HR: 0830h
AN: T41C-0229    [PDF]
TI: Vp/Vs Tomography of the Central Lau Spreading Center and Tonga Volcanic Arc
AU: * Conder, J A
EM: conder@seismo.wustl.edu
AF: Washington University, Dept of Earth and Planetary Sciences. Campus Box 1169, St. Louis, MO 63130 United States
AU: Wiens, D A
EM: doug@kermadec.wustl.edu
AF: Washington University, Dept of Earth and Planetary Sciences. Campus Box 1169, St. Louis, MO 63130 United States
AB: Previous 3D P-wave tomography of the Lau Basin from the Labatts experiment shows a $>$100 km deep slow velocity anomaly beneath the arc, above and paralleling the slab, and an asymmetric slow velocity anomaly beneath the Central Lau back-arc spreading center (CLSC) with nearly all of the anomaly to the west of the axis [Zhao et al., 1997]. These anomalies may be indicative of the melt structure beneath the arc and back-arc or alternatively could reflect higher temperatures or a combination of the two in these regions. To distinguish between the two effects, we jointly use S-waves with the P-waves since P and S-wave velocities are affected to different degrees by temperature and the presence of melt. Because the back-arc region strongly attenuates S-waves, we focus on 2D inversions beneath the array rather than attempt a 3D inversion that is only sparsely constrained by S-waves, and also solve for Vp/Vs rather than directly solve for Vs. The Vp/Vs structure exhibits several distinct anomalies. As expected, the subducting slab has a low Vp/Vs (with fast Vp), consistent with the cold downgoing plate. The CLSC has a strong asymmetric Vp/Vs anomaly as well as an asymmetric Vp anomaly, probably indicating that melt is mostly present west of the axis. There is no obvious anomaly beneath the volcanic arc, but could be due to poor resolution and/or ray coverage in the uppermost wedge. The models also exhibit a couple of curious anomalies: one with slow Vp and low Vp/Vs (fast Vs relative to Vp) beneath the western Lau remnant arc and the other with fast Vp and high Vp/Vs (fast Vp, slow Vs) below the slab. The latter anomaly involves arrivals almost exclusively observed at the island of Niue, oceanward of the trench. As the models improve we will be able to better detail the robustness of each of the observed anomalies and map out significant variations within the sub-arc and back-arc mantle.
DE: 7218 Lithosphere and upper mantle
DE: 8120 Dynamics of lithosphere and mantle--general
DE: 8180 Tomography
SC: Tectonophysics [T]
MN: 2003 Fall Meeting