A simple toy model for coupled retreat and detachment of subducting slabs. (September 2019)

• Record Type: Journal Article
• Authors: Bercovici, David; Mulyukova, Elvira; Long, Maureen D.
• Is Part Of: Journal of geodynamics. Volume 129(2019)
• Page Start: 275
• Page End: 289
• Publication Date: 2019-09
• Subjects: Plate tectonics -- Trench retreat -- Slab detachment -- Damage mechanics; Geodynamics -- Periodicals; Earth movements -- Periodicals; Rock deformation -- Periodicals; Earth -- Internal structure -- Periodicals; 551.1
• DOI: 10.1016/j.jog.2018.03.002
• Languages: English
• ISSNs: 0264-3707

Abstract

Abstract: Subducting slabs are the primary drivers of plate tectonics and mantle circulation, but can also undergo various instabilities that cause dramatic adjustments in tectonic evolution and motion. Slab rollback or trench retreat is possibly a dominant form of time dependence in the plate-mantle system, causing plates to shrink and the mantle to undergo complex flow patterns. Likewise, slab detachment can induce abrupt adjustments in both plate motions and vertical displacement of continents. The arrival or accumulation of continental crust over a subduction zone induces high stresses on the plate and slab that can trigger either rollback or detachment or both. However, these processes necessarily interact because of how stress is relieved and plate motions altered. Here we present a simple boundary-layer like model of coupled trench retreat and slab detachment, induced by continent accumulation, and with slab necking augmented by grain-damage self-weakening (to allow for abrupt necking). With this model we find that, with continental accumulation, initial rollback is at first modest. However, as the stress from continental accumulation peaks, it triggers abrupt slab detachment. The subsequent slab loss causes the plate to lose its primary motive force and to thus undergo a more dramatic and rapid rollback event. After the larger rollback episode, the contracted continental mass re-expands partially. Plausible grain-damage parameters and 40 km thick crust cause abrupt detachment and major rollback to occur after a few hundred million years, which means the plates remain stable for that long, in agreement with the typical age for most large plates. While the complexity of some field areas with a well documented history of detachment and rollback, such as the Mediterranean, taxes the sophistication of our toy model, other simpler geological examples, such as on the western North American plate, show that episodes of rollback can follow detachment.