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

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

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

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

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

Seismic Velocity Structures of Marginal Basins Isotropic velocities SV and SH velocities (anisotropy) Characteristics of the Mariana Basin -- slow velocities between 40-100 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

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

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

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

Seismic Attenuation Tomography [Roth et al., 1999]

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

Wiens and Smith, 2002

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) 1.1 - 1.4 Partial melt 1.8 – 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

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

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

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

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

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

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

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

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