Fig. 5.9 contour map of Bouguer gravity anomaly extension 140km (unit: milligal)
Fig. 5. Isogram of Bouguer gravity anomaly of10 70 km (unit: milligal)
5.3. 1. 1 Meng Xing lithosphere block
This lithospheric block is called Ergon-Xing 'an block in China, which includes Russian Ergon block and Ren Gang block in the northwest and Mongolian Krulun block in the southwest. China lithosphere block is characterized by Daxing 'anling Mountains, with an average elevation of over 1000 meters, which corresponds to this place. This block is the region with the largest lithosphere and crust thickness in Meng Xing-Jihei area, with an average crust thickness of over 40 kilometers. Although the lithospheric thickness reflected by electrical structure and velocity structure is quite different, which is about 1 10km and 150km respectively, it shows that the maximum lithospheric thickness in this area is clear. Another important feature of this area on the lithospheric scale is the complexity of Moho surface (Figure 5. 12). The outstanding performance is that Moho is not a simple and continuous interface, but a transition zone with a certain thickness and internal structure, which is discontinuous in the transverse direction. The most complex Moho surface appears under the main ridge of Daxing 'anling, which is lenticular, about 200km long, with the maximum thickness of 12km. This part corresponds to the area with the largest crust thickness in this area, reaching 43km, and the crust thickness gradually decreases to both sides, especially to the east, the thinnest in the west is about 37km, and the thinnest in the east is about 32km. The seismic wave velocity in the crust has a relatively stable velocity layer above 6km, and there is no obvious horizontal velocity layer below 6km. The deeper the velocity layer, the greater the fluctuation. There is a low-speed wedge inclined to the west in the depth of 6 ~ 15 km west of boketu in the central part of this area and Qiqihar in the east. These two low-speed wedges respectively correspond to the Xinlin-Xiguitu ophiolite belt (Li Ruishan, 199 1) and the Nenjiang-Kailu fault zone exposed on the surface. Especially, the position, occurrence and depth of the low-speed wedge in the west of Qiqihar are completely consistent with the low-resistance body lying to the west revealed by MT in the west of Qiqihar (fig. 2. 16). Therefore, it is certain that there is an ancient subduction zone that subducts westward between Songnen block and Xing 'an block. Combined with the gravity data in this zone, it is obvious, but the Xinlin-Xiguitu zone is not obvious. The Xinlin-Xiguitu zone was formed earlier, and the deep material was basically balanced, while the Nenjiang-Kailu subduction zone was formed later. This is one of the reasons why we regard the Ergon-Xing 'an plot as a unit.
Fig. 5. Structural division of11Meng Xing-Jihei lithospheric block.
Fig. 5. 12 Lithospheric velocity structure profile in Ergon-Xing 'an area (according to Manzhouli-Suifenhe geological profile, Zhang Yixia et al., 1988).
1) crustal structural characteristics. With the increase of depth, the crustal structure of Meng Xing lithosphere block becomes more and more complicated. Although the vertical velocity stratification of the middle and lower crust in this area is not obvious, from the overall velocity structure, the crust in this area can be roughly divided into three velocity structure units in the vertical direction. The lower unit extends from 20km to Moho surface, indicating that the complex changes of the top shape of the upper mantle and the transition zone of Moho surface correspond to the violent fluctuations of the crustal velocity in the middle and lower parts. The middle unit is from 6km to 20km, which is characterized by alternating high-speed blocks and low-speed blocks along the horizontal direction and being divided by two low-speed wedges inclined to the west; The upper unit is from the surface to the depth of 6 kilometers, which is characterized by a relatively stable velocity layer. This three-factor velocity structure stratification reflects the basic characteristics of crustal structure in this area. According to the analysis of the gradual and stable change trend of the velocity structure of the three units, the formation of the shallow crust structure and even the surface morphology in this area is closely related to the deep structure, and the strength of the deep structure is greater than that of the horizontal structure, which also shows that the formation time of the deep lithospheric structure in this area is later than that of the shallow structure.
