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 100 meters of slip. The fault continues to propagate through the trishear zone at an angle of 30 degrees. The purple lines represent the particle trajectories. Note how the foreland (leading) limb of the fold is becoming overturned. The hinterland limb continues to lengthen. Note also how the fault tip has continued to propagate, and how the strain in the immediate hanging wall and footwall of the ramp is substantially different. Click on the image to see what happens with 150 meters of slip on the decollement.