Geodynamic Modelling of Passive Margin Stability with Grain Damage: Conditions for Subduction Initiation

Subduction initiation remains a key open problem in geodynamics. One hypothesis for the spontaneous initiation of subduction is passive margin collapse triggered by grain damage: a rapid plunge in grain size in the lower parts of the lithosphere leads to strong rheological weakening and the formation of a localised shear zone that facilitates subduction. This mechanism has been proposed and tested in 1D models (Mulyukova & Bercovici, 2018), but has not been incorporated into fully dynamic subduction models because grain-size-dependent rheologies have a high complexity and computational cost. As a result, its viability as a trigger for subduction initiation remains uncertain.

Here we present high-resolution 2-D thermo-mechanical models that test whether grain damage can enable passive margin collapse and subduction initiation. We model the life cycle of an entire oceanic plate from mid-ocean ridge formation to the potential collapse at the passive margin (or stable evolution if no collapse occurs). The lithosphere is represented as a two-phase assemblage of 60% olivine and 40% pyroxene, which are well-mixed at the grain scale. Because grains of each phase impede the growth of the other through Zener pinning, grain growth is suppressed relative to single-phase compositions. This promotes strain localisation due to grain size reduction. Simulating this process requires accurate tracking of the mineral grain size, which is both history-dependent and sensitive to stress changes. Recent advancements in the community code ASPECT, including a higher-order particle method and adaptive time stepping for the grain-size evolution equation via the ARKode solver, now make this feasible.

Our models demonstrate that subduction initiation by grain damage is possible, but only within a narrow range of grain size evolution parameters. Passive margin collapse requires that a large fraction of deformational work in cold lithospheric regions is partitioned into interface damage rather than dissipated as shear heating. Even under these favourable conditions, additional weakening is needed to break the upper ≥15 km of the plate. In our models, we impose a narrow, weak zone to represent this shallow weakening. Elevated stresses in and around the weak zone promote grain damage, producing a grain size plunge and associated viscosity drop at mid- to lower-lithosphere depths. The resulting zone of small grain size propagates downward through the lower lithosphere until a narrow, continuous shear zone forms that enables passive margin collapse. However, the same imposed weak zone does not lead to subduction initiation in otherwise identical models with a fixed grain size.

These results indicate that grain damage alone is unlikely to be the primary trigger for passive margin collapse, but that it can substantially enhance strain localisation and modulate the conditions for subduction initiation when combined with additional weakening mechanisms.

 

References: Mulyukova, E., & Bercovici, D. (2018). Collapse of passive margins by lithospheric damage and plunging grain size. Earth and Planetary Science Letters, 484, 341-352.