Fig. 5. 13 Rb-Sr diagram of Mesozoic volcanic rocks in Daxinganling (simplified according to Xu, 1994)
2) Tectonic-thermal evolution characteristics of the crust. There are mainly two types of rocks exposed on the surface in Meng Xing area, which can reflect the deep material composition and tectonic-thermal evolution conditions of lithospheric blocks, including metamorphic rocks, late Paleozoic granites and Mesozoic volcanic rocks. A set of metamorphic rock series, called Xinghua Dukou Group, mainly exposed in the northwest of this area, is mainly composed of amphibolite plagioclase gneiss, amphibolite and amphibolite schist, with medium-low amphibolite facies conditions and belongs to active continental margin structure. The density of this set of metamorphic rocks is 2.68 ~ 2.84 g/cm3, the formation temperature reflected by metamorphic mineral assemblage is about 650℃, the pressure is 0.5 ~ 0.55 GPA, and the estimated formation depth is about 17km. The Zalantun Group, another metamorphic rock series, is mainly exposed in the east of this area, and consists of a set of sericite quartz schist, chlorite schist, quartzite, phyllite, marble, acidic and basic tuff, which was formed under low greenschist facies metamorphic conditions and belongs to passive continental margin sedimentary formation. The density of this set of rocks is 2.6 1 ~ 2.7 1g/cm3, the formation temperature reflected by metamorphic mineral assemblage is about 450℃, the pressure is 0.2~0.35GPa, and the estimated formation depth is about 10km. The characteristics of structure and formation conditions of these two sets of metamorphic rocks show that they are two sets of rocks formed in completely different environments. According to the discontinuity of their composition at the formation depth of about 7km, it shows that although they are associated on the surface, they are not in the upper and lower sequence relationship. The volcanic rocks distributed in this area are mainly Mesozoic basaltic trachyandesite-trachyandesite and trachyandesite-rhyolite. According to the rubidium-strontium binary diagram (Figure 5. 13), the magma source depth of the former combination is more than 30 kilometers, and the magma source depth of the latter combination is 20-30 kilometers (Xu, 1994). At this depth, the P-T condition of the middle and lower crust in this area should belong to amphibolite facies-granulite facies. These two sets of volcanic rocks belong to alkaline and calc-alkaline series, with rich distribution of rare earth elements, and the total rare earth elements increase with time (JBOY3 -K 1). The initial Sr value of volcanic rocks is between 0.7063 and 0.7064, which is obviously lower than that of metamorphic sedimentary rocks, indicating that the secondary metamorphic rocks in the deep crust of this area are not dominant. Combined with the experimental results of rock melting and the test data of rock density and wave velocity, the middle and lower crust (20 ~ 30km) in this area is mainly composed of granite and dioritic gneiss and amphibole, in which dioritic gneiss is the source rock of rhyolitic volcanic rocks, and granulite facies metamorphic rocks with the same composition as amphibole below 30km, such as amphibole-amphibole granulite, resulting in basaltic coarse andesite in this area.
