Čížková, H., and C. R. Bina, Rheological effects on slab stagnation in the transition zone and uppermost lower mantle, Abstracts of the XV International Workshop on Modelling of Mantle and Lithosphere Dynamics (NetherMod 2017), Putten, Netherlands, 36, 2017.
Abstract
An endothermic phase transition in mantle material at 660-km depth constitutes a barrier that in most cases prevents the direct penetration of subducted slabs. Seismic tomography shows that subducted material is in many subduction zones trapped at the bottom of the transition zone, just above the 660-km phase boundary. Recent tomographic models however also report subducted material that penetrates to the shallow lower mantle, and there it is observed to flatten at about 1000-km depth. Models of slab dynamics that generally assume a sharp rheological transition at 660-km depth, however, mostly predict slab stagnation at the bottom of the transition zone. Recent deformation experiments on ferropericlase indicate that viscosity may gradually increase in the uppermost 300 km of the lower mantle, rather than changing abruptly at the upper-lower mantle boundary. Here we present the results of a modelling study focused on the effects of rheological transition between the upper and lower mantle material on slab deformation and stagnation. We test the effects of smoothing the viscosity increase over 200 km and of shifting it to the depth of 1000 km or even deeper. We show that slab ability to penetrate to the lower mantle is mainly controlled by the trench migration ratio and that in turn is affected by the crustal viscosity. The lubrication of the contact between the subducting and overriding plates thus plays a key role in controlling slab penetration to the lower mantle and stagnation at the bottom of the transition zone or in the shallow lower mantle. The models with strong crust and consequently negligible rollback display penetration to the lower mantle without much hindrance and display no stagnation above or below the 660-km interface regardless of the viscosity stratification of the shallow lower mantle. The models with weak crust are characterised by fast rollback, and penetration is thus very limited. Such slabs generally buckle and flatten above the 660-km boundary. The most interesting models from the point of view of shallow lower mantle stagnation are models with intermediate crustal viscosity. Here rollback is efficient, though slower than in weak-crust cases. Horizontally lying slab segments are trapped in the transition zone, if the sharp viscosity increase occurs at 660 km, but shifting the viscosity increase to 1000 km depth allows for efficient sinking of the flat-lying part and results in stagnation below the upper-lower mantle boundary at about 1000 km depth.