Can the slab morphology control the volcanic evolution of and arc system?


Southwest Japan is a volcanic island arc formed by subduction of the Philippine Sea (PHS) plate via the Nankai trench. The magmatism in this arc has been active during the last 12 Myr and has produced variable volcanic rocks distributed in the Kyushu and Chugoku districts.

To explain spatial and temporal variations in magma types (Figure 1), we proposed the model for the evolution of subducting PHS plate in the last 12 Myrs (Figure). In early stage (12–8 Ma), the PHS slab would have been located at a greater depth (>100 km), and thus the slab likely released fluids dominated by an aqueous component (Fig. a). Subsequently, as the boundary of PHS and PAC (Pacific Plate) migrated to the northeast, the PAC Plate sank into deep asthenosphere beneath the PHS, allowing injection of asthenosphere beneath the PHS plate. Consequently, the PHS plate would have been impinged by asthenospheric flow at its base, eventually leading to its flattening and melting (Fig. b). The reaction of slab-derived melt and peridotite within the wedge mantle also led to the production of high-Sr IAB and ADK magmas in late-stage volcanism (Fig. c). As recognized in the other arcs (Aleutians and Kamchatka), our study also demonstrates that the subducting slab and sub-arc mantle mutually interact and the feedback effect of this interaction plays a major role in the evolution of island-arc volcanism. If you want to know more about it, please read the full article!

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Figure: Schematic illustrations for the evolution of magmatism in the Chugoku district. (a) 12–8 Ma. The Pacific (PAC) Plate is subducted at a shallower depth beneath the Philippine Sea (PHS) Plate so as to prevent injection of hot asthenospheric flow beneath the PHS. (b) 8–4 Ma. The trench junction (TTT) moved away from the Chugoku district to the northeast. The PAC subducted to a greater depth, allowing asthenospheric injection beneath the PHS Plate. The collision of Shikoku Basin spreading center (SBSC) and Kyushu-Palau Ridge (KPR) with the Nankai trench led to tearing of the PHS slab and partial fusion of its crustal section. (c) Progressive interaction with asthenospheric mantle led to further flattening of the subducting slab and propagation of tears. Increasing amounts of slab-derived melt facilitated its transport to the surface and resulted in emplacement of adakitic (ADK) magmas in close temporal and spatial proximity with mafic magmas.