Response of an orogenic wedge to climatic and tectonic perturbation

We investigate the feedbacks between tectonic and erosional forcing in convergent orogens by developing a numerical model of orogenic growth that couples a 2D cross-sectional model of crustal deformation with a 2D planform model of surface erosion. Tectonic deformation is calculated by a finite element model of deformation which responds to subduction-driven accretion of continental crust and to removal of material by surface processes. Erosion of surface topography occurs by fluvial incision within an emergent 2D channel network and by threshold landsliding. The tectonic and erosional systems are allowed to evolve in concert until the extent and pattern of erosion balances that of tectonic uplift, thereby reaching a large-scale topographic steady-state. We first examine orogenic growth with a base model that matches the general design of simple analog sandbox experiments. Growth of the orogenic wedge initiates as a pop-up structure centered above the point at which subduction occurs, continues with outward propagation of shear zones, and gradually ceases as the width and total relief of the wedge achieve steady values. At steady state, the mean elevation profile in the direction of convergence has a constant slope, consistent with the predictions of critical wedge theory. Short-wavelength topography, however, is continually advected across the orogen such that a point-by-point steady state is never achieved. Basin capture at both the outlet and divide is an important consequence of this lateral advection. Tectonic uplift within the wedge, as calculated in an Eulerian reference frame, is relatively uniform. We also examine the response of wedge width, erosional flux, and mean uplift rate to tectonic and climatic perturbations to the steady-state base model. Changes in accretionary flux and precipitation rate result in either growth or contraction of the wedge to a new steady-state. The response of a coupled wedge to a tectonic perturbation differs from its response to a climatic perturbation in two significant ways: i) permanent changes in tectonic flux lead to permanent changes in erosional flux, whereas changes in precipitation rate only temporarily influence the erosional flux, and ii) the mean uplift rate gradually achieves a new steady value in response to a change in precipitation rate whereas, for an increase in tectonic flux, the mean uplift rate increases rapidly and then asymptotically decreases to a new steady value. These results are potentially useful in differentiating between tectonic and climatic causes for changes in orogenic erosion rates.