The effect of a hydrous phase on P-wave velocity anisotropy within a detachment shear zone in the slow-spreading oceanic crust: A case study from the Godzilla Megamullion, Philippine Sea

摘要

We studied the contributions of plagioclase, clinopyroxene, and amphibole to the P-wave velocity properties of gabbroic mylonites of the Godzilla Megamullion (site KH07-02-D18) in the Parece Vela Rift of the central Parece Vela Basin, Philippine Sea, based on their crystal-preferred orientations (CPOs), mineral modes, and elastic constants and densities of single crystals. The gabbroic mylonites have been classified into three types based on their microstructures and temperature conditions: HT1, HT2 and medium-temperature (MT) mylonites. The P-wave velocity properties of the HT1 mylonite are dominantly influenced by plagioclase CPOs. Secondary amphibole occurred after deformation in the HT1 mylonite, so that its effect on P-wave velocity anisotropy is minimal due to weak CPOs. Although the HT2 mylonite developed deformation microstructures in the three minerals, the P-wave velocity properties of the HT2 mylonite are essentially isotropic, resulting from the destructive interference of different P-wave velocity anisotropy patterns produced by the distinct CPOs of the three constituent minerals (i.e., plagioclase, clinopyroxene, and amphibole). The P-wave velocity properties of the MT mylonite are influenced mainly by amphibole CPOs, whereas the effect of plagioclase CPOs on P-wave velocity anisotropy becomes very small with a decrease in the intensity of plagioclase CPOs. As a result, the gabbroic mylonites tend to have weak P-wave velocity anisotropy in seismic velocity, although their constituent minerals show distinct CPOs. Such weakness in the whole-rock P-wave velocity anisotropy could result from the destructive contributions of the different mineral CPOs with respect to the structural framework (foliation and lineation). These results show that amphibole has a high potential for P-wave velocity anisotropy by aligning both crystallographically and dimensionally during deformation in the hydrous oceanic crust. The results also suggest that the effect of a hydrous phase on P-wave velocity anisotropy within the detachment shear zone in a slow-spreading oceanic crust varies depending on the degree of deformation and on the timing of hydrothermal activity.