Deep Process Of Thinning And Destruction Of Heterogeneity In The North China Craton:Constraints From P-wave Tomography

The North China Craton(NCC)is located in Northeast Asia,which is the Early Cambrian landmass with the largest area,the longest history of formation and evolution,and the most complete geological record in China.The NCC that formed from 18.5Ga to 2Ga is relatively stable tectonic unit on Earth.However,since the Late Mesozoic,the NCC has undergone large-scale structural deformation,magmatism,seismic activity,and mineralization.The NCC experienced widespread reactivations,resulting in not only reduction in the lithosphere thickness but also marked change in the nature and thermal state of the lithosphere.Seismic tomography is by far the most powerful tool to obtain the deep velocity structure of the Earth.In this study,we used 82342 P-wave traveltimes to invert for the 3D P-wave velocity structure of the mantle beneath the NCC.These data are recorded by 330 permanent seismic stations from Chinese provincial seismic networks and 250 portable seismic stations from Institute of Geophysics,China Earthquake Administration.The results show that:(1)The upper mantle of the eastern NCC has small-scale velocity structure characteristics,and the velocity structure has obvious heterogeneity.The spatial heterogeneity of the upper-mantle velocity structure is the result of the uneven distribution of upwellings caused by the dehydration of the subducting Pacific plate and their interactions to the weak zone in the Cratonic interior.(2)There are two separated low-velocity anomalies beneath the northern and southern Trans-North China Orogen(TNCO)respectively.The northern low-velocity anomaly is Northwest-Southeast trending with a larger size and amplitude than the southern one.The large-scale low-velocity anomaly ascends from the top of the mantle transition zone and rises to the shallow mantle beneath Quaternary volcanoes.The low-velocity anomaly beneath the northern TNCO was mainly derived from the heterogeneous dehydration of the stagnant Pacific slab.The southern low-velocity anomaly shows a shallow origin depth,which is about 200-300 km,perhaps associated with the expansion of the Tibetan Plateau or local mantle upwelling.(3)An obvious high-velocity anomaly is revealed down to about 300 km depth under the Ordos block,indicating the thick lithospheric root.There is a low-velocity anomaly in the southwestern of the Ordos block,which may be related to the eastward asthenospheric flow from the Tibetan Plateau.(4)Under the NCC high-velocity anomalies are visible in the mantle transition zone,representing the stagnant Pacific slab.The shape of the stagnant Pacific plate shows spatial heterogeneity,which indicates that the stagnant Pacific plate may be bent and deformed,or even broken or fractured.Some parts of the stagnant Pacific plate have passed through the 660 km discontinuity and descended to the lower mantle.