Slab Breakoff Induced by Weak Crustal-Scale Heterogeneities

Slab breakoff is most commonly associated with continental collision. However, recent geodynamic studies have documented slab breakoff in non-collisional subduction settings, indicating that additional mechanisms may facilitate slab failure. The processes enabling breakoff in the absence of pronounced buoyancy contrasts remain poorly understood. Here, we use two-dimensional thermo-mechanical numerical models to investigate the role of weak crustal-scale heterogeneities embedded within a subducting oceanic plate on slab breakoff dynamics. The models are developed using the ASPECT code coupled with the Geodynamic World Builder for the setting of the initial geometry of the models. We systematically vary the viscosity, length, and distance to trench of weak crustal strips representing inherited compositional heterogeneities, such as sedimentary depocenters. Our results suggest that in models where the subducting slab is fixed or subjected to slow push from the lateral boundary, low-viscosity heterogeneities strongly localize deformation at the subduction interface. Meanwhile, the slab may stretch within the asthenosphere and accelerate as it sinks, ultimately leading to slab necking and breakoff. We identify a clear relationship between slab breakoff depth and the distance of the weak strip from the trench, with breakoff occurring at shallower depths for more trench-distal heterogeneities. This behaviour arises from the combined effects of enhanced slab pull and the presence of weak material farther from the trench, which localizes deformation at shallower depths and promotes shallow slab breakoff. Following slab breakoff, subduction commonly resumes when remnants of the weak strip remain at the plate interface, initiating a second phase of subduction. In addition, we find that the presence of a weak strip increases trench retreat velocities by up to a factor of two compared to a homogeneous reference model. These results demonstrate that relatively small-scale variations in oceanic crustal strength can precondition subducting slabs for breakoff without the need for continental collision, providing a viable explanation for episodic slab detachment observed in natural subduction zones.