This diagram was created using the software program Trishear 4.5 by Richard Allmendinger (1997-2000), to whom we are grateful for making this and other software so widely and easily accessible to the geocommunity. The algorithm used in the software is described in the following two references:
Here is the geometry after 50 meters of slip. The fault is propagating through the trishear zone at an angle of 30 degrees. Not only the shape, but the shade of red indicates the magnitude of strain. The purple lines represent the particle trajectories. A flat topped, broad fold forms. Click on the image to see what happens with 100 meters of slip on the decollement.