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