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Abrupt tectonics and rapid slab detachment with grain damage

Significance Subduction zones are delineated by Earth’s ocean trenches, and are where tectonic plates sink into the mantle as cold heavy slabs, which in turn drive plate motion. But slabs can detach from their surface plates, thus altering tectonic driving forces. Slab detachment can occur if thick crust from continents or oceanic plateaux is swept by plate motion into the subduction zone, thus plugging it up. Detachment is also accelerated because mineral grains in the slab become smaller during deformation, causing the slab to weaken rapidly while being stretched. The combination of crustal plugs with weakening causes abrupt slab detachment in a few million years, which can account for observed precipitous changes in plate tectonic motion and rapid continental uplift. A simple model for necking and detachment of subducting slabs is developed to include the coupling between grain-sensitive rheology and grain-size evolution with damage. Necking is triggered by thickened buoyant crust entrained into a subduction zone, in which case grain damage accelerates necking and allows for relatively rapid slab detachment, i.e., within 1 My, depending on the size of the crustal plug. Thick continental crustal plugs can cause rapid necking while smaller plugs characteristic of ocean plateaux cause slower necking; oceanic lithosphere with normal or slightly thickened crust subducts without necking. The model potentially explains how large plateaux or continental crust drawn into subduction zones can cause slab loss and rapid changes in plate motion and/or induce abrupt continental rebound.