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:

- Allmendinger, R. W., 1998, Inverse and Forward numerical modeling of trishear fault-propagation folds. Tectonics, v. 17, 640-656.
- Zehnder, A. T. & Allmendinger, R. W., 2000, Velocity field for the trishear model: Journal of Structural Geology, v. 22, p. 1009-1014.

This is the initial configuration of four beds, each 40 m thick. The dark line is the decollment. It stops at a tip point from which it will ramp up at a 30 degree angle in a trishear zone whose upper boundary is 60 degrees from the layering. The circles are used to track the strain pattern. The program allows you to control many different parameters and in doing so produce a great array of fold geometries. We have used the default values here. Click on the image to see what happens with 50 meters of slip on the decollement.