Anisotropic Tomography and Mantle Dynamics of the North China Craton

We present high-resolution 3-D images of isotropic P-wave velocity (Vp), azimuthal anisotropy (AAN) and radial anisotropy (RAN) down to 700 km depth beneath the North China Craton (NCC) and adjacent areas, which are obtained by inverting a great number of high-quality arrival time data recorded at 1374 portable seismic stations and 635 permanent stations in the study region. Our results reveal new and detailed features of the upper mantle structure beneath the NCC. Varying structural heterogeneities are revealed beneath different tectonic blocks, and differences also exist between northern and southern parts of each block. The fast velocity directions (FVDs) of azimuthal anisotropy are mainly NW-SE under the Alaxa block, and NE-SW beneath the Tibetan Plateau. The FVDs present an arc transition along the boundary faults separating the Tibetan Plateau, the Alaxa block, the western NCC, and the Sichuan basin. Low-Vp anomalies with positive RANs (i.e. horizontal Vp > vertical Vp) are revealed at 100-200 km depths under the Tibetan Plateau, reflecting frozen-in anisotropy in the thick lithosphere. Significant low-Vp anomalies with a circular AAN pattern exist at 0-700 km depths beneath the Datong volcano. In addition, negative RAN occurs right below the volcano, whereas positive RANs appear around it, suggesting that the Datong volcano is fed by hot upwelling flow from the lower mantle associated with collapsing of subducted slab materials down to the lower mantle. The eastern NCC shows complex Vp AANs and RANs. Seismic anisotropy exhibits east-west variations in the upper mantle across the Tanlu fault zone. The west of the Tanlu fault shows negative RANs (vertical Vp > horizontal Vp), whereas its east shows positive RANs at 300-500 km depths. The low-Vp anomaly under the Datong volcano is connected with a large low-Vp anomaly beneath the eastern NCC above similar to 250 km depth, suggesting that the hot upwelling flow under Datong may migrate laterally to the asthenosphere under the eastern NCC and contribute to the lithospheric delamination and destruction there.