Teleseismic P-wave Tomography Study Of Sichuan Basin And Ordos Block

The Tibetan Plateau is surrounded by the rigid craton,with the Tarim Basin and the Alashan Block to its north and the Sichuan Basin and the Ordos Block to its east,forming a complex tectonic contact relationship with strong tectonic activity at the boundary.Some large-scale active faults,such as the Longmenshan fault and Xianshuihe fault,are distributed on the west side of the Sichuan Basin,and a series of moderate to strong earthquakes have occurred in the history,which caused significant economic losses and casualties.The seismic activity inside the Ordos Block has been weak for a long time,and the seismic activity at its periphery is strong.Therefore these areas have been receiving key attention from the field of geophysics in China.The Sichuan Basin and Ordos region play an important role in the eastward extrusion of the Tibetan plateau.The study of the upper mantle velocity structure in these regions is important for understanding the dynamics of the Tibetan plateau and for exploring the background of seismic activity.Many tomographic imaging studies have been carried out in the Sichuan Basin and Ordos region,but the ray coverage is poor and the imaging resolution is relatively low due to the previous sparse and uneven station distribution in these regions.There are few stations inside the Sichuan Basin.In previous studies,the data from fixed stations were usually used,and the lateral resolution mostly weaker than 0.8~°×0.8~°,therefore these studies can only distinguish a rough upper mantle velocity structure.In the Ordos Block,there is also a lack of dense stations in the past,and the lateral resolution of imaging results is low.The distribution of high-velocity anomalies beneath the Ordos Block is controversial,which affects our understanding of the lithospheric structure and stability of this region.In order to further study the tectonic evolution and dynamics process of the Sichuan Basin and Ordos region,the Institute of Geophysics,China Earthquake Administration has carried out a series of large-scale dense temporary seismic stations in these areas in recent years,and the deployment of these stations has greatly improved the station density in the study area,which provides the possibility to obtain higher resolution 3D upper mantle velocity structure.Wu Jianping's research team from the Institute of Geophysics,China Earthquake Administration,deployed 25broadband temporary stations inside the Sichuan Basin from October 2014 to December 2016,and the observation period exceeded two years.The 472 temporary stations of the Phase III of the Chin Array Project were deployed inside of the Ordos Block and the central block of the North China Craton from 2016,which greatly improved the previous sparse station distribution in these areas.In this study,we make full use of the data from the newly deployed temporary seismic stations and the existing fixed stations in the study area to obtain the 3D upper mantle velocity structure in the Sichuan Basin and Ordos region,using 747 teleseismic events recorded by the stations in the Sichuan region and 404 teleseismic events recorded by the stations in the Ordos region.The lateral resolution of the new imaging results can reach 0.5~°×0.5~°in most of the study area,which is a significant improvement compared with the previous imaging results.In this study,the influence of shallow sedimentary layers and the uneven distribution of epicenter is considered to improve the imaging quality.The influence of thick sedimentary layers in the Sichuan Basin and Ordos region cannot be ignored.The previous calculation of the traveltime residuals is based on the IASP91 model,but the model does not include the velocity and thickness of the sedimentary layers,so the calculated traveltime residuals may contain some errors.To reduce the influence of the sediment layer,the sediment layer model of the study area was extracted from the CRUST1.0 model and combined with the IASP91 model to include the travel time of the ray in the sediment layer and correct the traveltime residuals.In addition,the distribution of seismic stations and the teleseismic events are usually uneven,especially the teleseismic events are densely distributed to the southeast of the study area.This uneven distribution of stations and events leads to uneven ray distribution,and the final imaging results will be stretched and distorted in the direction where the stations and events are densely distributed.In order to reduce the effect,the areas containing stations and events are divided into grids,and the weight of each station and earthquake in the calculation is adjusted to reduce the influence of the over-concentrated distribution.The above method is applied to the Sichuan Basin and Ordos region to obtain the fine velocity structure of the upper mantle,and explore the tectonic evolution and dynamics process of the study area by combining the previous imaging and other geophysical studies,and the main results and understandings obtained are as follows.The Sichuan Basin is located east of the Tibetan Plateau and is surrounded by mountains that have undergone complex deformation and uplift since the Cenozoic.So far,most velocity models have considered that a deep cratonic root exists beneath the Sichuan Basin,blocking the eastward extrusion of mantle materials from the Tibetan Plateau.In contrast to previous results,the imaging results in this study show high-velocity anomalies that dip to the east and southeast,and these velocity anomalies in the Sichuan Basin extend from 150 km eastward into the upper mantle to about 400 km depth.We speculate that southeastward subduction of the Yangtze block occurred in the Mesozoic and may have been reactivated in the Cenozoic.The subduction produced a relatively weak lithosphere east of the Sichuan Basin,and surface uplift occurred in the Hubei-Hunan-Guizhou fold belt and the eastern Sichuan fold belt in response to the eastward expansion of the Tibetan Plateau.A localized westward dipping high-velocity anomaly exists beneath the eastern Tibet,which can be interpreted as lithospheric delamination.This delamination may have led to the formation of high-altitude peaks in and around the Gongga Mountain and may have accelerated the formation of the Xianshuihe fault belt and the lateral extrusion of the Tibetan Plateau.The eastward extrusion of the Tibetan Plateau material is not obvious under the East Qinling orogenic belt,which indicates a limited lithospheric extrusion north of the Sichuan Basin.In the imaging results of this study,a thick high-velocity anomaly exists beneath the Ordos Block,with a depth of about 180 km in the west,100 km in the north,and up to 300 km in parts of the central and eastern parts,indicating that the Ordos Block still maintains cratonic stability as a whole.The lithosphere in the northern part of the Ordos is relatively thin,which may be related to the heating and modification of the lithosphere by the mantle upwelling.The lithosphere thinning in western Ordos may be influenced by the lateral expansion of relatively warm upper mantle material in northeastern Tibet,and the lithospheric mantle may have been partially modified.There are widespread low-velocity anomalies in the upper mantle of the eastern North China Craton,central North China Craton and northeastern Ordos,which may be related to the retreat of the Pacific plate,the dehydration of the stagnant slab and the extensional environment caused by local mantle convection at the front edge of the slab.Mantle convection can transfer heat to the base of the lithosphere,and in the extensional setting of this region,melts from the lithosphere or asthenosphere upwelled along the weak zone and formed the volcanoes including Datong.