Open Geosci. 2017; 9:174–185

Research Article Open Access

Jianming Guo*, Xuebing Wei, Guohui Long, Bo Wang, Hailong Fan, and Shiyang Xu Three-dimensional structural model of the : Implications for crustal shortening and growth of the northeast Tibet

DOI 10.1515/geo-2017-0015 ponent along the strike-slip , the block Received Sep 09, 2016; accepted Feb 28, 2017 oblique slip shows one more growth mechanism of the northeast Tibet. Abstract: The Qaidam basin, bounded by the Altyn Tagh fault in the north, is located in the northeast of the Ti- Keywords: Qaidam basin; 3D modeling; Structral restora- bet plateau, and it has important implications for under- tion; Altyn Tagh fault; Crust-buckling model standing the history and mechanism of formation during the Indo-Eurasia collision. In this study, we constructed the main geological structures and surfaces in three dimensions through the interpola- 1 Introduction tion of regularly spaced 2D seismic sections, constrained The high elevation and great crustal thickness of the Ti- by wells data and surface geology of the Qaidam basin bet plateau are generally assumed to be resulted from the in northeast Tibet. Meanwhile the Cenozoic tectonic his- India–Eurasia collision, but the processes that have led to tory of the Qaidam basin was reconstructed and the up- the state observed today remain unclear [1–5]. The Qaidam lift mechanism of the Tibetan plateau was discussed. This basin is located in a complex system of compressive struc- study presents the subsurface data in conjunction with ob- tures in the northeast Tibet, and is the largest topographic servations and analysis of the stratigraphic and sedimen- depression inside the Tibetan plateau (Figure 1). Because tary evolution. The Cenozoic deformation history of the the tectonic evolution of the Qaidam basin provides an Qaidam basin shows geologic synchroneity with uplifting observational basis for the processes of largescale conti- history of the Tibet Plateau. It is therefore proposed that nental deformation, The structural modelling and recov- the deformation and uplifting in the south and north edges ery studies have important implications for unraveling the of the Tibet Plateau was almost synchronous. The total formation mechanism and growth history of the Tibetan shortening and shortening rate during Cenozoic reached plateau [6–15]. Although much of its late Cenozoic defor- 25.5 km and 11.2% respectively across the Qaidam basin, mation is explained by the collision and subsequent pen- indicating that the loss of the left-lateral strike slip rates etration of India into Eurasia, how Eurasia deforms in of the Altyn Tagh fault has been structurally transformed response to the collision is still subject to debate. Most into local crustal thickening across NW-trending folds and of the papers in fact analyzed just single sectors of the thrust faults. Meanwhile there is an about 11∘ vertical com- 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 deforma- *Corresponding Author: Jianming Guo: Key Laboratory tion chronology that can be highlighted by taking into ac- of Resources, Province / Key Laboratory of count a bigger area of investigation and by the 3D recon- Petroleum Resources Research, Institute of Geology and Geo- struction to provide new insights into the kinematics of physics, Chinese Academy of Sciences, 730000, ; Email: [email protected] Eurasia. Xuebing Wei, Guohui Long, Bo Wang: Research Institute of The development of software for 3D modeling has Petroleum Exploration and Development, PetroChina Oil- opened a new frontier in the earth sciences, leading to a field Company, Dunhuang 736202, China more accurate spatial analysis of geological structure and Hailong Fan, Shiyang Xu: Key Laboratory of Petroleum Resources, to 3D models. Integrating geophysical and geological data, Gansu Province / Key Laboratory of Petroleum Resources Research, Institute of Geology and Geophysics, Chinese Academy of Sciences, from seismic available database and by geological maps, is Lanzhou 730000, China; University of Chinese Academy of Sci- possible to define geometrical and geological constraints ences, Beijing 100049, China

© 2017 J. Guo et al., published by De Gruyter Open. This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 License. Three-dimensional structural model of the Qaidam basin Ë 175

