Slab weakening: Mechanical and thermal-mechanical consequences for slab detachment

Abstract Slab detachment is a geophysical instability whose manifestation can be revealed by seismic tomography. Evidence of this phenomenon is in the Dinarides/Hellenic and the New Hebrides subduction zones. Subducted slabs in these regions are torn horizontally at depths ranging from 100 to 300 km. We constructed a viscoelastic three-dimensional finite element model and investigated the state of stress. We found that an area with high stress concentration of the order of several hundred MPa is formed near the tip of the tear inside the slab, which can cause lateral migration of the tear. Favorable conditions for slab detachment are characterized by large interplate frictional force at a subduction zone and small slab resistance force deeper down. Stress concentration increases with the down-dip tension inside the slab. The phenomenon of slab weakening has also been studied from a thermal-mechanical standpoint, using a two-dimensional convection model with non-Newtonian, temperature-dependent rheology. The stress-dependent rheology plays an important role in causing local weakening of the descending slab. In strongly time-dependent situations the fast descending slab is not strong everywhere but has a weak region in the middle, making it vulnerable to slab detachment. The presence of viscous heating will enhance slab detachment tendency by further weakening the interior by the frictional heating. Besides these effects, there are other mechanisms which can also weaken the slab interior and help to make slabs more pliable and susceptible to detachment.