19 12, Wei Gena put forward the hypothesis of continental drift, which caused great controversy. Until the early 1950s, British geophysicists quantitatively proved that the continent had drifted in geological age in paleomagnetism research, thus reviving the theory of continental drift. Paleomagnetism is one of the three pillars of plate theory.

In the long geological period, the geomagnetic field has the characteristics of axial geocentric dipole field. Therefore, the position of ancient magnetic poles can be calculated by using the residual magnetization direction of rocks, that is, the position of geographical poles at that time. Because the earth's magnetic field has the characteristics of axial geocentric dipole magnetic field, the earth has only one geomagnetic pole or geographical pole at the same time, just as the geomagnetic pole calculated from lava in modern continents is located near the geographical pole. On the contrary, the obvious unconformity between continents on the magnetic pole indicates that there has been translation or rotation between continents.

Polar drift route is an important basis for studying continental drift. From the polar drift curve, we can not only know the movement and direction of the continents, but also know the main relationship between them and the age of separation and drift from the polar drift route of the continents.

Draw the epipolar shift line (south magnetic pole) of South America and Africa on the same picture. As shown in figure 3-8- 13, the two obvious pole shift routes obviously do not overlap. The trends of the two routes are very similar, both of them gradually approach from the equator with age, and finally intersect at the south magnetic pole. The obvious pole shift in South America is always in the west of Africa, just as the South American continent is in the west of Africa. If the African continent is fixed, according to the shape of the continental shelf, the American continent will move eastward to fit the African continent, and its apparent pole shift route will also move eastward, as shown in Figure 3-8- 13. Before Mesozoic, the apparent pole shift routes of the two continents were basically the same, but after Mesozoic, the apparent pole shift routes diverged. This paleomagnetic study proves that the South American continent was connected in Paleozoic, when there was no Atlantic Ocean. The Mesozoic (Jurassic) began to split, and the South American continent drifted westward and rotated clockwise, forming the distribution of the two continents today.

This paper analyzes two road maps of obvious pole shift in Europe and North America. These two road maps are different, but the trend is similar. If North America rotates eastward along its obvious pole shift route at the center of the Arctic, the continental shelves of North America and Europe will close and the North Pacific will disappear. There is a good overlap from Silurian to Permian; However, after Triassic, two obvious pole shift routes were separated. It can be seen that before Triassic, Europe and North America were linked together, forming the ancient continent of Europe and America. After Jurassic, Europe and North America split to form the Atlantic Ocean and drifted to its present position.

Figure 3-8- 13 obvious pole shift routes in South America and Africa

(Shen Ninghua Guan Zhining 1985)

∑, S, D, C, P, T, K, Tr and Q are Cambrian, Silurian, Devonian, Carboniferous, Permian, Triassic, Jurassic, Cretaceous, Tertiary and Quaternary respectively; Mz is Mesozoic; L stands for bottom and u stands for top.

(2) Paleomagnetic evidence of seafloor spreading

Wilson comprehensively explained the mechanism of continental drift by using mantle convection and submarine expansion hypothesis, and its effect is simply shown in Figure 3-8- 14A. The newly formed crust and the volcano covering it gradually split into two sides and moved westward (Figure 3-8- 14B). When an advancing continent encounters a descending countercurrent, the movement will inevitably stop and pile up in front of the lighter continental crust to form mountains; At the same time, because the seabed was dragged down by falling fluid, a deep ditch was formed.

Figure 3-8- 14 Schematic Diagram of Mantle Convection and Submarine Expansion

The discovery and interpretation of seafloor zonal magnetic anomalies strongly support the hypothesis of seafloor spreading. The reversal of geomagnetic polarity quantitatively explains the hypothesis of zonal anomaly of the ocean and submarine expansion.

