Title:

Rheological modeling to assess the effects of seamount subduction

Abstract:
The Kyushu–Palau Ridge (KPR) acts as a structural boundary, separating two distinct subduction zones: the Ryukyu Trench, where the old West Philippine Basin (WPB) is subducting, from the Nankai Trough, where the younger Shikoku Basin of the Philippine Sea Plate descends. This contrast is reflected in a sharp thermal gradient, with high heat flow toward the Shikoku Basin and markedly lower values toward the WPB. Notably, the lowest heat flow occurs seaward of the subducting KPR, a region that also exhibits tectonic tremors and low-frequency earthquakes. It has been hypothesized that these correlated features result from stress and strain modulation caused by seamount subduction. To test this hypothesis, we conducted two-dimensional numerical simulations with realistic rheology for the Hyuga-Nada region. Our aim was to evaluate how seamount subduction affects the overlying plate and its potential links to both slow earthquake conditions and the observed heat-flow anomalies. We developed four numerical models to analyze the sensitivity of the system to seamount subduction as a function of the incoming plate’s age: an old plate (47 Myr) without (Model 1) and with (Model 2) a seamount, and a young plate (25 Myr) without (Model 3) and with (Model 4) a seamount. Forearc sediments were assigned a depth-dependent density, ranging from approximately 1600 kg/m³ to 2600 kg/m³ based on data from a nearby IODP site, assuming an exponential increase with depth. These sediments exhibit strain-weakening behavior, with an initial internal friction angle of 10° and cohesion of 1 × 10⁶ Pa, decreasing to 5° and 1 × 10⁵ Pa after deformation. In contrast, the oceanic and continental crust maintain constant values (30° and 1 × 10⁶ Pa, respectively). The modeling results indicate that seamount subduction enhances forearc deformation for both old and young plates, manifested as shear bands in the upper crust. The spatial distribution and intensity of these shear bands reflect the combined influence of plate age and the presence of a seamount. Models without a seamount (1 and 3) display nearly constant deviatoric horizontal stress and systematic shear-band development, indicative of strong interplate mechanical coupling. Conversely, models that include a seamount (2 and 4) show significant stress variations controlled by the seamount’s position, which locally suppress forearc fracturing within a specific depth interval. This suppression coincides with the seamount entering high-density sediments. Prior to this stage, interaction with low-density sediments generates permanent deformation at relatively low stress. After the seamount passage weakens the sediments, shear-band development resumes and subduction dynamics regain dominance. Plate age further modulates this response: Model 2 (old plate) produces greater forearc deformation and higher strain rates than Model 4 (young plate).