Figure 1: (a): Structural map of the Tibet Plateau within the Eurasia/India continental collision. Black arrow indicates present-day displace- ment of India continental block. (b): Shaded relief map of the Tibet and Qaidam basin. Main faults are indicated by white lines in order to create 3D surfaces, closed volumes and grids gyback basin model, and Qaidam upper-crustal structures from the constructed objects. 3D modeling allows to de- can be explained by thrusting along a mid-crustal décolle- tect and analyze complex spatial relationships, leading to ment [21]. The fourth is the crust-buckling model, which a better characterization of both exposed and subsurface is generated under horizontal compression [9]. The main geology [16–19]. It is especially useful in cases where the models were generally studied by using 2D structures. In structures are not cylindrical and 2D visualization may be this study, we used the 3D method to establish a com- not completely useful. plete structure to reveal the spatial distribution of faults There exist different views on the structure and evo- and strata of the basin, and with geologic section restora- lution of the Qaidam Basin, and the corresponding tec- tion, we measured the total shortening and shortening ra- tonic evolution models have been established[13]. The first tio during Cenozoic. 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 2 Regional geology occurred in the late Cretaceous - early Tertiary, while squeezing construction began in late [14]. The The Qaidam basin has a rhombic shape that is located on second is the flexure subsidence model of foreland basin, the north-eastern margin of the Tibetan plateau. The basin that at present is generally accepted, the main reason is covers an area of approximately 1.2×105 km2 and has ele- that Qaidam basin is currently defined by thrust faults at vations of 2500–3000 m (Figure 1). The deformation of ma- northeast and southwest sides [20]. The third is the pig- 176 Ë J. Guo et al. jor faults surrounding the basin and the Cenozoic strata and strong uplifting of the thrust sheets to form the NW- within the basin provides a record of the basin–mountain trending Qilian mountain [23]. 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- 3 Methods west, the Kunlun mountain thrust belt on the southern margin, and the Qilian mountain thrust belt to the north- east of the basin. The basin is divided into two contrasting 3.1 Datasets sectors, and the northwest sector has more folds, indicat- ing significantly stronger deformation than the southeast The typical input data for a 3D structural modeling project sector (Figure 1), containing a series of NW–SE trending can be quite diverse and may include field observations, thrust-fold belts. remote sensing pictures, seismic profiles and borehole Cenozoic strata in the Qaidam basin, over 15 km in to- data. Each data type has its specific features, which will tal thickness, mainly contains a continuous sequence of act upon how it is integrated in the modeling process and lacustrine, fluvial, alluvial and eolian sediments, and the affect the quality of the model [16]. depositional center of the basin shifted progressively to- Due to the vegetation is sparse around the Qaidam wards the east during Cenozoic times [12, 21]. The shift basin, surface linear structure is clear. In this study, we was accompanied by uplift in the west and subsidence collected SRTM DEM, Landsat ETM and QuickBird remote in the east of the basin. Cenozoic stratigraphic division sensing data to identify faults and folds on the surface and age assignments across Qaidam basin are based on (Figs 1, 2 and 3). The Shuttle Radar Topography Mission outcrop geology and its correlation with subsurface well (SRTM) is an international research effort that obtained digital elevation models (DEM) on a near-global scale to data. The units are: Paleocene to early (E1+2) of 1 generate the most complete digital topographic database the Lulehe formation (65–52.5 Ma); middle Eocene (E3) of the lower Ganchaigou formation (52.5–42.8 Ma); late of Earth based on the spaceborne interferometric syn- 2 thetic aperture radar, and the ground resolution of one Eocene (E3) of the lower Ganchaigou formation (42.8– 40.5 Ma); late Eocene to Oligocene of the upper Gan- pixel is 90 m. The Landsat Enhanced Thematic Mapper chaigou formation (40.5–24.6 Ma); early to middle (ETM) data cover the visible, near-infrared, shortwave, 1 and thermal infrared spectral bands of the electromag- (N2) of the lower Youshashan formation (24.6–12 Ma); 2 netic spectrum. The panchromatic resolution is 15 m, and late Miocene (N2) of the upper Youshashan formation 3 multispectral resolution is 30 m. QuickBird was a high- (12–5.1 Ma); (N2) of the Shizigou formation (5.1– 2.8 Ma); and late Pliocene to Quaternary (Q) of the Qige- resolution commercial earth observation satellite, and its quan formation (2.8 Ma–present) [8, 12]. Paleocene and panchromatic resolution is 0.6 m, multispectral resolution Eocene strata has a few small outcrops in northern Qaidam is 2.4 m. DEM and ETM data covering the Qaidam basin but are not widespread in the rest of the basin. Oligocene were downloaded freely from Global Land Cover Facility strata are more widespread and include lacustrine rocks (http://glcf.umd.edu/). The linear features of fault and fold within the lower Ganchaigou formation in north-western in remote sensing images are clear, according to the results Qaidam. Lacustrine deposition was also widespread in of remote sensing image interpretation, in the field we se- Qaidam during the Miocene, whereas lesser amounts of la- lected a number of representative points to observe distri- custrine strata are present in lower Pliocene sections. Up- bution of faults and folds (Figs. 2, 3). per Pliocene and Quaternary sections are generally coarser A total of 17 seismic lines were gathered, and the to- grained fluvial and alluvial deposits, which are quite thick tal length is over 3400 km, from a single length of 80 km and reflect high sedimentation rates [22]. to 450 km. Two-way time of seismic reflection is 6 seconds The Altyn Tagh fault is the major boundary fault on the (Figure 4), and seismic lines are parallel and perpendicu- northern margin of Tibet, and the left slip on the fault zone lar respectively to the structural direction. Seismic project is related to and absorbed by crustal shortening within the datum is 2772 m. Qilian mountain, the Qaidam basin, and other convergent A total of 9 wells were used to time-depth conversion, structures south of the Altyn Tagh fault zone [6]. Slip vector and these wells are located in the vicinity of seismic lines. analyses also indicate that the loss of the sinistral slip rates The depth of wells ranges from 3000 to 6000 m. Well log- from west to east has structurally transformed into local ging data are mainly sonic logs, density logs, and Geolog- crustal shortening perpendicular to the active thrust faults ical strata data. Three-dimensional structural model of the Qaidam basin Ë 177

