The initiation of subduction by crustal extension at a continental margin

We investigate how subduction may be triggered by continental crust extension at a continental margin. The large topography contrast between continental and oceanic domains drives the spreading of continental crust over oceanic basement. Subduction requires the oceanic plate to get submerged in mantle, so that negative buoyancy forces may take over and drive further descent. This is promoted by two mechanisms. Loading by continental crust bends the oceanic plate downwards. Extension in the continental domain induces crustal thinning, which acts to raise mantle above the oceanic plate. In this model, the width of the continental region undergoing extension is an important control parameter. The main physical controls are illustrated by laboratory experiments and simple theory for elastic flexure coupled to viscous crustal spreading. Three governing dimensionless parameters are identified. One involves the poorly constrained oceanic plate buoyancy. We find that the oceanic plate can be thrust to depths larger than 40 km even if it is buoyant, enabling metamorphic reactions and density increase in the oceanic crust. Another parameter is the ratio between the width of the continental extension region and the flexural parameter for the oceanic plate. Initiating subduction is easier if the continent thins over a short lateral distance or if the oceanic plate is strong. The third important parameter is the ratio of oceanic plate thickness to initial continental crust thickness, such that a weak plate and a thick crust do not favour subduction. Thus, the change from a passive to an active margin depends on the local characteristics of the continental crust and is not determined solely by the age and properties of the oceanic lithosphere. It is shown that the spreading of continental crust induces uplift of the margin as the adjacent seafloor subsides. Evidence for the emplacement of continental crust over oceanic basement at passive margins is reviewed.