[S-CG59] Oceanic plate as inputs to subduction zone: evolution process of the oceanic plate

[SCG59-04] Effect of fracture zones on bending of subducting oceanic lithosphere

*Nobukazu Seama^1,2, Tomoeki Nakakuki³, Kazuya Kitada⁴, Gou Fujie⁴ (1.Department of Planetology, Graduate School of Science, Kobe University, 2.Kobe Ocean-Bottom Exploration Center (KOBEC), 3.Earth and Planetary System Science Program, Graduate School of Advanced Science and Engnineering, Hiroshima University, 4.Japan Agency for Marine-Earth Science and Technology (JAMSTEC))

Keywords:fracture zone, oceanic lithosphere, subduction

One model used to explain the bending shape of the oceanic lithosphere as it subducts into a trench is a thin elastic plate bending model (Turcotte et al., 1978). This model could well explain almost all shapes of oceanic lithosphere subducting in trenches, such as the Mariana Trough and the Kuril Trench. An exception that cannot be represented by a thin elastic plate bending model is the oceanic lithosphere subducting into the Tonga Trench, which deviates significantly from the model result. Turcotte et al. (1978) explained this large deviation by using an elastic-perfectly plastic bending model, in which part of the plate becomes plastic. In this study, we present the characteristics of bending in the oceanic lithosphere when the fracture zone in the subducting oceanic lithosphere is parallel to the trench. These characteristics, which differ from the thin elastic plate bending model, allow us to propose the hypothesis that the fracture zone causes a decrease in the strength of the oceanic lithosphere. The research target is the oceanic lithosphere subducting offshore of the Izu-Ogasawara Trench. Three fracture zones have been identified in the area from the trench axis to 300 km offshore, and their strike is almost parallel to the trench. First, we reviewed the geomagnetic anomaly data, which allowed us to discover that there is an undiscovered fracture zone about 40km from the trench axis. Then, we superimposed several cross sections of the seafloor topography of the oceanic lithosphere subducting into the Izu-Ogasawara trench in the study area. The results showed the following three characteristic features of the flexure of the oceanic lithosphere: 1) The part corresponding to the outer rise appears as a height difference in the fracture zone. 2) The angle of subduction of the oceanic lithosphere is large, and the newly discovered fracture zone makes the bending angle changes larger. 3) The trench is deep (the difference in depth between the trench and the pre-bending seafloor is large). These features cannot be explained by a thin elastic plate bending model, but they can be explained if the strength of the oceanic lithosphere in the fracture zone decreases: the elastic bending stress associated with subduction is resolved as a step or angular change in the fracture zone. As a result, topographic changes in this fracture zone appear as differences in elevation and sudden changes in angle, and the trench becomes deeper. These results suggest that the fracture zone is the place where the strength of the oceanic lithosphere is reduced, leading us to propose our hypothesis that the fracture zone causes a reduction in the strength of the oceanic lithosphere. We will also review the results of previous seismic reflection surveys carried out in the area and will consider them as additional information to this result. We found that the case of the Tonga Trench, which is an exception to the thin elastic plate bending model, can be explained by our hypothesis. Tectonic research following the proposal of the elastic-perfectly plastic bending model has shown that the fracture zone of the oceanic lithosphere subducting here is parallel to the trench (Taylor, 2006). Therefore, if we can confirm our hypothesis that the fracture zone causes a reduction in the strength of the oceanic lithosphere, it will become a more general model without introducing the specific elastic-perfectly plastic bending model.