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 ofafold

1 The synthetic seismograms were used for the seismic T5 is the bottom reflection of3 E . It is characterized by and geologic horizon calibration. Combined with drilling the high amplitude, continuous features. 2 data, the geological horizon system of the basin was estab- T4 is the bottom reflection of3 E . It is a standard re- lished, and the seismic reflection characteristics of each flection in the basin. It is characterized by 2 phase, strong layer was basically defined. amplitude and continuous reflection under a set of blank

T6 is the bottom of Mesozoic (M) and the top reflec- reflection in the western basin. In the eastern basin, itlies tion of bedrock. It is an unconformity surface and the at the top of a group strong reflection. low frequency is continuous, when the waveform changes T3 is the bottom reflection of N1. It shows medium- strongly in the west of the basin. Whereas it is clear and high amplitude and continuous reflection. In the west of reliable in the east of the basin. the basin, the wave energy becomes weak. 1 TR is the bottom reflection of E1+2. It is major angular T2 is the bottom reflection of2 N . It is a 2 phase, strong unconformity in the basin, and is a phase axis under 2-3 amplitude and continuous reflection. The variation of am- continuous strong phases. Due to distinct characteristics, plitude becomes large and continuity becomes poor in the

TR is a standard reflection in the basin. eastern margin of the basin. 178 Ë J. Guo et al.

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 tothe 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

2 T2 is the bottom reflection of2 N . It is a regional uncon- depression, whereas the reflection is not stable at the mar- formity, and reflection is continuous except margin and gin, and at some area it is missing due to the erosion. eroded area. After completing seismic and geologic horizon cali- 3 T1 is the bottom reflection of N2. In the depression, it bration, the results were taken into the seismic profiles has good continuity, whereas it becomes unrest at the mar- to establish the interpretation network. Through repeated gin. comparison, repeated modification of the drilling and seis- T0 is the bottom reflection of Q. It shows 1-2 phase, mic data, the seismic and geological layer system of the medium-high amplitude, and continuous features in the whole basin was built up. Three-dimensional structural model of the Qaidam basin Ë 179

