Experimental Studies On Influence Of Shape Preferred Orientation On The Anisotropy Of Crustal Rocks

Elastic wave anisotropy is an essential physical property of crust. Due to the complexity of crust rock composition and structure, many factors can influence the elastic wave anisotropy.Many studies have been done on the influencing factors of anisotropy in the middle-lower crust, suggesting that anisotropy does not vary significantly with temperature (T< 700℃) while pressure has notable effects. At low confining pressure (P< 200MPa), the majority of rock anisotropy decreases rapidly with increasing pressure, indicating that it is closely related to oriented micro-cracks. At high confining pressure (P> 200MPa), because of micro-cracks closure, the rock anisotropy value tends to be a constant that is associated with the lattice preferred orientation (LPO) and shape preferred orientation (SPO) of minerals.Rock anisotropy in middle-lower crust correlates well with mineral contents. For intermediate and acidic rock, anisotropy is positively correlated with mica content, and negatively correlated with plagioclase content. There appears to be no systematic relation between the anisotropy and quartz content. In addition, for basic rocks, anisotropy is in good agreement with hornblende content, while it can be weakened by plagioclase.The experimental studies at high pressure and room temperature demonstrate that the degree of anisotropy is a function of mica content and rock fabric strength. Rocks with equal amounts of mica exhibit a range of anisotropy depending on the orientation of mica in the sample. Gneisses with significant anisotropy have well developed rock fabric, i.e. the (001) planes of mica are nearly parallel to one another and mica is segregated into distinct bands. By contrast, low anisotropy occurs in gneisses that lack well developed fabric. The mica distribution tends to be relatively disseminated and the (001) planes of mica are not obviously aligned.Results of analogue material experiments show that the contributions from mineral shape preferred orientation to anisotropy appears to be appreciable. When mineral compositions remain fixed, the higher aspect ratio is, the stronger anisotropy tends to be. Additionally, the anisotropy of analogue material with banded fabric is higher than that with homogeneous distributed fabric.