16:45 〜 17:00
[S16-1-02] ON THE NATURE OF LARGE ULTRA-LOW VELOCITY ZONES AT THE ROOT OF MAJOR HOTSPOT PLUMES
invited
Ultra-low-velocity-zones (ULVZs) are localized regions of extreme properties detected seismologically at the base of Earth's mantle, and have been found preferentially within or on the border of large low shear velocity provinces (LLSVPs). They may come in different shapes and forms. The nature and role of ULVZs in lower mantle dynamics is still poorly understood, in particular whether they are due to partial melting or the presence of solid state iron. Depending on assumptions made in geodynamic simulations, they may occur preferentially at the edges or in the middle of the LLSVPs.
We here discuss a particular class of very large ULVZ's (>800 km in lateral extent) that have been detected at the roots of broad plumes imaged in the lower mantle by seismic tomography, specifically Samoa (Thorne et al., 2013), Hawaii (Cottaar and Romanowicz, 2012) and most recently Iceland (Yuan and Romanowicz, 2017). In the case of Hawaii, a simple cylindrical ULVZ shape matched the observed shear diffracted (Sdiff) waveforms, but couldn't be uniquely constrained because of the limited illumination afforded by available data. The Iceland geometry is favorable for analyzing the effects of the ULVZ on Sdiff on paths that illuminate the root of the plume from different directions. Waveform modeling of data from 8 different earthquakes observed at arrays in north America and in China shows the presence of a large ULVZ, which, to a very good first approximation, has a regular, circular-shaped base. This shape suggests that this ULVZ is closely related to the dynamic upwelling at the base of the Iceland plume, and is therefore most likely due to partial melting of a denser and hotter than average core of the plume root, rather than to a solid state process. We also show that such structures are few and far apart, and may be characteristic of the dynamics of flow at the roots of the twenty-or-so broad mantle plumes imaged in the lower mantle.
We here discuss a particular class of very large ULVZ's (>800 km in lateral extent) that have been detected at the roots of broad plumes imaged in the lower mantle by seismic tomography, specifically Samoa (Thorne et al., 2013), Hawaii (Cottaar and Romanowicz, 2012) and most recently Iceland (Yuan and Romanowicz, 2017). In the case of Hawaii, a simple cylindrical ULVZ shape matched the observed shear diffracted (Sdiff) waveforms, but couldn't be uniquely constrained because of the limited illumination afforded by available data. The Iceland geometry is favorable for analyzing the effects of the ULVZ on Sdiff on paths that illuminate the root of the plume from different directions. Waveform modeling of data from 8 different earthquakes observed at arrays in north America and in China shows the presence of a large ULVZ, which, to a very good first approximation, has a regular, circular-shaped base. This shape suggests that this ULVZ is closely related to the dynamic upwelling at the base of the Iceland plume, and is therefore most likely due to partial melting of a denser and hotter than average core of the plume root, rather than to a solid state process. We also show that such structures are few and far apart, and may be characteristic of the dynamics of flow at the roots of the twenty-or-so broad mantle plumes imaged in the lower mantle.