Ab initio two-phase molecular dynamics simulations were performed on silica at pressures of 20-160 GPa and temperatures of 2 500-6 000 K to examine its solid-liquid phase boundary.Results indicate a melting temperature (T m ) of 5 900 K at 135 GPa. This is 1 100 K higher than the temperature considered for the core-mantle boundary (CMB) of about 3 800 K. The calculated melting temperature is fairly consistent with classical MD (molecular dynamics) simulations. For liquid silica, the O-O coordination number is found to be 12 along the T m and is almost unchanged with increasing pressure. The self-diffusion coefficients of O and Si atoms are determined to be 1.3×10 -9 -3.3×10 -9 m 2 /s, and the viscosity is 0.02-0.03 Pa·s along the T m . We find that these transport properties depend less on pressure in the wide range up of more than 135 GPa. The eutectic temperatures in the MgO-SiO 2 systems were evaluated and found to be 700 K higher than the CMB temperature, though they would decrease considerably in more realistic mantle compositions.

Waveform modeling and travel times analyses of S, ScS and SKS phases recorded at the broad‐band permanent station SYO in the Antarctic are used to determine the shear wave velocity structure and transverse isotropy in the D″ layer beneath the Antarctic Ocean. The SH wave structure has a discontinuity with the velocity increase of 2.0% at 2550 km. The SV structure is similar to PREM model. The magnitude of the anisotropy is highest at the top of D″ layer and lowest at the core‐mantle boundary. The D″ layer beneath the Antarctic Ocean is significantly thicker than those beneath Alaska and the Caribbean Sea. We attribute this anisotropic D″ layer to paleo‐slab materials. The subduction in and around the Antarctic Ocean has started ∼180 Ma and is the one of the oldest in the world. It has provided a large amount of the slab materials in the lowermost mantle.

Investigations of SKS wave splitting of teleseismic events from digital seismographs recorded at eight stations around the Lützow-Holm Bay Region have lead to understanding the evolution of the Antarctic Plate. The observed delay times of SKS splitting are up to 1.3 s, which are generally equal to the global average. A two-layer model reveals that the lower layer anisotropy is caused by the recent asthenospheric flow, as compared with the Absolute Plate Motion by the HS3-NUVEL1 model. The upper layer anisotropy corresponds well to polarization of NE-SW convergence direction between East and West Gondwana in Pan-African age. We suggest that the upper layer anisotropy was formed during Pan-African orogeny and was possibly influenced by the preexisting structure during Gondwana break-up. The origin of anisotropy is the Lattice Preferred Orientation of olivine which was caused by both paleo-tectonic events and the recent asthenospheric flow.Citation: Usui, Y., M. Kanao, A. Kubo, Y. Hiramatsu, and H. Negishi (2007), Upper mantle anisotropy from teleseismic SKS splitting beneath

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Broadband seismic deployments have been carried out in the Lützow-Holm Bay region (LHB), Dronning Maud Land, East Antarctica. The recorded teleseismic and local events are of sufficient quality to image the structure and dynamics of the crust and mantle of the terrain. Passive seismic studies by receiver functions and shear wave splitting suggest a heterogeneous upper mantle. Depth variations in topography for upper mantle discontinuities were derived from long period receiver function, indicating a shallow depth discontinuity at 660 km beneath the continental area of LHB. These results provide evidence of paleo upwelling of the mantle plume associated with Gondwana break-up. SKS splitting analysis anticipated a relationship between “fossil” anisotropy in lithospheric mantle and past tectonics. Moreover, active source surveys (DSSs) imaged lithospheric mantle reflections involving regional tectonic stress during Pan-African and succeeding extension regime at the break-up. By combining the active and passive source studies of the mantle structure, we propose an evolution model of LHB for constructing the present mantle structure.