Upper mantle structure of the Tonga‐Lau‐Fiji region from Rayleigh wave tomography

Abstract

AbstractWe investigate the upper mantle seismic structure in the Tonga‐Lau‐Fiji region by jointly fitting the phase velocities of Rayleigh waves from ambient‐noise and two‐plane‐wave tomography. The results suggest a wide low‐velocity zone beneath the Lau Basin, with a minimum SV‐velocity of about 3.7 ± 0.1 km/s, indicating upwelling hot asthenosphere with extensive partial melting. The variations of velocity anomalies along the Central and Eastern Lau Spreading Centers suggest varying mantle porosity filled with melt. In the north where the spreading centers are distant from the Tonga slab, the inferred melting commences at about 70 km depth, and forms an inclined zone in the mantle, dipping to the west away from the arc. This pattern suggests a passive decompression melting process supplied by the Australian plate mantle from the west. In the south, as the supply from the Australian mantle is impeded by the Lau Ridge lithosphere, flux melting controlled by water from the nearby slab dominates in the back‐arc. This source change results in the rapid transition in geochemistry and axial morphology along the spreading centers. The remnant Lau Ridge and the Fiji Plateau are characterized by a 60–80 km thick lithosphere underlain by a low‐velocity asthenosphere. Our results suggest the removal of the lithosphere of the northeastern Fiji Plateau‐Lau Ridge beneath the active Taveuni Volcano. Azimuthal anisotropy shows that the mantle flow direction rotates from trench‐perpendicular beneath Fiji to spreading‐perpendicular beneath the Lau Basin, which provides evidence for the southward flow of the mantle wedge and the Samoan plume.