In the figure, A, B, C, D and E indicate the positions of the following paragraphs: Figure 6.9, Figure 6. 10, Figure 6.1,Figure 6. 14, Figure 6. 15.
Ailaoshan-Honghe-Yinggehai fault is a strike-slip fault between Indo-China Peninsula and South China (Figure 6.8), which runs northwest-southeast, and is a suture zone between South China block and South China block. It runs through China, Yunnan Province, northern Vietnam, Beibu Gulf and Yinggehai, with a total length of 1000km or more. The Cenozoic has been developing dextral strike-slip activities for a long time (Allen, 1984). However, Tapponer (1986, 1990) thinks that after the collision of India-Eurasia plate, the Indo-China block was pushed southward by 1000 kilometers, so the Red River-Yinggehai fault has left-lateral strike-slip activity at 23Ma±0.2ma, and the strike-slip distance exceeds 100. However, according to the geological characteristics of this strike-slip zone, Leloup( 199 1) and Lacassin( 1993) think that the left-hand strike-slip distance is between 300 and 700 km. Rangin( 1995), after studying the geological and geophysical characteristics of the fault zone, thinks that the Honghe-Yinggehai fault underwent structural inversion before 30Ma, and before that, there was a large-scale NE-SW extension activity, that is, dextral strike-slip movement. Between 30 ~ 5.5 Ma, it moves left-handed intermittently in a 30km wide zone, and the left-handed distance is very small. After 5.5Ma, there is a slight right-lateral strike-slip activity. Figure 6.9 is the seismic profile of the Red River estuary. As can be seen from the figure, the red river fault has a negative flower-like structure on the profile, indicating that it is a strike-slip fault. The time of unconformity surface is 5.5Ma, which is determined by drilling data. After 5.5Ma, the Honghe fault is basically inactive.
Fig. 6.9 Seismic profile of the Red River fault crossing the mouth of the Red River.
See fig. 6.8 a for location. In the figure, Tg is the basement reflection wave and T3 is the interface reflection wave at 5.5Ma, and the time is determined by drilling data.
The Honghe-Yinggehai fault extends southeast to the sea area south of Hainan Island, and intersects with the NE-trending Xisha Trough fault and the NW margin fault of the South China Sea. Taylor et al. (1980, 1983) believe that during Cenozoic seafloor spreading in the South China Sea, the faults in the western margin of the South China Sea are the western boundary of the South China Sea tectonic activities, which they call the transformation margin. Hayes( 1985) used a simple tensile model to explain the history of the Cenozoic seafloor spreading in the South China Sea. It is considered that the eastern part of the South China Sea has undergone submarine expansion and the western part has undergone crustal extension. Between 30 ~1000 ma, the length of the new ocean basin in the east is 600km, so the elongation coefficient of the crust in the west margin is 5. This elongation is mainly along the eastern fault plate of the fault in the western margin of the South China Sea, so the fault in the western margin of the South China Sea has undergone dextral strike-slip activities during this period.
We believe that during the Shenhu movement in the northern South China Sea from the end of Cretaceous to the early Cenozoic, the crust in the southern margin of the South China Block extended to the southeast, and its western boundary was the fault in the western margin of the South China Sea. At this time, there is no tectonic activity in the crust of Indochina block. Therefore, during this period, dextral strike-slip activities occurred in the western margin of the South China Sea. During the seafloor spreading in the South China Sea (during 42 ~ 35 Ma, the seafloor spreading occurred in the southwest basin of the South China Sea, and during 32 ~ 17 Ma), the faults in the western margin of the South China Sea were still dextral strike-slip activities during the first seafloor spreading. Fig. 6. 10 is the seismic profile of the Indo-China continental shelf across the western margin of the South China Sea (Yao Bochu, 1999). As can be seen from the figure, the fault zone has a negative flower-like structure on the profile. According to the chromatographic data in Chapter 3, the fault reaches the bottom of the lithosphere. The fault zone consists of two faults, the width from east to west is 10km. The western faults affected the seabed, indicating that there was still activity in Quaternary. The eastern fault stopped active at the end of Miocene.
Fig. 6. Seismic profile of10 fault in the western margin of the South China Sea passing through the eastern continental margin of Indo-China Peninsula (see B in Figure 6.8 for location).
From the above discussion, it can be seen that the Ailaoshan-Honghe-Yinggehai-western margin of the South China Sea Fault is the Cenozoic tectonic boundary between the Indosinian block and the South China block, and it is also the western boundary of the Cenozoic tectonic activity of the South China block. During the Shenhu movement and submarine expansion in the South China Sea (Late Cretaceous-Early Paleocene, 42 ~17ma), the fault developed dextral strike-slip activity. After 17Ma, the fault may have experienced left-lateral strike-slip movement. After the middle Miocene, the fault changed to dextral strike-slip activity.