Magnetotelluric Evidence of Regional Lithospheric Modification in Ordos Block, North China

The Ordos Block (OB), which is located in the western part of the North China Craton (NCC), is one of the oldest and most stable cratonic blocks in Asia. To study the electrical structure of Ordos block, long period magnetotelluric (MT) array data from project SINOPROBE were acquired and modeled using three-dimensional (3D) MT inversion method. For the first time a high resolution 3D resistivity model of the lithosphere is acquired in the region. The lithosphere beneath the northern part of the OB and Hetao Graben is revealed to be geo-electrically distinct from that beneath the middle and southern Ordos. Contrary to what would be expected for a stable cratonic block, compelling evidence is presented of a large scale conductive complex in the lower crust and upper mantle beneath north Ordos and Hetao Graben. The abnormally conductive structure indicates the presence of aqueous fluids and/or inter-connected sulphides, possibly resulting from an upwelling of mantle material beneath the region. Correlating well with results of seismic studies, the evidence from independent magnetotelluric data supports a regional modification and thinning of the lithosphere, which might contribute to the extension of Hetao Graben started in late Miocene to Pliocene, as well as the uplifting of the north the Ordos Block since Neogene.