Investigation Of Lithospheric Electrical Anisotropy In Western Junggar

In recent years, with the development of observation techniques and theoretical methods of geophysics, more and more evidence shows geophysical properties of the Earth have obvious anisotropy in many cases. Magnetotelluric (MT) data can image the electrical resistivity of the entire lithospheric column and are therefore one of the most important data sources for understanding the structure, composition and evolution of the lithosphere.Three magnetotelluric profiles were set up crossing the NE-SW Darbut fault belt in Western Junggar, NW China. The analyses of phase tensors and induction arrows indicate a 3D resistivity structure where anisotropy may be present. The separate 3D isotropic inversions of impedance tensors and tipper arrows, respectively result in two extremely different models underlining that obviously a simple isotropy model explaining all the response functions together does not exist. Based on the results of 1D anisotropy,2D and 3D isotropy inversion, consequently an anisotropic 3D resistivity model was constructed containing an azimuthally anisotropic crustal layer (minimum resistivity striking east-west) beneath Darbut to Karamay-Urho faults in the depth of 5 to 40 km and an azimuthally anisotropic lithosphere mantle (minimum resistivity striking N20°E). The anisotropic resistivity model improves the fit of both phase tensors and induction arrows significantly. The azimuthally anisotropic crustal layer was inferred as the basaltic-oceanic-crustal remnant, which had extremely been serpentinized in terms of massive fluids added from the dehydration in the subduction plate during its underthrusting process in the late Paleozoic. The oblique subduction may attribute to the difference between the trench strike (～N40°E) and the minimum resistivity direction in the relics of subducted oceanic slab. Besides, the minimum resistivity direction of the crustal anisotropic layer and the trench direction deviate by separate 70° and 20° clockwise from that of the lithospheric mantle, indicating a decoupled oceanic-crustal movement from its underlying upper mantle in this area.