An Experimental Study Of Crustal Rock Anisotropy

Elastic wave anisotropy of natural rooks contains indicative information of crustal deformation. With the development of seismic sounding technology in recent years, it can be well detected and become an important physical parameter for understanding deep structural features and corresponding geodynamic mechanism. As an important branch of modern geophysical research, physical experiment at high temperature and high pressure (HTP) has done great favors for geoscientists gaining insight into the Earth's interior. Measuring physical parameters of rocks at HTP, experiments can provide effective constraints on interpreting geophysical field surveys.This thesis mainly includes the following three parts: 1) measuring mineral content and structural fabric of rocks under microscope,2) measuring anisotropic intensity of rocks with variable mica contents under HTP container to determine the relationship between mineral contents and anisotropy and 3) measuring anisotropic intensity of rocks with different deformation degrees under HTP container, determining the relationship between structural deformation and anisotropy.The main conclusions are as follows:1) At the low-pressure stage, elastic wave velocity (EWV) increases non-linearly and rapidly with growing pressure, reflecting a process of crevasse closing. Rock is changing from hole to non-hole state. When the pressure comes to 200 MPa, pores have been closed completely and the EWV begins to increase linearly with pressure, representing the eigen-velocity of rocks.2) Except for sample Y-02-1 (mica schist, about 30% mica) with about 7% anisotropy, anisotropic intensity of the other samples is very low. So macroscopic anisotropy of natural rocks is much lower than single crystal anisotropy due to the comprehensive effect of lattice preference orientation (LPO), usually not surpass 5% (weighted average of different rocks).3) A statistics is made to the mica content in thin sections and the result clearly shows that rock's anisotropic intensity and mica content are positively correlated, which implies that mid-acid rock's anisotropy is largely controlled by its mica content and orientating degree. For instance, Y-02-1 mica schist (mica content 30%) has strong anisotropy of 7%, and Y-04-3 garnet-bearing mylonite (mica content 2-3%) has weak anisotropy of 1%. 4) Strong deformation samples may have weak anisotropic intensity(Y-04-3, 1%), while weak deformed samples may be of strong anisotropy(Y-02-1, 7%). So as for crustal rocks, anisotropy can not be inferred just through the macroscopic deformed directly.5) Recent seismic detections reveal that there exist widespread strong anisotropy layers within the crust (20~50km) of the Tibeten plateau, and the intensity might come to 10%, even higher, this remarkable anisotropy could not be completely accounted for by the LPO of minerals, so it needs other appropriate mechanisms for interpretation.