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At subduction zones, sinking of the downgoing lithosphere is thought to enable a return flow of asthenospheric mantle around the slab edges, so that the asthenosphere from underneath the slab invades the ambient mantle flowing underneath the volcanic arc and the backarc basin. For instance at the northern end of the Lau Basin, trench retreat and slab rollback enable toroidal return flow of Samoan mantle beneath a transform margin to provide a supply of fresh, undepleted Indian mantle that feeds the backarc spreading center. Questions, however, arise about the sense of mantle flow when plate kinematics predict that the trench is advancing, as seen in the Mariana convergent margin. Does the mantle flow in or does it escape outward through slab tears or gaps? Here, we address the origin and sense of asthenospheric mantle flow supplying the southern Mariana convergent margin, a region of strong extension occurring above the subducting Pacific plate. Does the asthenosphere flow northward, from underneath the Pacific plate and Caroline hotspot through a slab tear or gap, or does it flow outward from the Mariana Trough, which possesses a characteristic Indian Ocean isotopic signature? To address these questions, we integrate geodetic data along with new Hf–Nd isotopic data for fresh basaltic lavas from three tectonic provinces in the southernmost Marianas: the Fina Nagu volcanic complex, the Malaguana–Gadao backarc spreading ridge and the SE Mariana forearc rift. Our results indicate that Indian mantle flows outward and likely escapes through slab tears or gaps to accommodate shrinking of the Philippine Sea plate. We thus predict that asthenospheric flow around the Pacific slab at the southern Mariana Trench is opposite to that predicted by most subduction-driven mantle flow models. [ABSTRACT FROM AUTHOR]

The eruption of South Sarigan Seamount in the southern Mariana arc in May 2010 is a reminder of how little we know about the hazards associated with submarine explosive eruptions or how to predict these types of eruptions. Monitored by local seismometers and distant hydrophones, the eruption from ~ 200 m water depth produced a gas and ash plume that breached the sea surface and rose ~ 12 km into the atmosphere. This is one of the first instances for which a wide range of pre- and post-eruption observations allow characterization of such an event on a shallow submarine volcanic arc volcano. Comparison of bathymetric surveys before and later the eruptions of the South Sarigan Seamount reveals the eruption produced a 350 m diameter crater, deeply breached on the west side, and a broad apron downslope with deposits > 50 m thick. he breached summit crater formed within a pre-eruption dome-shaped summit composed of andesite lavas. Dives with the Japan Agency for Marine-Earth Science and Technology Hyper-Dolphin remotely operated vehicle sampled the wall of the crater and the downslope deposits, which consist of andesite lava blocks lying on pumiceous gravel and sand. Chemical analyses show that the andesite pumice is probably juvenile material from the eruption. he unexpected eruption of this seamount, one of many poorly studied shallow seamounts of comparable size along the Mariana and other volcanic arcs, underscores our lack of understanding of submarine hazards associated with submarine volcanism. [ABSTRACT FROM AUTHOR]