Slab Stagnation and Buckling in the Mantle Transition Zone: Petrology, Rheology, and the Geodynamics of Trench Migration

Abstract:

Recent work indicates that subducting slabs may exhibit buckling instabilities and consequent folding behavior in the mantle transition zone for various dynamical parameters, accompanied by temporal variations in dip angle, plate velocity, and trench retreat. Governing parameters include both viscous (rheological) and buoyancy (thermo-petrological) forces. 2D numerical experiments show that many parameter sets lead to slab deflection at the base of the transition zone, typically accompanied by quasi-periodic oscillations in largely anticorrelated plate and rollback velocities, resulting in undulating stagnant slabs as buckle folds accumulate subhorizontally atop the lower mantle. Slab petrology, of mantle phase transitions and hydrated crust, is a dominant factor in this process (Čížková and Bina, 2013).

For terrestrial parameter sets, trench retreat is found to be nearly ubiquitous and trench advance quite rare, largely due to rheological and ridge-push effects. Recently updated analyses of global plate motions indicate that significant trench advance is also rare on Earth, being largely restricted to the Izu-Bonin arc (Matthews et al., 2013). Thus, we explore conditions necessary for terrestrial trench advance through dynamical models involving the unusual geometry of the Philippine Sea region. Our 2D modeling of such geometries, in which distal subduction of the overriding plate overprints an opposed slab-pull force on the usual ridge-push at the trench, yields persistent trench advance interrupted by episodes of back-arc extension, demonstrating that trench advance can occur for terrestrial rheologies in such special geometries (Čížková and Bina, 2015).