Topography of the 660-km seismic discontinuity beneath Izu-Bonin: Implications for tectonic history and slab deformation

We analyze the P wave codas of 65 paths from deep northwestern Pacific earthquakes recorded by arrays of stations in Germany, the western United States, India, and Turkmenistan. We identify a phase resulting from a near-source S-to-P conversion at a nearly horizontal discontinuity ranging in depth from 650 to 730 km, which we interpret as a thermally depressed spinel to perovskite and magnesiowustite phase transition. We migrate these data along with 39 more from Wicks and Richards [1993], accounting for three-dimensional ray bending by the sloping discontinuity, to produce a high-resolution topography map of the 660-km discontinuity in the Izu-Bonin region. Assuming an equilibrium phase transition, we interpret the discontinuity depth in terms of local temperatures. The slab, if defined by a thermal anomaly greater than -400 degrees K, is only about 100 km thick near 28 degrees N suggesting the slab is penetrating into the lower mantle with little or no advective thickening. Farther to the north, however, cold material appears Spread out over a wide region, consistent with the slab having been laid down flat on the 660-km discontinuity as the trench retreated 2000 km eastward. Both the narrow slab to the south and the flat-lying slab to the north are consistent with recent high-resolution tomographic images. The depression to 745 km along the are is consistent with a maximum thermal anomaly of about 1100 degrees K. Along the entire are, the depression occurs directly beneath the deepest earthquakes, even where seismicity is dipping at 45 degrees and stops at 450 km depth, suggesting that the slab steepens to a vertical dip at the deepest seismicity. This change to a vertical orientation suggests that the slab loses strength temporarily through a physical process which causes the seismicity to increase dramatically and then abruptly cease.