Load and unload as interference factors on cyclical behavior and kinematics of Coulomb wedges: Insights from sandbox experiments

Modes of accretion in orogenic wedges are strongly controlled by mass balance and the efficiency of basal detachment. Sandbox models of accretionary wedges have demonstrated that fault systems grow episodically via cycles of alternating wedge thickening and lengthening. Generally, a new thrust plane generates within the footwall of the previous one, following a piggy-back mode of accretion, whereas redistribution of the mass balance in the wedge is associated to underthrusting and the reactivation of previous thrusts.We present the results of five sandbox experiments that model the interaction between tectonic accretion, sedimentation in foredeep area and erosion in the axial zone, in order to analyze the influence of these events on the cyclical behavior of a growing wedge. Modifications of the initial setup were made to analyze the effect on wedge development of syntectonic denudation, syntectonic sedimentation and coeval sedimentation and erosion, which markedly altered mass transfer within the wedge. In particular, lowering the surface slope by syntectonic erosion triggered synchronous accretion and underthrusting modes; by contrast, a sudden syntectonic sediment load in the prowedge region promoted prolonged phases of underthrusting, retarding the accretion of new imbricates at the prowedge toe, whether wedge volume was increased or not.The high length/thickness ratio of the model (120 cm/2.5 cm) allowed us to monitor a complete cycle of recovery of the system after each episode of sedimentation and/or erosion, observing how the wedge reacted to modification both altering its own cyclicity and progressively recovering it.