Lithospheric Structure Near Jiuyishan, South China: Implications for Asthenospheric Upwelling and Lithospheric Modification

In this study, a high-resolution 3D lithospheric S wave velocity model of the Jiuyishan region is constructed by joint inversion of receiver function and ambient noise. Our Moho depth model reveals relatively thick crust (similar to 35 km) beneath the Yangtze block and thin crust (similar to 27 km) beneath the Cathaysia block. The model depicts a complex boundary between these blocks, roughly following the Chenzhou-Linwu fault in the northern part but gradually curving westward in the southern part between the two blocks. The 3D S wave velocity model shows low-velocity anomalies in the middle crust, as well as in the upper mantle, interpreted as evidence of asthenospheric upwellings in the upper mantle. These upwellings modify lithosphere structures by changing their temperatures and compositions, and may provide a heat source through the Chenzhou-Linwu fault for the low-velocity anomaly in the shallow part beneath Jiuyishan region. Plain Language Summary The South China block, which is composed of the Yangtze and Cathaysia blocks, has undergone a series of intraplate orogenic episodes since the Neoproterozoic. Location of the southwestern boundary of the Yangtze and Cathaysia blocks has been ambiguous, and the local lithospheric structure is not well constrained. We construct a high-resolution 3D lithospheric shear velocity model, which provides the first evidence suggesting that the Jiuyishan region is undergoing lithospheric modification. Our model depicts a complex boundary between these blocks, roughly following the Chenzhou-Linwu fault in northern part but gradually curving westward in the southern part between the two blocks. Moreover, the shear wave velocity structure shows prominent low-velocity anomalies in the middle crust as well as in the upper mantle. We interpret the low anomalies in mantle as asthenospheric upwellings, which may provide a heat source for the shallow low-velocity anomaly.