Anomalous Subduction Initiation: Young Under Old Oceanic Lithosphere

When subduction initiates along a transform fault, it is often interpreted that the older and therefore denser oceanic lithosphere should evolve into the downgoing plate. However, in the western Pacific, several geological cases are probably characterized by young oceanic plates underthrusting older and denser plates. Using numerical simulations of a transform fault under compression, we propose that subduction polarity depends mainly on the age offset between the two adjacent plates, the strength of the transform fault, and the plasticity of the plates. A small age offset across the plate interface favors the subduction initiation of the younger plate. Furthermore, weak transform faults promote strain localization along the plate boundary and trigger the underthrusting of the older and denser plate. Plastic deformation is prone to localize within the thinner and weaker plates in a location far from the transition zone of the two plates if the rate of strain weakening of plates is fast enough. When plates have a large age offset the younger plate may still underthrust if plasticity plays a role. Our numerical models explore factors controlling the induced subduction polarities and the potential mechanism of young plate subduction and explain certain related subduction geological settings.