Electrical resistivity structure of the Trans-North China Orogen: implications for lithospheric deformation history

Abstract:

The North China Craton (NCC) is composed of the Eastern Block (WB) and Western Block (EB) amalgamated along the Central Orogenic Belt (Trans-North China Orogen, TNCO). A magnetotelluric (MT) profile crossing the eastern margin of WB, southern TNCO and western margin of EB, combined with a subset of SinoProbe MT array near this profile is used in our study. Dimensionality analysis indicates that (a) the resistivity structure is nearly 2-D with geoelectric strike directions varying in different tectonic units, and that (b) electrical anisotropy may exist in the mid-lower crust beneath the Shanxi Rift and Qinshui Basin. Isotropic and anisotropic 2-D inversion and isotropic 3-D inversions have been conducted. The most significant resistivity features in the inversion models are a series of mid-lower crustal sub-horizontal blob-like conductors beneath Shanxi Rift and an east-dipping conductor extending from mid crust to a depth greater than 150 km beneath the southern Taihang Mountains. Inversions show that the crustal conductor series may represent an anisotropic layer, likely caused by conductive materials filling a network of fractures with preferred spatial orientation. The dipping conductor may represent a conduit of hot asthenospheric materials transporting upwards, which is interpreted to the leading edge of the eastward escape mantle flow caused by Indian-Asia collision. The hot materials are inferred to be resulted from the westward mantle convection associated with the westward subducting Pacific Slab, going upward into the mid-lower crustal detachment zone and heat the rock, generating partial melts filling the fractures beneath Shanxi rift. The rift system is believed to be associated with NW–SE-trending extensional stress field resulted from the rapid convergence of India–Eurasia. Thus, conclusion can be drawn that the deformation of the lithosphere beneath NCC, especially the TNCO, may be a far-field effect of Indian-Asia collision.