5.3. 1.2 Jihei lithosphere block
This block starts from Nenjiang-Kailu fault in the west, reaches Wusuli River, the Sino-Russian boundary river in the east, and adjoins the North China plate with Xilamulun River fault-Changchun-Yanji fault in the south. In terms of regional structure, there are three representative secondary tectonic units in turn from west to east: namely, the western tectonic unit represented by Songliao Basin-Xiaoxing 'anling and Zhangguangcailing basin-mountain system, the eastern tectonic unit represented by Sanjiang Basin, Jiamusi massif and Wandashan terrane, and the southern tectonic unit represented by Yanji Basin, Taipingling and Laoyeling. These three tectonic units have similar structures on the scale of lithosphere, the thickness of lithosphere is 80 ~ 60 km, and the thickness of crust is 29 ~ 40 km, which is obviously different from that of Meng Xing lithosphere block. Especially in this lithospheric unit, the layered characteristics of velocity structure in the crust are very obvious. The crustal structural features of the main structural units are as follows:
1) Songnen block. The structural and geomorphological features of this block are mainly manifested in the basin-mountain system composed of Songliao Basin and Xiaoxing 'anling-Zhangguangcailing Mountains. Previous studies have shown that Songliao basin is a craton composite basin above mantle uplift, and the basin itself is characterized by one uplift and two depressions in the horizontal direction, and a double-layer structure of downward rifting in the vertical direction. According to the research results of Manzhouli-Suifenhe geoscience transect, there are a series of structural detachment planes inclined eastward on the east side of the central uplift in Songliao Basin, which converge into a nearly horizontal reflection interface with a depth of about 15km. The discovery and determination of this reflection interface provides important evidence for understanding the formation of basin-mountain system represented by Songliao basin and Zhangguangcailing mountain range and its deep crustal structure characteristics. The results of Manzhouli-Suifenhe geoscience section show that the velocity in this area increases with the depth in the range of 15km, but at the depth of 15km, the velocity structure of basin-mountain system changes obviously laterally. From 15km to 25km below Zhangguangcailing, the velocity increased from 6. 10km/s to 6.50km/s, while in the transition zone between Acheng and Shangzhi, a low-velocity layer appeared at the same depth with a velocity of only 6.0 km/s. According to the drilling core data, the central uplift of the basin is mainly strongly deformed granite rocks. These characteristics indicate that the basement of Songliao basin may be a composite basement composed of rocks or slices of different structural levels. The results of reflection seismic profile and magnetotelluric sounding profile of Manzhouli-Suifenhe geoscience section also show that the center of Songliao basin is a mirror image of asthenosphere uplift, but not of Moho surface. The largest Moho uplift in Songliao basin is located near Harbin in the east of the basin, and the Moho surface is only 29 kilometers deep. According to the data of Li Sitian et al. (1989), the buried depth of the lower Moho surface in Songliao Basin has the evolutionary characteristics of gradually moving eastward and shallowing with time (Figure 5. 14). According to our research on the deep structural characteristics of the main basins in eastern Chinese mainland in recent years, the maximum uplift of the lower Moho surface in the main basins in eastern Chinese mainland does not correspond to the center of the basin, but inclines to the east. This phenomenon may be of great significance to explain the dynamic evolution of the eastern continent of China. For Songliao basin, the formation of a series of eastward inclined detachment zones in the crust may be related to the eastward movement of Moho uplift.
Fig. 5. 14 Variation of Moho surface with time in Songer earthquake profile (compiled according to data of Li Sitian et al. 1989)
2) Jiamusi plot. Jiamusi block is a very important geological structural unit in Jihei lithosphere block, and its tectonic significance has been widely concerned by people. Previous studies have considered that this tectonic unit is a stable block in the giant Hercynian fold belt, and its composition is mainly composed of Archean Mashan Group, Proterozoic Heilongjiang Group and large area Proterozoic granitic rocks. However, a series of studies since 1990s have proved (Zhang Xingzhou et al.,1991; Cao et al.,1992; WildeS.et.al., 2000), two sets of so-called early Precambrian metamorphic stratigraphic units in Jiamusi block, namely the so-called Heilongjiang Group and Mashan Group, are quite different from the traditional understanding in age and nature. The so-called Heilongjiang Group is not a normal metamorphic stratigraphic unit, but a set of structural melange containing disintegrated ophiolite fragments and subjected to high-pressure metamorphism. The so-called granulite facies metamorphism of Mashan Group occurred not in the early Precambrian, but in the early Paleozoic. The seismic sounding data of Manzhouli-Suifenhe geoscience section show that the tectonic melange in Mudanjiang area can extend at least 25km to the west and wedge westward under Zhangguangcailing, reflecting that it was a subduction zone in its early stage (Figure 5. 15). Due to the location of the section, Manzhouli-Suifenhe geoscience section only reveals a small part of the southern end of Jiamusi block, so it is impossible to understand the lithospheric thickness and crustal velocity structure characteristics of Jiamusi block from the perspective of seismology.
