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# Open Geosciences

### formerly Central European Journal of Geosciences

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Volume 9, Issue 1

# Three-dimensional structural model of the Qaidam basin: Implications for crustal shortening and growth of the northeast Tibet

Jianming Guo
• Corresponding author
• Key Laboratory of Petroleum Resources, Gansu Province / Key Laboratory of Petroleum Resources Research, Institute of Geology and Geophysics, Chinese Academy of Sciences, Lanzhou 730000, china
• Email
• Other articles by this author:
/ Xuebing Wei
• Research Institute of Petroleum Exploration and Development, PetroChina Qinghai Oil-field Company, Dunhuang 736202,China
• Other articles by this author:
/ Guohui Long
• Research Institute of Petroleum Exploration and Development, PetroChina Qinghai Oil-field Company, Dunhuang 736202,China
• Other articles by this author:
/ Bo Wang
• Research Institute of Petroleum Exploration and Development, PetroChina Qinghai Oil-field Company, Dunhuang 736202,China
• Other articles by this author:
/ Hailong Fan
• Key Laboratory of Petroleum Resources, Gansu Province / Key Laboratory of Petroleum Resources Research, Institute of Geology and Geophysics, Chinese Academy of Sciences, Lanzhou 730000, china
• University of Chinese Academy of Sciences, Beijing 100049, China
• Other articles by this author:
/ Shiyang Xu
• Key Laboratory of Petroleum Resources, Gansu Province / Key Laboratory of Petroleum Resources Research, Institute of Geology and Geophysics, Chinese Academy of Sciences, Lanzhou 730000, china
• University of Chinese Academy of Sciences, Beijing 100049, China
• Other articles by this author:
Published Online: 2017-06-02 | DOI: https://doi.org/10.1515/geo-2017-0015

## Abstract

The Qaidam basin, bounded by the Altyn Tagh fault in the north, is located in the northeast of the Tibet plateau, and it has important implications for understanding the history and mechanism of Tibetan plateau formation during the Cenozoic Indo-Eurasia collision. In this study, we constructed the main geological structures and surfaces in three dimensions through the interpolation of regularly spaced 2D seismic sections, constrained by wells data and surface geology of the Qaidam basin in northeast Tibet. Meanwhile the Cenozoic tectonic history of the Qaidam basin was reconstructed and the uplift mechanism of the Tibetan plateau was discussed. This study presents the subsurface data in conjunction with observations and analysis of the stratigraphic and sedimentary evolution. The Cenozoic deformation history of the Qaidam basin shows geologic synchroneity with uplifting history of the Tibet Plateau. It is therefore proposed that the deformation and uplifting in the south and north edges of the Tibet Plateau was almost synchronous. The total shortening and shortening rate during Cenozoic reached 25.5 km and 11.2% respectively across the Qaidam basin, indicating that the loss of the left-lateral strike slip rates of the Altyn Tagh fault has been structurally transformed into local crustal thickening across NW-trending folds and thrust faults. Meanwhile there is an about 11° vertical component along the strike-slip Altyn Tagh fault, the block oblique slip shows one more growth mechanism of the northeast Tibet.

## 1 Introduction

The high elevation and great crustal thickness of the Tibet plateau are generally assumed to be resulted from the India-Eurasia collision, but the processes that have led to the state observed today remain unclear [1-5]. The Qaidam basin is located in a complex system of compressive structures in the northeast Tibet, and is the largest topographic depression inside the Tibetan plateau (Figure 1). Because the tectonic evolution of the Qaidam basin provides an observational basis for the processes of largescale continental deformation, The structural modelling and recovery studies have important implications for unraveling the formation mechanism and growth history of the Tibetan plateau [6-15]. Although much of its late Cenozoic deformation is explained by the collision and subsequent penetration of India into Eurasia, how Eurasia deforms in response to the collision is still subject to debate. Most of the papers in fact analyzed just single sectors of the basin, whereas few works dealt with the correlation of each thrust fronts along strike. There are in fact unsolved aspects in terms of geometry, displacements and deformation chronology that can be highlighted by taking into account a bigger area of investigation and by the 3D reconstruction to provide new insights into the kinematics of Eurasia.

