchuan Basin，which also upholds this theory；many strong earthquakes that occurred in this area also offer direct evidence.The northeastern margin of the Qingzang-Tibet Plateau spatially and is the boundary of the active block，exhibiting comprehensive anomalous gradient zones with gravity，aeromagnetism，and crust thickness.Natural and artificial earthquake research reveals significant changes in crust thickness and several low-velocity bodies in the crust.There is obvious underplating in the crust and upper mantle transition zone.Geothermal data shows the same result.In short，strong earthquakes that occurred in this area are connected with a deep fault in the crust，a low-velocity body in the middle-lower crust，uplift of the upper mantle，high gravity-magnetic anomalies and a complex crust and upper-mantle transition zone.