Since the Late Cenozoic, the remote effect of India-Eurasia plate collision has strongly transformed Tianshan area (Tapponnier et al.,1979; Avouac et al.,1993; Hendricks et al.,1992; 1994; 1995; Suo Beier et al.,1997; Yin et al.,1998; Burchfiel et al.,1999; Brun et al., 2001; Guo Lingzhi, et al., 1992), Tianshan Mountain once again moved to form an intracontinental orogenic belt and regenerate the foreland basin (Lu et al.,1994; Jia Chengzao et al., 1997, 2003). The fine structural analysis of foreland thrust belt in the northern and southern margin of Tianshan Mountain provides a solid geological foundation for oil and gas exploration (Lu Huafu et al.,1999,2000; Wang Xin et al., 2002). Existing research and oil and gas exploration show that the oil and gas source areas of foreland thrust belts on both sides of Tianshan Mountain are closely related to Mesozoic coal measures strata (Jia Chengzao et al., 1997, 2003), but there are still different understandings about Mesozoic basin characteristics and basin-mountain relationship in Tianshan Mountain and its adjacent areas (Graham et al.,1993; Jia Chengzao et al., 1997, 2003; Wu Chaodong et al., 2004). As a key stage of the transformation from Paleozoic collision orogeny to Cenozoic intracontinental reconstruction orogeny, it is of great significance to determine the Mesozoic Tianshan tectonic attributes and their relationship with adjacent basins for analyzing the tectonic evolution of Tianshan orogenic belt and oil and gas exploration in its adjacent basins.

Xia Hou is a small intermountain depression near Urumqi in the northern margin of Tianshan Mountains (Figure 1-2-24), where Jurassic is well exposed and it is also an important coal base (Figure 1-2-25). Based on the systematic analysis of Jurassic sedimentary characteristics and stripping process in Xia Hou area, a new understanding of the relationship between Mesozoic and Cenozoic basins and mountains is put forward.

1. Jurassic sedimentary characteristics in Xia Hou

Jurassic in Junggar Basin on the northern margin of Tianshan Mountains is divided into six groups from bottom to top: Lower Jurassic Badaowan Formation (J 1b) and Sangonghe Formation (J 1s), Middle Jurassic Xishanyao Formation (J2x) and Toutunhe Formation (J2t), Upper Jurassic Guqi Formation (J3q) and Caraza Formation (J3Q). Badaowan Formation is completely exposed in Xia Hou area (Figure 1-2-25). The whole structure is upper and lower coal-bearing strata, and the middle part is lacustrine or lacustrine-delta dark mudstone with thin sandstone. Generally speaking, the Sangonghe Formation is dominated by lacustrine-delta facies, and there is no industrial coal seam. Xishanyao Formation is a set of lacustrine-bog coal measures strata with well-developed coal seams. Thick coal seams are generally concentrated in the middle and lower parts, and the upper part is multi-layer thin coal seams or coal lines. The main lithology is gray, dark gray, gray-green mudstone, siltstone mixed with yellow-green, light gray, gray-white sandstone or conglomerate, gray-black carbonaceous mudstone, coal seam and rhombic ore, and the bottom is generally gray-white or light gray thick layered massive sandstone or glutenite, with mottled conglomerate locally, with obvious lithological marks at the bottom. Toutunhe Formation is all exposed in Toutunhe area to the west of Urumqi, and the lower member is dominated by yellow-green and gray-green glutenite, mixed with fluvial variegated mudstone, fine sandstone and siltstone with different thicknesses. The upper section is grayish green, gray and dark gray mudstone, fine sandstone, siltstone mixed with marl and calcareous sandstone, carbonaceous mudstone or coal line near the bottom, and the upper section is purplish red, maroon mudstone and siltstone strip. In Xia Hou area, the Toutunhe Formation has only a few tens of meters of debris left at the top of the mountain. The Guqi Formation is a set of red clastic rocks with a small amount of tuff and tuffaceous sandstone deposits. Caraza Formation is grayish brown conglomerate of piedmont fluvial facies, with brown mudstone and gravelly sandstone. Gucci Formation and Caraza Formation are not exposed in Xia Hou area. For comparison, Figure 1-2-26 shows the Jurassic columnar profile and sedimentary sequence characteristics in the northern and southern margin of Xia Hou Basin, from which it can be seen that thick alluvial fan deposits developed at the bottom of Badaowan Formation (J 1b) in the southern margin after summer, and the thickness of Badaowan Formation (J 1b) was also greater than that in the northern margin.

