SLAB STAGNATION AND BUCKLING IN THE MANTLE TRANSITION ZONE: PETROLOGY, RHEOLOGY, TRENCH MIGRATION, AND SEISMIC STRUCTURE

Early work by Wood and Yuen (1983) showed the importance of petrology to lithospheric dynamics. 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 (Bina and Kawakatsu, 2010; Čížková and Bina, 2013).

For terrestrial parameter sets, trench retreat is found to be nearly ubiquitous and trench advance quite rare – due to rheological and ridge-push effects (Čížková and Bina, 2013). 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.

Buckled stagnant slabs are difficult to image due to smoothing effects inherent in seismic tomography, but velocity structures computed for petrologically layered slabs, spatially low-pass filtered for comparison with tomography of corresponding resolution, yield a better fit for undulating than flat-lying slabs when compared to P-wave velocity anomalies from stagnant slab material beneath northeast China (Zhang et al., 2013). Earthquake hypocentral distributions and focal mechanisms may provide clearer insights into slab buckling, as they vary across regions of slab stagnation (Fukao and Obayashi, 2013; Fukao et al., 2014). Stress fields computed from our dynamical models may help to further illuminate such observations.