3.2 3D structural modeling the pin are not translated. The pin should correspond to the axial surface of the fold. In the flexural slip algorithm, The data used in the study include well logs, checkshot only slip between bedding planes was permitted so that data, 2D seismic sections and base map of the study only the area of cross-section and the length of the beds area, and all data were imported into the Landmark’s parallel to the chosen reference surface were preserved. OpenWorks data management environment. Using Syn- The fault parallel flow algorithm is used to restore Tool software the time interpretation with wells via a one- faults and is based on particulate laminar flow over a fault dimensional synthetic seismogram was tied to match seis- ramp. The fault plane is divided into discrete dip domains mic time data with geologic depth data. After completing where a change in the fault’s dip is marked by a dip bisec- seismic and geologic horizon calibration, the results were tor. Flow lines are constructed by connecting points on dif- taken into the seismic profiles to establish the interpre- ferent dip bisectors of equal distance from the fault plane. tation network. Through repeated comparison, repeated Particles in the hanging wall translate along the flow lines, modification of the drilling and seismic data, the seismic which are parallel to the fault plane, by a defined distance. and geological layer system of the whole basin was built Decompaction is an essential step to obtain an ac- up. SeisWorks software was used to interpret T1-T6 hori- curate geometry of the reconstructed structures. Because zons and faults on seismic sections. Because the seismic the lithology of most of the stratigraphic units in Qaidam data were acquired and processed by different teams, lead- basin commonly has complex lateral variations, the de- ing to the appearance of misties, we applied the Interac- compaction has not been taken into account in the section tive Seismic Balance tools to analyze and resolve misties in restoration [12]. time, phase, and amplitude. After the interpretation, hori- zons and faults were converted from time to depth domain using TDQ software, and were exported out from Open- 4 Results Works for structural modeling. Then the data were loaded into the Petrel software to Using the remote sensing satellite data analysis and in- define the faults in the geological model that form the basis terpretation, we can identify the distinct structural fea- for generating the 3D grid. These faults define breaks in the tures including fold, fault, and lineament (Figure 2, 3). The grid, lines along which the horizons inserted later may be measurements denoted a good agreement with field data, offset. The offset which occurs is entirely dependent upon confirming the potentiality of satellite analyses in geolog- the input data. The Make horizons process generates in- ical studies. As a major boundary fault between two large dependent geological horizons from input data. To gen- geomorphic units, the Tibet and the , the Al- erate additional horizons using relative distance to exist- tyn Tagh fault displays a remarkable topographic contrast ing horizons use the Make zones process. These two pro- from the northern Plateau to the southern Tarim basin with cesses are used to create the geological zones within the a vertical throw up to 3500 m [23]. The Altyn Tagh fault model. Layering inserts the fine scale grid cells which will extends for at least 1,500 km from the west Kunlun thrust describe vertical variation within each geological zone. zone in the southwest to the edge of the in the northeast. It is divided into three main sections: 3.3 Geologic section restoration southwestern, central and northeastern. There is one ma- jor splay fault, the north Altyn fault. The main active fault trace of the fault lies within a zone of secondary structures The section is sequentially restored back in time to its ini- that is about 100 km wide in the central section. The fault tial stage before the deposition of Cenozoic strata using cuts various geomorphic surfaces, such as Quaternary flu- the computer section balancing software Move. The sec- vial fans and terraces, has developed distinct linear traces tion is divided into discrete blocks bounded by faults and along the fault (Figure 3). The horizontally offset gully, bedding planes, which are then individually manipulated which have been displaced from several meters to several using different restoration algorithms. The flexural slip un- hundred meters, and vertical fault plane, which has an 82∘ folding algorithm was applied to restore folds. The algo- dip and 76∘ strike, indicates it is a left-lateral strike-slip rithm works by rotating the limbs of a fold to a datum, fault (Figure 3b, 3d), meanwhile the uplifted fault scarp an assumed regional, or template geometry. Layer parallel and 11∘ oblique striations on the fault plane means that the shear is then applied to the rotated fold limbs in order to fault has a vertical component (Figure 3c, 3e). The thick- remove the effects of the flexural slip component of fold- ness of the fault is over several hundred meters on the out- ing. Unfolding occurs about a pin line and points along 180 Ë J. Guo et al.

