1. Douglas A. Wiens & James Conder
Mantle Structure and Flow Patterns in Island Arcs: Implications for the Mariana Arc/Backarc System
Douglas A. Wiens & James Conder

2. Structure of the Mariana mantle wedge
• What we know
large scale tomography
regional velocity structure
seismic anisotropy
• Comparison with other arcs/ Key questions
Low velocity zone beneath the arc – hydration and/or decompression?
Spatial relationship between arc and backarc magma sources
Depth range of slab dehydration
Mantle flow pattern

3. Large-scale tomography
Izu - Bonin
Mariana
From Widiyantoro et al., 1999

4. Seismic Anisotropy and Mantle Flow
P anisotropy of olivine
Relationship of anisotropy and strain
Shear wave splitting
Mainprice and Silver, 1993

5. Structure from regional waveform inversion
• Invert waveforms traversing
the Mariana basin, recorded
at Guam
• Use a synthetic seismograms
computed with reflectivity and
a genetic inversion algorithm
• Invert for separate SH and
SV structures, with a penalty
function to minimize
anisotropy

6. Seismic Velocity Structures of Marginal Basins
Isotropic velocities
SV and SH velocities (anisotropy)
Characteristics of the Mariana Basin
-- slow velocities between km depth
-- slower than inactive marginal basins, not as
slow as other fast-spreading basins
-- strong anisotropy at depths 80 km to the
moho, perhaps due to strong counterflow
-- little anistropy at depths > 80 km

7. Azimuthal Anisotropy from Shear Wave Splitting
Map view showing anisotropy
fast directions
Schematic cross-section
showing ray paths
Limited data suggest fast
Anisotropy direction
approximately parallel to the
convergence direction
From Fouch and Fisher, 1998

8. 1993-1995 SPASE and LABATTS experiments Washington University
Scripps Inst. of Oceanography
• 12 Land Broadband
Seismographs - 2 years
• 30 Ocean Bottom
Seismographs – 3 mo.

9. P wave tomography - Tonga – Lau system
Zhao et al., 1997

10. Seismic Attenuation Tomography [Roth et al., 1999]

11. The Effect of Partial Melt on Seismic Velocities
Finite Element Calculations -- Hammond and Humphries [2000]
P velocity
S velocity
Vp/Vs ratio

12. Wiens and Smith, 2002

13. S to P anomaly ratio
• The relative variation of S and P anomalies,
=  ln Vs /  ln Vp, provides important clues about the process causing the anomalies.
• Mineral physics experiments and calculations suggest values for  at upper mantle pressures:
Thermal anomalies (olivine)
Partial melt – 2.3
•  can be determined from the slope on a plot of S travel time anomalies to P travel times anomalies (a), using the formula
= a (Vs / Vp)
• We observe an S to P traveltime slope of 2.1 for phases traversing the Lau backarc, giving  = 1.2
• This suggests little melt resides in the upper mantle. The melt is extracted rapidly, and the velocity anomalies result mostly from high temperatures.
S anomaly as a function of P anomaly
Lau Basin
Koper et al., 1999

14. P wave tomography - Tonga – Lau system
Zhao et al., 1997

15. P wave tomography - Tonga – Lau system
Zhao et al., 1997

16. What produces low velocities beneath the volcanic arc?
Island arc volcanism and low velocities
probably result partially from slab
Dehydration.
Geodynamic modeling suggests
that decompression melting may also
be expected
The model incorporates temperature
dependent viscosity and assumes
a MORB source composition
Ablation of the lithosphere near the
wedge corner draws asthenosphere
upward, producing melting (black
contours)
The model also produces a melt
distribution (upper figure, gray curve)
similar to actual arcs (histogram)
Conder et al., 2002

17. Effect of mantle convection pattern on melt localization
This model includes a
Backarc spreading center
Melting region in backarc
Is asymmetric due to the
Mantle flow driven by the
Subducting slab
Magma production in the
Arc is much reduced due to
Depletion of the incoming
mantle
Conder et al., 2002

18. Shear wave splitting in the Lau Basin
Smith et al., [2001]

19. Lau Basin mantle flow from geochemical evidence
After Turner and Hawkesworth, 1998

20. Shear Wave Splitting in Arcs
Aleutians
New Zealand
Yang et al., 1995
Marson-Pidgeon et al., 1999

22. 2003-2004 Passive Ocean Bottom Seismograph Deployment 60-80 OBSs
20 Land Broadband
Seismographs
1 year duration