The Hidden Propagator: How Free-Slip Boundaries Corrupt 3D Simulations

Free-slip boundary conditions are routinely used in 3D geodynamic modelling because they reduce computational cost, avoid artificial shear zones at domain edges, and simplify the implementation of large-scale kinematic forcing. However, despite their apparent neutrality, our experiments show that free-slip boundaries systematically generate first-order artefacts that propagate deep into the model interior and can severely distort the interpretation of continental rifting simulations.

Here we present a set of 3D visco-plastic models inspired by the South China Sea (SCS) that were originally designed to study the effect of steady-state thermal inheritance and pluton-controlled crustal weakening. Unexpectedly, in all simulations except those with a very particular inverted rheological profile (POLC), the free-slip boundary on the “Vietnam side” of the domain generated a persistent secondary propagator, producing unrealistic amounts of lithospheric thinning in the southwest corner. This artefact appeared irrespective of crustal rheology, seeding strategy, or the presence of thermal heterogeneities.

We identify three systematic behaviours induced by free-slip boundaries in 3D:
(1) forced rift nucleation at boundary-adjacent thermal gradients,
(2) artificial propagator formation that competes with the intended first-order rifting, and
(3) rotation or shearing of micro-blocks not predicted by tectonic reconstructions.

These artefacts originate from the inability of free-slip boundaries to transmit shear traction, which artificially channels deformation parallel to the boundary when lateral thermal or mechanical contrasts exist. In 3D, unlike in 2D, the combination of oblique extension and boundary-parallel velocity freedom leads to emergent pseudo-transform behaviour that is entirely numerical.

Our results highlight a key negative outcome: free-slip boundaries cannot be assumed neutral in 3D rift models, especially when studying localisation, obliquity, multi-propagator dynamics, or the competition between structural and thermal inheritance. We argue that many published 3D rift models may unknowingly include such artefacts.