Multiple Phase Changes in the Mantle Transition Zone Beneath Northeast Asia: Constraints From Teleseismic Reflected and Converted Body Waves
Résumé
We reassess the mantle transition zone structure below the northeast Asia margin in the context of subduction of the Pacific plate below the Eurasian continent. We use two independent approaches of teleseismic imaging, namely, compressional-to-shear converted waves (receiver functions) and shear wave underside reflections (SS precursors), and compare them within their statistical uncertainties. We find localized complexity in the interfaces marking solid phase changes in mantle minerals, in terms of both apparent topography and reflectivity. The 660-km discontinuity is doubled, with approximate to 80-km maximum vertical distance between the interfaces, over an 890 x 350 km(2) region between 36-44 degrees N and 130-133 degrees E at the tip of the subducted Pacific plate. A similar complexity exists on the 410-km discontinuity, coinciding with the presence of a deep cluster of seismicity below the Japan Sea. Both methods suggest the presence of low-velocity zones atop the 410, within the mantle transition zone, and below the 660. This complex seismic signature is related to the Pacific plate and interpreted in light of the subduction thermal regime and phase equilibria for a pyrolitic mantle composition. Phase changes manifest themselves as broad zones of velocity gradients with localized doubled or multiple first-order discontinuities, associated with transitions in the olivine, pyroxene, and garnet systems. An average pyrolitic composition and local temperatures of 1000-1300K can explain the observed velocity gradients and multiple discontinuities. We show that the dissolution of stishovite, a high-pressure polymorph of SiO2, into the higher-pressure perovskite mineral, is a possible explanation for the low-velocity zones at the top of the lower mantle. Plain Language Summary In the early 1990s, the idea emerged that the study of seismic discontinuities near 410- and 660-km depths could inform us about the vertical circulation of material in the mantle. These two discontinuities were assigned to solid phase changes of a silicate mineral rich in magnesium (90%) and iron (10%), olivine. Advances in the field of mineralogy and high-pressure physics suggested, however, that other minerals, garnet and pyroxenes, should contribute to the general seismic signature. Due to the limited imaging capability of seismological methods and station deployments, however, a one-to-one correspondence between observed seismic structure and mineralogy has remained elusive, at best controversial. We provide here from two independent approaches compelling evidence for localized complexity in the seismic structure near 410- and 660-km depths, which we relate, through mineralogical modeling, to phase changes in olivine, garnet, pyroxene, and silica systems. This work provides the observational basis for assessing the viability of geodynamic and thermochemical models of the mantle beneath the northeast Asia margin.
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