Figure 1-2-24 Distribution of Mesozoic and Cenozoic outcrops and apatite fission track age samples in the northern margin of Tianshan Mountain near Urumqi.

1- four yuan; Level 2- 3; 3- Cretaceous; 4- Jurassic; 5- Triassic; 6- Permian; 7- Carboniferous and Pre-Carboniferous; 8- Ophiolite; 9— Main fault; ★——AFT sampling position and sample number

Figure 1-2-25 Geological Map of Xia Hou Area

1-Devonian; 2- Carboniferous; 3- Badaowan Formation; 4- Sangonghe Formation; 5— Xishanyao Formation; 6—— Toutunhe Formation; 7- 4 yuan; 8- thrust fault; 9— Normal fault; ★—— Post-sampling position

Figure 1-2-26 Sedimentary sequence and paleocurrent map of the northern and southern margin of Xia Hou.

Paleo-flow direction is an effective method to determine provenance and basin comparative analysis. In the field, oblique bedding, asymmetric ripple marks and pebble gravel are mainly used to judge the ancient flow direction. The paleocurrent data of Jurassic profiles in Xia Hou, southern margin and northern margin of Toutunhe River were counted respectively, and more than 20 data were obtained at each point. The paleocurrent in the north-south margin of the post-summer profile in the early and middle Jurassic was northward (figure 1-2-26), which indicated that at least in the early and middle Jurassic, Xia Hou was not an independent sedimentary basin, but the same basin connected with Toutunhe area in the southern margin of Junggar. According to the comparison of sedimentary sequences (Figure 1-2-26), marginal facies deposits, represented by thick alluvial fans, developed in the southern margin of summer, which should be close to the basin edge, that is to say, the southern margin of the sedimentary basin in the southern margin of Junggar in the early and middle Jurassic was at least in Xia Hou area, which was much larger than what is seen now. The remaining intermountain depressions in Xia Hou today are caused by the separation of late structures.

2. Analysis of Jurassic stripping process in Xia Hou area.

(a) Ro analysis of coal and rocks

Coal seams of Lower Jurassic Badaowan Formation (J 1b) and Middle Jurassic Xishanyao Formation (J2x) in intermountain depression of Xia Hou are also the main mining targets. In order to understand the burial degree and outcropping process of Jurassic, vitrinite reflectance (Ro) of coal samples from Xishanyao Formation (J2x) was analyzed, and the fission track age of sandstone in roof or floor of coal samples was compared (table 1-2- 1 1). From the table 1-2- 1 1, it can be seen that the vitrinite reflectance (Ro) of most coal seams is around 0.7, referring to the relationship between Ro and Tmax (Barker et al.,1986; Li Rongxi et al, 2001; Wang Liang et al., 2003) calculated Tmax as 105 ~ 1 15℃ (table 1-2- 1 1), indicating that the evolution degree of Xishan kiln coal seam in Xia Hou area is high, but it has not reached apatite fission track. According to the geothermal gradient of 35℃/km (Qiu Nansheng et al., 2002), there is at least 3km of stratum covering above the sample. Field investigation shows that there are only tens of meters of Toutunhe Formation above Xishanyao Formation, and the remaining Jurassic overburden on coal samples is less than 500 meters, indicating that at least 2.5 kilometers of erosion has occurred on it.

Table 1-2- 1 1 vitrinite reflectance (Ro) analysis of Jurassic coal samples in Xia Hou area and corresponding apatite fission track analysis data.

Note: Ro test was completed in China University of Mining and Technology (Beijing); See table 1-2- 12 for the detailed data of AFT.

