Cracked on-axis and pristine off-axis crust formed during forearc evolution at a nascent subduction zone

Forearc crust that formed at nascent subduction zones contains key evidence regarding Earth’s tectonic evolution, but its nature and underlying formation processes remain unclear. Key geological records are hidden within the deep oceanic lithosphere, highlighting the need to extract information from geophysical data by establishing clear links between rock physical properties and geological processes. Here we show that the petrophysical properties of drillcores from the Izu–Bonin arc, including forearc basalt and boninite, can be linked to the tectonic and magmatic processes experienced by these rocks. Our results indicate that the rocks which formed during forearc spreading have elevated crack densities due to intensive brittle deformation, whereas those produced by subsequent off-axis volcanism remain comparatively pristine. Petrophysics-based interpretation of the seismic velocity structure reveals that off-axis volcanic units intruded the cracked forearc crust. This provides geophysical evidence for distinct volcanic processes during nascent forearc evolution, thereby highlighting the importance of combining petrophysical and geophysical data when investigating crustal geological processes. The Izu–Bonin–Mariana forearc indicates two distinct types of forearc crust formation during subduction initiation, including a damaged crust linked to early seafloor spreading and a pristine crust that forms later due to off-axis volcanism, according to petrophysical and geochemical analysis of drillcores.