Seismic Analysis of the Tonga Subduction Zone and Implications on the Thermo-Petrologic Evolution of Deep Subduction

Utilizing high-resolution waveform analysis of triplicate P and S waves recorded by a broadband seismic array, we show that the subhorizontal, leading-edge (“toe”) of the Wadati-Benioff zone (WBZ) in the Tonga subduction zone exhibits characteristics of petrologic and petrofabric anomaly. Based on seismicity alone, the “toe” clearly connects with the rest of the WBZ and extends westward by about 300 km. The source region of earthquakes exhibits ~1% polarization anisotropy, a phenomenon rarely observed in the mantle transition zone, indicating the existence of a localized petrofabric. High P and S wave speeds (~3%) are expected to accompany the low temperature in the source region of seismicity, yet velocities are low within the anisotropic zone of earthquakes itself, requiring a petrologic anomaly that counteracts the effect of low temperature. In addition, sporadic fault plane solutions from earthquakes defining the toe show no clear pattern, so a localized stress must be responsible for the deep seismicity. Each component in this unusual collection of observations has also been documented for a zone of so-called outboard earthquakes - a remarkable swath of subhorizontal seismicity that extends over 1,000 km further to the west of the toe. Given the fact that the toe is still connected with the WBZ or the trajectory of active-subducting slab, there seems little doubt that shared properties between the toe and the outboard earthquakes reflect consecutive stages in deep subduction, with the toe being the immediate predecessor of a large-scale remnant of detached lithosphere. As documented in previous work, the only candidate for a petrologic and petrofabric anomaly that satisfies all available observations is metastable olivine - a buoyant material when present in the transition zone, promoting slab deflection near the 660-km discontinuity and acting as a barrier to deep-slab penetration. Over time, a positively-buoyant, anisotropic thermo-petrologic anomaly would evolve into a negatively-buoyant, pure thermal anomaly. As the petrologic anomaly dissipates with rising temperature over time, so does its anisotropy and seismicity - a prediction borne out by observations of high wave speed, isotropic slab remnants in the aureole surrounding the outboard earthquakes in Tonga, and in many aseismic slab anomalies elsewhere in the mantle transition zone of the western Pacific.