(2) Evidence of fission track chronology

Fission track dating (FT) is an isotope dating method applied to geology in 1960s. It is a technique to determine the thermal history of rocks at low temperature (less than (110/kloc-0)℃) (Green et al.,1989; Gunnell, 2000; Zhang Zhicheng et al., 2004). Fission track method can not only give the age simply, but also study track annealing and give the thermal history information of geological bodies, thus making fission track age more geological significance. Previous studies on Mesozoic and Cenozoic tectonic activities in Tianshan Mountains have accumulated some dating data (Yang Geng et al.,1995; Suo Beier et al.,1997; Wang Yanbin et al., 2000; Dumitru et al, 2001; Guo et al., 2005).

The measurement of apatite fission track age in this book was completed in the Institute of High Energy Physics, Chinese Academy of Sciences, and the flow method is basically consistent with the literature (Dumitru et al., 2001; Zhang Zhicheng et al., 2004). In the track length analysis, 100 horizontal and narrow tracks (if there are 100 tracks) are measured in each sample, and about 20 particles with good quality are randomly selected from each sample for dating (assuming there are enough particles). Traditionally, the statistical error of age and average length is 1 σ, but the error of 2 σ should be considered in geological interpretation. The sample location is shown in Figure 1-2-24, the fission track age analysis results of apatite are shown in Table 1-2- 12, and the fission track single event age radiation diagram is shown in Figure 1-2-27.

Two granite samples (WK 18-2, 19- 1) from Ullas Station in Bingdaban belong to the Early Cretaceous, with long fission tracks and many short tracks (Table 1-2- 12, Figure 65438+. Eight samples (HJ02-0 1 ~ WK43-2) were taken from the Jurassic-Neogene system in Toutunhe area west of Urumqi (table 1-2- 12, figure 1-2-27). The track length ranges from [(13.41.4) ~ (11.1.5438+0.7)] microns, and the fission track assemblage era is mainly from late Jurassic to early Cretaceous [(65438)].

There are six Jurassic samples (WK28- 1 ~ HX 13-06) in Xia Hou area, and the fission track ages mainly show Late Cretaceous [(83.7 6.9) ~ (68.4 4.6) Ma] and Oligocene [(37 3.0) Ma]. Four Jurassic samples calibrated by Ro were tested by Ketcham et al. (1999) annealing model. T-curve simulation (Figure 1-2-28) shows that the Middle Jurassic sandstone in Xia Hou area began to slowly denude and rise from the Late Cretaceous (about 90Ma), but rose rapidly since Miocene (mainly from 10Ma). It can also be seen from the figure that the time limit of rapid uplift from south to north is gradually becoming new, which may be related to the extension of thrust structure from south to north.

The above fission track analysis results are consistent with the development time of foreland thrust structure shown by the growth strata in front of Tianshan Mountain. Fang Shihu et al. (2004a) pointed out that the Caraza anticline separating the intermountain depression and the piedmont basin in Xia Hou was formed around 100 Ma, that is to say, the Xia Hou depression originally belonging to the same Jurassic basin was separated from Toutunhe area around 100 Ma.

Three. Discussion and conclusion

(1) discussion

There are still different views on the tectonic attributes and geomorphological characteristics of the Mesozoic Tianshan Mountains. One view is that the Tianshan Mountains developed a remarkable normal terrain separating the northern and southern basins in Mesozoic, and the Jurassic basins on both sides of the Tianshan Mountains were post-collision inheritance basins (Graham et al.,1993; Hendrix et al., 1992, 1994, 1995), which further shows that Tianshan Mountain and its adjacent areas were still in the background of regional compression. Another point of view is that Tianshan Mountain basically did not exist in Mesozoic, or the terrain fluctuated little (Fang Shihu et al., 2004b Wu Chaodong et al., 2004), and the leveling made the Paleozoic Tianshan orogenic belt close to the quasi-plain state, while the Jurassic basin was a rift basin dominated by extension, but there is still a lack of clear understanding of the scope of Jurassic basin and the relationship between basin and mountain. In the oil and gas exploration of Cenozoic foreland thrust belt on both sides of Tianshan Mountain, the original distribution of Jurassic coal measures strata as the main source rocks is of great significance for future exploration (Jia Chengzao et al., 2003).

