THE LITHOSPHERIC ELECTRICAL STRUCTURE OF JI'AN-FUZHOU PROFILE IN THE EAST PART OF SOUTH CHINA

As having experienced multi-stage tectonic magmatic activity, the structure of the lithosphere in the east part of South China is very complicated and the distribution of magma has obvious regularity. In order to study the deep tectonic background of magmatic activity of the different blocks in the east part of South China, this paper made a series of qualitative and quantitative analysis based on the Ji'an-Fuzhou magnetotelluric sounding profile data crossing the east part of South China. The subsurface dimensionality was analyzed by the Bahr phase decomposition, the geoelectric strike with different frequencies was obtained by the single-site multifrequency Groom-Bailey decomposition. Finally, the non-linear conjugate gradients (NLCG) was used to calculate the 2D resistivity structure in our research area.

The electrical structure model shows that there are significant differences between the two blocks-Wuyi uplift belt and Southeast coastal fold belt. It can be vertically divided into four electrical layers of the high resistivity layer in upper crust, the low resistivity layer in mid-lower crust, the sub-high resistivity layer in the lithosphere mantle, the low resistivity in the asthenosphere. In the upper crust, the high resistivity layer of more than 10000 Ωm indicates the distribution of granite whose bottom interface is about 15∼20 km deep. In the mid-lower crust, the high-conductivity layer in the Wuyi uplift belt is thin and of small scale. It is associated with the thrust faults. However, in the Southeast coastal fold belt, the high-conductivity layer is thicker and of larger scale. It is uplifted in a mushroom-shape. The resistivity in lithosphere mantle gradually reduces from inland to coast. Due to the limited detecting depth, the lithosphere-asthenosphere boundary (LAB) doesn't show in the Wuyi uplift belt which indicates the depth of the LAB is more than 100 km. In the Southeast coastal fold belt, the thickness of the lithosphere is reduced to 60 km, and the asthenosphere has an uplift tendency.

In the east part of South China, there are a series of discontinuous high-conductivity layers of different scales in the crust. The scale and burial depth of the high-conductivity layers are closely related to the deep tectonic environment and fault distribution. Combining with gravity and magnetic results, we discussed the formation mechanism of high-conductivity layer. It is inferred that the high-conductivity layer in the crust of the Southeast coastal fold belt is the result of partial melting by asthenosphere upwelling and basaltic magma underplating. While the high-conductivity layer in the Wuyi uplift belt is the result of incomplete condensation of the magma chamber after crust material remelting in the early compression environment, and the continuous heating from the deep heat flow in an extensional environment.

The lithosphere structure in the east part of South China has a marked zoning, and the high-conductivity layers with different causes are widespread within the crust. It shows that the magmatic activities in different tectonic units in the east part of South China have different diagenetic tectonic background. In the Southeast coastal fold belt, the deep lithospheric thermal erosion is active, and the lithosphere structure and material are strongly remoulded. The asthenosphere upwelling and the basaltic magma underplating caused the thinning of lithosphere. In the Wuyi uplift belt, the tectonic features of the intra-continental deformation pattern during the Indo-early Yanshanian period are recorded clearly, while the late Mesozoic extension tectonics has transformed the lithosphere material to some degree.