Undulating electrical lithosphere–asthenosphere boundary beneath Northeast China; as revealed by long–period magnetotelluric data

The geometry of continental lithosphere–asthenosphere boundary (LAB) significantly affects the movements of tectonic plates and flow of the upper mantle. However, the properties and geometry of this boundary beneath tectonically active regions in Phanerozoic continents are mysterious and elusive. Ninety–three long–period magnetotelluric (LMT) observations across Northeast China (NEC) provide a unique opportunity to image the shape of the LAB. Probabilistic resistivity–depth model inversion was employed to obtain the one–dimensional resistivity structure beneath each LMT site. The depth of the electrical LAB is constrained by the most probable interface at which the resistivity changes rapidly from high to low. The inversion results show that the electrical LAB is undulating beneath NEC. The North–South Gravity Lineament marks the transition from deep to shallow LAB. The thickest lithosphere occurs beneath the Great Xing'an Range (GXAR), where the LAB is ∼200 km deep. The electrical lithosphere is relatively thin in the east, with the thinnest lithosphere (∼50 km) being found beneath the Changbai Mountain volcano. We speculate that melt depletion, together with possible mantle downwelling are the likely causes for the formation of the thick GXAR lithosphere. The unresolved lithosphere beneath the northern Songliao Basin could be ascribed to the strong attenuation of electromagnetic waves in highly conductive sedimentary layers or the weak difference in the resistivity of the lithosphere and asthenosphere. The lithosphere steps that can form the edge-driven convection in the asthenosphere may play key roles in forming the widely-spread intraplate magmatism in NEC.