Quantifying Crustal Growth in Arc‐Backarc Systems: Gravity Inversion Modeling of the Lau Basin

The formation and evolution of arc‐backarc systems govern crustal production in some of the most volcanically and hydrothermally active environments on Earth. This study presents the first complete three‐dimensional density model of the active arc‐backarc system in the southwest Pacific comprising the Lau Basin and Tofua arc. Seafloor density and crustal thickness maps reveal changes in crustal composition and growth rates throughout the basin and along the volcanic arc. Crustal thickness varies significantly between the different centers of accretion (i.e., assemblages), resulting from 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. This study shows that the inverse modeling approach can be applied to global gravity data sets to characterize and quantify the density and thickness of the crust anywhere in the oceans. Plain Language Summary This study examines how certain volcanic and hydrothermal areas on Earth are formed and change over time. It focuses on the Lau Basin and Tofua volcanic arc which are part of an arc‐backarc system in the southwest Pacific Ocean, north of New Zealand. A detailed three‐dimensional density model was created, allowing us to produce seafloor density and thickness maps of the Earth's crust in the study region. This model was used to understand changes in crustal composition and growth rates in the basin and along the volcanic arc. The thickness of the crust varies significantly in different areas, which is caused by the spreading of the crust and the accumulation of sub‐crustal magma. Growth rates calculated for the arc‐backarc system's components suggest that areas of the densest seafloor and fastest growth might be influenced by melted rock from the Earth's mantle. Key Points 3D density modeling of an active arc‐backarc system Seafloor density varies across the Lau Basin, reflecting changes in crustal composition and mantle contamination Crustal thickness can be used to identify microplate boundaries and calculated crustal growth rates