Figure 1

(a): Structural map of the Tibet Plateau within the Eurasia/India continental collision. Black arrow indicates present-day displacement of India continental block. (b): Shaded relief map of the Tibet and Qaidam basin. Main faults are indicated by white lines

The development of software for 3D modeling has opened a new frontier in the earth sciences, leading to a more accurate spatial analysis of geological structure and to 3D models. Integrating geophysical and geological data, from seismic available database and by geological maps, is possible to define geometrical and geological constraints in order to create 3D surfaces, closed volumes and grids from the constructed objects. 3D modeling allows to detect and analyze complex spatial relationships, leading to a better characterization of both exposed and subsurface geology [16-19]. It is especially useful in cases where the structures are not cylindrical and 2D visualization may be not completely useful.

There exist different views on the structure and evolution of the Qaidam Basin, and the corresponding tectonic evolution models have been established[13]. The first model is the extension-compression two-stage model, and extensional tectonics of high-angle normal faults along the southeastern margins of the major rifted depression occurred in the late Cretaceous - early Tertiary, while squeezing construction began in late Oligocene [14]. The second is the flexure subsidence model of foreland basin, that at present is generally accepted, the main reason is that Qaidam basin is currently defined by thrust faults at northeast and southwest sides [20]. The third is the piggyback basin model, and Qaidam upper-crustal structures can be explained by thrusting along a mid-crustal décollement [21]. The fourth is the crust-buckling model, which is generated under horizontal compression [9]. The main models were generally studied by using 2D structures. In this study, we used the 3D method to establish a complete structure to reveal the spatial distribution of faults and strata of the basin, and with geologic section restoration, we measured the total shortening and shortening ratio during Cenozoic.

## 2 Regional geology

The Qaidam basin has a rhombic shape that is located on the northeastern margin of the Tibetan plateau. The basin covers an area of approximately 1.2x105 km2 and has elevations of 2500-3000 m (Figure 1). The deformation of major faults surrounding the basin and the Cenozoic strata within the basin provides a record of the basin-mountain tectonic framework and geodynamic setting. Specifically, four thrust belts control the evolution of the basin: the Kunlun fault to the south, the Altyn Tagh fault to the north-west, the Kunlun mountain thrust belt on the southern margin, and the Qilian mountain thrust belt to the northeast of the basin. The basin is divided into two contrasting sectors, and the northwest sector has more folds, indicating significantly stronger deformation than the southeast sector (Figure 1), containing a series of NW-SE trending thrust-fold belts.

Cenozoic strata in the Qaidam basin, over 15 km in total thickness, mainly contains a continuous sequence of lacustrine, fluvial, alluvial and eolian sediments, and the depositional center of the basin shifted progressively towards the east during Cenozoic times [12, 21]. The shift was accompanied by uplift in the west and subsidence in the east of the basin. Cenozoic stratigraphic division and age assignments across Qaidam basin are based on outcrop geology and its correlation with subsurface well data. The units are: Paleocene to early Eocene (E1+2) of the Lulehe formation (65–52.5 Ma); middle Eocene $\left({\text{E}}_{3}^{1}\right)$ of the lower Ganchaigou formation (52.5–42.8 Ma); late Eocene$\left({\text{E}}_{3}^{2}\right)$of the lower Ganchaigou formation (42.8–40.5 Ma); late Eocene to Oligocene of the upper Ganchaigou formation (40.5–24.6 Ma); early to middle Miocene $\left({\text{N}}_{2}^{1}\right)$ of the lower Youshashan formation (24.6–12 Ma); late Miocene$\left({\text{N}}_{2}^{2}\right)$ of the upper Youshashan formation (12–5.1 Ma); Pliocene $\left({\text{N}}_{2}^{3}\right)$ of the Shizigou formation (5.1–2.8 Ma); and late Pliocene to Quaternary (Q) of the Qigequan formation (2.8 Ma–present) [8, 12]. Paleocene and Eocene strata has a few small outcrops in northern Qaidam but are not widespread in the rest of the basin. Oligocene strata are more widespread and include lacustrine rocks within the lower Ganchaigou formation in north-western Qaidam. Lacustrine deposition was also widespread in Qaidam during the Miocene, whereas lesser amounts of lacustrine strata are present in lower Pliocene sections. Upper Pliocene and Quaternary sections are generally coarser grained fluvial and alluvial deposits, which are quite thick and reflect high sedimentation rates [22].

