The incompressible continuity equation is broadly used in numerical geodynamic modelling,although in many cases it is a rather big simplification(e.g.inthecaseof the whole Earth mantle convection, e.g., Tackley, 2008). Typical examples of geo-dynamic settings where deformations are strongly defined by the incompressibility condition are, for example, corner flow in the mantle wedge above subducting slabs (e.g. Turcotte and Schubert, 2002) and circulation of tectonic melanges in subduction channels (e.g. Cloos, 1982).