The Karakorum-Jiali fault zone was proposed by Armijo( 1989) and is considered as the southern boundary of the Qinghai-Tibet Plateau. However, there are few studies on the spatial distribution and Quaternary activity of such an important regional fault zone. In Tapponiner (1982) Cenozoic tectonic deformation model, the south boundary of plateau compression is Karakorum-Kerry fault, which consists of three fault zones with different evolution histories. The western section of Bangong Lake is along the northwest section of the right-lateral strike-slip fault in Karakorum, the middle section is distributed along the Bangong Lake-Nujiang suture zone along the Gaize and Dongcuo lines, and the eastern section is from Dongcuo to the east, passing through Chiayi. There are many questions about the spatial position of the fault zone extending to the southeast. Amizhuo (1989) thinks that there are two possibilities: one is to cross the north of Chayu along the Palong Zangbo Valley and then go straight into the Nujiang Valley; The other is that the turning SE is connected with Xiachayu fault (Figure 5-26); Ren Jinwei et al. (2000) and Shen Jun et al. (200 1) advocated the latter connection scheme, arguing that there is no continuous south boundary fault with high right-lateral strike-slip rate in the plateau squeezed eastward.
Figure 5-26 Distribution Map of the Eastern Section of Jiali Fault Zone
Jiali fault zone refers to the eastern part of Karakorum-Jiali fault zone and its eastern extension. The fault starts from the south of Naqu in the northwest, passes through Sangdi, Maidi Zangbo, Aza and Laojiali in the southeast, and reaches Tongmai along the Zangbo River in Gong Yi. Then walk southeast along Gongriggebuqu and leave the country through Shangchayu and Xiachayu. According to the fault structure, it can be divided into three parts from west to east: one is the NW-trending fault in Naqu South-Samba section; The second is the NWW fault along the Zangbo River in Gong Yi; The third is the NW-trending Tongmai-Xiachayu fault. Generally speaking, the fault belongs to Cenozoic strong deformation zone, and some faults have strong Quaternary activity, which is of great significance to railway engineering safety. We made a field investigation of Jiali fault zone, focusing on the tectonic activities from Naqu (Dangxiong) to Xiachayu, and gained some new knowledge.
Second, the characteristics of Quaternary activities before the fault
Between Nyainqentanglha Mountain and Samba, a brittle-ductile deformation zone with a width of 2 ~ 5 km is distributed in the east-west direction in the middle of Lhasa block. Armijo et al. (1989) and the new generation1:1.5000 geological maps of the Qinghai-Tibet Plateau and its adjacent areas all connect this fault zone with the ne-trending fault in the southeast of Nyainqentanglha Mountain, and the latter continues to connect with NWW to the west. In order to distinguish it from Quaternary Kerry Fault, Armijo et al. (1989) named the east-west Cenozoic shear zone between Nyainqentanglha Mountain and Samba as "Dangxiong-Kerry Fault". Due to the limited data, this book only discusses the fault characteristics east of Yangbajing, and adopts the fault name of Armijo et al.
1. Yangbajing-Samba section
On the southeast side of Nyainqentanglha Mountain, a large ductile deformation zone gently dipping to the south is formed, which is generally called Nyainqentanglha ductile shear zone (NSZ), generally trending northeast and southeast with an inclination of 20 ~ 30. It extends more than 95 kilometers along the strike, forming the dividing line between Nyainqentanglha Mountain and the southeast of Dangxiong-Yangbajing Basin. Nyainqentanglha ductile shear zone is mainly developed in the southeast of Nyainqentanglha Miocene granite, partially passing through the remnants of middle-deep metamorphic rocks and Carboniferous-Permian shallow metamorphic rock series in Guyu Nyainqentanglha Group of Upper Proterozoic (photo 5-7- 1), passing through Yangbajing in the southwest, then turning to the west to connect with NWW-trending faults, extending to NE-EW-trending foliation zone and cleavage zone, and finally extending to Sangba in the northeast. The exposed width of mylonite in Nyainqentanglha ductile shear zone is generally 1 ~ 3 km, and the exposed width of mylonite in the west of Yangbajing basin is 4 ~ 5 km, which is mainly composed of granitic primary mylonite, mylonite and mylonite schist. The early mylonite in Nyainqentanglha ductile shear zone is equivalent to the eyeball-shaped granite gneiss described by predecessors, mainly distributed in the northwest of the ductile shear zone, with σ-shaped felsic eyeball-shaped residual spots, 2-6 cm long and 0.5~2.5 cm wide, and mylonite schistosity between the residual spots. The size of the eyeball-shaped residue of felsic is generally 3 ~ 10 mm, and its content is about 50% ~ 75%. Aggregate is eyeball-shaped, and together with shear foliation, it constitutes S-C fabric.
