Diachronous slab break-off in oppositely-dipping double subduction system: Insights from 3D numerical experiments

Diachronous slab break-off around the Adriatic microplate is inferred to occur with contrasting timing and kinematics on its western (Apennine) and eastern (Dinaride–Hellenide) margins. While the Apennines exhibit long-lived slab rollback followed by laterally migrating slab break-off, the eastern margin appears to have experienced earlier continental-collision-related shortening and slab break-off. To investigate the controlling factors on slab break-off and tearing in such a double-sided, oppositely dipping subduction system, we conduct 3D thermo-mechanical numerical experiments in which two subduction zones interact through a shared lower plate. We vary three key parameters: (1) the initial length of the oceanic lithosphere, (2) the initial subduction trench obliquities on each side (symmetric vs. asymmetric), and (3) oceanic plate ages, which collectively control the slab rollback velocity, trench rotation, interacting mantle flow, slab break-off, and tear propagation. In a symmetric reference experiment (with identical initial trench obliquity and oceanic plate length on both sides), closure of the short oceanic segment does not immediately trigger slab break-off. Instead, oceanic subduction evolves into intra-continental subduction, followed by a late-stage slab break-off. In contrast, on the longer oceanic segment, slab rollback drives trench retreat and rotation, causing progressive lateral plate decoupling that propagates along strike, and slab break-off initiates after the retreating trench meets the continent, long before continental collision. Asymmetric experiments (with different initial trench obliquity and oceanic plate length) demonstrate diachronous slab break-off on opposite sides. Here, on the shorter oceanic domain with lower trench obliquity, earlier continental collision and slab break-off occur, whereas on the longer oceanic domain with higher trench obliquity, slab rollback persists for a longer duration, accompanied by pronounced trench rotation, resulting in delayed slab break-off and tear propagation. Overall, our results indicate that (1) oceanic closure alone is not always sufficient to trigger slab break-off, (2) trench rotation linked to obliquity is a key factor controlling delayed slab break-off and tear propagation, and (3) a shorter oceanic domain with lower margin obliquity facilitates earlier continental collision and slab break-off. We propose that the tectonics around the Adriatic microplate can be interpreted as an interactive two-sided asymmetric subduction system in which the western margin evolves through obliquity-driven trench rotation and delayed slab break-off propagation, whereas the eastern margin experiences earlier slab break-off due to continental collision.