Since 1950s, large-scale aeromagnetic surveys have found that marine magnetic anomalies have the following characteristics: ① The magnetic anomalies are distributed in strips, and the strip strike is parallel to the ocean ridge. ② Positive and negative anomalies alternate, with a bandwidth of 20km~30km, a length of several hundred kilometers and an anomaly amplitude of several hundred NAT. ③ Magnetic anomalies are symmetrically distributed on ocean ridges. The above anomalies are called marine zonal magnetic anomalies, which are quite different from those on the mainland. Fig. 3-8- 15 magnetic anomaly map of Reykjavik ridge in southern Iceland, where black represents positive anomaly and white represents negative anomaly. Fig. 3-8- 15 is a plane section, where AA' is the location of Reykjavik Ridge, and the section curve records the magnetic field intensity of each point on the section. The strong magnetic field and weak magnetic field are symmetrically distributed on the profile and can be tracked continuously on adjacent profiles. The strong magnetic field corresponding to each cross section is connected into some strips by dotted lines, which is equivalent to the black part on the plan. Seen from the plane section, the magnetic anomaly intensity on both sides of the ridge is also basically symmetrical.

This symmetric positive and negative zonal magnetic anomaly in the ocean is observed not only on the Atlantic ridge, but also on the ridges of the Pacific Ocean, the Indian Ocean and the Antarctic Sea. This phenomenon can be reasonably explained by the theory of seafloor spreading and geomagnetic polarity inversion.

Fig. 3-8- 15 zonal magnetic anomaly in Reykjavik Ridge

(quoted from Hai Zi Le, 1966)

1963, Vain and Matthews put forward a hypothesis: the hot matter in the mantle rises to the ocean ridge by convection, and when it cools to Curie point, it obtains the thermal remanence in the same direction as the geomagnetic field at that time. The fluid keeps upwelling, pushing the old seabed to expand to both sides and forming a new ocean floor on the mid-ocean ridge. In the process of submarine expansion, the geomagnetic field turns over many times, and the seabed formed by normal geomagnetic field has positive magnetization; The seabed formed by the reverse geomagnetic field has reverse magnetization. Therefore, the seabed at different distances from the ridge is composed of magnetic rock layers with alternating positive and negative magnetization. Fig. 3-8- 16 is a schematic diagram of seafloor spreading and earth magnetic pole overturning. The symmetry of magnetic anomalies on both sides of the ridge is the result of the equal expansion speed on both sides of the seabed.

(3) applying paleomagnetism to study regional geological structure

After primary remanence (TRM or DRM) is obtained in the process of rock formation, if tectonic movement occurs, the relative positions of rocks in different parts of the structure will change during the formation process. In this way, the stable primary remanence preserved in the rock also changes its spatial position with the rock carrier. If we measure the stable remanence direction in rocks in different parts of modern structures and find out the relative variation law of their directions, we can infer and verify the way and direction of tectonic movement in turn.

Most scholars believe that the famous Tancheng-Lujiang deep fault in eastern China is a left-lateral translation fault. However, there are different views on the time and distance of translation. The paleomagnetic investigation of CAMBRIAN and Jurassic strata on the east and west sides of the fault zone by the Institute of Geology of the State Seismological Bureau provides meaningful data for solving the above problems. As shown in Figure 3-8- 17, in the east of the fault zone, the magnetic declination of Fuxian in the early Cambrian is 338, and that of Wulian in the late Jurassic is 7, indicating that the latter rotates 29 clockwise relative to the former, while the magnetic declination of Suxian in the early Cambrian and Huoshan in the late Jurassic is 42 and17 respectively. The above data show that the crust on both sides of the fault zone has its own independent movement mode, and at least before Jurassic, the strata on both sides have undergone relative movement.

Figure 3-8- 16 Comparison of seafloor spreading and geomagnetic overturning chronology

A —— Comparison between the measured geomagnetic anomaly profile and the estimated profile at the eastern Pacific uplift. The horizontal bars in the above picture are geomagnetic chronologies; B- submarine expansion indication

Fig. 3-8- 17 paleomagnetic declination map on both sides of Tanlu deep fault

According to the paleolatitude data of Early Cambrian on both sides of the fault zone, Fuxian area in the east is 39.2, and Suxian area in the west is 40.8, indicating that the two places are basically at the same latitude during this period. Today, the latitude of Fu County is 39.5, while that of Su County is 34. The comparison shows that the west side of the fault may have moved southward by 6.8, about 800 km. If the error in determining paleolatitude (about 5 ~ 6) is taken into account, the west side of the fault zone has moved at least 100km southward since Cambrian.