The Altyn Tagh fault is the major boundary fault on the northern margin of Tibet, and the left slip on the fault zone is related to and absorbed by crustal shortening within the Qilian mountain, the Qaidam basin, and other convergent structures south of the Altyn Tagh fault zone [6]. Slip vector analyses also indicate that the loss of the sinistral slip rates from west to east has structurally transformed into local crustal shortening perpendicular to the active thrust faults and strong uplifting of the thrust sheets to form the NW-trending Qilian mountain [23].

## 3.1 Datasets

The typical input data for a 3D structural modeling project can be quite diverse and may include field observations, remote sensing pictures, seismic profiles and borehole data. Each data type has its specific features, which will act upon how it is integrated in the modeling process and affect the quality of the model [16].

Due to the vegetation is sparse around the Qaidam basin, surface linear structure is clear. In this study, we collected SRTM DEM, Landsat ETM and QuickBird remote sensing data to identify faults and folds on the surface (Figs 1, 2 and 3). The Shuttle Radar Topography Mission (SRTM) is an international research effort that obtained digital elevation models (DEM) on a near-global scale to generate the most complete digital topographic database of Earth based on the spaceborne interferometric synthetic aperture radar, and the ground resolution of one pixel is 90 m. The Landsat Enhanced Thematic Mapper (ETM) data cover the visible, near-infrared, shortwave, and thermal infrared spectral bands of the electromagnetic spectrum. The panchromatic resolution is 15 m, and multispectral resolution is 30 m. QuickBird was a high-resolution commercial earth observation satellite, and its panchromatic resolution is 0.6 m, multispectral resolution is 2.4 m. DEM and ETM data covering the Qaidam basin were downloaded freely from Global Land Cover Facility (http://glcf.umd.edu/). The linear features of fault and fold in remote sensing images are clear, according to the results of remote sensing image interpretation, in the field we selected a number of representative points to observe distribution of faults and folds (Figs. 2,3).

Figure 2

Folds in the Qaidam basin. (a-c) 3D remote sensing images of folds, (d, e) field photos of the limbs of the folds, (f) core of a fold

Figure 3

Remote sensing images (a, b) and field photos (c, d, e) of the Altyn Tagh fault. (a): Folds in Qaidam basin direct obliquely to the Altyn Tagh fault. (b): A river is left-laterally offset by the fault. (c): The fault cuts the Quaternary alluvial sediments and fault scarp shows the fault has a vertical component. (d): an outcrop of the fault on the side of the river. (e): The striations of the Altyn Tagh fault, the strike of the fault plane is 76°, dip is 82°, and the striations plunging direction is 257°, plunging angle is 11°. (f): The piedmont thrust of the Kunlun mountain

A total of 17 seismic lines were gathered, and the total length is over 3400 km, from a single length of 80 km to 450 km. Two-way time of seismic reflection is 6 seconds (Figure 4), and seismic lines are parallel and perpendicular respectively to the structural direction. Seismic project datum is 2772 m.

