Backlimb trishear: a kinematic model for curved folds developed over angular fault bends

Numerous analog models and some natural examples display smoothly curving backlimb fold hinges over sharp, angular bends in the underlying thrust surface. We present a new kinematic model that can reproduce this geometry by defining a triangular zone focused on the fault bend. The model presupposes incompressible flow in the triangular zone and makes common assumptions about velocities on either side of the fault bend. Either symmetric or asymmetric triangular zones can be defined, with parallel kink folding and similar folding being special cases of the two, respectively, where the apical angle of the triangular zone is zero. The results of the numerical model compare well to analog experiments. The model is conceptually analogous to forelimb trishear at the tip of a propagating fault and hence we refer to it as ‘backlimb trishear’.