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

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. 論文表題:低速拡大海洋地殻のデタッチメント剪断帯のP波速度異方性に対する含水鉱物の効果〜フィリピン海ゴジラメガムリオンを例として〜 フィリピン海パレスベラ海盆のゴジラメガムリオンから採取されたハンレイ岩質マイロナイトは主に斜長石・単斜輝石・角閃石 から構成されており,その微細構造から3種類に区別される.本研究では,これら3鉱物の結晶方位ファブリックを元にP波速度を計算して3種類の微細構造との関係性を調べた.その結果,特に単斜輝石を置換して形成された角閃石の形状と結晶方位がマイロナイト全体のP波速度異方性に大きな影響を与えることがわかった.このことは,海洋地殻への流体付加によって2次的に生成される含水鉱物