Table 1-2- 12 apatite fission track analysis data table of Wula station section in southern Urumqi-Bingdaban.

Note: No(n): the number of apatite particles measured; ρd: standard track density (×105/cm2); ρs: spontaneous track density (×105/cm2); Ns: the number of spontaneous trajectories; ρi: induction track density of external detector (×105/cm2); Ni: number of induced tracks; γSi is the linear correlation coefficient of ρs and ρi; P(%): test probability; T 1σ: aggregation age); Fission track; L (micron) (n): fission track length (micron, n is the number of tracks measured). The age of WK sample was calculated with CN5 standard glass, ξ CN5 = 465,438+0065,438+00, while ξ CN5 = 357.8 6.9 for HX and HJ samples.

Figure 1-2-27 Single event age radiation map and fission track length distribution map of Urumqi-Bingdaban section

[The median age and sample formation age (thick line and short line) are marked in the radiation diagram, and M is suitable for thermal simulation]

Figure 1-2-28 Simulated t-T curve of some samples in Xia Hou.

[Ketcham et al. (1999) model is adopted]

Acceptable fitting-acceptable simulation results; Good fitting-in line with good simulation results; Path modeled- in line with better t? T curve; Constraints-restrictive conditions; The curve in the fission track length diagram is an ideal fission track length distribution curve, which is consistent with the simulation results.

The study of Jurassic sedimentary characteristics in Xia Hou shows that the present Xia Hou sag is not an independent basin in Jurassic period, and its paleocurrent data show that the paleocurrent in the northern and southern margins all point to the north, so the sag should be connected with the Jurassic basin in the southern margin of Junggar at that time. The analysis of sedimentary sequence shows that thick alluvial fan facies deposits (Figure 1-2-26) are developed at the bottom of Jurassic in the southern margin after summer, which should be close to the edge of prototype basin, but Jurassic marginal facies deposits are generally not found in the foreland areas on the north and south sides of Tianshan Mountain (Wu Chaodong et al., 2004). This shows that the area of Jurassic basin is larger than that seen now, and the piedmont area is only a part of Jurassic basin deposition. From the perspective of Xia Hou area and the southern margin of Junggar, the original sedimentary range of Jurassic has crossed the suture boundary of Paleozoic plates (as shown in figure 1-2-24, the suture zone represented by Bayingou ophiolite belt), so the Jurassic basin should not be an inherited basin after Paleozoic collision (Graham et al., 1993). It can also be seen from the field investigation and figure 1-2-24 that there are Jurassic deposits containing coal seams near Bingdaban, Bayinbuluke and Yuxi Molegaidaban in the south of Xia Hou, indicating that the above areas should have been in a similar coal marsh environment at that time (Wu Chaodong et al., 2004), so there will be no Tianshan Mountain with a similar scale as today, and Tianshan Mountain and its adjacent areas are basically quasi-plains. However, the original Jurassic sedimentary range of the above points and whether they are connected with the Jurassic basins on the north and south sides deserve further study.

Due to late tectonism, Xia Hou sag was separated from Jurassic in the northern margin of Tianshan Mountains. The analysis of vitrinite reflectance (Ro) of coal samples in Xia Hou shows that there is at least 3km thick sedimentary cover above the middle Jurassic Xishanyao Formation (J2x), that is to say, the coal seam is exposed to the surface at least 2.5km later. The study on the southern margin of Junggar shows that although the Tianshan area has undergone different types of structural changes from Mesozoic to Paleogene, the Mesozoic-Paleogene sediments are basically complete, continuous and coordinated, and there is no obvious angular unconformity between them (Fang Shihu et al., 2004a, b; Wu Chaodong et al., 2004), indicating that the intensity and amplitude of tectonic activity in this period were much weaker than that of intracontinental orogeny in the Late Cenozoic. The growth stratum of the foreland thrust belt shows that the first row of thrust structures in the northern margin of Tianshan Mountains was formed around 100 Ma (Fang Shihu et al., 2004 a). The fission track chronology in this book shows that the burial depth of Xishanyao Formation (J2x) in Xia Hou reached the maximum between 100 ~ 60ma, and then it began to rise slowly, and rose rapidly since the late Cenozoic (100~60Ma). We believe that the rapid uplift of Tianshan Mountains and foreland thrust structures since the Late Cenozoic separated the Xia Hou Depression in the northern margin of Tianshan Mountains from Jurassic, forming the present structural features.

