Buckling instabilities of subducted lithosphere beneath the transition zone

A sheet of viscous fluid poured onto a surface buckles periodically to generate a pile of regular folds. Recent tomographic images beneath subduction zones, together with quantitative fluid mechanical scaling laws, suggest that a similar instability can occur when slabs of subducted oceanic lithosphere encounter enhanced resistance to penetration at 660 km depth. Beneath subduction zones such as Central America and Java, the fast anomalies in elastic wavespeed are too broad (≥ 400 km) to be explained by simple compressional thickening of subducted lithosphere that is initially 50-100 km thick. By contrast, a periodic buckling mechanism can account for both the characteristic 'wedge' shapes of the anomalies and for their widths, which agree closely with those predicted by the scaling laws. In addition to providing a plausible physical interpretation of tomographic observations beneath some subduction zones, buckling instabilities have important consequences for other aspects of global geodynamics such as tectonic plate reconstructions, interpretation of geoid anomalies, and modeling of secular changes in Earth's moment of inertia.