Crustal structure and extensional deformation of thinned lithosphere in Northern China

We herein present an interpretation of a 320 km-long wide-angle seismic profile between Anxin and Kuancheng, which was obtained in 2002. The profile runs from the North China Plain (NCP), where the lithosphere is just 70 km thick; to the Yanshan Mountain Folded Belt (YMFB), where the lithosphere is180 km thick. Our model shows a crustal thickness that varies from 31 km under the NCP to 36 km under the YMFB. The observed thinning of the crust in the NCP is about 14%, which compares with an average extension of 24–41% at basin-scale and 25% at lithosphere-scale. This finding suggests that the extensional deformation of the lithosphere in the North China block depends on depth. The thin, high-velocity crust–mantle transition zone has most likely originated after a delamination of the bottom of the crust and a concomitant intrusion of materials from the mantle. The lower velocity of the lower crust may be attributed to the destruction of the lithosphere, which permitted the lateral flow of melting materials above the Moho from the NCP to the YMFB. The differences found between the crust and the lithospheric mantle help to dispel any of the remaining uncertainty in the extensional factors, and they may be attributed to detachment of middle crust and an intrusion of magma that originated in either lithosphere or asthenosphere. We infer that the detachment of the middle crust, lower-crustal flow and magma intrusion probably lead to the underestimation of the crustal-scale extensional factor, and may represent the crustal response to the thinning of the lithosphere.

Research Highlights
► Crust thins from 36 km under the Yanshan Mountain Folded Belt (with 180-km-thick lithosphere) to 31km under North China Plain (with 70km thick lithosphere) in North China.
► The thin, high-velocity crust–mantle transition zone has most likely originated after a delamination of the bottom of the crust and a concomitant intrusion of materials from the mantle.
► The lower velocity of the lower crust may be attributed to the destruction of the lithosphere, which permitted the lateral flow of melting materials above the Moho from the North China Plain to the Yanshan Mountain Folded Belt.
► The extensional deformation of the lithosphere in the North China block depends on depth (the extensional factor is about 14% at crust-scale, 24–41% at basin-scale and 25% at lithosphere-scale in the North China Plain).
► The detachment of the middle crust, lower-crustal flow and magma intrusion probably lead to the depth-dependent extension, and may represent the crustal response to the thinning of the lithosphere.