(2) Conclusion

1) sedimentology research and paleocurrent measurement show that the Jurassic in Xia Hou area was not an independent basin when it was deposited, but the same basin was connected with the northern margin of Tianshan Mountain, but closer to the southern margin of the basin, and developed a good marginal facies sequence. At that time, the sedimentary range of Jurassic basin was larger than that of Jurassic in the northern margin of Tianshan Mountain today.

2) Since the Late Cenozoic (100 Ma), the rapid uplift of Tianshan Mountains and foreland thrust structures have separated Xia Hou Depression from Jurassic in the northern margin of Tianshan Mountains, and the Xia Hou area has experienced a process of nearly 2.5km.

refer to

, Guo,,, Wu Chaodong,, Yuan,,. Preliminary determination of the formation time of foreland thrust belt in the northern margin of Tianshan Mountains. Geology of Xinjiang, 2004a,22 (1): 24 ~ 29.

Guo, Wu Chaodong. 2004b。 The nature and evolution of Mesozoic-Cenozoic Tianshan Mountains and its basins on both sides. Journal of Peking University, 40 (6): 886 ~ 897.

Guo, Wu Chaodong,,, Yuan. Characteristics of Jurassic detrital components in the southern margin of Junggar basin in 2006 and their implications for structural attributes and basin-mountain pattern. Journal of Geology, 80 (in press).

Guo Lingzhi, Shi, Lu Huafu, et al. Two long-distance tectonic effects of India-Tibet collision. Essays on Modern Geology (Volume I). Nanjing: Nanjing University Press, 1 ~ 12.

Guo, Wu Chaodong,,. In 2005, the uplift process of Tianshan Mountain in Mesozoic and Cenozoic and its comparative study with Junggar and Altai Mountains. Journal of Geology, 1979 (in press).

Jia Chengzao. 1997. Structural characteristics and oil and gas in Tarim Basin. Beijing: Petroleum Industry Press.

Jia Chengzao, Wei, et al. 2003. Formation and gas control of two foreland basins in central and western China. Acta petrolei sinica, 24 (2): 13 ~ 17.

Li Rongxi, Jin. 200 1. Reconstruction of structural evolution characteristics of sedimentary basins by vitrinite reflectance. Journal of Mineralogy, 2 1 (4): 705 ~ 709.

Lu Huafu, Jia Dong, Chen Chuming et al. 1999. Cenozoic tectonic characteristics and deformation time of Kuqa. Frontier of Earth Science, 6 (4): 2 15 ~ 22 1.

Lu Huafu, Chen Chuming, Liu Zhihong, etc. 2000. Structural characteristics and genesis of Kuqa regenerated foreland thrust belt. Acta petrolei sinica, 2 1 (3): 18 ~ 24.

Qiu Nansheng, Yang Haibo and Wang Xulong. 2002. Tectonic-thermal evolution characteristics of Junggar Basin. Geological Science, 37 (4): 423 ~ 429.

Wang, Li Cheng, Li Cheng, et al. 2003. Distribution characteristics of geothermal gradient in Kuqa foreland basin on the northern margin of Tarim Basin. Journal of Geophysics, 46 (3): 403 ~ 407.

Wang Xin, Jia Chengzao, Yang Shufeng, et al. 2002. Tectonic deformation time of Kuqa thrust fold belt in southern Tianshan Mountains. Journal of Geology, 76 (1): 55 ~ 63.