In view of this, during the research period of this project, Huanan-Raohe MT sounding was carried out in the middle of Jiamusi block east of Wandashan terrane, with a total length of 240km. The results show that Jiamusi block has a regular two-dimensional shape and a stable lithospheric thickness (about 90km), and there is a stable high conductivity layer in the crust with a depth of 15km. There are several high resistivity blocks and slices above the high conductivity layer, which are separated by high conductors, especially at the eastern edge of the block (see Figure 2.20 and Figure 2.2 1). The characteristics of electrical structure show that the position of the eastern boundary of Jiamusi block is obviously different between the surface and the deep. On the surface, it is located in the east of Baoqing, while the nearly vertical deep boundary is located in the west of Baoqing, with a difference of about 40 kilometers. MT data show that there is an obvious low-resistivity layer under Huanan Uplift. Combined with the obvious thrust nappe characteristics of Huanan Uplift in Jiamusi Block after the Early Cretaceous (Zhao et al., 20 1 1), it shows that the difference between the eastern boundary of Jiamusi Block on the surface and the deep part may be related to the eastward thrust structure of the shallow crust. The existence of thrust nappe structure in this area has been confirmed by exact geological evidence. In the south of Jiamusi massif, granulite-facies high-grade metamorphic rocks invaded from the coal measures strata of Muling Formation in the early Cretaceous.
Fig. 5. Crustal velocity structure in the eastern section of15 Mansui geoscience section (unit: km/s)
The study of metamorphic rock assemblage and metamorphic mineral assemblage also proves that there is a strong thrust structure in Jiamusi block The main manifestation is that two sets of metamorphic rock series exposed in this area have undergone granulite facies and amphibolite facies metamorphism respectively. Isotopic dating evidence shows that the last regional metamorphism of the two occurred in the early Paleozoic, with an age of about 500Ma. Although they are all exposed on the surface and associated together at present, their regional occurrence is not consistent. Granite-facies metamorphic rocks are mainly exposed in the south, spreading in the east-west direction, and amphibole-facies metamorphic rocks are mainly exposed in the vast area north of granulite, spreading in the north-south or north-north direction. Taking metamorphic argillaceous rocks as an example, the granulite facies metamorphic mineral assemblage in the south reflects the formation temperature of 800 ~ 850℃ and the pressure of 0.70 ~ 0.5 GPA, the high amphibolite facies metamorphic mineral assemblage in the north reflects the formation temperature of 600 ~ 700℃ and the pressure of 0.4 ~ 0.5 GPA, and the low amphibolite facies metamorphic mineral assemblage reflects the formation temperature and pressure of 530 ~ 560℃. It can be seen that the same protolith suffered different degrees of metamorphism at the same time, which reflects their differences in formation depth. According to the temperature and pressure conditions of metamorphic mineral assemblage, the depth results show that the formation depth of granulite facies rocks is about 25 ~ 27 km, high amphibolite facies rocks are about 15 ~ 18 km, and low amphibolite facies rocks are about 9 ~ 15 km. This relationship shows that the rocks of high amphibolite facies and low amphibolite facies are continuous in formation depth, which can represent the crustal profile with a depth of 9 ~ 18 km when Jiamusi block is metamorphic, while the rocks of granulite facies and amphibolite facies are discontinuous in formation depth, with a gap of about 7km between them. This undoubtedly reflects a structural relationship. Both granulite facies metamorphic sedimentary rocks and basic intrusive rocks have high density, the former is 2.89, and the latter is 3.0 1, which not only reflects that they were formed at a deep depth, but also experienced rapid tectonic uplift. The P-T-t-T evolution track of metamorphic rock series also reflects that the metamorphic rock series has undergone a rapid decompression process (Figure 5.438+06).
Fig. 5. P-T-t-t evolution track of khondalite series metamorphism in16 Jiamusi block.
