MT Array Observation Network And Three Dimensional Lithospheric Conductivity Structure Of North China

The North China Block is generally considered to be assembled by the western and eastern blocks along the Trans-North China orogenic belt at early Proterozoic.It has experienced destruction in the Mesozoic,which lead to the thinning of the lithosphere,although the mechanism of lithospheric thinning and the thickness of the lithosphere are still under debate.The electrical property of the North China Block has barely been investigated by previous studies.In this thesis,a Magnetotelluric(MT)observation network from the project "Deep Exploration Technology and Experimentation"(SinoProbe)covering the North China Block is introduced.Technologies for field data acquisition,data processing,analysis and inversion are discussed.Based on the electrical model obtained through inversion,the lithospheric electrical structure of the North China Block is investigated,and some related issues such as the thickness of the lithosphere and mechanism of lithospheric thinning are discussed.The SinoProbe project has determined a series of technical regulations for station deployment,data acquisition and processing.Data analysis based on tensor decomposition,induction vector and phase tensor analysis showed that the shallow sediments of the Ordos Block are generally of 1D characteristics.The mid-lower crust of the Ordos Block displays dominant principal electrical axis of E-W and N-S directions,which indicates a regional strike direction of E-W or N-S direction.The Bohai Bay Basin shows a consistent direction of shallow principal electrical axis with the surface geological features,whereas this direction turns clockwise from NE20°/SE110° to NE60°/SE150° in the depth range of mid-lower crust.Direction of principal electrical axis displays a relatively complex pattern in the shallow Trans-North China Orogen,but exhibits a distinct E-W/N-S distribution in the lower crust.According to regional geological structures,the deep regional strike direction is determined as N-S direction.Furthermore,induction vector maps indicate significant conductive regions beneath the northern Ordos Block and the Bohai Bay Basin.The electrical properties of major geologic units in North China were analyzed based on results from Bostick transformation and 3D inversions on both lithospheric and crustal scales.The Ordos Block in the west can be divided into two parts along latitude 37.5°.The northern part is characterized by a large scale mushroom shaped conductor in the mid-lower crust.The Trans-North China Orogen is generally resistive,except a relatively large scale N-S stretching conductive belt in the depth range of 30-50 km,which is probably the electrical indicator of basement assemblage of the North China Block.The lithosphere thickness of the North China Block was discussed on the basis of lithospheric 3D inversion model.The lithosphere beneath the Alxa Block is about 200km thick.The northeastern Qinling Orogen has a lithosphere of similar thickness whereas the thickness of its southwestern part reduces sharply to about 130km.The lithosphere thickness is about 100-150km and 120-150km for the Ordos Block and the Trans-North China Orogen separately.Lithosphere beneath the southern Bohai Bay Basin and Luxi Uplift is only 60-80km thick.This thickness increases to about 120km beneath the northern Bohai Bay Basin,but reduces again to 100km beneath the region where the northern margin of the North China Block meets the Yanshan uplift.