ELSC Project Summary

Two fundamental questions posed in the RIDGE2000 (R2K) Science plan deal with melt formation and transport, and its relationship to mantle flow patterns and composition. The Eastern Lau Spreading Center (ELSC) was selected as a R2K ISS site in large part because of its tremendous along- strike variation in geochemistry, petrology, spreading rate, crustal structure, and morphology. Because of this variation, as well as the presence of slab earthquakes to function as a source array, the ELSC represents the optimum location for an experiment to study the relationship between mantle melt production, mantle flow and spreading center processes. We propose a combined active and passive seismic experiment along the Eastern Lau Spreading Center (ELSC) to test the following previously proposed hypotheses:

1. Circulation in the mantle wedge is dominated by slab driven flow.

TEST: comparison of seismic anisotropy results to predictions of slab driven and other flow models

2. Interaction of the arc and backarc magma production regions controls the character of the ridge by influencing melt flux, petrology, and geochemistry.

TEST: the along-strike pattern of spatial separation between arc and spreading center melt production zones as determined by P, S, surface wave, and attenuation tomography

3. Variations in the mantle melt supply control ridge crest features such as morphology, thermal structure, and hydrothermal venting.

TEST: the observed relationship between ridge crest features and melt content imaged in the crust and uppermost mantle

The passive experiment consists of 55 broadband ocean bottom seismographs (OBS) and 5 land seismographs deployed for 10 months to image the larger-scale structure of the melt production region and the mantle flow pattern. Interaction between the spreading center and arc magmas and fluids may control many processes along the ELSC. Therefore the OBS array will extend across the active Tonga volcanic arc to allow imaging of the spatial relationship between the backarc ridge magma source region and fluid rich regions near the slab and arc magma source. The deployment will also provide an exceptional record of ELSC seismic activity, including the potential to accurately determine depths and focal mechanisms. The active source experiment consists of 100 OBS deployments along a 250 km section of the ELSC extending from the inflated Valu Fa region to the magma-starved northern ELSC where the axial melt lens is absent. This experiment will image structure on a scale of 1-3 km, and will provide detailed constraints on thermal structure and melt distribution immediately beneath the ELSC. The active survey will be carried out during the first part of the passive deployment cruise using the same OBSs; this single-leg approach saves ship time and OBS costs compared to separate experiments.

Seismic data will be analyzed using body and surface wave tomographic methods and shear wave splitting analysis to obtain seismic P and S and attenuation images, and constraints on seismic anisotropy throughout the mantle wedge. These results will be compared to predictions of flow models incorporating temperature-dependent viscosity, where seismic anisotropy is computed from the orientation distribution of modeled olivine/enstatite aggregates. The combination of active and passive seismic techniques will place powerful constraints on seismic properties at different depths and resolution scales, and enable tracking of melt from initial production at depths of 20->100 km up to the axial magma chamber. If funded, this project will occur in 2006; however, due to the ever-lengthening waiting list to obtain broadband OBSs it needs to be scheduled during the current year for results to be obtained before the end of the R2K program.

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