The S-C fabric of mylonite shows that the upper wall of ductile shear zone generally moves in the east-west direction, which belongs to a regionally important extensional detachment structure. In the strong deformation zone, greenschist facies-low amphibolite facies dynamic metamorphism occurred, forming dynamic metamorphic rocks such as chlorite schist, sericite schist, feldspar quartz schist, biotite schist, metamorphic rock, biotite plagioclase schist and biotite quartz schist, and the early formed Leng Qing granite gneiss and Nyainqentanglha group had obvious greenschist facies degeneration.
There are a large number of weakly deformed granites and metamorphic rocks in Nyainqentanglha ductile shear zone. In some areas, strongly deformed mylonite zones and weakly deformed rocks are alternately distributed, and felsic dikes with different thicknesses are filled along some mylonite schists, forming a layered structural landscape. Early mylonite and mylonitized granite are developed in the northwest of ductile shear zone, which belongs to weak ductile shear deformation; It gradually transforms into felsic mylonite and mylonite schist with different compositions to the southeast, and develops wiredrawing structure and core-mantle structure, which belongs to the part with strong ductile shear deformation.
Nyainqentanglha ductile shear zone extends to ne direction, and its width gradually decreases until the mylonite zone of Dangxiong Power Station disappears and becomes a physical-chemical zone and cleavage zone. With the NE-trending inclination and extrusion, the exposed area of granite in Nyainqentanglha Mountain gradually decreased until the northeast section of Nyainqentanglha Mountain disappeared, and the overlying Carboniferous strata were widely exposed. In the cleavage and foliation zone on the northeast side of NSZ, Carboniferous sedimentary strata have undergone strong dynamic metamorphism and solid plastic rheology, forming a large number of structural foliations and fault steps (photo 5-7- 1) and structural lineations dipping in ne direction, as well as staurolite, sillimanite, garnet, hard chlorite, chlorite, sericite and muscovite. Along the strike, NSZ shows a trend of rising in the southwest and falling in the northeast. Nyainqentanglha ductile shear zone extends to the basement rock series in Dangxiong-Yangbajing basin along the SE direction, with an inclination angle of 20 ~ 30.
Figure 5-7 Field Geological Characteristics of Kerry Fault
With regard to the age of Nyainqentanglha mylonite, Harrison et al. (1995) conducted systematic sampling in Gulunqu, and completed 39Ar/40Ar thermal dating. The results show that the formation and rapid uplift time of NDS is 4 ~ 9 Ma BP, mainly occurring at 5 ~ 8 Ma BP, which corresponds to an important regional tectonic thermal event after the plateau uplift period.
On the western edge of Yangbajing-Dangxiong basin, the late high-angle normal fault cuts the early ductile shear zone and mylonite, which constitutes the boundary fault of Yangbajing-Dangxiong rift basin, forming a steep cliff and a linear fault triangle. The ductile shear zone in the eastern part of Nyainqentanglha Mountain has a dynamic genetic relationship with the high-angle boundary normal fault and the Yangbajing-Dangxiong Basin rift.
2. Samba-Tongmai Section
NWW-trending faults along Gongyi Zangbo River are characterized by linear valleys (Figure 5-27a and Figure 5-27b). The fault structure is single and consists of two new and old faults distributed in parallel. The geomorphological features of old faults are a series of passes or cliffs, which are distributed on the right bank or half slope of Zangbo River in Gong Yi. Thick limestone and Paleogene purplish red sandstone lenses distributed continuously along the fault zone (Figure 5-27a). The fault occurrence and lens are steep or vertical, and this landform extends nearly 100 km from east to west. In the south of Bata village, the structural slices of purple sandstone and limestone interbedded almost vertically along the fault zone, and limestone weathered into normal terrain. Purple glutenite can be seen in the east as a structural slice sandwiched in the vertical structural zone. The formation time should be since the late Cenozoic.
