| Tanlu fault is a deep penetrating fault with an NNE strike in eastern China.Its Anhui-Jiangsu segment and the adjacent area traverse the Dabie orogenic belt,the Sulu orogenic belt,the Middle-Lower Yangtze metallogenic belt,and the Hefei Basin.In order to study the crustal velocity structure and deformation characteristics in this region,we used continuous waveform data from May 2014 to July 2015,a total of 14 months,of 105 stations(98 permanent stations and 7 temporary stations)in Anhui and Jiangsu Provinces and its surrounding area(115°E-121°E,29.5°N-35°N).With the ambient noise cross-correlation method,we finally extracted 2590 surface Rayleigh wave phase velocity dispersion curves from the vertical component cross-correlation function,and then obtained an azimuthally anisotropic Rayleigh wave phase velocity maps in a period band of 5-30s.Then we inverted the Rayleigh wave phase velocity dispersion at each grid point for a one-dimensional layered shear velocity model,which is combined into a three-dimensional shear velocity model.In order to obtain the three-dimensional velocity structure of the Shandong-Anhui-Jiangsu segment of Tanlu fault zone and its adjacent area(113°E-123°E,28.5°N-38.5°N),the stations in Shandong province,the south side of the Dabie orogenic belt and Jiangnan Orogenic Belt were added.Therefore,there are 216 stations including previous stations in Jiangsu and Anhui,with time ranging from January 2015 to June or December 2016,a total of 18 or 24 months.The new region of the Luxi block and a series of small basins in southern North China have more complex geological settings.The three-dimensional S-wave velocity structure in the larger area directly inverted the Rayleigh wave phase velocity dispersion to the three-dimensional velocity structure without the phase velocity map of each period.The three-dimensional shear wave velocity model of these two overlapping regions obtained by different methods and data are quite similar,which demonstrates the reliability of our inversion.Based on the inversion results as well as other data,we draw the following conclusions.(I)The Northern Dabie orogenic belt,the Middle-Lower Yangtze metallogenic belt,the Bengbu uplift,the northern Luxi blcock,the Taikang uplift and the Daqiao-Dingyuan depression in the northern Hefei Basin exhibit higher shear velocity anomalies in the middle crust,which may be directly related to the lithosphere delamination and the Mesozoic volcanic magmatism in eastern China,revealed from the dating of igneous rocks(about 130-120Ma).The Mesozoic igneous rocks in the Northern Dabie,the Qianlong uplift,and the Middle-Lower Yangtze metallogenic belt are due to the lithosphere and lower crust delamination.In the North China Craton including the Bengbu uplift,the northern part of the Luxi block,and the Taikang uplift,the first-order dynamic mechanism of is the subduction of the paleo-Pacific plate while the lithosphere and lower crust delamination is secondary.Considering that the Mesozoic igneous rocks in the region have almost the same age,together with the specific origin of the Bengbu uplift,we suggest that the Mesozoic magmatism in the northern Dabie and Middle-Lower Yangtze metallogenic belt is also related to the subduction of the paleo-Pacific plate.(2)The southern part of the Luxi area exhibits lower shear velocity anomalies in the middle crust.The low-velocity zone of the middle crust is considered to be the detachment surface of the brittle-ductile transformation.The magma rises up from the mantle to the crust through the fault near the Moho,forming a high-velocity uplift in northwestern Shandong,and the underplating magma further dehydrates the minerals in the lower crust,the water moving to the middle crust to form the low-velocity zone of the middle crust.Earthquakes with magnitude greater than 3 in the Luxi area mainly occur near the low-velocity zone,and the two most destructive earthquakes(M>7)in Shandong are also at the depth of the low-velocity layer.So it is interpreted that the detachment surface is prone to stress accumulation,thus may be the underlying cause of the two destructive earthquakes in the area.(3)Perpendicular to the Jiashui-Xiangshui fault,the low velocity zone deepens from south to north in the upper-middle crust through the Sulu orogenic belt.Combined with the imbricate high-pressure metamorphic rocks on the surface and the South China plate subducting toward the North China plate,the low velocity zone may be a fracture zone where the high pressure metamorphic rocks come into contact with the Yangtze plate or a contact boundary between the Yangtze plate and the North China plate caused by dehydration of oceanic crust in the contact zone.(4)The fast-axis direction of the short period Rayleigh wave phase velocity on both sides of the Hefei-Jiashan section of the Tanlu fault and the eastern boundary of the Dabie orogenic belt is basically consistent with strike of the Tanu fault,which may be the result of the large-scale sinistral strike-slip activity of the Tanlu fault during the Triassic collision period and the Cretaceous period;(5)The fast-axis direction of Rayleigh wave in the period band 15-20s is northwest-southeast in the south of the Hefei Basin,indicating a northwest-southeast fast wave direction in the middle and lower crust in this area,presumably the trace of the exhumation of the Dabie orogenic belt.(6)The structure and seismicity of the Tanlu fault zone are distinctly segmented.The seismicity of the Tanlu fault from Jiashan to Tancheng is weak at present,but strong paleo-earthquakes used to occur.In addition,the low-velocity layer in the middle crust of the Luxi area,in which stress is easy to accumulate,can extend to the Suqian area,suggesting that the crust of this segment of Tanlu fault is prone to accumulate stress.It is inferred that the Suqian section of the Tanlu fault may be in a locked state.In the long run,more attention should be paid to the possibility of large earthquakes occurring in this area. |
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