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.