Orogenic wedges are expressions of dynamic equilibrium between tectonic and gravitational stresses. Thus, surficial mass transport by climate-driven erosion changes the morphology of, and the state of stress within, the orogenic wedge. The orogenic wedge adjusts itself to the new stress conditions, inducing changes in the pre-existing morphology, which may modify the pre-existing drainage pattern, and therefore changes also the erosion rate. Furthermore, depending on the amount of vertical alteration in wedge topography the climate is modified, which may lead to a different denudation rate. The aim of this study is to describe the particle flow pattern in a bivergent sand wedge as a function of (i) the ratio between tectonic input and denudation, (ii) the ratio between pro- and retro-wedge denudation, (iii) the ratio between frontal and basal accretion and (iv) the 2D lateral distribution of denudation. Sifted sand with a friction-controlled elastic/plastic rheology with strain hardening and softening is used to simulate upper crustal rock deformation. The experiments were run in a 2D shearbox with two converging sand layers which build up a bivergent sand-wedge. A thin glass-bead layer which simulates a mid-level detachment was incorporated to allow frontal and basal accretion. Using a vacuum cleaner, incremental denudation, which decreases linearly from the top of the sand-orogen to the toe of either the pro-wedge or retro-wedge, was simulated. A 3D camera system - Particle Imaging Velocimetry - was used to obtain high resolution images of the particle flow, which permits an accurate calculation of the velocity field through time and space and the computation of the incremental and the bulk strain field. Results can be then compared with numerical models, e.g. from the European Alps and the Southern Alps (N.Z.). Preliminary results are: Denudation has a first-order control on the acceleration (facto 0.1 - 11) of the bulk and incremental particle-flow rate with respect to the convergence rate. Individual particle flow paths suggest a damped oscillation which is most prominent in the experiments with denudation. However, it remains to be answered, if this flow pattern is either regular or episodic. Also, denudation leads to vertically and horizontally diverging particle paths, and thus enhances the juxtaposition of formerly separated crustal units. In addition, particle flow path analysis concerning the influence of the mid-level detachment suggests: in the non denudation experiment, particles flow almost parallel, and in the pro-wedge (retro-wedge) denudation experiment, particles above (below) the mid- level detachment show generally higher displacements than particles from below (above). Together with structural analysis, these results suggest that denudation changes the rates of the mass-transfer processes, but not the general style of frontal and basal accretion. During pro- and retro-wedge denudation material is frontally- and basally-accreted, however, the area of maximum uplift shifts either towards the pro-wedge or retro-wedge side, respectively. For retro-wedge denudation, exhumation is very localized and has its maximum on the retro-wedge side of the central part of the orogen. During pro-wedge denudation, the exhumation profile is broadened and spans nearly the entire pro-wedge. Finally, the amount of uplift is ca.~2.5 times the thickness of the descending sand-layer in the pro-wedge denudation case, whereas it is ca.~3 times the thickness of the descending sand-layer in the retro-wedge denudation case. According to these results retro-wedge denudation is more effective at exhuming material than pro-wedge denudation.
(2002): Climate control on particle flow patterns in bivergent orogenic wedges: Insights from scaled sandbox models
. Western Pacific Geophysics Meeting (Wellington, New Zealand 2002)