Phanerozoic orogenic belt
Orogenic belts formed by plate collision or unidirectional subduction have always been the focus of deep reflection seismic surveys. The orogenic belts before the Proterozoic Era have been discussed in the previous section. They have been cratonized, and traces of the orogenic shell roots can still be retained in the lower crust (Figure 6.3, top), but the mountains on the ground have long been flattened and no longer exist. Only by revealing the crustal structure of the young orogenic belts in the Mesozoic and Cenozoic can we have a complete understanding of this complex tectonic activity. Therefore, large-scale reflection seismic surveys have been carried out in the Alps and Tibetan Plateau in recent years.
Giant orogenic belts were formed in a compressional environment. Oblique reflectors produced by lithospheric collision and subduction are often combined to form some common reflection patterns in the orogenic crust, as shown in Figure 6.9 Show. Figure 6.9(a) shows a set of polygonal reflectors that look similar to the duplex building blocks that appear in extrusion environments. Some people call them "multi-story buildings", or duplexes. Figure 6.9(b) shows typical wedge-shaped reflectors (wedges), which reflect the collision and interpenetration of rock blocks and the thickening and shortening of the crust. If the wedge is large in scale and appears below the plate suture zone in the shape of a crocodile mouth, it is called a crocodile mouth reflector (Figure 6.9c). In actual seismic profiles, the above reflection modes often appear in combination. For example, in the lower crust of the Sulu terrane at the eastern end of my country's central orogenic belt (Figure 6.10), multiple wedge-shaped reflectors and a compound reflector appeared between 5 and 10 s, indicating that the Yangtze Plate in the south and the Sulu terrane are in Collision here (see Lecture 10 for details). There are multiple sets of southward-dipping reflectors in the upper crust (0~4.5s), which are consistent with the mylonite ductile shear zones seen on the surface. This section is located in the Dabie-Sulu ultrahigh-pressure metamorphic belt. Ultrahigh-pressure metamorphic eclogites and epicrustal rocks that have been exhumed from the mantle are exposed on the surface. Internal ductile shear zones are also well developed.
Figure 6.9 Three common combined reflection modes in extruded crust
(a) Compound reflector; (b) Wedge reflector; (c) Alligator mouth-shaped reflection The body, the circle represents the position of the crocodile's mouth
As can be seen in Figure 6.10, the Moho surface reflection of the young collision orogenic belt shows an inverted figure of eight, with an unsealed shell root in the middle, which seems to reflect the crust of a plate. subducting beneath another plate's crust. Chinese seismologist Mr. Zeng Rongsheng (1998) believes that this is a common reflection pattern on the Moho surface of orogenic belts. Figure 6.11 is a reflection seismic profile across the Pyrenees orogen (top) and across the Himalayas (bottom). The integrated geophysical profile of the Western Alps was shown in Figure 5.1. In the profile of the Pyrenees, the shallow mantle is relatively transparent, with a strongly reflective lower crust above it. The Moho surface is in an inverted splay, the lower crust is transparent along its axis, and there is a crocodile mouth reflector on its south side. European seismologists believe that the reflectors in the southern section of this section all tilt northward, reflecting the northward subduction of the African plate beneath the Eurasian plate, while the lower crust of the Eurasian plate is inserted above the subduction zone, forming a crocodile mouth. . The northern section of the profile has shear zones and reverse faults reflecting collision, which are divided into two groups: south-dipping and north-dipping. The axis of the collision zone shows transparent and weak reflection due to the vertical intrusion of magma, and is associated with foreland or backland basins on both sides. As for why there is a strong reflective layer in the lower crust, some people believe that it is caused by magma sheet intrusions and metamorphism after the orogenic period.
Figure 6.10 A section of the reflection seismic profile in the Sulu area
From Niushan to Zhaoji, Donghai County, Jiangsu Province, the southward-dipping reflection and surface in the upper crust (0~4.5s) The ductile shear zones are consistent; there are multiple wedge-shaped reflectors and a compound reflector in the middle and lower crust; the Moho surface is inverted splay
The seismic profiles passing through the Himalayas also have many similarities. On the south side of the Brahmaputra suture zone, there is a series of strong reflectors dipping northward, as shown by numbers (2) (4) (5) in the figure, reflecting the reverse faults formed by the subduction of the Indian plate. Unlike the Pyrenean section, there is a strong near-horizontal reflection below the Brahmaputra suture zone, which likely reflects the top interface of an ultramafic intrusion, or a modern magma chamber. There is a strong reflection of the Moho plane at 22s at the southern end of the profile (left). It is not reflected clearly north of Kangma, so the inverted figure-eight pattern cannot be seen. According to refraction and other data, the wave speed increases under the reflector (2). Mr. Zhao Wenjin and others believe that this may be a new Moho surface being formed.
