Quantifying crustal growth in the Lau arc-backarc system through gravity inverse modelling

The formation and evolution of arc-backarc systems govern crustal production in some of the most volcanically and hydrothermally active environments on Earth. Geologic mapping of these systems is increasingly possible by interpretation of emerging ship-based and global geophysical datasets. Although specific rock types cannot be confidently identified from a single physical property, the relative density changes across a region can provide information about the composition of the crust and how it was formed, for example, indicating whether old crust was produced along the volcanic arc or at a back-arc spreading center. This study presents the first complete three-dimensional density model of the Lau Basin and Tofua arc-backarc system in the southwest Pacific Ocean. Seafloor density and crustal thickness maps were produced that reveal changes in crustal composition and growth rates throughout the basin and along the volcanic arc. Crustal thickness varies greatly between the different centers of accretion (indicated here by assemblages), reflecting seafloor spreading and subsurface melt accumulation below volcanic fields. Volumetric growth rates were calculated for each assemblage, corresponding to their respective contribution to basin expansion. The highest crustal density and growth rates are thought to be related to a mantle-derived melt source entering the basin from the north around the edge of the subducting Pacific Plate. The inverse modelling approach used in this study can be applied to global gravity datasets to characterize and quantify the density and thickness of the crust anywhere in the oceans.