Physical models of transtensional folding

Physical models of transtensional deformation indicate the formation of folds for a range of plate divergence angles from 0° to 30°. Folds are upright and fold axial planes are vertical for all angles of oblique divergence. The transtensional fold hinges are initiated parallel to the direction of the maximum horizontal infinitesimal extension and rotate with the maximum horizontal finite extension toward parallelism with the oblique movement direction (not the shear-zone boundary). Large amounts of hinge-parallel extension occur during fold formation and material lines are observed to rotate through the fold hinges during progressive deformation. The transtensional folding experiments reinforce the pitfalls of assuming a simple coincidence between geological structures and strain or stress. For the 22° and 30° obliquely divergent experiments, the observed fold axial planes are not perpendicular to the maximum contraction direction (S₃) or the inferred maximum compressive stress direction (σ₁). Consequently, one must be cautious in making interpretations of principal strain directions, and especially principal stress directions, based on the orientations of folds formed in transtension.