Melt production and transport beneath the slow-spreading Mariana arc−back-arc system

Back-arc basin basalts are broadly similar in composition to mid-ocean ridge basalts, but also incorporate arc-like geochemical signatures. The processes governing melt transport from the arc to the back-arc spreading center remain enigmatic, particularly beneath slow-spreading systems with limited melt volumes, where imaging of small-scale structures has been challenging. Here, we use multimode Rayleigh-wave tomography, enhanced by increased raypath coverage through both two- and three-station interferometry, to resolve S-wave velocity structure down to ∼100 km depth beneath the central Mariana subduction zone. Our model reveals a triangle-like low-velocity anomaly at 15−30 km depth beneath the Mariana trough and an elliptical low-velocity anomaly at 50−90 km depth beneath the Mariana arc, both consistent with melt equilibration depths from thermobarometry. Additionally, a cylindrical low-velocity anomaly extends from the base of the triangle-like low-velocity anomaly to ∼80 km depth and appears weakly connected to the anomaly beneath the arc. We interpret this feature as diapiric upwelling that links melt sources beneath the arc and back-arc.