3. Tongmai-Xiachayu Section
The NW-trending Tongmai-Xiachayu fault is mainly distributed in Gongrigebuqu River Basin and along Gongrigebuqu. On the right bank of Gongrigabuqu, 7 km northwest of Xiachayu Town, the gray biotite plagioclase gneiss is gneiss-like. After ductile deformation, gneiss forms eyeball plagioclase fragments with directional arrangement, and its mylonite foliation occurrence is 60 ∠ 75, and the tensile lineation composed of biotite and other flaky minerals is150 ∠ 20. After passing through Shangchayu and Xiachayu, the fault turns to nearly north-south direction, extending to Myanmar and connecting with Sagaing fault, with a length of nearly 300 km in Tibet. The fault is characterized by the strong deformation and orientation of Neoproterozoic granite near Tongmai, and the gneiss of Gangdise Group is 20 ∠ 73. The rock consists of biotite dark mineral aggregate strips and felsic veins alternately arranged, mainly felsic, which have strong ductile shear deformation (photo 5-7-3) and extend to the SE complex. A 7 km long ductile shear zone is continuously exposed along the highway on the right bank of Sangqu River at 10 km east of Xiachayu Town. A huge ductile shear deformation zone consists of garnet biotite mylonite, eyeball granite mylonite and weak gneiss granite (photo 5-7-4). The original rocks are metamorphic Guyu metamorphic rocks and Mesozoic granite, and mylonite foliation and gneiss granite are stable, with a strike of 365,438+. The structural deformation characteristics show that the Tongmai-Xiachayu fault is a late Cenozoic dextral oblique thrust strike-slip fault.
Three. Quaternary tectonic activity of faults
1. The south Naqu-Samba section is NW-trending.
To the west of Samba, the main structural line is NW-trending, and the fault zone consists of NW-trending faults with obvious right-lateral strike-slip characteristics and nearly SN- trending extensional normal faults. The NW-trending faults are mostly basin-mountain boundary faults, which have obvious staggered landforms. The latest paleoseismic fault zone has been found and very fresh fault scarps can be seen in different periods. During the late Pleistocene, a fault scarp with a height of 0.5 ~ 3m was formed on the moraine between the east of Luorma and Zhekongma and Chongchanglang, and the spring water distributed linearly along the fault strike (100). The NNE fault and the NW-trending dextral strike-slip fault are closely related in genesis, which together constitute a special structural combination of the NNE extensional graben and the NW-trending dextral strike-slip fault at its end. Normal faults extending in NNE direction are developed at the boundary of graben or semi-graben, such as the normal fault scarp on the east side of Daren Basin, which is similar to the steep slope at the rear edge of landslide (Shen Jun et al., 200 1).
2. NWW fault along the Zangbo River in Gong Yi.
In Azaxi, there are class III river terraces on the east and west sides of the SN- trending river, and some terraces on the east side, but they have shifted eastward by1.500m.. The gray limestone on the west side of the bedrock stands upright, and the east-west Kerry fault runs through its north slope. Most of the new faults along Gongyi Zangbo River extend along the river valley, but the exact location is difficult to determine, and the signs of deep river being reformed by faults are not obvious. New fault activity is only found locally. Armijo( 1989) found a very clear seismic deformation zone on the south side of Lafen Basin in Aza District. During our field investigation, we also found an earthquake uplift in Dongerhe terrace in Aza District, but the scale was small.
Paleogene purplish red sandstone and conglomerate are exposed in the west of Laojiali County, in which there are many S-trending faults. Scratches and steps are densely developed on the section, indicating that the fault movement is normal. Purple sandstone and conglomerate are both fragments and lenses, and extremely thick limestone lenses are sandwiched between purple gravel. The landform is normal, the fault bandwidth is 1040 m, and the south side is in contact with the Upper Paleozoic sandstone fault. Black fault gouge with a thickness of 4 m is formed in the contact zone (Figure 5-27e). There are many gray-white timely sandstone lenses with strong fold deformation left in the fault gouge, and the granite on the north side forms a wide 10 m broken rock zone, but there is no ductile deformation, which may belong to Quaternary deformation.
