Degree of serpentinization in the forearc mantle wedge of Kyushu subduction zone: quantitative evaluations from seismic
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ORIGINAL RESEARCH PAPER
Degree of serpentinization in the forearc mantle wedge of Kyushu subduction zone: quantitative evaluations from seismic velocity Shaohong Xia • Jinlong Sun • Haibo Huang
Received: 9 May 2014 / Accepted: 29 September 2014 Ó Springer Science+Business Media Dordrecht 2014
Abstract Serpentinization is an important phenomenon for understanding the water cycle and geodynamics of subduction zones in the upper mantle. In this study, we evaluate quantitatively the degree of serpentinization using the seismic velocity. The results show that serpentinization mainly occurs in the forearc mantle wedge along the subducted oceanic crust, and the degree of serpentinization in the forearc mantle wedge of Kyushu is strongly heterogeneous and varies from 0 to 12 %, containing about 0–1.8 % water contents. In general, the degree of serpentinization gradually decreases with depth from 40 to 80 km and the largest degree usually occur in about 40–50 km depth. Localized high anomalies of serpentinization are revealed in the northern and southern portions of Kyushu, respectively. We suggest that in the northern portion of the forearc mantle wedge, the water contents are relatively large, which might result from the abundant fractures and cracks with more fluids in the subducting slab because of the subduction of Kyushu-Palau ridge and the sudden change in its subduction angle of Philippine Sea lithosphere. But the high degree of serpentinization in the southern portion is closely associated with the active leftlateral shear zone revealed by global positioning system site velocities and earthquake focal mechanisms. In addition, the present results also display that the low degree of serpentinization in the central domain of the forearc mantle wedge is consistent with the location of anomalous arc
S. Xia J. Sun (&) H. Huang CAS Key Laboratory of Marginal Sea Geology, South China Sea Institute of Oceanology, Chinese Academy of Sciences, Guangzhou 510301, China e-mail: [email protected] S. Xia e-mail: [email protected]
volcano. The distribution of water contents is closely associated with the degree of serpentinization in the forearc mantle wedge. Keywords Serpentinization Mantle wedge Subduction Seismic velocity Kyushu
Introduction Subduction zones are the dominant physical and chemical system of Earth’s interior (e.g., Zhao 2012, and references therein) and play a significant role in water and carbon cycles (e.g., Ohtani et al. 2004). A large number of studies suggest that water from dehydration of subducting slab could cause intensive and extensive magmatism and volcanism, and alter the petrology of the mantle wedge and overlying plate to induce mantle wedge serpentinization and subduction zone seismicity (e.g., Zhao et al. 2002; Hyndman and Peacock 2003; Hacker et al. 2003). It has been repeatedly proposed that fluids fluxes from the subducting plate are sufficient to hydrate the entire forearc mantle wedge during several tens of millions of years according to the previous studies (e.g., Peacock 19
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