Abstract
Abstract. Subduction modelling in regional model domains, in 2-D or 3-D, is commonly performed using closed (impermeable) vertical boundaries. Here we investigate the merits of using open boundaries for 2-D modelling of lithosphere subduction. Our experiments are focused on using open and closed (free slip) sidewalls while comparing results for two model aspect ratios of 3:1 and 6:1. Slab buoyancy driven subduction with open boundaries and free plates immediately develops into strong rollback with high trench retreat velocities and predominantly laminar asthenospheric flow. In contrast, free-slip sidewalls prove highly restrictive on subduction rollback evolution, unless the lithosphere plates are allowed to move away from the sidewalls. This initiates return flows pushing both plates toward the subduction zone speeding up subduction. Increasing the aspect ratio to 6:1 does not change the overall flow pattern when using open sidewalls but only the flow magnitude. In contrast, for free-slip boundaries, the slab evolution does change with respect to the 3:1 aspect ratio model and slab evolution does not resemble the evolution obtained with open boundaries using 6:1 aspect ratio. For models with open side boundaries, we could develop a flow-speed scaling based on energy dissipation arguments to convert between flow fields of different model aspect ratios. We have also investigated incorporating the effect of far-field generated lithosphere stress in our open boundary models. By applying realistic normal stress conditions to the strong part of the overriding plate at the sidewalls, we can transfer intraplate stress to influence subduction dynamics varying from slab roll-back, stationary subduction, to advancing subduction. The relative independence of the flow field on model aspect ratio allows for a smaller modelling domain. Open boundaries allow for subduction to evolve freely and avoid the adverse effects (e.g. forced return flows) of free-slip boundaries. We conclude that open boundaries in combination with intraplate stress conditions are to be preferred for modelling subduction evolution (rollback, stationary or advancing) using regional model domains.
Highlights
In the past decades, numerical modelling of lithosphere subduction has advanced considerably by incorporating coupling between plates, between plates and mantle, and by incorporating the complexity of detailed subduction zone processes
Our results show that with open sidewalls increasing the model aspect ratio does not change the overall evolution of subduction and mantle flow
The presence/absence of the accretionary wedge only leads to small differences in dip angle and stress field of the trench zone and does not affect the overall subducting slab evolution for the boundary conditions we consider in our models, we conclude from various tests
Summary
Numerical modelling of lithosphere subduction has advanced considerably by incorporating coupling between plates, between plates and mantle, and by incorporating the complexity of detailed subduction zone processes (see Gerya, 2011, for a review and references therein). The limited spatial domain requires prescribing boundary conditions on the vertical sidewalls of the domain These conditions are an important influence on the development of the model interior (Quinquis et al, 2011; Duretz et al, 2011, 2012; Ueda et al, 2012). The primary reason is that closed sidewalls basically cause return flows from both sides towards the centre of the model, which feeds back artificially into the evolving subduction process We expect this to hold for 3-D models despite the larger degree of freedom to develop lateral flow
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