From Gongyi Lake to Suotong, there are 8 hot springs or boiling springs distributed in a straight line along the fault. The temperature of Changqing boiling spring in Tongmai can reach 94℃, and the water volume can reach 6L/s. At the same time, this section is also an earthquake-prone area, especially the Yanqiao area where the right step of Gongyihu No.2 fault intersects obliquely (Figure 5-26). Fault activity and seismic activity often lead to the emergence and revival of landslides, so this section is highly active at present. This shows that since Holocene, although the activity of Jiali fault zone is not strong, it has shown strong activity in some areas adjacent to the extensional basin, and the activity outside the basin has obviously weakened. This phenomenon is very similar to Fengyi-Dingxiling fault in the northern section of Honghe fault in northwest Yunnan and other fault zones connecting basins and canyons. From the above facts, the strike-slip activity of Jiali fault is accompanied by extensional activity.
3. The northwest Tongmai-Xiachayu fault.
According to the interpretation of satellite images, predecessors thought that Jiali fault extended southeast along Gongrigebuqu. Due to the strong downward cutting and stable extension of the river, the direction of water flow is consistent with the direction of regional tectonic line. This phenomenon makes people believe that the Jiali fault or its branches, which are still active in the Quaternary, may extend more than 300 kilometers along Gongrigebuqu (Armijo et al., 1989). In order to confirm the above understanding, we traced it from Xiachayu to the north of Shangchayu along Gongrigebuqu in the summer of 2006. Due to military control, we only observed the part 50 kilometers north of the river.
Along Gongrigebuqu to the north, a tug-of-war gravel layer of 150 ~ 200m with a thickness of 3 ~ 5m can be seen along the way. Gravel is round, round-sub-round. Gravel is mainly composed of gray granite, with gravel particle size 10 ~ 40cm, with gravel layer locally and thickness of 0.5 ~1m. These gravels are undoubtedly the remnants of Gongrigabuqu highland, and the TL sample taken from the fine sand interlayer is 27.912.37 kabp, which indicates that Gongrigabuqu has suffered a strong undercut of about 200 m since the late Pleistocene, and the undercut rate is as high as 7.2 mm/a, which is consistent with the uplift rate since the Pliocene of the East Himalayan structure. The fission track age of apatite shows that the uplift rate in this area has reached 5 ~ 65,438 00 mm/a since 65438±0ma BP, and it still remains at 65,438 000 mm/a (Ding Lin et al., 65,438 0995). Therefore, it can be inferred that the rapid uplift of the East Himalayan structure since Pliocene is the main reason for the strong undercut of Gongrigabuqu. Because this area is located in the southeast of the East Himalayan tectonic knot, the tectonic uplift rate is high, and the rainfall in this area is large and the river action is strong, it is difficult to see the preservation of river terraces in most areas. Tug-of-war 10 m, 25 m class I and II river terraces are occasionally seen on the left bank of Gongrigabuqu. These terraces, together with the alluvial and diluvial fans superimposed on them, are intact, and no signs of fault dislocation are found (Figure 5-27c). However, the limited investigation at present can not rule out the possibility of Quaternary activity in this fault section. On the one hand, there are 45 ~ 63℃ hot springs along the fault in Bendui and Bicun, indicating that the fault is still active. On the other hand, the earthquake with M = 8.6 of 1950 is located less than 30 kilometers west of the fault, which Molnar and Deng (1984) think is a thrust fault along the Himalayas. If so, this fault should be an important seismogenic fault in the eastern Himalayan tectonic junction area.
Figure 5-27 Structural Deformation Profile and Photos of Kerry Fault Zone
To sum up, Jiali fault belongs to late Cenozoic and Quaternary active faults, and it is an important regional fault structure in the southeast of Qinghai-Tibet Plateau. Its quaternary activity is segmented, and the strong activity is limited to the fault oblique bridge area and the extension basin area, which controls the seismic activity, the distribution of hot springs and the formation and evolution of landslide disasters. Gongyi Lake and Tongmai section are the most affected areas.