Physical experiments of transpressional folding

In order to understand the process of folding in obliquely convergent settings, we formed folds within a shear box capable of creating homogeneous transpressional deformations. Folds were created in a single layer of stiff mixed plasticine and silicone that overlay a Newtonian silicone, for a variety of plate convergence angles. As small amplitude folds became visible, they were parallel to the long axis of the horizontal finite strain ellipse. With increasing deformation, the fold hinges rotated parallel with the long axis of the horizontal finite strain ellipse for all angles of convergence. This parallelism indicates that fold hinges, once formed, rotate with the horizontal strain ellipse rather than as material lines. The experiments highlight several interesting effects of transpression dynamics. The fold hinges initiate parallel to either S-1 or S-2 and are parallel to either S-1 or S-2 with increasing deformation. Neither infinitesimal strain (stress) nor finite strain is resolvable solely from fold geometry. Further, the net amount of contraction determined by folding across the zone was overestimated in all cases except pure contraction. This effect is obvious for the case of wrenching, where folding implies that the zone contracts if elongation parallel to the fold hinge is not considered. Therefore, attempts to balance cross-sections in transpressional zones will tend to overestimate contraction unless the wrench component of deformation is addressed. This result is validated by applying the modeling results in folding in central California adjacent to the San Andreas fault, where cross-section balancing indicates higher amounts of contraction than predicted by plate motion. (C) 1998 Elsevier Science Ltd. All rights reserved.