PROGRESS IN NATURAL SCIENCE, Vol. 8, No. 5, p. 597–603, October 1998
 

Subsidence characteristics of the Turpan Basin (NW China) and its tectonic implications

Shao Lei1, K. Stattegger2, LI Wenhou3, B. J. Haupt2  and LIU Yinqun3

1Laboratory of Marine Geology, Tongji University, Shanghai 200092, China;
2Geological and Paleontological Institute, University Kiel, Germany;
3Department of Geology, Northwest University, Xi'an 710069, China
 
Received January 7, 1998, revised January 23, 1998

 
Abstract   The Turpan Basin, a back-arc basin formed during the Late Permian, underwent first thermal subsidence and then flexure subsidence. The thermal subsidence took place during the Late Permian and Early Triassic following the period of magmatic activities in this region. The flexural subsidence was throughout the Middle Triassic to Early Tertiary induced by orogenic movements which produced periods of high subsidence rates. Accelerated subsided periods occurred during the Late Triassic/Early Jurassic, Late Jurassic, Latest Jurassic/Earliest Cretaceous, and Latest Cretaceous/ Early Cenozoic, indicating the effect of the collision and accretion onto the south Asian continental margin of the Qiangtang block in the Late Triassic/Early Jurassic, the Gangdise block in the Late Jurassic and Latest Jurassic/Earliest Cretaceous, and the Indian Subcontinent in the Latest Cretaceous/ Early Cenozoic. There are relatively large breaks in the variation of the petrologic and geochemical data among these events. The Turpan Basin evolved from a back-arc basin in the Late Paleozoic into a foreland basin in the Mesozoic, and a large intermontane basin of the Tian Shan in the Cenozoic.

Keywords: sedimentary history, basin analysis, tectonic development, Turpan Basin

The Turpan Basin is a relatively great intermontane basin of the Tian Shan in Northwest China, and about 5x104 sq. km overall. It is connected to the north and northeast with the Bogda Shan and Haerlike Shan and towards the south with the Jueluotage Shan (fig. 1), and located in the foreland region of the Himalayan orogenic belts formed by the collision between the Indian and Eurasian plates in the Cenozoic. Moreover it has thick and well developed sedimentary sequences. Therefore, this basin is one of the sedimentary basins which is best viewed as a Mesozoic basin with respect to the pre-Himalayan convergent margin of southern Asia. However, there is always controversy and differing views on the basin classification and the exiting of the thermal subsidence in its evolutional history[1–6]. Some believe that the Turpan Basin is a foreland basin[3, 4]. This paper will discuss about basin classification of the Turpan Basin and its special evolutional characteristics through the analysis of petrologic and geochemical data, and the comparative analysis with the typical subsidence curve of foreland basins in other areas.

1   Method and data

The modal analyses of sandstone were implemented on the 162 thin sections by counting over 300 points per thin section, using Gazzi-Dickinson point-count method[7]. The major elements were analyzed by conventional X-ray fluorescence (XRF) techniques and the trace and rare earth elements (REE) by inductively coupled plasma emission mass spectrometry (ICP-MS). The utilized decompaction method for subsidence analysis was the backstripping method [8, 10]. Due to the limited extension of individual sections, this analysis used the composite sections of the Taoshuyuan section (from the Upper Permian to Lower Jurassic) and Lianmuqin section (from the Middle Jurassic to Tertiary). The absolute ages for stratigraphic divisions were derived from Harland[9]. The sedimentary facies are almost continental environment such as lacustrine and fluvial facies through time. The paleowater depth has little affection in the total subsidence of the basin, therefore, we haven’t considered its influence. The important data of the subsidence analysis are listed in table 1.
 
Table 1: Data list for the subsidence analysis of the TURPAN BASIN
table 1
 

2   Exploratory data analysis

The petrological and chemical data are divided into six groups. The general differences between the groups are described by multivariable analysis and shown in star symbol plots (fig. 2). In Permian, the contents of Fe2O3, MgO, TiO2, Al2O3, Na2O and P2O5 are relatively high, while in the Triassic, the group T3+T2 has high MnO contents. It is interesting to note that groups J1 and J3+J2 are the same, indicating a similar element distribution at this time. In the Lower Cretaceous, TiO2, K2O, P2O5, Na2O and MgO are increasing. In the Upper Cretaceous and Tertiary (group Tertiary + K2) all elements are almost equipollent, reflecting the relatively well mixed source rock types.

