Asthenospheric outflow from the shrinking Philippine Sea Plate: Evidence from Hf–Nd isotopes of southern Mariana lavas

• Published in: Earth and planetary science letters Vol. 478; pp. 258 - 271
• Main Authors: Ribeiro, Julia M., Stern, Robert J., Martinez, Fernando, Woodhead, Jon, Chen, Min, Ohara, Yasuhiko
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 15.11.2017
• Subjects: Hf–Nd isotopic systematics; Indian mantle; Pacific mantle; Philippine Sea Plate kinematics; Southern Marianas; subduction-driven mantle flow; subduction-driven mantle flow; Hf–Nd isotopic systematics; Indian mantle; Southern Marianas; Pacific mantle; Philippine Sea Plate kinematics
• ISSN: 0012-821X; 1385-013X
• DOI: 10.1016/j.epsl.2017.08.022

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. •We examine Hf–Nd isotopic ratios and geodetic data from the southern Marianas.•We seek to constrain whether the mantle is flowing in or out through slab tears or gaps.•The mantle must flow out to accommodate shrinking of the Philippine Sea plate.•We predict a mantle flow in the opposite sense to that predicted by slab-driven mantle flow models.