[S-IT18] GEOPHYSICAL PROPERTIES AND TRANSPORT PROCESSES IN THE DEEP CRUST AND MANTLE

[SIT18-02] Big mantle wedgeの3次元構造、異方性とダイナミクス

*趙 大鵬¹、Jia Ruo¹ (1.東北大学大学院理学研究科附属地震・噴火予知研究観測センター)

キーワード：マントルダイナミクス、地震波異方性、プレート内部火山

A big mantle wedge (BMW) has formed in the upper mantle (the Moho to 410 km depth) and the upper part of the mantle transition zone (MTZ; 410-660 km depths) above the subducting Pacific slab that is stagnant in the lower part of the MTZ beneath East Asia (Zhao et al., 2004, 2007; Lei & Zhao, 2005). Although many previous studies have been made to investigate the BMW (see Zhao, 2021 for a recent review), its detailed structure and dynamics are still not very clear. In this work, we determine robust 3-D P-wave anisotropic tomography of the crust and upper mantle beneath NE China using high-quality travel-time data of local earthquakes and teleseismic events recorded at 334 network and portable stations. In the upper crust, nearly E-W fast-velocity directions (FVDs) of azimuthal anisotropy are revealed in the central Songliao basin, which is surrounded by circular-shaped FVDs along the basin edges. The E-W FVDs may reflect micro-cracks or fractures in the upper crust, which are aligned under the control of regional tectonic stress. In the lower crust, low-velocity (low-V) anomalies with NE-SW FVDs exist along the Tanlu fault zone, which may reflect NE-SW trending ductile deformation or viscous flow along the fault zone (Jia et al., 2022). The FVDs are mainly NNW-SSE to N-S in the uppermost mantle beneath most of the study region, which may reflect fossil deformation of the mantle lithosphere caused by the Paleo-Pacific plate subduction. High-velocity anomalies with NE-SW FVDs are revealed at ~100 km depth directly beneath the Great XingAn Mountain, which may reflect an old lithospheric remnant. In the deep upper mantle, the FVDs are mostly NW-SE, consistent with SKS splitting measurements, which may be closely related to the stagnancy and retreating of the Pacific slab beneath East Asia. Under the Changbai and other six intraplate volcanoes in and around NE China, prominent low-V anomalies exist in the BMW above the slab in the MTZ, suggesting that these intraplate volcanoes are caused by hot and wet upwelling flows in the BMW closely associated with the deep subduction and dehydration of the western Pacific plate (Zhao et al., 2009; Zhao & Tian, 2013).

References

Jia, R., D. Zhao, J. Wu (2022). P-wave anisotropic tomography of NE China: insight into lithospheric deformation, mantle dynamics and intraplate volcanism. Geophys. J. Int. 229, 1372-1391.
Lei, J., D. Zhao (2005). P-wave tomography and origin of the Changbai intraplate volcano in Northeast Asia. Tectonophysics 397, 281-295.
Zhao, D. (2021). Seismic imaging of Northwest Pacific and East Asia: New insight into volcanism, seismogenesis and geodynamics. Earth Sci. Rev. 214, 103507.
Zhao, D., Y. Tian (2013). Changbai intraplate volcanism and deep earthquakes in East Asia: a possible link? Geophy. J. Int. 195, 706-724.
Zhao, D., J. Lei, Y. Tang (2004). Origin of the Changbai volcano in northeast China: Evidence from seismic tomography. Chinese Sci. Bull. 49, 1401-1408.
Zhao, D., S. Maruyama, S. Omori (2007). Mantle dynamics of western Pacific to East Asia: New insight from seismic tomography and mineral physics. Gondwana Res. 11, 120-131.
Zhao, D., Y. Tian, J. Lei, L. Liu, S. Zheng (2009). Seismic image and origin of the Changbai intraplate volcano in East Asia: Role of big mantle wedge above the stagnant Pacific slab. Phys. Earth Planet. Inter. 173, 197-206.