14:15 〜 14:30
[S04-04] ケルマデク・トンガ沈み込み温度場と地震分布変動
1. Introduction
The calamitous phreatomagmatic eruption of the Hunga Tonga-Hunga Ha’apai (HTHH) volcano in January 2022 has been reported to have had a volcanic explosivity index (VEI) of 5-6 and a column height of 20 km and to have caused powerful tsunamis across the Pacific Ocean, suggesting that destructive arc magmatism evolved from an active and large subduction complex below Kermadec-Tonga. Along the heterogeneous and tightly coupled subvolcanic megathrusts, the hydrothermal structure and slab petrological metamorphism are considered essential to the deep arc volcanism and catalysis of focused interplate earthquakes. The mechanism of fast-slow earthquakes is still uncertain due to the heterogeneity of megathrusts, evidenced by the transition between the depleted arc lavas in northern Kermadec (and Tonga) and the mildly depleted southern Kermadec arc lavas and the slab geometries.
2. Model and method
With the focus on the Kermadec–Tonga megathrust and its changes in hydrothermal state along the strike and with subduction, we constructed a 3D thermomechanical kinematic model on the basis of the finite difference method (FDM) and the code Stag 3D. The dimensions of our model were 800 × 1800 × 400 km (length × width × depth, Fig. 1), which was designed to simulate subduction of the Pacific Plate northwestward over 20 Myr. We applied an anelastic approximation and the equations of conservation of mass, momentum, and energy. The plate geometry adopted Slab 2 within the modeled domain to ensure full subduction and steady thermal conditions. Observations of surface heat flow and Curie depth estimation were utilized to constrain the model thermal results along eight trench-perpendicular profiles.
3. Results and Discussion
Through modeling, we obtained the evolution of the subduction thermal regime at each time step and the final steady thermal state as well as the induced mantle viscous flow field. We predominantly focused on the steady thermal state of the subducted plate, which is largely influenced by the slab geometry and heterogenic intraslab structure. The results suggest that the subduction of the Pacific Plate beneath the Kermadec–Tonga microplate results in a cold thermal transition from 300 °C to > 900 °C between the mantle edge and subvolcanic interface, which changes greatly from Tonga to Kermadec along strike due to slab geometry heterogeneity. The distribution of seismicity is associated with the dehydration of subducted water-bearing minerals, which promotes the occurrence of both fast and slow subduction earthquakes. In addition, the ultramafic layer is much thicker than the MORB layer, suggesting that intraslab harzburgitization could be the dominant dehydration-derived fluid source for mantle melting.
Therefore, slab metamorphism released large amounts of fluids, especially those from intraslab harzburgitization, which are key to influencing mantle melting and arc volcanism in Kermadec-Tonga.
The calamitous phreatomagmatic eruption of the Hunga Tonga-Hunga Ha’apai (HTHH) volcano in January 2022 has been reported to have had a volcanic explosivity index (VEI) of 5-6 and a column height of 20 km and to have caused powerful tsunamis across the Pacific Ocean, suggesting that destructive arc magmatism evolved from an active and large subduction complex below Kermadec-Tonga. Along the heterogeneous and tightly coupled subvolcanic megathrusts, the hydrothermal structure and slab petrological metamorphism are considered essential to the deep arc volcanism and catalysis of focused interplate earthquakes. The mechanism of fast-slow earthquakes is still uncertain due to the heterogeneity of megathrusts, evidenced by the transition between the depleted arc lavas in northern Kermadec (and Tonga) and the mildly depleted southern Kermadec arc lavas and the slab geometries.
2. Model and method
With the focus on the Kermadec–Tonga megathrust and its changes in hydrothermal state along the strike and with subduction, we constructed a 3D thermomechanical kinematic model on the basis of the finite difference method (FDM) and the code Stag 3D. The dimensions of our model were 800 × 1800 × 400 km (length × width × depth, Fig. 1), which was designed to simulate subduction of the Pacific Plate northwestward over 20 Myr. We applied an anelastic approximation and the equations of conservation of mass, momentum, and energy. The plate geometry adopted Slab 2 within the modeled domain to ensure full subduction and steady thermal conditions. Observations of surface heat flow and Curie depth estimation were utilized to constrain the model thermal results along eight trench-perpendicular profiles.
3. Results and Discussion
Through modeling, we obtained the evolution of the subduction thermal regime at each time step and the final steady thermal state as well as the induced mantle viscous flow field. We predominantly focused on the steady thermal state of the subducted plate, which is largely influenced by the slab geometry and heterogenic intraslab structure. The results suggest that the subduction of the Pacific Plate beneath the Kermadec–Tonga microplate results in a cold thermal transition from 300 °C to > 900 °C between the mantle edge and subvolcanic interface, which changes greatly from Tonga to Kermadec along strike due to slab geometry heterogeneity. The distribution of seismicity is associated with the dehydration of subducted water-bearing minerals, which promotes the occurrence of both fast and slow subduction earthquakes. In addition, the ultramafic layer is much thicker than the MORB layer, suggesting that intraslab harzburgitization could be the dominant dehydration-derived fluid source for mantle melting.
Therefore, slab metamorphism released large amounts of fluids, especially those from intraslab harzburgitization, which are key to influencing mantle melting and arc volcanism in Kermadec-Tonga.