Geodynamic Evolution of the Lau Basin

The formation of Lau Basin records an extreme event of plate tectonics, with the associated Tonga trench exhibiting the fastest retreat in the world (16 cm/yr). Yet paleogeographic reconstructions suggest that seafloor spreading in the Lau Basin only initiated around 6 Ma. This kinematics is difficult to reconcile with our present understanding of how subduction drives plate motions. Using numerical models, we propose that eastward migration of the Lau Ridge concurrent with trench retreat explains both the narrow width and thickened crust of the Lau Basin. To match the slab geometry and basin width along the Tonga‐Kermadec trench, our models suggest that fast trench retreat rate of 16 cm/yr might start ~15 Ma. Tonga slab rollback induced vigorous mantle flow underneath the South Fiji Basin which is driving the extension and thinning of the basin and contributing to its observed deeper bathymetry compared to neighboring basins. Plain Language Summary The opening of the Lau Basin is complex. In this study, we used numerical models to examine the connection between Tonga subduction and the opening of the Lau Basin. Our models correctly predicted the observed slab structure and size of the Lau Basin. This challenges the current idea that the Lau Basin floor started spreading apart about 6 million years ago. We proposed a new theory. When the Tonga trench is retreating eastward, the Lau Ridge on its western boundary of the Lau Basin is moving toward the trench. This movement could explain the width of the Lau Basin width and the fact that its curst is thicker on the western side than the eastern side. We also suggest that the South Fiji Basin must have stretched and thinned during this process due to the vigorous flow induced by the Tonga slab rollback. The thinned crust making it deeper than nearby basins. The eastward movement of the Lau Ridge could also mean that the amount of trench retreat is larger than previously thought. This might require a revision of plate reconstruction in this region to account for the stretching in the South Fiji Basin. Key Points Trench retreat rate exerts primary control on the Tonga‐Kermadec slab morphology and back arc basin formation Opening of the Lau Basin is dynamic with eastward migration of both the Tonga Ridge and Lau Ridge The South Fiji Basin experienced lithospheric thinning due to the fast mantle flow induced by the Tonga slab