# 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 (Wortel and Spakman 2000; van Hunen and Allen 2011; Garzanti, Radeff, and Malusà 2018).

This benchmark is based on the setup by S. Schmalholtz (Schmalholz 2011), which was subsequently run with by A. Glerum (Glerum et al. 2018). The computational domain is a $$1000 \si{km}\times 660 \si{km}$$ 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.

Two materials are present in the domain: the lithosphere and the mantle as shown in Figure 1. The gravity acceleration is Earth-like with $$g=9.81 \si{m}\si{s}^2$$. The overriding plate is $$80\si{km}$$ thick and is placed at the top of the domain. The already subducted lithosphere extends vertically into the mantle for $$250 \si{km}$$. This slab has a density $$\rho_s=3300\si{kg}/\si{m}^3$$ 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 $$\dot\varepsilon$$: $$$\eta_{eff} = \eta_0 \, \dot\varepsilon^{1/n-1}$$$$with$$n=4$$and$$\eta_0=\SI{4.75e11}{Pa . s}$$. The mantle occupies the rest of the domain and has a constant viscosity$$\eta_m=\SI{1e21}{Pa . s}$$and a density$$\rho_m=\SI{3150}{kg/m^3}$$. Viscosity is capped between$$\SI{1e21}{Pa . s}$$and$$\SI{1e25}{Pa . s}$. Figure [fig:slab_detachment_evolution] shows the various fields and their evolution through time. As shown in (Schmalholz 2011; Glerum et al. 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 (Schmalholz 2011) in (Glerum et al. 2018; Hillebrand et al. 2014) and the effect of viscosity and material averaging was explored in (Glerum et al. 2018).

Garzanti, E., G. Radeff, and M. G. Malusà. 2018. “Slab Breakoff: A Critical Appraisal of a Geological Theory as Applied in Space and Time.” Earth-Science Reviews 177: 303–19. https://doi.org/10.1016/j.earscirev.2017.11.012.

Glerum, A., C. Thieulot, M. Fraters, C. Blom, and W. Spakman. 2018. “Nonlinear Viscoplasticity in ASPECT: Benchmarking and Applications to Subduction.” Solid Earth 9 (2): 267–94. https://doi.org/10.5194/se-9-267-2018.

Hillebrand, B., C. Thieulot, T. Geenen, A. P. van den Berg, and W. Spakman. 2014. “Using the Level Set Method in Geodynamical Modeling of Multi-Material Flows and Earth’s Free Surface.” Solid Earth 5 (2): 1087–98. https://doi.org/10.5194/se-5-1087-2014.

Schmalholz, S. M. 2011. “A simple analytical solution for slab detachment.” Earth Planet. Sci. Lett. 304: 45–54.

van Hunen, J., and M. B. Allen. 2011. “Continental collision and slab break-off: A comparison of 3-D numerical models with observations.” Earth Planet. Sci. Lett. 302: 27–37.

Wortel, M. J. R., and W. Spakman. 2000. “Subduction and slab detachment in the Mediterranean-Carpathian region.” Science 290: 1910–17.