HR: 1340h
AN: DI53A-1092 INVITED    [Abstracts]
TI: Detailed Structure of Upper Mantle Discontinuities in the Tonga and Mariana Subduction Zones
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: Recordings of deep Tonga earthquakes from two arrays of 12 broadband seismographs each in the Fiji and Tonga islands were stacked and searched for reflections and conversions from upper mantle discontinuities in the Tonga subduction zone. For the 410-km discontinuity, the results show no systematic variations in depth with distance to the cold slab. The 660-km discontinuity varies between 656 and 714 km in depth. For the southern and central parts of the subduction zone, the largest depths occur in the core of the Tonga slab. For the northern part, two separate depressions of the 660 are observed. These anomalies are interpreted as being induced by the active, steeply subducting Tonga deep zone and a subhorizontally lying remnant of subducted lithosphere from the fossil Vityaz trench, respectively. Interpreting the deflections of the 660 in terms of local temperatures implies a thermal anomaly of -800K to -1200K at 660 km depth. Except for the southern region where it may thicken, the width of the depressed 660 region implies that the Tonga slab seems to penetrate the 660 with little deformation. Waveform modeling suggests that the 660-km discontinuity is at most 2 km thick in many parts of the region, and a first-order discontinuity cannot be precluded. The 410-km discontinuity thickness shows somewhat more variability and range from 2 to 10 km ouside the slab and is at most 10 km thick within the slab.\par We use P-to-S converted phases from teleseims recorded at island and ocean bottom stations in Mariana to investigate the upper mantle structure in the region. We find evidence for double seismic discontinuities at the base of the transition zone near the Mariana slab. A shallower discontinuity is imaged at depths of ~650-- 715 km, and a deeper interface lies at ~740--770 km depth. The large lateral extent at near constant depths for both features is consistent with horizontal interfaces rather than small-scale scatterers. The amplitude ratios of the seismic signals suggest that the shear velocity contrast across the two interfaces is comparable. These characteristics support the notion that the discontinuities are the results of phase transformations in olivine (ringwoodite to post-spinel) and non-olivine component (ilminite to perovskite), respectively, for the pyrolite model of mantle composition.
DE: 1011 Thermodynamics (0766, 3611, 8411)
DE: 1012 Reactions and phase equilibria (3612, 8412)
DE: 3621 Mantle processes (1038)
DE: 3909 Elasticity and anelasticity
DE: 8124 Earth's interior: composition and state (1212, 7207, 7208, 8105)
SC: Study of the Earth's Deep Interior [DI]
MN: 2007 Fall Meeting