The North China Craton has undergone intensely tectonic reactivation since the Mesozoic, which resulted in lithosphere modification, thinning and destruction, and accompanied by large amount of magmatic activity. The neotectonic movement is strong and destructive earthquakes occur frequently in this region. It is of great significance to obtain medium deformation and structure information in the crust and upper mantle for understanding these process. High-resolution lithosphere structure will provide important basis for understanding a series of scientific issues such as the tectonic deformation of crust-mantle media, the interaction between structural blocks, the deep environment of strong earthquakes, the spatial distribution range and dynamic mechanism of lithosphere thinning and destruction.
Many researches about seismic tomography have been carried out in the North China Craton. But, the results of high-resolution of the lithosphere across the whole North China Craton are still few, due to the limitation of observation conditions or range, which limits further analysis on a series of scientific issues. As the rapid development of seismic array observation technology, some high-resolution seismic tomography techniques appear which suitable for dense arrays. The Eikonal surface wave tomography takes into account the bending phenomenon of seismic wave propagation path in complex media, and is suitable for both ambient noise and seismic surface wave. Its lateral resolution is equivalent to the average spacing of stations for arrays with relatively uniform distribution of stations. Considering that the ambient noise tomography is limited to medium and short periods （generally below 40 s）, which mainly restricts the depth range from the crust to the top of the upper mantle so that it is difficult to carry out research and discussion on deeper depths. In this study, we choose the surface wave observation data to extract Rayleigh wave phase velocity of medium and long period by Eikonal surface wave tomograph, which can provide constraints on the entire lithosphere depth range.
We collected the surface wave data of teleseismic events from 1513 seismic stations in this study, including 670 portable stations from ChinArray PhaseⅡ, 361 portable stations from ChinArray Phase Ⅲ-1 , 324 portable stations from ChinArray Phase Ⅲ-2 , and 158 permanent stations from China National Seismic Network （CSN）. This is the most intensive seismic station bservations in the study region, with an average station spacing of about 35 km. Their corresponding observation periods are September 2013 to June 2016, November 2016 to January 2019, November 2017 to November 2020, and April 2016 to January 2019. The 3D high-resolution S-wave velocity model was obtained by two-step method in the depth range of 200 km below the study region. Firstly, We obtained the phase velocity of Rayleigh surface wave at 10−120 s by Eikonal surface wave tomography and extracted the pure path dispersion curves of each grid node on the surface. Secondly, we obtained these one-dimensional S-wave velocity models at these corresponding nodes by linear inversion method, then all the one-dimensional S-wave velocity models are combined to obtain the 3D S-wave velocity model. The lithosphere thickness is estimated by the empirical relationship between upper mantle S-wave velocity and pressure and temperature based on this model.
The results showed that there are some smaller scale variations of the lithosphere thickness in the North China Craton in addition to the first-order distribution characteristics of ‘thick in the west and thin in the east’. Which includes （1） within the Ordos block, the lithosphere is thinner in the north than that in the south, （2） within the peripheral rift zone around Ordos block, it is characterized by significantly heterogeneous thinned lithosphere, and （3） there is significant difference between Yanshan Orogenic Belt and North China plain on its south side．In Shanxi rift zone, both the northern and southern regions exhibit varying degrees of low velocity anomalies in the upper mantle （＜100 km）, which are separated by a high velocity anomaly zone in the central area．At depth of more than 150 km, a remarkable low-velocity anomaly belt oriented NNE is observed from the southern edge of Taihang Mountain to the northern edge of Shanxi rift zone, indicating that the shallow upper mantle low-velocity anomalies are connected in the deep．Combined with some other research findings, we speculated that these low-velocity anomalies may stem from a greater depth （＞200 km）, potentially linked with the stagnant dehydration of the subducted Pacific plate and consequent upwelling of thermal material in the upper mantle, as well as small-scale mantle convection. The lithospheric structures of the Yanshan Orogenic belt is significantly different from North China Plain, with former experienced much less destruction and reconstruction. Zhangjiakou-Bohai seismic zone is located at the transitional region between these two distinct crust-mantle structures, and characterized by intense seismic activity． We concluded that the combination of significant differences in deep structure and thermal action, as well as the far-field extrusion effect of the Qinghai-Tibet Plateau, mainly contributes to the intense seismic activity in this zone. There are some significant high velocity anomalies near the depth of 200 km in Yanshan Orogenic belt, northern part of the North China Plain and around Bohai Bay. It is speculated that these anomalies may be related to the local delamination of the lithosphere, which represent the remnants of the Archean cratonic lithosphere sinking into the asthenosphere.
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