Current tectonics of the Tonga–New Hebrides region

This paper proposes a present-day tectonic map of the Tonga–New Hebrides region, the result of combining data from bathymetry, satellite-derived gravity, seismicity, magnetic measurements and GPS observations. Focal mechanisms and a large amount of bathymetric data collected during the last decade in the Lau and North Fiji back-arc basins indicate a deformation distributed over numerous spreading ridges rather than diffuse and shear-dominated as was previously thought. The region is also characterised by a large variation of both consumption rate along the arcs and opening rate along the back-arc basins spreading centres. Geometry and rate of spreading in the Lau and North Fiji back-arc basins, and segmentation of the Tonga and New Hebrides arcs, are closely related to, and significantly influenced by, the subduction of aseismic ridges, namely the Louisville, DʹEntrecasteaux and Loyalty ridges. Where subducting aseismic ridges enter the trenches, we observe slower convergence, arc-transverse strike-slip faulting, thrusting at the rear of arcs, and slow or absent back-arc spreading. Rapid subduction correlates with fast back-arc opening. Sometimes parallel spreading ridges in the back-arc domain are required to accommodate the fast motion of convergence.