Expanding shear bands form at the Coulomb angle

What is it about?

One way of creating faults or shear bands in tectonic modeling (simulation of the formation and evolution of geological structures, usually of a large scale) is to assume a "strain-weakening" Mohr-Coulomb model in which yield stress becomes smaller as the amount of permanent deformation increases. However, many numerical tectonic models fail to produce faults at an angle predicted by the very plasticity model they employ. We find that letting a fault expand as it slips is the key to making it form at the expected orientation. Our solution works well even in low-resolution meshes and in different modeling codes. We also explain why it works in the light of the classical theory for strain localization.

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Why is it important?

First of all, it is nice to have models that are consistent with a powerful, simple theory, which is the Coulomb failure criterion in this case. Also, in order to understand the long-term evolution of faults, it is often useful to quantify the amount of rotation they went through. It is very convenient if a fault is created at an theoretically expected orientation. Many numerical models failed to consistently and reliably predict fault orientations. Our solution addresses this problem and turns out to be robust.