Experimental studies on seismic anisotropy enhanced by alignment of melt: Implication for the origin of abnormal anisotropy in Tibetan crust

Abstract

It is commonly agreed that seismic anisotropy, most likely caused by lattice preferred orientation (LPO) of major minerals, is a very important indicator of intracrustal deformation. Ultra- sonic velocity measurements on the rocks from higher Himalayan crystallines (HHC) and Honghe (红河) strike-slip fault zone in Southwest China showed an average anisotropic magnitude of about 5%. How- ever, a series of seismic measurements conducted in Tibet indicated marked anisotropy with a magni- tude ranging from 8% to 18% within middle to lower crust. What causes the anomalously strong ani- sotropy within Tibetan crust? Parts of HHC rocks, to some extent, had undergone granulitic-grade metamorphism, the temperature and pressure of which were in excess of their solidus. Additionally, oriented leucocratic portions, which are accepted to be products crystallized from localized melt bands and aligned melt pocket (AMP), are present in HHC pervasively. If melt is oriented, it is expected to be an extremely important factor to influence anisotropy behavior. Experiments performed on analogue materials composed of plexiglass matrix and chocolate demonstrated that aligned melt could result in an extra anisotropy whose magnitude might increase two to three times. The contribution of AMP on