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[SCG50-01] 3D numerical simulation of island arc deformation due to steady plate subduction: Effects of a bend along a trench axis
Keywords:island arc-trench system, plate subduction, free-air gravity anomaly, crustal deformation, arc-arc junction
Island arc-trench systems are characterised by a pair of topography and free-air gravity anomaly that is high on the island arc and low around the trench. These features are observed worldwide without exceptions. However, the physical mechanism of island-arc deformation has not been well understood. To this problem, based on the elastic dislocation theory, Fukahata and Matsu'ura (2016, GJI) calculated the deformation of the island arc lithosphere due to steady plate subduction for a two-dimensional elastic-viscoelastic layered half space. They explained the deformation mechanism of the island arc-trench system by the combination of block rotations of the oceanic and island-arc lithospheres and gravitational restoring of them. However, we cannot neglect the effect of three-dimensional geometry for actual island arc-trench systems. For example, we observe the trench axis has a bend at the junction of the Japan Trench and the Kuril Trench (off the Shimokita Peninsula), where large negative free-air gravity anomalies are observed (Sandwell and Smith. 1997, JGR). The large negative free-air gravity anomalies have been reproduced by numerical models (Hashimoto et.al., 2004, 2008, PAGEOPH), but the physical mechanism of them has not been well understood.
In this study, developing a 3-D numerical model for an elastic-viscoelastic layered half-space (Fukahata and Matsu’ura, 2005, 2006, GJI), we compute displacement rates in the lithosphere caused by steady slip on a plate interface, which has a bend along the trench axis. By changing the bending angle of the trench axis, we investigated the effect of bending on island-arc deformation when the bend of a trench axis is convex toward the island arc. The computation shows that a significant subsidence area appears on the island arc lithosphere around the bend and that the amount and spatial scale of the subsidence become larger as the bend of the trench axis is larger. On the other hand, when the trench axis bends convexly toward the ocean, the significant uplift occurs on the island arc lithosphere around the bend. We also observed that the uplift became larger as the bend of the trench axis is larger.
These results can be reasonably explained by the following mechanism. When the oceanic plate subducts across the trench axis, with a convex bend toward the island arc, the surface mass of the subducting oceanic plate becomes excessive, as understood from the analogy of a tablecloth. Since the upper surface of the oceanic plate and the lower surface of the island arc plate move exactly in the opposite direction due to dislocation, mass deficit inevitably occurs in the island arc plate, which lead to subsidence. When the trench axis bends convexly toward the ocean, the same idea can be applied, that is to say mass deficit on the upper surface of the oceanic plate. Based on this mechanism, the larger the bend angle of the trench axis is, the larger the excess or deficit of mass on the top of the oceanic plate. This is consistent with the results of numerical computation in this model.
In this study, developing a 3-D numerical model for an elastic-viscoelastic layered half-space (Fukahata and Matsu’ura, 2005, 2006, GJI), we compute displacement rates in the lithosphere caused by steady slip on a plate interface, which has a bend along the trench axis. By changing the bending angle of the trench axis, we investigated the effect of bending on island-arc deformation when the bend of a trench axis is convex toward the island arc. The computation shows that a significant subsidence area appears on the island arc lithosphere around the bend and that the amount and spatial scale of the subsidence become larger as the bend of the trench axis is larger. On the other hand, when the trench axis bends convexly toward the ocean, the significant uplift occurs on the island arc lithosphere around the bend. We also observed that the uplift became larger as the bend of the trench axis is larger.
These results can be reasonably explained by the following mechanism. When the oceanic plate subducts across the trench axis, with a convex bend toward the island arc, the surface mass of the subducting oceanic plate becomes excessive, as understood from the analogy of a tablecloth. Since the upper surface of the oceanic plate and the lower surface of the island arc plate move exactly in the opposite direction due to dislocation, mass deficit inevitably occurs in the island arc plate, which lead to subsidence. When the trench axis bends convexly toward the ocean, the same idea can be applied, that is to say mass deficit on the upper surface of the oceanic plate. Based on this mechanism, the larger the bend angle of the trench axis is, the larger the excess or deficit of mass on the top of the oceanic plate. This is consistent with the results of numerical computation in this model.