Varying mechanical coupling along the Andean margin: Implications for trench curvature, shortening and topography

""Convergent margins often exhibit spatial and temporal correlations between trench curvature, overriding plate shortening and topography uplift that provide insights into the dynamics of subduction. The Andean system, where the Nazca plate plunges beneath continental South America, is commonly regarded as the archetype of this class of tectonics systems. There is distinctive evidence that the degree of mechanical coupling between converging plates, i.e. the amount of resistive force mutually transmitted in the direction opposite to their motions, may be at the present-day significantly higher along the central Andean margin compared to the northern and southern limbs. However quantitative estimates of such resistance are still missing and would be desirable. Here we present laboratory models of subduction performed to investigate quantitatively how strong lateral coupling variations need to be to result in trench curvature, tectonic shortening and distribution of topography comparable to estimates from the Andean margin. The analogue of a two-layers Newtonian lithosphere\\\/upper mantle system is established in a silicone putty\\\/glucose syrup tank-model where lateral coupling variations along the interface between subducting and overriding plates are pre-imposed. Despite the simplicity of our setup, we estimate that coupling in the central margin as large as 20% of the driving force is sufficient to significantly inhibit the ability of the experimental overriding plate to slide above the subducting one. As a consequence, the central margin deforms and shortens more than elsewhere while the trench remains stationary, as opposed to the advancing lateral limbs. This causes the margin to evolve into a peculiar shape similar to the present-day trench of the Andean system. (C) 2011 Elsevier B.V. All rights reserved.""