3) Wandashan terrane. Wandashan terrane, located in the east of Jiamusi massif, is the only tectonic unit with Mesozoic marine strata in Meng Xing-Jihei area. As a part of the huge Schott-Alin terrane in the east, this tectonic unit is of great significance to understand the formation and dynamic evolution of the current lithospheric structure in the eastern part of Jilin and Heilongjiang and even the continental margin of Northeast Asia. Many researchers have done detailed research on biostratigraphy, petrology, tectonic geology and paleomagnetism in this area, and proved that Wandashan terrane is mainly composed of a set of ultramafic, mafic accumulation rocks, basic lava, siliceous rocks and argillaceous rocks. Radiolarians and other fossils in siliceous rocks and argillaceous rocks belong to middle-late Triassic-early-middle Jurassic (regional geology of Heilongjiang Province, 1993). Paleomagnetic data also show that the Wandashan terrane was located near the equator in the Middle Triassic (KojimaS.et.al,1987; kojimas . 1989； Ji Shaoan et al.,1995; Yang Huixin et al., 1989), the new carbonaceous limestone blocks in terrane and the brown corals, corals and radiolarians in Triassic siliceous rocks belong to low-latitude warm-water fauna; However, the paleoecology of radiolarians in the early-middle Jurassic siliceous rocks reflects that the latitude of the water area has obviously increased at this time, which has obvious ecological characteristics of cold water type and middle and high latitude (KojimaS.et.al,1987; Ji Shaoan et al.,1995; Zhang Qinglong et al., 1997). By the late Jurassic, the Wandashan terrane had been located near 43.5 north latitude (KojimaSandMizutaniS. , 1987; kojimas . 1989； Zhang Shihong et al.,1991; Zhang Qinglong et al., 1997). The above evidence shows that the Wandashan terrane migrated rapidly from low latitude to middle latitude in the early and middle Jurassic, and was located in the eastern margin of Jiamusi block in the late Jurassic. At present, the consensus is that the assemblage is an exotic terrane with ophiolite properties. Based on the results of MT sounding profile of Huanan-Raohe River (Figure 2.20 and Figure 2.2 1), this paper makes a possible analysis of the structure and evolution of Wandashan terrane. The results of magnetotelluric sounding show that there is no regular electrical structure under the Wandashan terrane east of Jiamusi block, and it is basically impossible to determine the top and bottom boundaries of the low resistivity layer. Generally speaking, the whole area is dominated by high conductivity, and there are four relatively high resistance blocks with a scale of 20 ~ 30 kilometers in the high conductivity area. The low resistance layer between high resistance blocks is almost vertical, and it is cut by a nearly horizontal low resistance layer from 50 ~ 6~8km deep to 6~8km deep. Combined with the surface geological and structural characteristics, the 6 ~ 8 km thick high conductivity layer may be an important structural interface in the Wandashan terrane. If this understanding is correct, then the Wandashan ophiolite assemblage exposed in the east may be a westward thrust rock sheet with a depth of less than 8 kilometers. Recently, we found that cordierite was contained in granite that invaded the ophiolite distribution area of Wandashan Mountain. Zircon U-Pb age is 124 ~ 13 1 Ma. The significance of this important discovery is that the Wandashan terrane was thrust onto the aluminum-rich crust before 13 1Ma (early Early Cretaceous). It is worth noting that cordierite granite has geochemical characteristics of peraluminous-strongly peraluminous granite and belongs to S-type granite, in which zircon is sometimes captured as 463Ma (Cheng Ruiyu et al., 2006), which is the most important age for advanced metamorphism and granitic magmatism in Jiamusi block (WildeS et al., 2000,2003), indicating that the formation and emplacement process of magma is different from that in Jiamusi block. However, because the source rocks of S-type granite containing cordierite mainly come from weathered sedimentary rocks, and zircon in granite has a high ratio of 176Hf/ 177Hf and a positive value of εHf(t), it shows that the source area of cordierite granite is not entirely Mashan Group with basement, and there may be pre-Mesozoic sedimentary strata. According to regional geological data, there are two sets of continental margin deposits in the eastern margin of Jiamusi block, one is Devonian-Carboniferous; The other set is Upper Triassic-Lower Jurassic. These two combinations are distributed in the eastern margin of brea block in northern Russia and the eastern margin of Xingkai block in southern Russia (KirillovaGL. , 2003, 2005), indicating that continental margin deposits were widely developed in the eastern margin of the Breya-Jiamusi-Xingkai block at that time. Therefore, it cannot be ruled out that the formation of peraluminous-strong peraluminous granite is related to these two sets of sedimentary rocks. According to this idea, there may be an upper Paleozoic or early Mesozoic under the Wandashan terrane. The high-resistivity block under the terrane may be a concealed terrane collaged during the proliferation of the oceanic plate to the eastern margin of Jiamusi block, but it is not excluded that it is a block that cracked early in the eastern margin of Jiamusi block. The low-resistivity body between them may be the early oceanic sediments between accretionary blocks, and the nearly vertical low-resistivity zone may be related to the late strike-slip structure and Cenozoic basalt eruption.
