DI023-0010
Upper mantle hydration indicated by decreased shear velocity near the Southern Mariana Trench from Rayleigh wave tomography

Tuesday, 15 December 2020
Poster
Gaohua Zhu, Chinese University of Hong Kong, Hong Kong, Hong Kong, Douglas Wiens, Washington University in St Louis, St. Louis, MO, United States, Hongfeng Yang, Georgia Institute of Technology, Atlanta, GA, United States, Jian Lin, Woods Hole Oceanographic Institution, Department of Geology and Geophysics, Woods Hole, MA, United States, Min Xu, CAS Key Laboratory of Ocean and Marginal Sea Geology, South China Sea Institute of Oceanology, Guangzhou, China and Qingyu You, Key Laboratory of Petroleum Resources Research, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, 100029, China
Abstract:
Extensional faults within the incoming plates at subduction zones provide an important pathway to transport water into the Earth, leading to a pervasively serpentinized region. The extent and degree of mantle hydration within the incoming plate have been employed to estimate the quantity of water carried into the subduction zone. Widespread serpentinization of the upper mantle in the central Mariana subduction zone is indicated by both slow seismic velocities and fore-arc serpentinite mud volcanos. However, few constraints have been obtained in the southern Mariana.

In order to investigate the extent of mantle hydration in the Southern Mariana subduction zone, we conduct Rayleigh wave tomographic studies to derive the SV-wave velocity structure across the Challenger Deep using data recorded by 12 broadband OBSs. The seismograms from both teleseismic earthquakes and ambient noise cross-correlations are used to measure the group and phase velocities of Rayleigh waves. Using the ambient noise data, group and phase velocities for periods of 6-17 s are estimated using the frequency-time analysis method. Using teleseismic Rayleigh waves, phase velocities for periods of 20-36 s are measured through Eikonal tomography. The measured group and phase velocities as a function of period are then combined to invert for the SV-wave velocity using a Bayesian Monte Carlo algorithm. The tomographic results show low velocities (3.5-4.0 km/s) within the upper ~30 km of the mantle near the trench, indicating extensive mantle hydration in the southern Mariana. Based on the experimental relationship between seismic velocities and degree of serpentinization, shear wave velocities of 3.5-4.0 km/s indicate serpentinization percentages of ~20-40 vol%. Compared with the central Mariana, the estimated amount of water transported into the subduction zone in the subducting mantle is on the same order, much higher than previous estimates.

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