Andean Growth and the Deceleration of South American Subduction: Time Evolution of a Coupled Orogen-subduction System

Abstract

Present-day orography at the Andean margin is a result of isostasy, tectonic accretion, and erosional processes. The resulting excess mass of the Andes gives rise to frictional stresses on the seismogenic plate interface that resist the sinking of the subducting slab into the upper mantle. Thus, subduction rates should be sensitive to the time-dependent dynamics of a back-arc orogen, as well as erosional or accretional processes that affect orogen growth. Here we develop a two-dimensional coupled orogen–slab model that allows for the prediction of orogen size and plate motion in response to both tectonic and erosional forcing. We find that the frictional force exerted by the orogen on the subducting slab grows quadratically with orogen width and that the frictional resistance typically balances 10–50% of the slab pull force. The time evolution of the coupled orogen-subduction zone system is largely controlled by the rate of orogen growth, which is controlled by the rate of convergence and the erosivity of the climate state. In the case of the Andean margin, our models show that Miocene aridification leads to reduced erosion, increased orogen growth, greater frictional resistance to subduction, and, ultimately, to a ~50% reduction in the convergence rate between the Nazca and South American plates.