Anomalous Subduction Initiation: Young under Old Oceanic Lithosphere
Author (s): Zhang Lulu, S. Zlotnik, Chun-Feng Li
Journal: Geochemistry, Geophysics, Geosystems
When subduction initiates along a transform fault, it is thought that the older and therefore denser oceanic lithosphere should evolve into the downgoing plate if density anomaly dominates. However, in the western Pacific, several geological cases are likely characterized by underthrusting of young oceanic plates beneath old ones, and how they formed remains unknown. We perform a 2D thermomechanical numerical modeling to explore such subduction initiation along a transform fault under compression. The results suggest that subduction polarity depends mainly on age contrast between the two plates. A smaller age offset across the plate interface favors the subduction initiation of the younger plate. Furthermore, weaker transform faults or contacts with adjacent plates magnify the effect of the buoyancy forces on subduction, and tend to trigger the underthrusting of the older and denser plate. Plastic deformation is prone to localize within the thinner and weaker plates in a location away from the transition zone of the two plates, if the rate of strain weakening of plates is fast enough. Subduction of the younger plate at intra-oceanic settings would occur along a strong transformational boundary under compression, when the age offset of the two plates is small (< 30 Ma). Our numerical models explore factors controlling the induced subduction polarities and the potential mechanism of young plate subduction, and explain certain controversial subduction geological settings.