Numerical modeling of deformation associated with seamounts subduction.Implications for the seismic cycle.

Subduction zones are frequently affected by the subduction of seamounts (Wessel et al., 2010). Numerous studies have proposed that seamount subduction could significantly influence the seismic cycle of subduction zones (Wang & Bilek, 2014). In recent years, seamounts have been increasingly linked to the induction of fluid overpressures that trigger shallow slow slip events (SSEs) (Saffer & Wallace, 2015), contributing to the aseismic behavior of subduction zones.

However, rigorously establishing a connection between the seismic cycle and seamount subduction remains challenging due to the limited availability of observations. Identifying subducted seamounts is particularly difficult: seismic reflection methods are limited to depths of a few kilometers, while gravimetric techniques rely on inverse modeling, which introduces substantial uncertainties.

In this study, we performed numerical simulations to investigate the deformations associated with multiple seamount subductions in accretionary wedges. Our objective is to improve our understanding of the relationship between seamounts and the seismic cycle by:

• Determining new structural criteria to better locate seamounts along mega-thrusts, thereby increasing the number of observations of wedges deformed by seamounts.
• Providing mechanical constraints on the link between the seismic cycle and the subduction of seamounts.

We used the pTatin2d thermo-mechanical code (May et al., 2014, 2015), considering lithospheric flexure and surface processes (Jourdon et al., 2018). Our simulations explored variations in basal friction, seamount size, and lithospheric elastic thickness.

We showed that, contrary to previous thought (Wang & Bilek, 2014; Ruh et al., 2016), seamounts can be cut off during their subduction. This primarily depends on their size, as only smaller seamounts can be cut off. More surprisingly, it also depends on the timing of the seamount's arrival at the deformation front relative to the backthrust-forethrust succession.

The tectonic structures of the wedge are strongly influenced by the deformation mode of the seamount. If it is cut off, the structural inheritances of the wedge are preserved, with slices and basins that reflect past seamount subductions. If it is not cut off, gravitational collapse occurs. Additionally, the structural inheritances are not preserved but deformed during seamount burial. Only the structures associated with the subduction of the most recent seamount remain visible, consisting of a basin, a slice, and mass transport deposits at the surface.

We also investigated the stress state within the wedge. Once cut off, seamounts have no influence on the stress state. On the other hand, non-cut off seamounts induce significant tectonic overpressure landward and underpressure seaward (Ruh et al., 2016). Landward of the seamount, an undeformed sediment zone is identified (Wang et al., 2021). This zone is favorable for fluid burial since it is not drained by faults. Additionally, the horizontal orientation of the principal stresses is also favorable for the buildup of fluid overpressure (Sibson, 1990), which may induce SSE nucleation (Leeman et al., 2018). This study provides mechanical explanations for the observations of shallow SSEs landward of seamounts, as observed at Hikurangi (Bell et al., 2014; Barker et al., 2018;  Todd et al., 2018) and Nankai (Takemura et al., 2023).