Plate tectonics on Earth takes place along faults zones such as the San Andreas fault. Many of these mega-shear zones are related to damaging earthquakes and tsunamis, as in Sumatra and Japan. Moreover, they act as a channelway for magma and other liquids that form the major ore deposits on which the world’s economy is based. The superficial part of major fault zones can easily be studied, but deeper parts, from 10 -40 km, are not well known. At depths between 10 and 40 km, rocks in fault zones flow in a ductile manner and form km-wide “shear zones”. Direct observation of these deep shear zones, between 10-40 km is not possible. “Fossil”, now inactive examples, however, can be found in areas with older, metamorphic rocks, where they have been made accessible by erosion. The rocks in such “fossil” shear zones show a lot of complex structures. Geologists study these fossil remains, but these are the end result of a long history of motion. Although we can study fossil shear zones, it is not possible to directly observe how the zones operate, since they do not flow ductile at the surface. The flow pattern in straight zones can be easily modelled, but many zones branch, join and split in a complex way. It is important to understand their functioning since the branching sites can be a source of major earthquakes, and a channelway for ore-forming fluids. Because of their complexity, computer modelling is an extremely helpful tool to understand their internal physics, particularly if combined with field observations. Here we will perform numerical modes of interacting and branching shear zones under both surface-near brittle and ductile conditions. This will help to better understand how multiple shear zones interact, and which branches are active at which point in time. This, combined with field observations, can help to understand their role in the formation of ore deposits and of major earthquakes.