| Utilizing waveform analysis, we show the subhorizontal leading edge ("toe") of the Wadati-Benioff zone (WBZ) of the Tonga subduction zone displays petrologic and petrofabric anomalies. While it appears connected to the WBZ, sporadic fault plane solutions require a localized stress for the deep seismicity. High P-wave speeds (∼3%) are absent in the seismogenic toe, requiring a petrologic anomaly to counteract low temperature, and considerable SH-SV splitting implies an anisotropic petrofabric anomaly. The same observations were previously documented for shallower outboard earthquakes in the transition zone extending over 1,000 km further west. Hence, these shared properties represent stages in deep subduction, the toe preceding a large-scale detachment of lithosphere. Metastable olivine satisfies all available observations, accounts for buoyant lithosphere in the transition zone, discouraging slab penetration, and provides a mechanism for deep seismicity. Rising temperature leaves only a thermal anomaly, as seen south of the toe and elsewhere in the Pacific. |
