Three-dimensional Seismic Velocity Structure Of Crust And Mantle Beneath Fujian

The Fujian region is located at the margin of southeastern China,it is the frontier zones of the interaction between the Eurasian plate and the Philippine Sea plate,and belongs to Wuyi-Yunkai orogenic system on geotectonic units.It is also an important part of the Cathaysia block and the southeast coastal Pacific tectono-magmatic belt.The tectonic evolution process in this region was extremely complex,and it had been undergone intense multi-stage volcanic-intrusive magmatic activities over a long geological period.The study of three-dimensional seismic velocity structure of crust and mantle is of significance in understanding the regional tectonic evolution process and deep seismogenic environment.In this paper,based on the observational data from the seismic network in the Fujian region and temporary seismic stations,the crustal and mantle velocity structure of the region was obtained using methods such as doubledifference tomography,receiver function H-κ stacking,and joint inversion of receiver functions and surface waves.By combining the results with geological structures,geochemical analyses of rocks,and other geophysical observations,this study explores the tectonic implications of the imaging results.The main findings and understandings obtained are as follows:(1)By utilizing near earthquake travel time observations from seismic permanent station in Fujian and surrounding areas,the three-dimensional crustal P-wave velocity structure of the study area was obtained using the double-difference tomography method.The results reveal that the Southwestern Fujian block exhibits relatively lowvelocity anomalies in the middle and lower crust.Combining these findings with other results such as Vp/Vs ratio,it is inferred that this anomaly is related to a higher content of felsic components in the crust of this region.The deep structural characteristics of the Fu’an-Nanjing fault zone are pronounced,particularly in the middle and lower crust.In comparison with the widely studied Zhenghe-Dapu fault zone,the velocity differences on both sides of the Fu’an-Nanjing fault zone are more significant.This aligns with the distinct Bouguer gravity anomaly and aeromagnetic data on both sides of the fault zone,indicating that the Fu’an-Nanjing fault zone is a major deep fault zone within the region.(2)A high-resolution image of crustal thickness and Poisson’s ratio distribution in the study area was obtained using the teleseismic receiver function H-κ method.The results indicate that the overall characteristic of the Moho depth in the Fujian region gradually becomes shallower from the inland to the coastal areas.The variation in crustal thickness ranges mainly between 28 to 34 km,suggesting a thinned continental crust in the coastal region.The Poisson’s ratio of the crust in the study area ranges from 0.22 to 0.29.The coastal areas generally exhibit higher Poisson’s ratios compared to the inland areas,with slightly higher values in the northern part than the southern part.This distribution is believed to be primarily associated with the Late Mesozoic magmatic underplating and the development of low-velocity thin layers within the coastal crust.The difference in crustal thickness on both sides of the Fu’an-Nanjing fault zone in the southern section indicates that the fault zone is a major deep fault cutting through the Moho.Along the Fu’an-Nanjing fault zone,the relationship between crustal thickness and Poisson’s ratio differs on each side.On the western side,the Cathaysia block shows a negative correlation,while the eastern side,the volcanic-intrusive rock belt along the Fujian-Zhejiang coastal region,exhibits a positive correlation.This reflects the differences between different blocks during various stages of tectonic evolution.(3)The three-dimensional S-wave crustal and mantle velocity structure of the study area was obtained through the joint inversion of receiver functions and surface waves.The results show that in the middle to lower crust,the Eastern Fujian block exhibits a relatively high S-wave velocity,while the Southwestern Fujian block shows a relatively low S-wave velocity.The velocity difference in the middle to lower crust on both sides of the Fu’an-Nanjing fault zone is more significant than that of the Zhenghe-Dapu fault zone.In the upper mantle,there is a prominent low-velocity anomaly beneath the Southwestern Fujian block within a depth range of 200 km,which may be related to the subduction and retreat processes of the Paleo-Pacific plate during the Mesozoic.The crustal and mantle S-wave velocity difference on both sides of the Nanping-Ninghua tectonic zone between the Southwestern Fujian.block and the Northwestern Fujian block is significant,which corresponds to an area of relatively active historical strong earthquakes in the inland area of Fujian.In the Eastern Fujian block,except for the Pingtan-Dongshan metamorphic belt in the eastern part,which exhibits a relatively low velocity,the rest of the area shows mainly high velocities.At depths of 70～120 km in the upper mantle of the Pingtan-Dongshan metamorphic belt,there is a significant S-wave low-velocity anomaly,suggesting that the heat source from the mantle is one of the important heat sources for the metamorphic process in this belt.