We use the two-plane wave method of Rayleigh wave phase velocity tomography including finite-frequency effects to analyze seismic data from 52 ocean bottom seismographs and 18 land-based seismic stations deployed in Fiji, Tonga, and across the Lau basin. Preliminary results show that the slow velocity anomalies extend along the spreading centers with a N-S striking variation. Although temperature dominates the seismic properties in the Lau basin, smaller-scale seismic anomalies are interpreted in terms of partial melting. The asymmetric and broad distribution of partial melt in the mantle across the Lau basin implies the passive decompression melting process. The variations along the spreading centers indicate the various depths and in-situ melt contents beneath the central Lau spreading center (CLSC) and the eastern Lau spreading center (ELSC). Deeper slow anomalies to the west of the ELSC and CLSC suggest initial partial melting during asymmetric upwelling governed by the backarc mantle flow pattern, and shallower slow anomalies showing melting along a continuous band to the east of the CLSC. The slowest velocities are found in the northern part of the study region, where the backarc spreading rate is faster. The Pacific subducting slab, the Fiji plateau, and the Lau ridge are all characterized by high velocity anomalies, with the latter two regions showing evidence of cold and refractory island arc lithosphere.

Acknoweldgements: We thank Patrick Shore and all other field workers of the EPSL project. David Heeszel, Garratt G. Euler, and Yingjie Yang provided great help on data processing. In addition, we thank Katherine A. Kelley, Philip Skemer, and Brandon Mahan for constructive discussions on seismic interpretations. This research was funded by NSF Grant OCE-0426408.

Keywords: rayleigh_wave_tomography, lau_backarc_basin, partial_melting