To sum up, the Meng Xing-Jihei lithosphere block consists of three early Paleozoic consolidated blocks, namely Ergon-Xing 'an block, Songnen block and Jiamusi block, and the easternmost Wandashan Mesozoic accretion terrane. Among them, Ergon-Xing 'an block and Songnen block are Heihe-Nenjiang-Kailu ancient suture zone, and Songnen block and Jiamusi block are Jiayin-Mudanjiang ancient suture zone. As relatively weak structural belts, these two ancient suture belts have the characteristics of multi-stage activity, which have the most important restrictive effect on the formation of lithospheric structure in this area and become important structural zoning boundaries within the Meng Xing-Jihei lithospheric block. Generally speaking, these two tectonic belts were characterized by subduction to the west in the early stage, which led to an increase in the thickness of the crust and lithosphere in the adjacent western regions and complicated deep structures.
1) The Ergon-Xing 'an block west of Heihe-Nenjiang-Kailugu suture zone is the block with the largest lithospheric thickness and crustal thickness in this area, but the Moho surface beneath it is complex in shape and fluctuates greatly, and obvious Moho surface appears in the suture zone. There is no stable velocity layer in the lower crust, and the velocity structure is consistent with Moho surface. The velocity structure of the middle and upper crust is relatively stable, characterized by the alternating distribution of high-speed bodies and low-speed bodies; The velocity layer in the shallow crust is continuous and stable. Because the Heihe-Nenjiang-Kailu ancient suture zone is covered by the late Carboniferous-Permian sediments widely distributed in this area, it shows that the suture zone was formed at least before the late Carboniferous. At the same time, according to the good coupling relationship between the subsidence of Songliao basin on the east and west sides of the belt and the uplift of Daxinganling, it shows that the fault is still an important boundary to control the uplift of Daxinganling and the subsidence of Songliao basin in Cretaceous. The complex crust-mantle structure in this area seems to reflect such a complex evolution process, and it also shows that the deep part of this area is in a severe stage of crust-mantle balance adjustment.
2) Xiaoxing 'anling-Zhangguangcailing area west of Jiayin-Mudanjiang ancient suture zone is also one of the areas with the largest crust and lithosphere thickness in Meng Xing-Jihei lithosphere block. The asthenosphere uplift corresponds to the Songliao basin in the west of the crustal thickening zone, and they are mirror images, but the largest uplift of Moho is located in the east of the basin, which does not show a good mirror image relationship with the basin. The lower crust of Songliao basin is generally low-speed (Vp is 6.00 ~ 6.08 km/s) to the depth of 23km, but there are high-speed bodies with the speed as high as 6.8km/s under the central uplift of the basin 13 ~ 18 km. On the east side of the high-speed body, a series of reflection interfaces inclined to the east appear, and the upper part of the interface is steep, and the deceleration converges to a depth of about 65438. The detachment interface of Zhangguangcailing is obviously steep eastward, which makes the thickness of Zhangguangcailing lower crust increase obviously. These evidences show that the Jiayin-Mudanjiang paleosubduction zone has also thickened the crust or lithosphere of Songnen block. According to the characteristics that the Devonian-Early Carboniferous fossil assemblage in the eastern Xiaoxing 'anling is different from the contemporaneous fossil assemblage in the eastern margin of Jiamusi block, it is speculated that the formation time of this suture zone should be after the Early Carboniferous. On the other hand, the crust thickening in Zhangguangcailing area may also be related to the eastward detachment of the crust in Songliao basin, because the eastward detachment interface of the basin basement has obviously extended below Zhangguangcailing, reaching a depth of 40km. Therefore, the eastward movement of the basement crust in Songliao basin has made an important contribution to the crustal thickening in Zhangguangcailing area.