Abstract
The effects of sphericity are regularly neglected in numerical and
laboratory studies that examine the factors controlling subduction
dynamics. Most existing studies have been executed in a Cartesian
domain, with the small number of simulations undertaken in a spherical
shell incorporating plates with an oversimplified rheology, limiting
their applicability. Here, we simulate free-subduction of composite
visco-plastic plates in 3-D Cartesian and spherical shell domains, to
examine the role of sphericity in dictating the dynamics of subduction,
and highlight the limitations of Cartesian models. We identify two
irreconcilable differences between Cartesian and spherical models, which
limit the suitability of Cartesian-based studies: (i) the presence of
sidewall boundaries in Cartesian models, which modify the flow regime;
and (ii) the reduction of space with depth in spherical shells,
alongside the radial gravity direction, which cannot be captured in
Cartesian domains. Although Cartesian models generally predict
comparable subduction regimes and slab morphologies to their spherical
counterparts, there are significant quantitative discrepancies. We find
that simulations in Cartesian domains that exceed Earth’s dimensions
overestimate trench retreat. Conversely, due to boundary effects,
simulations in smaller Cartesian domains overestimate the variation of
trench curvature driven by plate width. Importantly, spherical models
consistently predict higher sinking velocities and a reduction in slab
width with depth, particularly for wider subduction systems, enhancing
along-strike slab buckling and trench curvature. Results imply that
sphericity must be considered when simulating Earth’s subduction
systems, and that it is essential for accurately predicting the dynamics
of subduction zones of width ~2400 km or more.