Fig. 4-2 1 Dabie-Sulu published five refraction seismic sounding profiles, revealing the Vp wave velocity (km/s) structure of the crust and its comparison with the eastern China and the global continental crust.
The underlined number is the average crustal Vp.
Figure 4-22 Vp and Poisson's ratio structure of Dabie Xiaotian-Susong line (the first and second numbers in brackets respectively)
(According to Wang Chunyong and others, 1997)
4.4.2. 1 Determination of rock types in deep crust
Gao Shan et al. (2002) studied the general characteristics of rock seismic wave velocity based on the experimental results of rock wave velocity, and put forward the following methods to estimate the in-situ wave velocity of deep rocks and determine rock types. Gaoshan, 2002, petrophysical properties and crustal structure-rock model in Dabie-Sulu area (final report of natural science foundation project).
：
(1) At a constant temperature, both Vp and Vs of rock show a nonlinear rapid increase with the increase of pressure when P < 200 MPa (about 8km), and then show a nearly linear slow increase. This characteristic corresponds to the characteristics of volume change, which proves that the nonlinear segment mainly reflects the closure effect of microcracks in rocks at low pressure, while the linear segment represents the intrinsic characteristics of rocks. Because of the existence of shallow micro-cracks in the same rock, it shows obvious anisotropy of wave velocity, which decreases obviously with the increase of depth. Therefore, the influence of rock micro-cracks must be considered in seismic sounding interpretation of shallow crust. In order to eliminate the influence of rock microcracks on wave velocity under shallow low pressure, the calculation method of in-situ wave velocity of deep rock is obtained. Gaoshan et al. developed and put forward the following calculation formula:
V(z)= V(0)+[(dV/DP)Tp+(dV/dT)pT]dz( 1)
Figure 4-23 shows the Vp and Poisson's ratio of ultra-high pressure rocks and related rocks at different depths calculated by formula (1) according to the current heat flow value and the variation coefficient of measured wave velocity with temperature and pressure in Dabie-Sulu area.
(2) The restriction of rock composition in the deep crust. According to the obtained average seismic wave velocities of different rocks and their coefficients varying with temperature and pressure, and the average surface heat flow in Dabie-Sulu area, the VP isolines of three possible main rocks in Dabie-Sulu middle crust and lower crust are calculated from (1) (Figure 4-24). This map can be used to quantitatively estimate the rock composition of the middle and lower crust in this area.
③ Poisson's ratio of eclogite. The Poisson's ratio of mantle peridotite (0.26) obtained by predecessors is difficult to distinguish from eclogite (0.27). Based on the systematic study of surface eclogites in Dabie-Sulu area, it is found that eclogites often deteriorate with the decrease of temperature and pressure during exhumation, and water-bearing minerals (including crystal water) are often produced in rocks, such as amphibole and chlorite, so the crystal water (H2O+) of rocks directly indicates the retrogradation degree of eclogites, that is, the higher the metamorphic degree, the higher the H2O+. At the same time, through the comparison of rock wave velocities, it is proved that metamorphism can improve the Poisson's ratio of eclogite, and the Poisson's ratio of fresh eclogite with H2O+< 1% is 0.24 ~ 0.25, which is obviously lower than that of peridotite. Therefore, Poisson's ratio provides an effective method to distinguish deep eclogite from peridotite. See Gao Yang (200 1) for a detailed discussion.
Crustal structure-rock and chemical composition model of Lu 'an-Huangshi section in Dabie orogenic belt, 4.4.2.2
Based on the above research results of crustal wave velocity structure and experimental data of rock wave velocity, along the sounding profile of Lu 'an-Huangshi earthquake completed by Yuan Xuecheng and others, the rock geochemical profile of Dabie collision orogenic belt as shown in Figure 4-25 is established. Pictured: ①VP isoline distribution; (2) The possible rock composition of each tectonic layer in the crust, especially the eclogite content range; ③ Because the content of SiO _ 2 determines the characteristics of major elements and trace elements in rocks, the average content of SiO _ 2 in each layer is given in this figure. Among them, the upper crust rock composition prediction is mainly based on Figure 4-23, and the middle and lower crust rock composition prediction is mainly based on Figure 4-23 and Figure 4-24. According to the above inferred rock composition and corresponding chemical composition, the calculation of SiO2 _ 2 content in each layer is obtained. Further explanation is as follows:
(1) As can be seen from Figure 4-25, the middle crust is a low-speed zone with a Vp of 5.6 ~ 5.9 km/s, even lower than that of felsic gneiss (6.30 km/s) under the condition of the middle crust, indicating that the amount of eclogite (Vp=7.7km/s) is very small even if it exists. At the same time, it shows that this layer is a structurally weak zone or contains fluid, which makes its wave velocity even lower than that of felsic gneiss. Lead and Neodymium Isotopes of Yanshanian Granites in North Huaiyang, North Dabie, South Dabie and Susong in Dabie Area (see Figure 4- 14 and Zhang Hongfei, etc. , 200 1). Although the structural properties of these four units are quite different, the Pb-Nd isotopic composition of their Yanshanian granites is surprisingly consistent, that is, they are all very consistent with gneiss of Peking University. Because granite comes from the middle and lower crust, it is fully proved that Nanda UHP unit is a thin-skinned structural unit confined to the shallow crust. Therefore, the restriction of rock physical properties on seismic data and isotope tracing of granite source region prove that the Dabie middle crust no longer contains eclogite.
Figure 4-23 Vp and Poisson's ratio of Dabie-Sulu UHP rocks and related rocks calculated at different depths
(2) The Vp of the upper Dabie lower crust is between felsic granulite and mafic granulite, which is closer to mafic granulite, and its Vp can be fitted by 80% mafic granulite +20% felsic granulite.
(3) The Vp and Poisson's ratio of Dabie lower crust are consistent with mafic granulites, so it is basically composed of mafic granulites. The study of global lower crust inclusions and the study of lower crust inclusions in Cenozoic volcanic rocks such as Hannuoba and Nvshan show that mafic granulite inclusions from the lower crust are mainly the products of mantle-derived basaltic magma underplating, and their ages are obviously later than those of granulites exposed on the surface. For example, the granulite exposed on the surface of Hannuoba area is 2.5 ~ 2.7 Ga, while the granulite inclusion is 120 ~ 140 Ma, which is the product of Mesozoic underplating (Fan Qicheng et al.,1998; Liu et al., 200 1). The study of lower crust inclusions and magmatic rocks in North China, Yangtze and South China shows that mantle-derived magma underplating of 120 ~ 140 Ma is widespread in eastern China. The age of the Dabie orogenic belt is 120 ~ 130ma (Hacker et al.,1998; Jahn et al., 1998) indicate that the underplating in this era may also exist widely in the Dabie orogenic belt. Therefore, geophysical, geological and chronological studies show that the lower crust of Dabie Flow is probably composed of mafic granulites formed by Mesozoic mantle-derived underplating. Of course, there is still no direct evidence to support the inference of the age of the invasion, and further research is needed.
Fig. 4-24 end-member mixed isolines of three possible main rock components in Dabie-Sulu Middle Crust and Lower Crust.
Figure 4-25 Model Diagram of Crustal Structure and Petrochemical Composition of Dabie Lu 'an-Huangshi Section (Vp structure is based on unpublished data of Yuan Xuecheng, etc. )
(4) The Poisson's ratio of the lower crust and upper mantle of Dabie orogenic belt is 0.265 ~ 0.280, which is obviously higher than eclogite and even higher than metamorphic eclogite, so even if eclogite exists in the lower crust and upper mantle, its content is very small.