Thermomechanical modelling of slab detachment

Slab detachment or breakoff is appreciated as an important geological process, as shown by recent tomographic imaging. Using a 2-D upper-mantle model 660-km deep and 2000-km wide, we have investigated with a 2-D finite-difference and marker-in-cell numerical technique the multi-resolutional character of the thermomechanical phenomena related to this complex geological process. Our experiments show that this process can be initiated in form of slab necking by a prolonged (8-30 My) period of slab weakening due to thermal diffusion (<20 degreesC/My) after cessation of active subduction. The rapid detachment process takes place over a few million years and is accelerated by non-Newtonian strain-rate softening and focused thermal erosion (>60 degreesC/My) due to strong positive thermal feedback from shear heating. Detached slab fragments sink rapidly with a tendency for coherent rotation. The influence of temperature- and pressure-dependent thermal conductivity on the process of thermal weakening of the slab is quite significant. This supports the idea that the breakoff process is triggered by thermal diffusion on a time scale linearly dependent on heat conductivity. Rapid topographic changes and increasing volcanic activities due to the melting of subducted oceanic crust are possible scenarios of this vigorously driven geodynamic process. (C) 2004 Elsevier B.V. All rights reserved.