We are used to draw fault related folds using graphic methods involving kink folding and parallel folding. This methods allow us to get an idea of the folding mechanism ruling some structures, but in general nature is more complicated than that...
Rick Allmendinger, structural geologist from the Department of Earth and Atmospheric Sciences at Cornell University explains it in full in his website :-)
I think a good understanding of this concept should be a requirement to anyone approaching structural geology in a degree. Unfortunately, it doesn't seem to be a very spread idea yet. So you know... Visit it, play it, learn it!
What the heck is "Trishear," anyway?
Eric Erslev (1991) deserves full credit for developing the concept of trishear. During his studies of the Laramide Rocky Mountain foreland in Colorado and Wyoming, he noticed that discrete fault zones within the basement diffuse outward and upward in a triangular zone of deformation in the overlying sedimentary section. He called these triangular zones of deformation "trishear." Trishear provides an alternative to the parallel kink fold description of fault-propagation folding. Unlike the simple kink fold model, trishear can produce footwall synclines, downward steepening dips and thickening and thinning of forelimb strata. Furthermore, trishear provides a richer description of heterogeneous strain distribution at the tips of propagating faults (see the above graphic), which may ultimately prove useful, for example, in studies of fracture distribution and orientation. However, whereas parallel kink fold angular relations can be determined graphically (e.g., Suppe & Medwedeff, 1990), trishear models can only be calculated numerically.
Since 1997, we have been studying trishear extensively at Cornell and have concluded that trishear structures are far more widely distributed than previously recognized. We have written general 2D and pseudo-3D trishear modeling programs which enable us to explore the complete broad range of trishear-associated deformation In these web pages, we give a brief, mostly graphical, introduction to trishear, with the intent of illustrating how it might be used. Be sure to check out the trishear movies!
This material in this site is partly based upon work supported by the National Science Foundation under Grant No. EAR-9814348. Any opinions, findings and conclusions or recomendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation (NSF). Additional support comes from the Donors to the Petroleum Research Fund of the American Chemical Society. And finally, there is a lot of unsupported stuff in here as well :-)