In box-and-whisker plots, most variables for the major element, trace element and petrological data sets are astonishingly consistent, i.e. these plots clearly show that the variational trends of the variables can be divided into three variational periods or parts. The group P2 is the first part; groups of the Triassic and Jurassic, including groups T3 + T2, J1 and J3 + J2, build the second part; groups K1 and Tertiary + K2 are the third part (fig. 3). There are relatively large breaks in the variation of the variables among these three parts, and in each part the variation of the variables of each group is harmonious or changes little from one another. This consistency reveals the tectonic evolutional regularity of the basin and surrounding areas.

3   Subsidence curve analysis

The primary subsidence mechanisms for sedimentary basins are generally two types: the thermal subsidence and flexural subsidence. The thermal subsidence is driven by thermal reequilibration after magmatic activities as magma cools, which leads contraction of the crust and builds a sedimentary basin. The best example for thermal subsidence is the ocean floor located between ocean ridge and convergent boundary. Its  subsidence curve at begin is strongly concave-down and then further smooth downward (fig. 4). This indicates that at begin the subsidence rate is very great, and the sedimentary basin is in a rapid subsidence period. After the cooling of magma the sedimentary basin is only slowly further subsidence on account of the weight of sediments and water load. Flexural subsidence is led by tectonic force, the emplacement of a thrust belt load and the weight of the load forming sedimentary basin such as foreland basin. The subsidence curves of these basins consist of segmented lines (fig. 4), reflecting the rate of thrusting in the adjacent orogenic belt and sedimentation rate in the basin. Segmented subsidence curves may reflect discontinuous movement of the thrust belt. When the stress is great, the subsidence rate of the basin is high, otherwise, the subsidence rate is small.

Due to the special geological position, the Turpan Basin, as well as other west Chinese basins, was classified as one basin type[8], and called also often foreland basin or in very general terms intermontane basin[2, 3, 6]. Fig. 5 includes curves of total subsidence, tectonic subsidence and paleowater depth. The total decompacted paleosediment thickness is over ten thousand meters from the Upper Permian to Miocene. The highest rates of subsidence were present throughout the Late Permian and Early Triassic periods, active tectonic subsidence throughout the Middle Triassic and Jurassic, relatively reduced activity during the Cretaceous period and increased activity in the Early Tertiary. In addition, periods of accelerated basin subsidence occurred during the Late Permian and Early Triassic, Late Triassic/Early Jurassic, Late Jurassic, Latest Jurassic/ Earliest Cretaceous, and Latest Cretaceous/Early Cenozoic. In general, high rates of subsidence coincide very well with peaks in coarse clastic deposition and there are commonly major unconformities before these periodic high rates of subsidence.

4   Discussion

Due to the Hercynian movement in the Late Paleozoic, volcanic and intrusive activities as well as flysch deposits were quite extensively developed in this region during the Carboniferous and Early Permian[11]. Since the Upper Permian these activities had slowly ceased, the basin was in a extensional situation[12]. The curves of basin subsidence from the Upper Permian to Early Triassic are strongly concave-down, forming the highest subsidence rate of the basin. Mckenzie[13] suggested that the greatest subsidence of theoretical models of thermal subsidence occurred within about 50 Ma after the onset of extension. This situation was presented also in the Turpan Basin. These segments of the curves reflect thermal subsidence following the period of magmatic activities in this region and mark the beginning of the evolution of the basin. According to the extensional situation, volcanic activities and the thermal subsidence, the Turpan Basin should be a backarc basin in this period. Moreover, the subsidence curve and the sandstone composition reflect that there was a tectonic movement at the Upper Permian/Early Triassic. At this time the Tarim Block transported toward the north and converged with the Kazakstan Plate[14]. This tectonic event disturbed the thermal subsidence curve of the basin.

During the Middle Triassic and Early Tertiary, the total subsidence rate of the basin is generally in a high scale, although the tectonic subsidence is obviously less than before. There are several accelerated subsided periods in these times. Their segments of the curves are typical of flexural loading subsidence. Accelerated subsided periods occurred during the Late Triassic/Early Jurassic, Late Jurassic, Latest Jurassic/Earliest Cretaceous, and Latest Cretaceous/Early Cenozoic. The variation of the sandstone composition has a great spring point between Jurassic and Cretaceous. The Jueluotage Shan, located in south of the basin, was the important provenance region for the basin in the Triassic and Jurassic, while the Bogda Shan (located north of the basin) had gradually folded and uplifted late, and was an important source region for the basin in the Cretaceous and Tertiary[3, 11].