Wang Yanbin, Wang Yong, Liu Xun, etc. 2000. Apatite fission track records of episodic activities in Tianshan and West Kunlun Mountains. Regional Geology of China, 20 (1): 94 ~ 99.

Wu Chaodong, Quan, Guo et al. 2004. Types of Jurassic prototype basins in Xinjiang. Geology of Xinjiang, 22 (1): 56 ~ 63.

Yang Geng, Qian Xianglin. 1995. Evidence of uplift of Mesozoic and Cenozoic Tianshan intraplate orogenic belt: zircon and apatite fission track dating. Journal of Peking University, 3 1 (4): 473 ~ 478.

Zhang Zhicheng, Wang Xuesong. 2004. Problems in the application of fission track dating data and their geological significance. Journal of Peking University, 40 (6): 898 ~ 905.

Awake ·J·P, Tapenier P, Bai M, et al .. 1993. Active thrusts and folds in the northern Tianshan Mountains and Late Cenozoic rotations of Tarim, Junggar and Kazakhstan. Journal of Geophysical Research 98:6755~6804

Barker C E，Pawlewicz M j 1986。 Correlation between vitrinite reflectance and maximum temperature in humus organic matter. Lecture notes on earth science, 5:79~8 1

Brun forensic medicine 200 1. Late Cenozoic tectonic evolution in northwestern Tianshan Mountains: a new age estimate of the beginning of orogeny. GSA Bull etin， 1 13( 12): 1544 ~ 1559

Burchfiel B.C., Brown E.T., Deng Q.D., et al .. 1999. Crustal shortening at the edge of Tianshan Mountains in Xinjiang. Geol.Reυ。 ,4 1:665~700

Dumitru Ta, Zhou Di, Chang Aizi, Graham Sa, Hendricks Mi Sa, Suo Beier Ayre and Carol A R.20065438+0. Uplift, denudation and deformation of Tianshan Mountains in China, in Hendrix M S and Davis G.A. (eds.), Paleozoic and Mesozoic tectonic evolution in Central Asia: from continental assemblage to intracontinental deformation: American Geological Society, pp. 7 1~99.

Graham, Hendricks, Wang, et al .. 1993. collision inheritance basin in western China: influence of structural inheritance on sandstone composition: bulletin of American Geological Society, 105:323~344.

Green ·P·F, Duddy I R, Laslet ·G·M, et al .. 1989. Thermal annealing of apatite fission track, 4. Quantitative simulation technology and its extension to geological time scale, Geochemistry, 79: 155~ 182.

Thermochronology of apatite fission track: a review of the potential and limitations of geomorphology. Basin study.12:115 ~132

Hendricks, Graham, Carol, Suo Beier, Mcknight, Shu Lin and Wang. Sedimentary Records of Periodic Deformation in Tianshan Mountains and Its Climatic Significance: Evidence from Mesozoic Strata in Northern Tarim Basin, Southern Junggar Basin and Turpan Basin

Hendricks M S, Dumitru T A and Graham S A. 1994. Late Oligocene-Early Miocene Topping of Tianshan Mountains in China: Early Effect of Indo-Asian Collision. Geology, 22:487~490

Hendricks M S, Blaser S C, Carol A R and Graham S A 1995. Sedimentology, organic geochemistry and petroleum potential of Jurassic coal measures: Tarim, Junggar and Turpan basins, AAPG, 79:929~959.

Carlson W. D.1999 Variability of apatite fission track annealing dynamics III: extrapolation of geological time scale. American mineralogist.84:1235 ~1255

Sobel E R and Dumitru T. A.1997. Thrust and exhumation of the western margin of Tarim basin during India-Asia collision. JGR. 102(B3)，5043~5063

Tapenier Pass, Monral Pass 1979. Active faults and Cenozoic structures in Tianshan, Mongolia and Baier areas of JGR, 84: 3425 ~ 3459.

Yin, Nie, Craig, Harrison, Reyerson, Zhang Xianglin, Geng .. 1998. Late Cenozoic tectonic evolution, geotectonics, 17: 1~27 in Tianshan, South China.

(Guo, Wu Chaodong,,,)