Figure 4: Seismic lines AB and CD. AB profile shows that thrusts control east and west sides of the basin, whereas the CD profile ismainly 1 affected by the Altyn Tagh fault on the north side. T0 indicates the base of (2.8 Ma), T1 Pliocene (5.1 Ma), T2 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 crop (Figure 3d). Faults and folds extending northward are The two-dimensional time–depth converted seismic cut and terminated by the Altyn Tagh fault. profile AB was used for the construction of the geological The axial traces of Qaidam folds trend NW-SE. Many section, which is perpendicular to the trends of structures. Qaidam anticlines are fault-propagation folds with steeper The length of the section is over 200 km, and entire section dipping fore-limbs and shallower dipping back-limbs. The shows a synclinorium shape. Strata on both sides is thin, fore-limbs are located on the northern sides of the ax- meanwhile in the middle is thick, and the thickest part is ial traces, indicating that the anticlines are propagating over 2 km. The uplifted formation on both sides has been northward to northeastward, and the fold geometries may eroded. In the basin, secondary anticlines and synclines be controlled by reactivation or inversion of preexisting developed. The Kunlun mountain fault and the Qilian south to southwest-dipping faults in the underlying base- mountain fault thrust to the basin and the maximum offset ment [24]. of the thrusts is over 8 km (Figure 4, 6). The restoration re- In 3D modeling, the main geological structure is that sults of the geological section AB (Figure 6), including to- the basin is bounded by the southwest-directed Kunlun tal shortening, average shortening rates, and shortening thrust belt in the west and the northeast-directed Qilian rates during different time periods, are listed in Table 1. mountain thrust belt in the east (Figure 5). The basin has The total shortening and shortening ratio during Cenozoic been compressed to a narrow irregular diamond shape. respectively reaches 25.5 km and 11.2% across the Qaidam The tectonic deformation in the north, where the Altyn basin, which is consistent with previous works [12, 21, 25]. Tagh fault lies, is stronger than in the south. 42.9% and 34% of total shortening took place in the peri- ods since 5.1 Ma and since 2.8 Ma, respectively. The short- Three-dimensional structural model of the Qaidam basin Ë 181

Table 1: Restoration measurements and shortening rates of section AB

Layer Length Age (yr) Shortening Time Interval Percentage of total Shortening (mm) Length (mm) (yr) shortening (%) Rate(mm/yr) Present 201802000 0 8652000 2800000 34.0 3.09 Q(T0) 210454000 2800000 2274000 2300000 8.9 0.99 3 N2 (T1) 212728000 5100000 4060000 6900000 15.9 0.59 2 N2 (T2) 216788000 12000000 1150000 12600000 4.5 0.09 1 N2 (T2) 217938000 24600000 3698000 15900000 14.5 0.23 N1 (T3) 221636000 40500000 2478000 2300000 9.7 1.08 2 E3 (T4) 224114000 42800000 1457000 9700000 5.7 0.15 1 E3 (T5) 225571000 52500000 1722000 12500000 6.8 0.14 E1+2 (TR) 227293000 65000000 Total 227293000 25491000 65000000 11.2 0.39

Figure 5: 3D modeling structures of Qaidam basin, and the data unit is elevation depth (meter). (a): The base of the Pleistocene (Q). (b): 2 2 The base of the upper Miocene (N2). (c): The base of the upper Eocene (E3). (d): The base of the Paleocene (E1+2). The magenta surface in- dicates 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 ening rates from the restoration of the section indicate to 0.09 mm/yr, and finally the rate increased to the maxi- that the Qaidam basin has been undergoing continuous mum in Quaternary. shortening since the beginning of Cenozoic with two rel- 2 atively fast shortening phases, the first during E3 layer (42.8–40.5 Ma) and the second during Q layer (2.8 Ma– 5 Discussion present), and the shortening rates reached the maximum values to 3.09 mm/yr since 2.8 Ma, whereas the average In this study, the Cenozoic deformation history of the rate during the Cenozoic is 0.39 mm/yr (Table 1). From Qaidam basin (Figure 6) shows that the basin experienced 65 Ma to 42.8 Ma, the shortening rate increased from 0.14 to continuous compression since the beginning of Cenozoic. 1.08 mm/yr, then from 40.5 Ma to 12 Ma, the rate decreased 182 Ë J. Guo et al.

