南北地震带北段速度结构与电阻率特性

SEISMIC VELOCITY STRUCTURE AND ELECTRIC RESISTIVITY ON NORTH SEGMENT OF THE NORTH-SOUTH SEISMIC ZONE

摘要:

建立了南北地震带北段下地壳部分熔融几何形状和分布的模型.熔融的几何分布和连通性由以下3个因素决定:①熔融的位置;②熔融所占总体积的比例;③固-固和固-液界面的最小自由能.熔融形状有薄版形、管道形和椭球包体形.用此模型计算出含有部分熔融岩石的体积、剪切弹性模量和电导率的一般表示式.选择参数并计算出南北地震带北段下地壳的v_p、v_s和电阻率ρ_s值,即在鄂尔多斯地台下地壳的速度为v_p ≈6.71 km/s,v_s≈3.96 km/s,电阻率为上百至几百欧姆米.而在祁连、秦岭褶皱系下地壳速度v_p约为6.4~6.6 km/s,v_s约为3.72~3.89 km/s,电阻率为几至几十欧姆米.数值模拟的结果与地球物理反演结果基本相符,并进行了定性的构造解释.用一个模型对下地壳中的速度特征与电性特性进行联合数值模拟,说明了此模型在解释下地壳中的v_p、v_s和ρ的有效性.

Abstract:

Partial melt models in different geometrical configuration and distribution are established.The geometrical distribution and the connectiveness of the melt are determined by three factors:① the location of melt;② the melt fraction;③the minimum total free energy of the solid-solid and the solid-liquid interfaces.The melt geometries are films,tubes and isolated ellipsoidal inclusions.The bulk and shear moduli and conductivity in partially melted rocks are calculated by using the models.The v_p,v_s and ρ are calculated by choosing parameters in the lower crust on both sides of the northern segment of North-South seismic zone,i.e.,in the lower crust of Ordos platform v_p≈ 6.71 km/s,v_s≈ 3.96 km/s,resistivities are from one hundred Ωm to several hundreds Ωm,in the fold systems of Qilian and Qinling 6.4 km/s ≤v_p ≤ 6.6 km/s,3.72 km/s ≤v_s ≤ 3.89 km/s,resistivities are from several Ωm to dozens Ωm.The results of numerical modelling agree with that of geophysical inversion for the most part,the geotectonics are explained qualitatively.The combined numerical modelling of velocity and resistivity in lower crust by using a model shows that the model is available in interpretation of v_p,v_s and ρ.