Volcano Clustering Promoted by the Cessation of Back‐Arc Spreading and Ensuing Nascent Lithospheric Drips

In Northeast Japan and Izu‐Bonin, arc volcanoes form in clusters or as cross‐arc chains. Their occurrence emphasizes the non‐uniform distributions of sub‐arc temperature and fluids that control the spacing of arc volcanoes. Here, using 3‐D numerical models, we show that the cessation of back‐arc spreading promotes volcano clustering by triggering the formation of nascent lithospheric drips – downward protrusions of cold and dense lithosphere‐adjacent to the thinned back‐arc lithosphere. The nascent drips interfere with the flow of the hot asthenospheric mantle from the back‐arc toward the arc, leading to gradual development of alternating hot and cold regions beneath the arc. The results indicate that along‐arc variation in the sub‐arc mantle temperature is largest not during back‐arc spreading but after its cessation, explaining the time offset by several million years between back‐arc spreading and volcano clustering in Northeast Japan and Izu‐Bonin. Plain Language Summary Arc volcanoes form in subduction zones because the mantle beneath them experiences partial melting due to high temperature and the presence of water. Arc volcanoes in Northeast Japan and Izu‐Bonin, cluster together, indicating that temperature and water are not distributed uniformly in the underlying mantle wedge. Behind these two arcs, the back‐arc basin formed through rifting and seafloor spreading in the past. We use 3‐D numerical models to study how the back‐arc spreading affects the temperature and flow conditions beneath the arc. We found that alternating hot and cold regions develop in the mantle beneath the arc after the back‐arc spreading stops. This occurs because some parts of the back‐arc lithosphere next to the newly formed thin back‐arc lithosphere become unstable and protrude into the underlying mantle. These protrusions interfere with moving mantle that transfers heat from back‐arc region to the arc, causing cold regions beneath the arc. Key Points Cessation of back‐arc spreading triggers the formation of nascent lithospheric drips adjacent to the thinned back‐arc lithosphere Nascent lithospheric drips interfere with the mantle wedge flow, leading to alternating hot and cold regions beneath the arc Clustering and cross‐arc chains of volcanoes tend to occur not during but after back‐arc spreading