3D velocity structure of upper mantle beneath South China and its tectonic implications : evidence from finite frequency seismic tomography

We present three-dimensional high-resolution V-p velocity model of the upper mantle beneath South China from finite-frequency tomography using seismic data recorded at 155 portable seismic stations in the region. The model shows a clear low-velocity anomaly in the upper mantle of most parts of the Cathaysia Block and a high-velocity anomaly in the upper mantle beneath the Middle Yangtze Craton, which are separated by the Jiangnan orogenic belt. Both these velocity anomalies can be traced down to the upper mantle transition zone. An important result is that an apparent upwelling (with a strong slow velocity anomaly) through the 410-km interface is observed in an area centered at latitude of 27 degrees north and longitude of 118 degrees east. This upwelling gradually expands westward and northward as it rises, resulting in a large-scale low-velocity anomaly at the depths of 200 similar to 400 km in the Cathaysia Block. This large-scale of low-velocity anomaly may provide a mantle source for the Cenozoic volcanism that was widely distributed in the Cathaysia Block. More importantly, the upwelling beneath the Cathaysia Block extends northward, beyond the Jiangnan orogenic belt, then into the southern part of the Yangtze Craton lithosphere. It caused the illusion that the Yangtze Craton's thicker lithosphere tilted southward. Finally, the eastern part of the Yangtze Craton lithosphere, east of longitude of 117 degrees east ( the southern end of the Tanlu fault zone) , has been "re-activated", replaced by the hot asthenosphere from the south. We speculate that the low velocity anomaly in the upper mantle transition zone below the Cathaysia Block may be related to the Hainan mantle plume. The relationship between the Hainan mantle plume and the (27 degrees N, 118 degrees E) upwelling zone at the 410-kilometer interface remains to be verified in the future by larger-scale seismic array studies.