Figure 6.11 Reflection seismic profile of a young orogenic belt
The unit of the vertical axis is two-way travel time (seconds). The upper picture shows a typical cross-section through the Pyrenees (Choukroune et al., 1989); the lower picture shows a cross-section through the Himalayas (Zhao Wenjin et al., 1996). Above it is the Buga gravity anomaly of the cross-section; the cross-section starts from Pali in the south and ends in the north. As far as Yangbajing, the numbers ① are the near-horizontal reflectors in the upper crust, ② are the "new" Moho plane, ③ are the Moho planes, ④ are the main Himalayan thrust faults, and ⑤ are the reflectors that cross the Brahmaputra River.
From the picture It can be seen from several deep reflection profiles such as Figure 5.1 and Figure 6.11 that the subduction of the plate is accompanied by more than one set of crustal thrust faults. For example, the Himalayan profile has two main thrust faults (4) and two sets of thrust faults crossing the Brahmaputra River in the north (5). This situation also occurs in the Sulu ultrahigh pressure metamorphic belt. Based on this, I proposed the evolution model of double subduction (Yang Wencai, 1997), which will be discussed in detail in Lecture 10.
The inverted splay feature of the Moho surface of a young orogenic belt, which reflects the crust root, may be preserved as the orogenic belt ages (the example above in Figure 6.3), or it may be denuded due to crust-mantle interaction. or uplifted to the lower crust due to tectonic movements. Deep reflection soundings indicate that in many Paleozoic or even early Mesozoic orogens, their shell roots no longer exist. For example, no shell roots were found in the long section through the East Qinling Mountains (Yuan Xuecheng, 1994). The Appalachian orogen in the eastern United States now also has no shell roots (Fig. 6.12). The Caledonian and Variscan orogens in Western Europe also have no shell roots. However, compound reflectors, wedges or crocodile mouths that reflect the characteristics of orogenic belts can often be retained in the lower crust (e.g. Figure 6.12b).
Figure 6.12 Three reflection seismic sections through the Appalachian orogen and their geological interpretation
(a) Southern Appalachian thin-skinned structure (Cook et al., 1981) ; (b) Through New England and central Green Mountain complex anticline (Brown et al., 1983); (c) Coastal eastern Appalachia, strong reflection of the lower crust (Phinney, 1986)
In the figure In 6.12(a), the reflection seismic pattern of the so-called "thin-skinned structure" is shown, that is, there is a very gentle detachment surface in the upper crust, which separates the basement below from the thin exotic rock sheet above, and the latter is formed by It is thrust from the southeast (see arrow in the picture). On the New England section (b), it can be seen that the thickness of the Green Mountain complex anticline and Taconic exosome is only about 5km, which comes from the overthrust of the middle and lower crust in the east, while the complex folds and thrust faults in the thin-skin structural belt are caused by the overthrust. formed by covering. The strong reflections from 4 to 10 s on the left side of the section reflect a series of nappe and thrust sheets. In Figure 6.12(c), the lower crust contains a large number of wedge-shaped bodies, compound reflectors and sub-horizontal layered reflectors, which are believed to be formed by the superposition of Paleozoic collisional orogeny and subsequent crustal extension. They also exist in the upper crust. The shear zone reflects the strong reflection of the suture zone. In all three profiles, the shallow mantle is relatively transparent, and the Moho reflection is discontinuous but relatively horizontal. Deep reflection seismic data revealed the existence of large-scale thrust-nappe and detachment detachment structures in the interior and edges of the continent, indicating that there are large-scale low-angle displacements and overlapping in the interior and edges of the plate. This is a common deformation of the continental crust. . What is the dynamic mechanism of this deformation? If the detachment of young orogenic belts is related to continental collision or plate subduction, then how did the thin-skinned structures formed by nappe formation in the middle and late Paleozoic orogenic belts form? These problems put the plate theory into trouble. For example, it has been suggested that many mountain ranges can form from horizontal stacking of intracontinental upper crust, independent of collisional subduction at plate edges. Therefore, in recent years, GPS has been used to conduct extensive displacement and deformation measurements on continental mountain chains. These data may reveal the connotation of modern continental deformation and related geological processes.