Distinct lateral variation of lithospheric thickness in the Northeastern North China Craton

Abstract

A detailed knowledge of the thickness of the lithosphere in the northeastern North China Craton (NCC) is important for understanding the significant tectonic reactivation of the craton in the Mesozoic and Cenozoic time. We achieve this goal by applying the newly proposed wave equation-based migration technique to the S-receiver functions recently collected in the region. Distinct negative signals are identified below the Moho in all the S-receiver function-migrated images and stacks, which we interpret as representing the S-to-P conversions from the lithosphere–asthenosphere boundary (LAB). The imaged LAB is as shallow as ∼60–70 km in the southeast basin and coastal areas and deepens to no more than 140 km in the northwest mountain ranges and continental interior. These observations indicate widespread lithospheric thinning in the study region in comparison with the >180-km lithospheric thicknesses typical of most cratonic regions. The revealed topography of the LAB generally agrees with the lateral variation in upper mantle seismic anisotropy previously measured through SKS splitting analysis. In particular, a sharp LAB step of ∼40 km is detected at the triple junction of the basin and mountains, at almost the same place where an abrupt change from NW–SE to NE–SW in fast polarization direction of shear waves was found. These findings suggest a close correlation between the seismic anisotropy and hence deformation of the upper mantle, the lithospheric thickness, and the surface tectonics of the northeastern NCC. While the thinned lithosphere and the NW–SE fast shear wave polarizations in the east areas probably are related to the dominant NW–SE tectonic extension in the late Mesozoic–Cenozoic time, the thicker lithosphere and the NE–SW fast polarization direction in the west mountain ranges may reflect earlier contractional deformations of the region. Synthetic tests indicate that the LAB beneath the northeastern NCC is a well-defined zone 10–20 km thick. Combined with seismic tomography results and geochemical and petrological data, this suggests that complex modification of the lithosphere probably accompanied significant lithospheric thinning during the tectonic reactivation of the old craton.