[S04] PM-1

[S04-01] 2D subduction models in the presence of a high-density rigid continental block: a case study for the slab geometry beneath the Kii Peninsula.

*Erika Moreno¹, Vlad Manea², Marina Manea², Nobuaki Suenaga³, Shoichi Yoshioka¹ (1. Kobe University, 2. CGEO, Universidad Nacional Autónoma de México, 3. Kyoto University)

Subduction along the Nankai Trough is characterized by great diversity in terms of slab geometry. For example, the Philippine Sea (PHS) plate exhibits shallow subduction beneath Shikoku and Chugoku regions, meanwhile the subduction beneath the Kii Peninsula occurs with a higher dip angle. However, considering a continuous and relatively uniform subduction process along the Nankai Trough since 15 Ma, the expected PHS slab between Shikoku and the Kii Peninsula would show little variation. However, seismic tomography and seismicity studies reveal two styles of subduction between both regions. The Kii Peninsula is characterized by the presence of the Kumano Pluton near the Nankai Trough. Seismic tomography studies reveal high seismic velocity zones adjacent to the slab beneath the Kii Peninsula, and a low seismic velocity zone beneath seismic swarm area in Wakayama. These high-velocity zones near the slab are currently associated with acidic rocks that consists of the Kumano Pluton. It is proposed that these blocks are characterized by rigid, impermeable, high-density materials that act as a barrier to rising the PHS slab fluids. Several hypotheses suggest that these rigid blocks induce a high dip angle subduction beneath the Kii Peninsula. In this study we employ 2D high-resolution visco-elasto-plastic numerical models of spontaneous subduction in order to better understand how the presence of a rigid and high-density tectonic block influences on the subduction dynamics of the PHS plate. We focus on a 2D profile passing through the Kii Peninsula. Our numerical models evolve over a period of 15 Myr, and the PHS plate initiates subduction beneath the Kii Peninsula at a high convergence rate of 7.69 cm/yr. In the vicinity of the Nankai Trough and between the upper and lower continental crust, high-density (HD) (rock density of r = 3200 kg/m³) rigid materials (rocks cohesion of c = 1x10 ⁶ MPa) are imposed. Results of our numerical models are evaluated in terms of the current PHS slab geometry beneath the Kii Peninsula and the location of the rigid materials which were estimated using seismic tomography studies. Our numerical modelling found that the presence of the HD rigid block plays a fundamental role in the evolution of subduction dip angle. Without the presence of the HD rigid block, the subduction is characterized by shallow dip angle as observed beneath Shikoku and Chugoku regions. On the other hand, including the HD block adjacent to the Nankai Trough, the final result became slab geometry with a higher dip angle as observed beneath the Kii Peninsula.