Geophysics
Geophysical data show the structure characteristics of the Qinling orogen that deep geophysical field is featured by nearly south-north trending anomalies, while the upper crust is dominated by east-west trending structure. Between them are the middle and lower crusts that are in a rheological state of the horizontally flattening.
1. Seismic Structure
Seismic studies (reflection and deflection) show that the Qinling lithospheric structure is extremely inhomogeneous (Fig. 8, Fig. 9, and Fig. 10).
The average of the crustal thickness is less than 40km(Zhang et al., 1995; Liu et al., 1995; Huang, 1992). But the crustal thickness gets thinner to the east as thin as 29 km and no mountain root exists. The crust becomes thicker toward the west up to 57 km and mountain root does exist there (Zhang et al., 1995).
The Moho is flat and shows a gentle south-dipping slope. The Moho in the east has been flattened and no mountain root survived, while the Moho in the west is now in the state of adjustment and thus mountain root can still be seen (Zhang et al., 1995).
The average velocities are 6.01 km s-1 in the upper crust, 6.24 km s-1 in the middle crust, and 6.89 km s-1 in the lower crust (Gao et al., 1998). And the average Vp of the total crust is 6.36 km s-1. From the south margin of the North China Block to the south Qinling, the velocity of the boundary of the crust and mantle all exhibits the clear shape of stairs. The velocity of the lower crust is 6.49-6.8 km/s and that of the upper mantle is 7.75-7.82km/s. The gradient is more than 1.0km/s. Therefore, there does not exist a mountain root in the East Qinling orogen (Jin et al., 1996).
The reflection at the Shangdan suture zone is listric-shaped and north-dipping in the profile. This area is actually a boundary separating different crustal velocity rates and seismically a transparent zone with few reflections.
Reflections in the upper crust are mostly continuous and overlapped, and show thin-skinned structures. The interweaving of opposite flat reflections occurs in the middle and lower crust, especially marked to the north of the Shangdan zone.
Fig. 8 Map showing location of the reflection seismic profiles (Yuan et al., 1994). 1. geological boundary; 2. fault; 3. inferred fault; 4. exploding seismic line; 5. reflection seismic line.
Fig. 9. Map showing the main reflection seismic interfaces (a) and crustal tectonic sketch (b) in the Qinling Orogen. The figures on the surface are the ordinal of reflection seismic poles. M = Moho; f1-f14 = inferred faults; Q = Quaternary; N = Neogene; E = Paleogene (Yuan, 1996).
The seismic reflection profile (Fig. 9) from Yexian of Henan across the Qinling Mountains to Nanzhang of Hubei shows that the crust of the Qinling orogen is composed of multiple crocodile-shaped structures wedging from south to north into the mid-crust. Two nappe systems, the South Qinling and North Qinling, may be recognized. The North Qinling nappe system was formed by decoupling, thrusting and stacking of upper crustal sheets of the North Qinling along a brittle-ductile transition zone. It is composed of the Luanchuan nappe, Waxuezi nappe, Erlangping nappe and Zhuxia nappe. The South Qinling Nappe system resulted from south-vergent thrusting of the upper crust of the South Qinling. It comprises the Douling nappe, Xingye nappe, Zaoyang Nappe, Shiyan Nappe and Xiangyang nappe. The subduction of the Yangtze block towards the North China block during the Palaeozoic had not caused the land in Qinling to be uplifted and become mountains. It was only when the Qinling-Yangtze block wedged into the mid-crust of North China in the Indosinian that Qinling was uplifted rapidly and became mountains. Therefore, the Qinling orogen is caused by wedging of the Yangtze block into the middle crust of North China block in crocodile type, and the lower crust of North China block subduced to the south, while the upper crust thrusted to the south (Yuan et al., 1994; Yuan, 1997).
2. Geoelectromagnetism
According to the figure 10, low-velocity high conductivity layers develop in the middle crust in the North and South Qinling orogen. In addition, in the northern part of the Qinling orogen there also appears a south-dipping low-resistivity layer at shallow level and a south-dipping high-resistivity body at deep level. Moreover, it is also demonstrated that a huge body with high resistivity exists in North Qinling crust while South Qinling is northward dipped systematically.
Geoelectromagnetic and geothermal flow studies show that the top surface of the asthenosphere fluctuates, deepening up to 250 km both in the North Qinling and in the southern edge of the Bashan arc and being at the depth of 110 km in the South Qinling. Considering that the Moho is relatively flat and 40 km deep on the average in the Qinling orogen, the lithospheric mantle thickness can be 150 km. Rapid elevation of the asthenosphere is undoubtedly of geodynamic significance.
Fig. 10. Geophysical cross-sections of the Qinling orogen (Zhang et al., 1995).
3. Gravity
Regional gravity fields of the Qinling show that it is just located between 2 NNE-trending gravity gradient zones of China, which respectively run along the Taihang-Wulingshan and the eastern flank of Tibet. Internally, it is an EW-trending gravitational low zone, which gets broader toward the east with the Pingdingshan area as its top. Furthermore, satellite gravity data show the gravitational variations on 2 sides of longitude 108° E in the Qinling. The east regions including of the Dabieshan is clearly regional gravitational high, but the west is gravitational low (Zhang et al., 1995).
4.Magnetic
Regional magnetic fields possess a two-layer structure. The shallow layer is complicated and local anomaly overlaps regional low magnetic anomaly field at depth. This situation is quite distinct from those of both the North China Block and the South China Block, but there still exist some NE-oriented magnetic anomaly zones in the Qinling, which penetrate from the North China Block and the South China Block.
5. Heat flow
The Qinling orogen is a region with high value heat flows, 72-109 mW/m2 on the average (Zhang et al., 1997). Geothermal gradient is an averagely 28°C/ km (Zhang et al., 1995).
