Discontinuity structure of the mantle transition zone beneath the North China Craton from receiver function migration

A better understanding of the significant Phanerozoic tectonic reactivation and destruction of the North China Craton (NCC) demands a detailed knowledge of the deep structural features of the region. We applied the wave equation-based poststack migration technique to a combined receiver function data set from more than 250 broadband seismic stations to construct the structural image of the mantle transition zone beneath the NCC. Our imaging results reveal a relatively simple and flat 410-km discontinuity but a structurally complicated 660-km discontinuity beneath the region. Double discontinuities and a similar to 30-km depression of the 660-km discontinuity are observed locally in the southern part of the eastern NCC, in contrast to the smoothly varying structure to the north and in the central and western parts of the craton. Distinctly rapid variations in both the 660-km discontinuity structure and mantle transition zone thickness were found across the north-south gravity lineament (NSGL) near the boundary between the eastern and central NCC, which probably reflects different thermal and probably chemical properties on the two sides of the NSGL. These differences are possibly associated with the Pacific slab, which is imaged tomographically as a flat-lying structure in the mantle transition zone under the region east of the NSGL. The structural variation in the deep upper mantle appears to coincide with the sudden changes in surface topography, gravity field, and crustal and lithospheric structures as well, indicating that the two domains may have tectonically deformed differently throughout the whole upper mantle during the Phanerozoic cratonic destruction. The mantle transition zone on the eastern side of the NSGL is up to 30 similar to 40 km thicker than the global average; this thickness and the complex structure of the 660-km discontinuity in this region may reflect the strong influence that the deep subduction and stagnancy of the Pacific slab, and its possible sporadic penetration into the lower mantle, have had on mantle dynamics and lithospheric reactivation in the eastern NCC since the Mesozoic time. On the other hand, the less variable structure and normal-to-thin mantle transition zone imaged beneath the central and western NCC may indicate that the India-Eurasia collision has had a relatively weak effect on the Cenozoic tectonics of these regions.