Figure 4

Seismic lines AB and CD. AB profile shows that thrusts control east and west sides of the basin, whereas the CD profile is mainly affected by the Altyn Tagh fault on the north side. T0 indicates the base of Pleistocene (2.8 Ma), T1 Pliocene (5.1 Ma), ${\text{T}}_{2}^{1}$ upper Miocene (12.0 Ma), T2 lower Miocene (24.6 Ma), T3 Oligocene (40.5 Ma), T4 upper Eocene (42.8 Ma), T5 lower Eocene (52.5 Ma), TR Paleocene (65.0 Ma), T6 Mesozoic. All red lines are thrusts in the basin

A total of 9 wells were used to time-depth conversion, and these wells are located in the vicinity of seismic lines. The depth of wells ranges from 3000 to 6000 m. Well logging data are mainly sonic logs, density logs, and Geological strata data.

The synthetic seismograms were used for the seismic and geologic horizon calibration. Combined with drilling data, the geological horizon system of the basin was established, and the seismic reflection characteristics of each layer was basically defined.

T6 is the bottom of Mesozoic (M) and the top reflection of bedrock. It is an unconformity surface and the low frequency is continuous, when the waveform changes strongly in the west of the basin. Whereas it is clear and reliable in the east of the basin.

TR is the bottom reflection of E1+2 . It is major angular unconformity in the basin, and is a phase axis under 2-3 continuous strong phases. Due to distinct characteristics, TR is a standard reflection in the basin.

T5 is the bottom reflection of${\text{E}}_{3}^{1}.$ It is characterized by the high amplitude, continuous features.

T4 is the bottom reflection of ${\text{E}}_{3}^{2}.$ It is a standard reflection in the basin. It is characterized by 2 phase, strong amplitude and continuous reflection under a set of blank reflection in the western basin. In the eastern basin, it lies at the top of a group strong reflection.

T3 is the bottom reflection of N1. It shows medium-high amplitude and continuous reflection. In the west of the basin, the wave energy becomes weak.

T2 is the bottom reflection of ${\text{N}}_{2}^{1}.$ It is a 2 phase, strong amplitude and continuous reflection. The variation of amplitude becomes large and continuity becomes poor in the eastern margin of the basin.

T2 is the bottom reflection of ${\text{N}}_{2}^{2}.$ It is a regional unconformity, and reflection is continuous except margin and eroded area.

T1 is the bottom reflection of ${\text{N}}_{2}^{3}.$ In the depression, it has good continuity, whereas it becomes unrest at the margin.

T0 is the bottom reflection of Q. It shows 1-2 phase, medium-high amplitude, and continuous features in the depression, whereas the reflection is not stable at the margin, and at some area it is missing due to the erosion.

After completing seismic and geologic horizon calibration, the results were taken into the seismic profiles to establish the interpretation network. Through repeated comparison, repeated modification of the drilling and seismic data, the seismic and geological layer system of the whole basin was built up.

## 3.2 3D structural modeling

The data used in the study include well logs, checkshot data, 2D seismic sections and base map of the study area, and all data were imported into the Landmark’s OpenWorks data management environment. Using Syn-Tool software the time interpretation with wells via a one-dimensional synthetic seismogram was tied to match seismic time data with geologic depth data. After completing seismic and geologic horizon calibration, the results were taken into the seismic profiles to establish the interpretation network. Through repeated comparison, repeated modification of the drilling and seismic data, the seismic and geological layer system of the whole basin was built up. SeisWorks software was used to interpret T1-T6 horizons and faults on seismic sections. Because the seismic data were acquired and processed by different teams, leading to the appearance of misties, we applied the Interactive Seismic Balance tools to analyze and resolve misties in time, phase, and amplitude. After the interpretation, horizons and faults were converted from time to depth domain using TDQ software, and were exported out from OpenWorks for structural modeling.

Then the data were loaded into the Petrel software to define the faults in the geological model that form the basis for generating the 3D grid. These faults define breaks in the grid, lines along which the horizons inserted later may be offset. The offset which occurs is entirely dependent upon the input data. The Make horizons process generates independent geological horizons from input data. To generate additional horizons using relative distance to existing horizons use the Make zones process. These two processes are used to create the geological zones within the model. Layering inserts the fine scale grid cells which will describe vertical variation within each geological zone.

