Subduction initiation: From the observational record to earth dynamics

Constraints from onshore and offshore show that ocean basins open and close. Subduction initiation is likely key during closure but understanding the kinematics and geodynamics has been difficult to constrain with no Mesozoic examples of subduction initiation at Atlantic-type margins. Consequently, our understanding of basin closure and subduction initiation has been informed by the records of ophiolites and Pacific Cenozoic evolution. Examples from the Pacific are especially important because we can link plate kinematics and antecedent tectonics to the nature of subduction initiation, including its causes and consequences. We review evidence for subduction initiation from four western Pacific examples: Izu-Bonin-Mariana (IBM), Tonga-Kermadec, Matthews-Hunter, and Puysegur subduction zones. Various factors may have influenced their initiation: pre-existing weak zones, plate compression, and jumps in plate/crust thicknesses either from composition or temperature (from pre-existing relic arcs and mantle plumes). Although IBM has often been argued as an example of spontaneous initiation at a fracture zone, interpretation of fore-arc volcanism suggests that a substantial vertical force must have been in operation to explain proximal plate extension. If so, this suggests that the fore-arc of IBM records subduction propagation, not initiation. Matthews-Hunter also points to a strong role for the existing main Vanuatu slab. Potentially significant, of these four Cenozoic examples three had a strong strike-slip component of motion during initiation (IBM, Matthews-Hunter, and Puysegur). Computations show that strike-slip motion enhances the propensity of a margin to initiate subduction. When a new subduction zone forms, there is likely a strong rearrangement in the force balance of plates, leading to changes in plate motion. Ultra-high resolution global models for plates at 50 Ma, show the role of the growing IBM slab on Pacific Plate motion, with IBM subduction propagation contributing only about 10-20% of the apparent direction change manifested in the Hawaiian-Emperor Seamount bend. This geological and geodynamic insight can be used to infer the likely causes and consequences of subduction initiation on closing ocean basins.