Electrical Anisotropy of Quaternary continental volcanic fields revealed by 3D MT survey: examples form Datong and Tengchong volcanic fields

In volcanic fields, MT transfer function can be effected by the electrical anisotropy that is mostly attributed to partial melting associated with localized deformation. With development of identification, modeling and inversion method of the electromagnetic fields with anisotropic structures, re-examination of previous MT responses could provide more geodynamic information. Here we report two cases of volcanic fields in China with anisotropy analysis of 3D MT data: the monogenetic Datong volcanoes located in the Shanxi Rift Zone (40.0-40.1°N and 113.6-113.9°E ) with the major 0.3-0.4 Ma basalt, and the Tengchong Volcanoes near the Gaoligong Shear Zone (25.0-25.3°N and 98.4-98.6°E) with youngest 0.007-0.01 Ma basaltic andesites. As shown by isotropic 3D inversion result, a conductor arising from the upper mantle connects with mid-crustal conductive zones beneath Datong Volcanic fields, and three mid-crustal conductors are situated under the Tengchong Volcanic fields. However, the mid-crust of Datong that has alternating stripy conductive and resistive zones cannot rule out the possibility of microscopic anisotropic medium being interpreted inappropriately. For Tengchong Volcanic fields, the uniform NE direction of induction arrows at long period and large disagreement of inversion results by using different dataset (with or without tippers) cannot be satisfactorily explained by isotropic model. In the current study, 3-D anisotropic forward modeling and 2-D anisotropic inversion method are utilized to evaluate the electric anisotropy in these two regions. The preliminary result suggests that the complex electric structure of mid-crust in Datong could be explained by an anisotropic layer at 15-20 depth, and both the impedance tensor and tipper could be fitted if a 3D anisotropic body is placed on the west part of study region at lower crust depth.