Uppermost mantle structure of the North China Craton: Constraints from interstation Pn travel time difference tomography

The uppermost mantle is the key area for exchange of heat flux and material convection between the crust and lithospheric mantle. Spatial variations of lithospheric thinning and dynamic processes in the North China Craton could inevitably induce the velocity heterogeneity in the uppermost mantle.In this study,we used Pn arrivals from permanent seismic stations in North China and surrounding regions to construct a tomographic image of the North China Craton.The tomographic method with Pn travel time difference data were used to study the velocity variations in the uppermost mantle.Pn velocities in the uppermost mantle varied significantly in the Eastern,Central and Western blocks of the North China Craton.This suggests that the lithosphere beneath different blocks of the North China Craton have experienced distinct tectonic evolutions and dynamic processes since the Paleo- zoic.The current uppermost mantle has been imprinted by these tectonic and dynamic processes.Fast Pn velocities are prominent beneath the Bohai Bay Basin in the Eastern Block of the North China Craton,suggesting residuals of the Archean lithospheric mantle.Beneath the Tanlu Fault Zone and Bohai Sea,slow Pn velocities are present in the uppermost mantle,which can be attributed to significant lithospheric thinning and asthenospheric upwelling.The newly formed lithospheric mantle beneath Yanshan Mountain may be the dominant reason for the existence of slow Pn velocities in this region.Conversely,the ancient lower crust and lithospheric mantle already have been delaminated.In the Central Block,significant slow Pn velocities are present in Taihangshan Mountain,which also extends northward to the Yinchuan-Hetao Rift on the northern margin of the Ordos Block and Yinshan Orogen.This characteristic probably is a result of hot asthenospheric upwelling along the active tectonic boundary on the margin of the Western Block.The protracted thermal erosion and underplating of hot asthenospheric upwelling may induce lithospheric thinning and significant slow velocities in the uppermost mantle.Fast velocities beneath the Western Block suggest that the thick,cold and refractory Archean lithospheric keel of craton still is retained without apparent destruction.