HR: 0800h
AN: T41C-0697    [Abstracts]
TI: Seismic Evidence for Widespread Serpentinized Forearc Mantle Along the Mariana Convergence Margin
AU: * Tibi, R
EM: tibi@wustl.edu
AF: Department of Earth and Planetary Sciences, Washington University, One Brookings Drive, St. Louis, MO 63130, United States
AU: Wiens, D A
EM: doug@wustl.edu
AF: Department of Earth and Planetary Sciences, Washington University, One Brookings Drive, St. Louis, MO 63130, United States
AB: We use P-to-S converted phases from teleseisms recorded at broadband stations in the Mariana Islands to image the forearc and arc regions of the Mariana convergence margin. The Moho in the subducting Pacific plate is observed at depths between 75 and 110 km beneath the region extending from Rota to Saipan. The S-wave velocity in the subducting crust is inferred to be ~10% slower than the surrounding mantle. This demonstrates that the crust has not yet undergone conversion to eclogite at these depths, in agreement with observations made for other arcs. A low velocity zone (LVZ), approximately 10--25 km thick, whose upper boundary is imaged at about 40--55 km depth, is detected in the forearc region of the mantle wedge along the entire margin. The anomaly is located too shallow to represent subducted oceanic crust. We interpret the LVZ as a serpentinized region in the forearc mantle, resulting from hydration by slab-expelled water. The occurrence of the serpentinized zone along the entire margin suggests that serpentinization of the forearc mantle is a widespread phenomenon in the Mariana arc. The inferred S wave velocity in the LVZ of as low as ~3.6 km/s represents a level of serpentinization of 30--50%, corresponding to a water content of about 4--6 wt%.
UR: http://epsc.wustl.edu/seismology/MARIANA DE: 3613 Subduction zone processes (1031, 3060, 8170, 8413)
DE: 8104 Continental margins: convergent
DE: 8170 Subduction zone processes (1031, 3060, 3613, 8413)
DE: 8178 Tectonics and magmatism
DE: 8413 Subduction zone processes (1031, 3060, 3613, 8170)
SC: Tectonophysics [T]
MN: 2007 Fall Meeting