Liu et al.[15] pointed out that the Qiangtang Block at the Late Triassic/Early Jurassic, the Gangdise Block during the Late Jurassic, Latest Jurassic/Earliest Cretaceous, and the Indian Subcontinent during the Early Cenozoic collided and converged with the Eurasian plate respectively. Therefore, the acceleratively subsided events can be regarded as the influence of these collision and convergence for this area.

Hendrix et al.[4] displayed a subsidence history diagram of the Turpan Basin in their paper. However, the rapid subsidence period during the Later Permian and Early Triassic has not been displayed in this diagram. Therefore, they believed that the Turpan Basin is only a flexural subsidence basin. In addition, the diagram shows that the higher rates of subsidence occurred shortly after coarse clastic deposition. However, this situation did not appear in this study. According to the subsidence curve of the basin (fig. 5) and detailed observations in the field, it is clear that the thermal subsidence existed in the basin and the higher rates of basin subsidence occurred with coarse clastic deposition at the same time, reflecting a coincidence of the rapid tectonic subsidence and the strong erosion in the nearby source regions.

5   Conclusions

The Turpan Basin formed during the Late Permian, underwent first thermal subsidence and then flexural subsidence. The evolution of the basin can be divided into four periods: the rapidly subsiding period (from the Upper Permian to Early Triassic); the actively subsiding period (from the Middle Triassic to Jurassic), reduced subsiding period (Cretaceous) and increased subsiding period (Early Tertiary). Due to the first thermal subsidence, it should not be a foreland basin, otherwise was a backarc basin in the Late Paleozoic. The thermal subsidence took place during the Late Permian and Early Triassic following the period of magmatic activities in this region. The flexural subsidence was throughout the Middle Triassic to Early Tertiary induced by orogenic movements which produced periods of high subsidence rates. Accelerated subsided periods occurred during the Late Triassic/Early Jurassic, Late Jurassic, Latest Jurassic/Earliest Cretaceous, and Latest Cretaceous/ Early Cenozoic, indicating the collision and accretion onto the south Asian continental margin of the Qiangtang Block in the Late Triassic/Early Jurassic, the Gangdise Block in Late Jurassic and Latest Jurassic/Earliest Cretaceous, and the Indian Subcontinent in the Latest Cretaceous/ Early Cenozoic. The Turpan Basin evolved from a backarc basin in the Late Paleozoic into a foreland basin in the Mesozoic, and a large intermontane basin of the Tian Shan in the Cenozoic.

Acknowledgement   We thank very much Prof. Wang Pinxian and Lao Qiuyuan for giving a lot of support and help in the writing of this paper. The chemical analyses were made in the Chemical Analytical Laboratory of the Geological and Palaeontological Institute of the Christian-Albrechts-University of Kiel, Germany.

References

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Figures

 
 
 figure 1
 
Fig. 1. Location map of the studied areas in Northwest China. 1, Aiweiergou; 2, Keyayi; 3, Meiyaogou; 4, Taoshuyuan; 5 Kekeya; 6, Lianmuqin; 7, Qiketai; 8, Qijiaojing; 9, Shisanjianfang; 10, Cheguquanxi. Core profiles: 1 Sheng 101 well. I , Outcrop section; * , core profile.
 
 
 
figure 2
 
Fig. 2. Star plots of major elements of the Turpan Basin show the distributive characteristic for different periods (Symbols: P2 = Upper Permian; T3+T2 = Middle and Upper Triassic; J1 and J3+J2 = Lower and Upper + Middle Jurassic; K1 = Lower Cretaceous; K2 = Upper Cretaceous).
 
 
 
 figure 3
 
Fig. 3. Box-and-whisker plots of some variables of the Turpan Basin show that there are three variational parts between the data sets: first part = Upper Permian; second part = Triassic + Jurassic; third part = Cretaceous + Tertiary. (Symbols: Q = quartz; L = lithic fragments; C = Carboniferous).
 
 
 
figure 4
 
Fig. 4. Subsidence curve diagrams for foreland basins and ocean floor (after Angevine et al. 1990), displaying that the flexural subsidence curve of foreland basin consist of segmented lines and the thermal subsidence curve of ocean floor is strongly concave-down and then further smooth downward.
 
 
 
figure 5
 
Fig. 5. Subsidence history diagrams for the Turpan Basin, displaying that the Turpan Basin underwent first thermal subsidence and then flexural subsidence. The thermal subsidence took place during the Upper Permian and Early Triassic. The flexural subsidence was throughout the Middle Triassic to Early Tertiary. There are several accelerated subsidence periods associated with the alluvial coarse clastic sediments commonly overlying major unconformities.
 
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  Bernd J. Haupt (bjhaupt@psu.edu)