5:15 PM - 6:45 PM
[SSS03-P13] Seismic and thermal structure imaging in NE Japan: New insights into slab melting and arc magmatism
Keywords:NE Japan subduction zone, slab melting and arc magmatism, joint seismic tomography, thermal structures of petrological models
Arc magmatism and slab melting in a subduction zone attributed to structural heterogeneity, mineral composition, fluid saturation, and thermal regime are speculated by numerous previous studies. However, how these factors affect them is far from clear. The extensive arc magmatism observed on the island arcs is considered to be an indicator of chemical exchange between the mantle wedge and the subduction zone interface. The northeastern (NE) Japan subduction zone is characterized by the presence of the Eurasian plate, the Okhotsk plate, and the Pacific plate, which subducts WNW under the Japan Island arc with a ~30° angle of dip and a rate of ~9 cm/yr from the Japan Trench. This subduction zone is regarded as one of the best laboratories to examine the processes of arc magmatism, slab melting, and other geodynamic phenomena due to the densest seismic networks recording numerous earthquakes with higher quality than the other subduction zones. In the last decades, a number of seismological studies were carried out to reveal the structural heterogeneities; however, few of them provided the seismic velocities (Vp, Vs), Vp/Vs ratio (or Poisson’s ratio), and thermal structures with considerations of the mineral compositions of the upper mantle simultaneously under the entire-arc region of the NE Japan subduction zone. Even though the role of fluids in arc magmatism in the mantle wedged due to slab dehydration in a subduction zone is widely accepted, fluids of the melting process with different mineral compositions and thermal regimes in the upper mantle are not yet well understood.
Although the previous studies revealed low-V and high-Vp/Vs or Poisson’s ratio anomalies in the mantle wedge under the NE Japan subduction zone, the mechanism of slab melting is enigmatic and widely debated. This is because slab melting is associated with multiple factors, including rock composition, temperature, depth of subducting oceanic crust, fluid content, etc. Some early studies suggest that the island-arc magmatism mainly contributed to the melting of peridotite in the mantle wedge due to fluids released from the dehydration process associated with the subducting oceanic crust. They further suggest that the oceanic plate could only provide water to the overlying mantle wedge for arc magmatism, but not melt itself since it is too cold to melt the underthrusting slab. On the contrary, some other studies revealed that the hydrated basalt derived from the mid-ocean ridges would be melted with high temperature (T) and water saturated on the upper interface of the subducting plate in the mantle wedge. Here we show high-resolution three-dimensional (3-D) velocities, Vp/Vs ratios, and T structures behind the Japan Trench. The T structure was calculated from two petrological models of the upper mantle: a peridotite assemblage and a pyrolite assemblage. A layer about 10 km thick with low-V, high-Vp/Vs, and slightly high-T anomalies was imaged along the upper boundary of the Pacific slab. This distinct layer implies partial melting of the oceanic crust due to deep-seated metamorphic reactions whose characteristics depend on the source of fluids and the thermal regime. Such a process in the mantle wedge enriches the peridotite content of the basalts underlying the island arc. Localized zones of significantly low-Vp and -Vs, high-Vp/Vs, and high-T anomalies were revealed in the mantle wedge along the volcanic front, indicating partial melting of peridotite-rich mantle material to produce tholeiitic magma. The T calculated using the peridotite model was found to match the mean geotherm and simulated temperature of the upper mantle more closely than those from the pyrolite model. The present study demonstrates that fluids released from slab dehydration, mineral composition, and the thermal regime play crucial roles in both arc magmatism and slab melting in the subduction zone.
Although the previous studies revealed low-V and high-Vp/Vs or Poisson’s ratio anomalies in the mantle wedge under the NE Japan subduction zone, the mechanism of slab melting is enigmatic and widely debated. This is because slab melting is associated with multiple factors, including rock composition, temperature, depth of subducting oceanic crust, fluid content, etc. Some early studies suggest that the island-arc magmatism mainly contributed to the melting of peridotite in the mantle wedge due to fluids released from the dehydration process associated with the subducting oceanic crust. They further suggest that the oceanic plate could only provide water to the overlying mantle wedge for arc magmatism, but not melt itself since it is too cold to melt the underthrusting slab. On the contrary, some other studies revealed that the hydrated basalt derived from the mid-ocean ridges would be melted with high temperature (T) and water saturated on the upper interface of the subducting plate in the mantle wedge. Here we show high-resolution three-dimensional (3-D) velocities, Vp/Vs ratios, and T structures behind the Japan Trench. The T structure was calculated from two petrological models of the upper mantle: a peridotite assemblage and a pyrolite assemblage. A layer about 10 km thick with low-V, high-Vp/Vs, and slightly high-T anomalies was imaged along the upper boundary of the Pacific slab. This distinct layer implies partial melting of the oceanic crust due to deep-seated metamorphic reactions whose characteristics depend on the source of fluids and the thermal regime. Such a process in the mantle wedge enriches the peridotite content of the basalts underlying the island arc. Localized zones of significantly low-Vp and -Vs, high-Vp/Vs, and high-T anomalies were revealed in the mantle wedge along the volcanic front, indicating partial melting of peridotite-rich mantle material to produce tholeiitic magma. The T calculated using the peridotite model was found to match the mean geotherm and simulated temperature of the upper mantle more closely than those from the pyrolite model. The present study demonstrates that fluids released from slab dehydration, mineral composition, and the thermal regime play crucial roles in both arc magmatism and slab melting in the subduction zone.