How the combination of local and tectonic forces affects the subduction zone initiation at passive margins

Although subduction is a well-known and widely studied process, the mechanisms leading to the initiation of a new subduction zone are poorly understood. Two main types of subduction zone initiation (SZI) are currently recognised: induced, when tectonic convergence is dominant, and spontaneous, when SZI is mainly driven by local forces, i.e., the negative buoyancy due to the gravitational instability of the plate (Stern 2004; Stern and Gerya 2018; Crameri et al. 2020). Potential sites for spontaneous SZI include passive margins, due to the natural local forces that characterise these geodynamic settings, i.e. lateral density, composition, strength and temperature contrasts, topographic discontinuity, sedimentary loading, and so gravitational instability. These local forces should favour the initiation of subduction (Stern and Gerya 2018; Lallemand and Arcay 2021; Arcay et al. 2020), but previous research suggests that local forces are not strong enough to develop a self-sustained subduction and that the collapse of a passive margin without horizontal tectonic forcing would require an unlikely coincidence of multiple weakening mechanisms, casting doubt on the feasibility of spontaneous subduction under present-day tectonic conditions (Lallemand and Arcay 2021; Arcay et al. 2020). We have explored how the weakening and the deformation induced by the local forces during an initial gravitational phase influence a subsequent SZI and the eventual subduction style once convergence begins to affect the margin (convergent phase). 2. Methods and first results We performed 225 2D simulations using the finite element code FALCON (Regorda et al. 2023) on a domain 3000 Km wide and 700 Km deep, representing a passive margin composed by a 20 Myr old oceanic lithosphere and a 90 Km thick continental lithosphere. The upper boundary is a free surface, and the lower boundary is free slip, while the lateral boundaries are free slip or convergent, depending on whether we are simulating a gravitational or a convergent phase. The numerical simulations have been developed in three phases (Fig 1): In the first phase, we built 9 gravitational models, by testing nine combinations of plastic and viscous strain weakening parameters under free-slip lateral boundary conditions over 30 Myr, to investigate how the weakening parameters affect the evolution of the system. In the second phase, we built 54 forced models, by applying external convergence, at six different rates (from 0.01 cm/yr to 1 cm/yr), to the same nine setups of the phase 1, to understand at which rates the evolution of the forced and the gravitational models are similar. In the third phase, we built 162 combined models, by adding convergence after 10, 20 and 30 Ma of gravitational evolution. Finally, we compared the evolution of the forced models with respect to the combined models, to obtain the effect that a gravitational phase on the evolution of the system. The first results show that even in scenarios where subduction does not initiate, and so a spontaneous SZI cannot be recognised, during the gravitational phase the system experiences deformation localisation, perturbation and weakening of the margin. In particular, comparing the combined models with respect to the forced models, that simulate a purely induced SZI, we observe that the initial gravitational phase affects the time of initiation of the subduction, the slab depth and the trench migration. References Arcay, Diane, Serge Lallemand, Sarah Abecassis, and Fanny Garel (2020). “Can subduction initiation at a transform fault be spontaneous?” In: Solid Earth 11. DOI: 10.5194/se-11-37-2020. Crameri, Fabio et al. (2020). “A transdisciplinary and community-driven database to unravel subduction zone initiation”. In: Nature Communications. DOI: 10.1038/s41467-020-17522-9. Lallemand, Serge and Diane Arcay (2021). “Subduction initiation from the earliest stages to self-sustained subduction: Insights from the analysis of 70 Cenozoic sites”. In: Earth-Science Reviews 221. DOI: 10.1016/j.earscirev.2021.103779. Regorda, Alessandro, Cedric Thieulot, Iris van Zelst, Zoltán Erdős, Julia Maia, and Susanne Buiter (2023). “Rifting Venus: Insights From Numerical Modeling”. In: Journal of Geophysical Research: Planets 128. DOI: 10.1029/2022JE007588. Stern, Robert J. (2004). “Subduction initiation: Spontaneous and induced”. In: Earth and Planetary Science Letters 226. DOI: 10.1016/j.epsl.2004.08.007. Stern, Robert J. and Taras Gerya (2018). “Subduction initiation in nature and models: A review”. In: Tectonophysics 746. DOI: 10.1016/j.tecto.2017.10.014.