Tearing of stagnant slab.

Subducted slabs of oceanic lithosphere below the western Pacific tend to be stagnant in the transition zone with poorly known mechanical properties. Typical examples are the Izu-Bonin and Japan slabs that meet each other to form a cusplike junction beneath southwest Japan. Here, we show that these two slabs are torn apart at their junction when they bend to flatten over the 660-kilometer discontinuity, as is expected from a simple geometric argument. We present three lines of evidence for this ongoing slab tear.