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

초록

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 similar 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 similar 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.

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