Pervasive Seismic Wave Reflectivity and Metasomatism of the Tonga Mantle Wedge

Mantle wedge diapirs detected by a dense seismic array in Northern Taiwan

It is conventionally believed that magma generation beneath the volcanic arc is triggered by the infiltration of fluids or melts derived from the subducted slab. However, recently geochemical analyses argue the arc magma may be formed by mélange diapirs that are physically mixed by sediment, altered oceanic crust, fluids, and mantle above the subducted slab. Further numerical modeling predicts that the mantle wedge diapirs have significant seismic velocity anomalies, even though these have not been observed yet. Here we show that unambiguously later P-waves scattered from some obstacles in the mantle wedge are well recorded at a dense seismic array (Formosa Array) in northern Taiwan. It is the first detection of seismic scattering obstacles in the mantle wedge. Although the exact shape and size of the scattered obstacles are not well constrained by the arrival-times of the later P-waves, the first order approximation of several spheres with radius of ~ 1 km provides a plausible interpretation. Since these obstacles were located just beneath the magma reservoirs around depths between 60 and 95 km, we conclude they may be mantle wedge diapirs that are likely associated with magma generation beneath active volcanoes.

Tomography of the subducting Pacific slab and the 2015 Bonin deepest earthquake (Mw 7.9)

On 30 May 2015 an isolated deep earthquake (~670 km, Mw 7.9) occurred to the west of the Bonin Islands. To clarify its causal mechanism and its relationship to the subducting Pacific slab, we determined a detailed P-wave tomography of the deep earthquake source zone using a large number of arrival-time data. Our results show that this large deep event occurred within the subducting Pacific slab which is penetrating into the lower mantle. In the Izu-Bonin region, the Pacific slab is split at ~28° north latitude, i.e., slightly north of the 2015 deep event hypocenter. In the north the slab becomes stagnant in the mantle transition zone, whereas in the south the slab is directly penetrating into the lower mantle. This deep earthquake was caused by joint effects of several factors, including the Pacific slab’s fast deep subduction, slab tearing, slab thermal variation, stress changes and phase transformations in the slab, and complex interactions between the slab and the ambient mantle.

Upper mantle discontinuity topography from thermal and chemical heterogeneity

Using high-resolution stacks of precursors to the seismic phase SS, we investigated seismic discontinuities associated with mineralogical phase changes approximately 410 and 660 kilometers (km) deep within Earth beneath South America and the surrounding oceans. Detailed maps of phase boundary topography revealed deep 410- and 660-km discontinuities in the down-dip direction of subduction, inconsistent with purely isochemical olivine phase transformation in response to lowered temperatures. Mechanisms invoking chemical heterogeneity within the mantle transition zone were explored to explain this feature. In some regions, multiple reflections from the discontinuities were detected, consistent with partial melt near 410-km depth and/or additional phase changes near 660-km depth. Thus, the origin of upper mantle heterogeneity has both chemical and thermal contributions and is associated with deeply rooted tectonic processes.