# The slab detachment benchmark *This section was contributed by Cedric Thieulot and Anne Glerum.* Slab detachment (slab break-off) may occur in the final stages of subduction as a consequence of the combination of a buoyant crust and strong slab pull. It is often invoked to explain geophysical and geological observations such as tomographic images of slab remnants and exhumed ultra-high-pressure rocks {cite}wortel:spakman:2000,vanhunen:allen:2011,garzanti:etal:2018. This benchmark is based on the setup by S. Schmalholtz {cite}schmalholz:2011, which was subsequently run with by A. Glerum {cite}glerum:etal:2018. The computational domain is a $1000 \text{ km}\times 660 \text{ km}$ box. No-slip boundary conditions are imposed on the sides of the system, while free-slip boundary conditions are imposed at the top and bottom. {figure-md} fig:slab_detachment_setup Slab detachment benchmark: Initial geometry  Two materials are present in the domain: the lithosphere and the mantle as shown in {numref}fig:slab_detachment_setup. The gravity acceleration is Earth-like with $g=9.81 \text{ m}\text{ s}^2$. The overriding plate is $80\text{ km}$ thick and is placed at the top of the domain. The already subducted lithosphere extends vertically into the mantle for $250 \text{ km}$. This slab has a density $\rho_s=3300\text{ kg/m}^3$ and is characterized by a power-law flow law so that its effective viscosity depends on the square root of the second invariant of the strainrate $\dot\varepsilon$: {math} \eta_{eff} = \eta_0 \, \dot\varepsilon^{1/n-1}  with $n=4$ and $\eta_0=4.75\times 10^{11}\text{ Pa . s}$. The mantle occupies the rest of the domain and has a constant viscosity $\eta_m=1\times 10^{21}\text{ Pa . s}$ and a density $\rho_m=3150\text{ kg/m}^3$. Viscosity is capped between $1\times10^{21}\text{ Pa . s}$ and $1\times 10^{25} \text{ Pa . s}$. {numref}fig:slab_detachment_evolution shows the various fields and their evolution through time. As shown in {cite:t}schmalholz:2011,glerum:etal:2018 the hanging slab necks, helped by the localizing effect of the nonlinear rheology. Model results were shown to compare favorably to the results of {cite:t}schmalholz:2011 in {cite:t}glerum:etal:2018,hillebrand:etal:2014 and the effect of viscosity and material averaging was explored in {cite}glerum:etal:2018. :::{figure-md} fig:slab_detachment_evolution Slab detachment benchmark: a,b) velocity and strain rate fields at $t=0$. c,d,e) and f,g,h) time evolution of the viscosity and slab composition fields at $t=0, 6, 12\text{ Myr}$. :::