Investigating Kinematics Of Folding From Sandbox Experiments

We analyze the kinematics of folding using sandbox experiments in order to provide guidelines to interpret uplift rates revealed by alluvial or fluvial terraces deformed by fault propagation folds. The experiment consists of sand layers intercalated with low friction glass beads layers over an horizontal rigid basement, pushed by a moving backstop. The total thickness, h, is 80 pixels (1m). The basal friction angle is estimated to 20°. Deformation along the cross section is monitored from a video system. The displacement field between two images is measured from the optical flow technique. This set-up allows describing quantitatively the development of a nascent fold formed at front of a propagating décollement. The experiment shows that in the early stage, when cumulative shortening is less than 2h/3, slip along the décollement tapers gradually to zero and the displacement gradient is absorbed by distributed deformation of the overlying medium. In this stage, horizontal displacements decrease linearly with distance. Vertical displacements reflect a nearly symmetrical mode of folding, with displacements varying linearly between relatively well defined hinges. When the cumulative slip on the décollement exceeds about 2h/3 deformation tends to become localized on a few discrete shear bands. When deformation exceeds h deformation is fully localized on a single frontal ramp reaching the surface. At this stage the fault geometry does not evolve any more and the hanging wall deforms by simple shear as it overthrusts the flat-ramp system. The ratio between maximum uplift rate and horizontal shortening rate increases until deformation gets fully localized, and then drops to a constant value imposed by the fault geometry. As long as strain localization is not fully established the sand layers experience a combination of thickening and horizontal shortening which induce gradual rotation of the backlimb and forelimb of the growing fold. During that stage the relationship between uplift rate and shortening rate depends on cumulative deformation. We propose an analytical expression that reproduces this behavior. The kinematics observed in these experiments might be used to relate finite deformation and incremental folding in the case of fault-propagation folds and hence derive shortening rate from deformed recent markers.