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. Ultrasonic 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%. However, a series of seismic measurements conducted in Tibet indicated marked anisotropy with a magnitude ranging from 8% to 18% within middle to lower crust. What causes the anomalously strong anisotropy 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 anisotropy is likely comparable to or larger than that induced by LPO of major minerals, possibly amphiboles and micas, in middle to lower crust. It is implied that aligned melt may be a potential factor to induce anomalously strong anisotropy within Tibetan middle to lower crust.

Keywords

partial melting / aligned melt pocket / anisotropy / experimental measurement / Tibetan crust

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