Figure 6: Sequential restorations of the geological section AB Three-dimensional structural model of the Qaidam basin Ë 183

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

It means that the deformation and uplifting in the south the crust-buckling model [9] are all support that the short- and north edges of the Tibet plateau were almost syn- ening along the mountain frontal thrust system accommo- chronous. The restoration results of this study is compa- dates much of the present-day convergence [27]. Whereas rable with previous studies by Zhou et al. [12] and Yin et how to answer why the Cenozoic sedimentary center is lo- al. [21] in the same area. The total shortening of this study cated in the central basin, and what a dynamic process is 25.5 km, and Zhou et al. gave a 36.6 shortening and Yin et leads to a 15 km deposition, are the keys to understand al. got a 20-80 km result. The difference may be caused by the formation of the basin [13]. The settlement of the base- different time-depth conversion and restoration methods, ment caused by the foreland basin requires the center of and the total shorting across the Qaidam basin should be subsidence to occur at the leading edge of the thrust fault, about 20-40 km. The restoration shows that the deforma- and the subsidence and sedimentation centers are usually tion of Qaidam basin was not uniform [26], and it experi- located at the edge of the basin, so the model of foreland enced an increase and a decrease, then an increase pro- basin could not answer why the sedimentary center is lo- cess. In later Eocene and Quaternary, the basin has two cated in the central basin. The piggyback basin model as- relatively fast shortening phases, and from later Miocene sumes that the formation of the Qaidam basin is related to to present, the rate has been increasing to the maximum the southward large-scale forward thrusting process, but value ever, suggesting the northeastern Tibet will keep it did not consider northward thrusting at the piedmont of strong deformation in the future. Correspondingly, the the Kunlun mountain (Figure 3f). Cenozoic deformation history of the Qaidam Basin has im- The crust-buckling model [9, 13] gives an explana- plications for the evolution of Altyn Tagh fault. The total tion of the initiation and development of the Qaidam strike-slip offset of the Altyn Tagh fault across the Qaidam basin, and It can explain that the sedimentary center of basin is about 25.5 km during Cenozoic. From west to east the Qaidam basin is located in the middle of the basin. of the Qaidam basin, the slip rate of the Altyn Tagh fault The model requires the low crustal of eastern Tibet to be decreased at about 3 mm/yr in Quaternary. The data pre- weak enough to flow outward from the interior of the Tibet sented in this study are more regular, with data from low Plateau [28]. The outward growth of eastern Tibet is proba- to high, then to low, and finally increased, consistent with bly driven by lower crustal flow under the lateral pressure long-term geological process characteristics. The thrusts gradients. Since the upper crust of the blocks is dominated and folds developed in Qaidam basin absorbed the move- by brittle deformation, the ductile flow of the lower crust ment of the Altyn Tagh fault. In the field work, we observed would drag the brittle upper crustal blocks to move with that the strike-slip Altyn Tagh fault has about 11∘ verti- respect to each other. The interactions among the brittle cal component (Figure 5e), which is supported by the up- upper crustal blocks cause strain accumulations among lifted young fault scarp at the south side of the fault on their bounding faults to generate large earthquakes [29]. the flood plain (Figure 5c), Indicating that the south block A recent study determined both the P- and S-wave veloc- has a whole oblique uplift along the fault (Figure 7). The ity structure across the central Qaidam basin along a 350- driving force may come from the southward subduction of km-long seismic refraction/wide-angle reflection profile lithospheric mantle overlying detached crust [13]. extending from the Kunlun mountain through the central The previous studies of the flexure subsidence model Qaidam basin into the Qilian mountain, and shows brit- of foreland basin [20], the piggyback basin model [21], and tle deformation in the upper crust through thrust folding, 184 Ë J. 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