## 3.3 Geologic section restoration

The section is sequentially restored back in time to its initial stage before the deposition of Cenozoic strata using the computer section balancing software Move. The section is divided into discrete blocks bounded by faults and bedding planes, which are then individually manipulated using different restoration algorithms. The flexural slip unfolding algorithm was applied to restore folds. The algorithm works by rotating the limbs of a fold to a datum, an assumed regional, or template geometry. Layer parallel shear is then applied to the rotated fold limbs in order to remove the effects of the flexural slip component of folding. Unfolding occurs about a pin line and points along the pin are not translated. The pin should correspond to the axial surface of the fold. In the flexural slip algorithm, only slip between bedding planes was permitted so that only the area of cross-section and the length of the beds parallel to the chosen reference surface were preserved.

The fault parallel flow algorithm is used to restore faults and is based on particulate laminar flow over a fault ramp. The fault plane is divided into discrete dip domains where a change in the fault’s dip is marked by a dip bisector. Flow lines are constructed by connecting points on different dip bisectors of equal distance from the fault plane. Particles in the hanging wall translate along the flow lines, which are parallel to the fault plane, by a defined distance.

Decompaction is an essential step to obtain an accurate geometry of the reconstructed structures. Because the lithology of most of the stratigraphic units in Qaidam basin commonly has complex lateral variations, the decompaction has not been taken into account in the section restoration [12].

## 4 Results

Using the remote sensing satellite data analysis and interpretation, we can identify the distinct structural features including fold, fault, and lineament (Figure 2,3). The measurements denoted a good agreement with field data, confirming the potentiality of satellite analyses in geological studies. As a major boundary fault between two large geomorphic units, the Tibet and the Tarim basin, the Altyn Tagh fault displays a remarkable topographic contrast from the northern Plateau to the southern Tarim basin with a vertical throw up to 3500 m [23]. The Altyn Tagh fault extends for at least 1,500 km from the west Kunlun thrust zone in the southwest to the edge of the Qilian mountains in the northeast. It is divided into three main sections: southwestern, central and northeastern. There is one major splay fault, the north Altyn fault. The main active fault trace of the fault lies within a zone of secondary structures that is about 100 km wide in the central section. The fault cuts various geomorphic surfaces, such as Quaternary fluvial fans and terraces, has developed distinct linear traces along the fault (Figure 3). The horizontally offset gully, which have been displaced from several meters to several hundred meters, and vertical fault plane, which has an 82° dip and 76° strike, indicates it is a left-lateral strike-slip fault (Figure 3b, 3d), meanwhile the uplifted fault scarp and 11° oblique striations on the fault plane means that the fault has a vertical component (Figure 3c, 3e). The thickness of the fault is over several hundred meters on the out crop (Figure 3d). Faults and folds extending northward are cut and terminated by the Altyn Tagh fault.

The axial traces of Qaidam folds trend NW-SE. Many Qaidam anticlines are fault-propagation folds with steeper dipping fore-limbs and shallower dipping back-limbs. The fore-limbs are located on the northern sides of the axial traces, indicating that the anticlines are propagating northward to northeastward, and the fold geometries may be controlled by reactivation or inversion of preexisting south to southwest-dipping faults in the underlying basement [24].

In 3D modeling, the main geological structure is that the basin is bounded by the southwest-directed Kunlun thrust belt in the west and the northeast-directed Qilian mountain thrust belt in the east (Figure 5). The basin has been compressed to a narrow irregular diamond shape. The tectonic deformation in the north, where the Altyn Tagh fault lies, is stronger than in the south.

