P-wave anisotropic tomography of NE China: insight into lithospheric deformation, mantle dynamics and intraplate volcanism

Abstract

SUMMARY We determine robust 3-D P-wave anisotropic tomography of the crust and upper mantle beneath NE China using high-quality traveltime 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 microcracks 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. 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 Palaeo-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 intermittently retreating stagnant Pacific slab beneath East Asia. Under the Changbai and other intraplate volcanoes in NE China, prominent low-V anomalies exist in the big mantle wedge (BMW) above the slab in the mantle transition zone, suggesting that these 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.