Finite-element models are used to investigate certain mechanical and deformational characteristics of an active triangle zone system. Both converging and nonconverging upper and lower detachment configurations are considered. The models suggest that the frictional characteristics of the upper and lower detachments have a major influence on the distribution of deformation in an evolving triangle zone. The core region of the triangle zone may be expected to be a zone of progressive deformation and structural thickening. Interaction between the upper detachment and active forethrusts in the core of the triangle zone can steepen the upper detachment and produce intense, local deformation. Backthrust displacement on an upper detachment that extends tens of kilometres to the foreland of the core region of a triangle zone requires very low resistance to sliding along both the upper and lower detachments through this foreland extension. The development of new faults in the footwall of an existing triangle zone may require a zone of relatively weak material and/or high pore pressure in the footwall.