Figure 5

3D modeling structures of Qaidam basin, and the data unit is elevation depth (meter). (a): The base of the Pleistocene (Q).(b): The base of the upper Miocene $\left({\text{N}}_{2}^{2}\right).$ (c): The base of the upper Eocene $\left({\text{E}}_{3}^{2}\right).$ (d): The base of the Paleocene (E1+2). The magenta surface indicates the site of the Altyn Tagh fault, and the Qaidam basin is bounded by the southwest-directed Kunlun thrust belt in the west and the northeast-directed Qilian mountain thrust belt in the east. The green arrow indicates north direction

The two-dimensional time-depth converted seismic profile AB was used for the construction of the geological section, which is perpendicular to the trends of structures. The length of the section is over 200 km, and entire section shows a synclinorium shape. Strata on both sides is thin, meanwhile in the middle is thick, and the thickest part is over 2 km. The uplifted formation on both sides has been eroded. In the basin, secondary anticlines and synclines developed. The Kunlun mountain fault and the Qilian mountain fault thrust to the basin and the maximum offset of the thrusts is over 8 km (Figure 4, 6). The restoration results of the geological section AB (Figure 6), including total shortening, average shortening rates, and shortening rates during different time periods, are listed in Table 1. The total shortening and shortening ratio during Cenozoic respectively reaches 25.5 km and 11.2% across the Qaidam basin, which is consistent with previous works [12, 21, 25]. 42.9% and 34% of total shortening took place in the periods since 5.1 Ma and since 2.8 Ma, respectively. The shortening rates from the restoration of the section indicate that the Qaidam basin has been undergoing continuous shortening since the beginning of Cenozoic with two relatively fast shortening phases, the first during ${\text{E}}_{3}^{2}$ layer (42.8-40.5 Ma) and the second during Q layer (2.8 Ma–present), and the shortening rates reached the maximum values to 3.09 mm/yr since 2.8 Ma, whereas the average rate during the Cenozoic is 0.39 mm/yr (Table 1). From 65 Ma to 42.8 Ma, the shortening rate increased from 0.14 to 1.08 mm/yr, then from 40.5 Ma to 12 Ma, the rate decreased to 0.09 mm/yr, and finally the rate increased to the maximum in Quaternary.

Table 1

Restoration measurements and shortening rates of section AB

Figure 6

Sequential restorations of the geological section AB

## 5 Discussion

In this study, the Cenozoic deformation history of the Qaidam basin (Figure 6) shows that the basin experienced continuous compression since the beginning of Cenozoic. It means that the deformation and uplifting in the south and north edges of the Tibet plateau were almost synchronous. The restoration results of this study is comparable with previous studies by Zhou et al. [12] and Yin et al. [21] in the same area. The total shortening of this study is 25.5 km, and Zhou et al. gave a 36.6 shortening and Yin et al. got a 20-80 km result. The difference may be caused by different time-depth conversion and restoration methods, and the total shorting across the Qaidam basin should be about 20-40 km. The restoration shows that the deformation of Qaidam basin was not uniform [26], and it experienced an increase and a decrease, then an increase process. In later Eocene and Quaternary, the basin has two relatively fast shortening phases, and from later Miocene to present, the rate has been increasing to the maximum value ever, suggesting the northeastern Tibet will keep strong deformation in the future. Correspondingly, the Cenozoic deformation history of the Qaidam Basin has implications for the evolution of Altyn Tagh fault. The total strike-slip offset of the Altyn Tagh fault across the Qaidam basin is about 25.5 km during Cenozoic. From west to east of the Qaidam basin, the slip rate of the Altyn Tagh fault decreased at about 3 mm/yr in Quaternary. The data presented in this study are more regular, with data from low to high, then to low, and finally increased, consistent with long-term geological process characteristics. The thrusts and folds developed in Qaidam basin absorbed the movement of the Altyn Tagh fault. In the field work, we observed that the strike-slip Altyn Tagh fault has about 11° vertical component (Figure 5e), which is supported by the uplifted young fault scarp at the south side of the fault on the flood plain (Figure 5c), Indicating that the south block has a whole oblique uplift along the fault (Figure 7). The driving force may come from the southward subduction of lithospheric mantle overlying detached crust [13].

