Anisotropic Seismic Tomography Study Of Crust And Upper Mantle Structure

Seismic anisotropy is an indicator of stress state and geodynamics of the Earth’s interior. Seismic tomography is a powerful method to map out the structural heterogeneities in Earth. P-wave anisotropic tomography can determine the anisotropies and the structural heterogeneities at the same time. Central Japan and northeastern margin of Tibetan Plateau were studied by using P-wave anisotropic tomography method and the results can be summarized as follows:Beneath central Japan, lots of low-velocity (low-V) and high Poisson’s ratio anomalies are visible beneath the arc volcanoes and within the mantel wedge and most of them origin from the subducting Pacific slab (PAC) and Philippine Sea slab (PHS). The partial melting of the upper mantle initialed by the dehydration of the underlain slab during subduction may be responsible for it. Anisotropy distribute widely beneath central Japan except for the central portion of the mantle wedge. The anisotropic mantle layer may exist on the top and the bottom of the mantle wedge. Partial melts and the domination of diffusion creep deformation which both weaken the olivine lattice-preferred orientation (LPO) may be responsible for the relatively isotropic central portion of mantle wedge. The fast-velocity directions (FVDs) of the mantle wedge beneath central Japan are generally trench-parallel in the fore-arc area and trench-normal in the back-arc area, which can be explained by the LPO of olivine changing from B-type under the fore-arc area to A-type under the back-arc area with the variation of water content and the occurrence of mantle flow. The subducting PHS slab beneath Tokai is revealed as a high-velocity (high-V) anomaly descending gently with relatively larger anisotropic amplitudes than the surroundings and with NEE to NE-SW FVDs which are consistent with the spreading direction of the PHS during its formation. The NEE to NE-SW FVDs of the PHS are inferred as the fossil anisotropy of the PHS. The subduction of the PHS beneath Tokai may have no influences on the anisotropy kept in the PHS slab. The anisotropy of the upper portion of the Pacific slab is revealed in this study. The FVDs of Japan slab and Izu-Bonin slab are sub-parallel to the Japan Trench and Izu-Bonin Trench respectively from～60to120km and change to trench-normal below100-120km. After comparisons with the Mesozoic magnetic anomaly lineations and the principle stresses in the Pacific slab, we propose that the fossil FVDs of the Pacific slab beneath central Japan were already rebuilt by the principle stresses in the slab.Obvious lateral velocity heterogeneities and widely distributed anisotropies manifest in the crust of northeastern margin of Tibetan Plateau. Along with the depth increasing, the anisotropic magnitude increases. Most of FVDs are sub-parallel to the strikes of tectonic lines (e.g., faults and mountains), which indicates the shape-preferred orientations (SPOs) are responsible for the anisotropies. Beneath Longmenshan sub-block, a thin high-V anomaly with NW-SE FVDs which is the same with FVDs of Songpan-Ganzi block has thrust into the west margin of Sichuan Basin block with low angles and has reached the lower crust at about25km depth beneath Longquanshan fault. It forms a thrust structure from Songpan-Ganzi block to Sichuan Basin block in crustal scale. The low-V anomaly nearby108°E with N-S FVDs corresponds to the Foping reverse thrust tectonic granite belt of Qinling orogen.The cylindrical low-V anomaly with variant FVDs manifests beneath the Wudang terrain and adjacent Huangling anticline and Shennongjia Group below25km depth may be attributed to the dome structure of Wudang terrain. Significant low-V anomalies with NE-SW FVDs are visible at lower crust to uppermost mantle depth beneath trans-North China orogen. Low-V anomaly may be related to the upwelling of asthenospheric material initiated by the delamination of the lithosphere of North China Craton. The NE-SW FVDs share the same orientation with regional normal faults, which indicates the SPOs are still the main reasons for the anisotropies beneath trans-North China orogen.The anisotropic tomography studies of central Japan and northeastern margin of Tibetan Plateau also indicate that with sufficient data the anisotropic tomography method is an effective way to study structures and anisotropies of crust and upper mantle.