The geological process that the Indo-Australian plate has the greatest influence on the tectonic evolution of the Eurasian plate is the collision between the Indian block and the Eurasian plate in the Qinghai-Tibet region. In the late Paleocene, with the convergence rate of India and Eurasia rapidly decreasing from 1.70mm/y to about 60mm/y, India and Eurasia began to collide at 56Ma (late Paleocene) until the end of Middle Eocene (about 43.5Ma), and they all collided and gradually wedged into Eurasia. On the one hand, this large-scale land-land collision led to the mutual thrusting, overlapping and thickening of the continental crust in the Himalayan collision zone, on the other hand, it led to the compression movement of Indo-China Peninsula (Tapponier et al.1986, 1990), forming a large-scale escape structure in Southeast Asia (as shown in Figure 2-2).
Figure 2-2 Regional Dynamic Background of Yinggehai Basin
There are many giant strike-slip faults in Indosinian block and its edge as the boundary of escape structure. On the northeast side of the block is the NW Ailaoshan-Honghe fault, which is now dextral strike-slip and separates the South China block from the Indosinian block. Its extension length on land is about 1000km, and the core area of this zone is a ductile shear zone with a width of 10km. The study on the microstructure and macro-structural kinematics of mylonite in this zone proves that the shear zone has the movement process of left-handed shear, and the displacement is over 300 ~ 700 km (Tapponnier et al.,1990; Leloup et al., 1995), the isotopic age value is between 35 and 22 Ma (Scharer et al.,1990; Leloup et al., 1993, 1995). 40Ar/39Ar thermal chronology shows that the Red River fault zone experienced a slow cooling period between 34 and 25ma, and then a rapid uplift period between 25 ~ 17ma, and there was a sinistral slip component in the uplift process (Leloup et al., 1995). Quaternary geomorphology and modern earthquake focal mechanism reveal that the northern section of the Red River fault zone has been dextral strike-slip since 5Ma, and the slip rate is 7 3 mm/y (Leloup et al.,1995; Allen, 1984). A great deal of evidence shows that the Indo-Eurasian plate has collided since the late Paleocene, and its most remarkable structural effect is the relative movement between Indo-China Peninsula and South China block. Through the comparative study of paleomagnetism of Mesozoic sedimentary basins in Indo-China Peninsula and South China Block, the results generally support that Indo-China Peninsula moved left relative to South China Block after Mesozoic (Funahara et al.,1993; Yang and Besse, 1993), revealing that the ancient land mass of Indo-China Peninsula has different clockwise rotations in the left-handed movement.
The red river fault zone extends into the sea area in SSE direction, and the extension length of the sea area is also about 1000km. The tectonic activity in the southern section of the Honghe fault zone is much more complicated than that in the northern section. The area near the southern section of the Red River fault zone is composed of several parallel NW-trending fault zones, such as Heishuihe fault zone, Zhaihe fault zone, Qijiang fault zone and Majiang fault zone. All extensions in Hanoi and its sea area are rift-shaped. Rangin( 1995) studied the development and evolution characteristics of the Red River Fault Zone in Tokyo Bay Depression, Vietnam. The study reveals that before 30Ma, the mutual dislocation between the Indo-China block and the South China block was composed of the left-lateral strike-slip movement of several NW-trending fault zones, and the crustal deformation was mainly tensile, forming a series of extensional rifts (Rangin et al., 1995). After 30Ma, the sinistral strike-slip amplitude of the Red River fault decreases, only a few tens of kilometers, not exceeding 100km, and the interface of 15.5ma is an important interface. From 30Ma to 15.5 Ma, the left-handed strike-slip movement shows transformation extension, and from 15.5Ma to 5.5Ma shows transformation extrusion.
Yingxi fault zone in Yinggehai basin extends northward and is connected with Honghe fault, Heishuihe fault and Majiang fault. Ran-gin et al. (1995) studied the structural deformation pattern and evolution in this area in detail. After the Yingxi fault zone extended northward to Tonkin Bay depression in Vietnam, the structural style changed into a set of wavy fold development areas with a width of about 30 kilometers, accompanied by obvious thrust faults. This strong inversion occurred in the north-south Yingxi fault and the arc transition section of the northwest Zhaihe-Honghe-Qijiang fault zone. Rangein (1995) identified it as the product of structural inversion in S50-S30 (15.5 ~ 5.5 Ma), and considered it as a set of fold and thrust structures formed under the mechanism of sinistral extrusion. Rangin( 1995) also proved that before 30Ma, the Tonkin Bay in Vietnam was mainly characterized by regional extensional rifting under the background of sinistral strike-slip, but after 30Ma, the sinistral displacement of the Red River fault was obviously reduced, not exceeding several tens of kilometers, and the range from 30Ma to 15.5Ma was a transformation extension in the sinistral strike-slip deformation field. However, from 15.5Ma to 5.5Ma, it is the transformation and compression in the sinistral strike-slip deformation field, and the deformation area is limited to the range less than 30km wide in the boundary fault zone, and the intensity of structural inversion gradually weakens from NW to SE, which does not affect the Yingxi fault zone. On the contrary, during S50~S30, the Lingao uplift area is in a weak extension state. The simple model shown in Figure 2-3 can explain the situation that different tectonic stress fields are displayed at the same time and in different positions (that is, during S50~S30, the Tokyo Bay area in the north is compressed and the Yingxi-Lingao area in the south is stretched). The profile in the picture is similar to the red river fault zone with a large dip angle. During S50~S30, although its displacement has been sharply reduced, it is still in a left-handed slip state, and during the slip process, it rotates clockwise around an axis perpendicular to the section (rotation arrow 1 in Figure 2-3), forming a "fulcrum fault". At the same time, paleomagnetic data show that the Indosinian block rotates clockwise and horizontally in the process of sliding to the southeast. The Indosinian block moving from the north side of the pivot line to the southeast and its clockwise horizontal rotation act on the arc transition section of the north-south Yingxi fault and the northwest Zhaihe-Honghe-Qijiang fault, which can lead to the left-handed compressive deformation field and form strong thrust faults and compressive folds. On the south side of the fulcrum line, the fault block rotates downward, and the clockwise and horizontal rotation of the Indosinian fault block leads to the tensile stress field, showing the normal fault (through or concealed). With the Indosinian block moving towards SE, the force source of inversion comes from NW side, so the intensity of tectonic inversion gradually weakens or even disappears to the south.
Figure 2-3 Simplified model of relative movement between Indosinian block and South China block near Honghe fault zone.
It is precisely because of the above dynamic background that Yinggehai Basin began to expand in Eocene. Although the Late Oligocene-Miocene has the characteristics of thermal subsidence stratum structure, the analysis of subsidence history shows that it has very rapid and episodic subsidence. The formation of these extensional basins is related to the extension caused by the clockwise rotation of Indosinian block and its secondary blocks in the process of large-scale compression and escape to the southeast.