Figure 7

Schematic shortening model of the northeast Tibet along the Altyn Tagh fault. The folding and thrusting on the south side of the Altyn Tagh fault lead to the thickening and uplift of the upper crust, meanwhile the south block has a whole oblique uplift along the fault

The previous studies of the flexure subsidence model of foreland basin [20], the piggyback basin model [21], and the crust-buckling model [9] are all support that the shortening along the mountain frontal thrust system accommodates much of the present-day convergence [27]. Whereas how to answer why the Cenozoic sedimentary center is located in the central basin, and what a dynamic process leads to a 15 km deposition, are the keys to understand the formation of the basin [13]. The settlement of the basement caused by the foreland basin requires the center of subsidence to occur at the leading edge of the thrust fault, and the subsidence and sedimentation centers are usually located at the edge of the basin, so the model of foreland basin could not answer why the sedimentary center is located in the central basin. The piggyback basin model assumes that the formation of the Qaidam basin is related to the southward large-scale forward thrusting process, but it did not consider northward thrusting at the piedmont of the Kunlun mountain (Figure 3f).

The crust-buckling model [9, 13] gives an explanation of the initiation and development of the Qaidam basin, and It can explain that the sedimentary center of the Qaidam basin is located in the middle of the basin. The model requires the low crustal of eastern Tibet to be weak enough to flow outward from the interior of the Tibet Plateau [28]. The outward growth of eastern Tibet is probably driven by lower crustal flow under the lateral pressure gradients. Since the upper crust of the blocks is dominated by brittle deformation, the ductile flow of the lower crust would drag the brittle upper crustal blocks to move with respect to each other. The interactions among the brittle upper crustal blocks cause strain accumulations among their bounding faults to generate large earthquakes [29]. A recent study determined both the P- and S-wave velocity structure across the central Qaidam basin along a 350-km-long seismic refraction/wide-angle reflection profile extending from the Kunlun mountain through the central Qaidam basin into the Qilian mountain, and shows brittle deformation in the upper crust through thrust folding, and ductile deformation of the lower crust though pure shear deformation [30]. The 3D model of the Qaidam basin shows overall a synclinorium, with a narrow NW-SE direction distribution, and on the east and west side of the basin, main piedmont thrusts control the boundary, and the strike-slip Altyn Tagh fault limits the north side of the basin (Figure 5). The synclinorium shape supports that the upper crust of Qaidam basin has been compressed to a depression, under the flow of lower crust.

## 6 Conclusions

3D modeling shows that both sides of the mountains thrust to the Qaidam basin, and the basin is squeezed to a narrow irregular diamond shape. The tectonic deformation in the north, where the Altyn Tagh fault lies, is stronger than in the south. The Altyn Tagh fault, as a boundary strike-slip fault, controls the formation and evolution of the Qaidam basin. The amount of slip southward transformed into thrusts and folds, and the total shortening and shortening ratio during Cenozoic respectively reaches 25.5 km and 11.2% across the basin. The compression of Qaidam basin was not uniform, and it experienced an increase and a decrease, then an increase process.

There are two uplift mechanisms of northeastern Tibet, one is the oblique uplift of the whole block along the Altyn Tagh fault, and another is thrusting and folding that caused thickening of the upper crust in the Qaidam basin and surrounding areas.

## Acknowledgement

We are grateful to two anonymous reviewers for their constructive reviews on the manuscript. This study is financially supported by the Important Direction Project of Knowledge Innovation in Resource and Environment Field, Chinese Academy of Sciences (KZCX2-EW-QN112), and the fund from the Key Laboratory of Petroleum Resources, Gansu Province, Institute of Geology and Geophysics, Chinese Academy of Sciences (No. 135CCJJ20160517). We thank Global Land Cover Facility (GLCF) for providing the SRTM DEM and Landsat ETM datasets (http://glcf.umd.edu/).

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Accepted: 2017-02-28

Published Online: 2017-06-02

Citation Information: Open Geosciences, Volume 9, Issue 1, Pages 174–185, ISSN (Online) 2391-5447,

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