Crust–mantle structure difference across the gravity gradient zone in North China Craton: Seismic image of the thinned continental crust

The Archean North China Craton (NCC) has been tectonically modified and lost its thick lithospheric keel during the Mesozoic and Cenozoic. The processes and mechanisms of the lithospheric modification and its appearance in and the relation between different subregions of the NCC are still poorly understood. Seismic data from 45 stations along a 470-km long profile cross the Bohai Bay Basin (BBB) and the Taihangshan Mountain Range (TMR) in the NCC were employed to construct a coherent structural image of the crust and uppermost mantle. An integrated receiver function imaging technique combining the common conversion point stacking approach with waveform inversion and forward modeling was proposed to extract the structural information beneath the study region. Modeling of Bouguer gravity anomalies was also applied to constrain the density distribution. The imaging result reveals distinct structural features between the mountain range and the basin area, and presents a picture of uneven crust thinning within the study region. In the east BBB the crust is significantly thinned due mainly to the reduction in the thickness of the lower crust including the crust–mantle transition zone, by up to ∼12 km. The west TMR, in contrast, is characterized by a relatively thick lower crust of ∼20 km. The teleseismic waveform data and the gravity observation suggest a thicker crust and a buoyant mantle lithosphere beneath the TMR compared with the BBB. The contrasting crustal structural features appear coupled with the lithospheric processes and possibly reflect that different tectonic mechanisms and deformation regimes dominated the evolution of the two regions. The North-South Gravity Lineament, lying between the TMR and BBB, might represent a deep intra-continental boundary separating the NCC into topographically and tectonically different regions.