Vol. 92 No. 2 pp.536–555 ACTA GEOLOGICA SINICA (English Edition) Apr. 2018

Late Cretaceous-Cenozoic Multi-Stage Denudation at the Western Ordos Block: Constraints by the Apatite Fission Track Dating on the Langshan

CUI Xianyue1, ZHAO Qihua1, ZHANG Jin2, *, WANG Yannan3, ZHANG Beihang2, NIE Fengjun4, QU Junfeng2 and ZHAO Heng2

1 State Key Laboratory of Geohazard Prevention and Geoenvironment Protection (Chengdu University of Technology), Chengdu 610059, 2 The Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China 3 Xinjiang Institute of Ecology and Geography, Chinese Academy of Sciences, Ürümqi 830011, China 4 Donghua Polytechnic University, Nanchang 330013, China

Abstract: The apatite fission track dating of samples from the Dabashan (i.e., the Langshan in the northeastern Alxa Block) by the laser ablation method and their thermal history modeling of AFT ages are conducted in this study. The obtained results and lines of geological evidence in the study region indicate that the Langshan has experienced complicated tectonic-thermal events during the the Late Cretaceous-Cenozoic. Firstly, it experienced a tectonic-thermal event in the Late Cretaceous (~90–70 Ma). The event had little relation with the oblique subduction of the Izanagi Plate along the eastern Eurasian Plate, but was related to the Neo-Tethys subduction and compression between the Lhasa Block and Qiangtang Block. Secondly, it underwent the dextral slip faulting in the Eocene (~50–45 Ma). The strike slip fault may develop in the same tectonic setting as sinistral slip faults in southern Mongolia and thrusts in West to the southwest Ordos Block in the same period, which is the remote far-field response to the India-Eurasia collision. Thirdly, the tectonic thermal event existed in the late Cenozoic (since ~10 Ma), thermal modeling shows that several samples began their denudation from upper region of partial annealing zone (PAZ), and the denudation may have a great relationship with the growth of Qinghai- to the northeast. In addition, the AFT ages of Langshan indicate that the main body of the Langshan may be an upper part of fossil PAZ of the Late Cretaceous (~70 Ma). The fossil PAZ were destroyed and deformed by tectonic events repeatedly in the Cenozoic along with the denudation.

Key words: apatite fission track, cooling history, thermal modeling, Late Cretaceous-Cenozoic, Langshan

1 Introduction Qinghai-Tibetan Plateau. The first one is that the plateau extended and grew gradually from south to north (Meyer With the India-Eurasia collision, the uplift of the et al., 1998; Tapponnier et al., 2001). The second one is Qinghai-Tibet Plateau is known to be a major geological that the northern plateau experienced deformation in the event occurred in the Cenozoic. This not only has same period of collision or shortly later (Yin et al., 2002; reshaped the landform pattern (De Grave et al., 2007; Fang Xiaomin et al., 2004; Clark et al., 2010; Duvall et al., Vassallo et al., 2007; Jolivet, 2015) of long-term stability 2011, 2013; Zhang Kexin et al., 2010, 2013). The third in East and Central Asia since the Late Jurassic-Late one is that the plateau extended and grew from the middle Cretaceous epoch, but has also impacted the climate part to the north and south sides (Wang et al., 2008, 2014). change, leading to the formation and enhancement of the Moreover, in recent years, there are different opinions on Asian monsoon (An et al., 2001; Guo et al., 2002). There the scope of the far field influence and the epoch of India- are roughly three models about the growth pattern of the Eurasia collision. For example, Yue and Liou (1999) and Darby et al. (2005) thought that the Altyn Tagh fault zone * Corresponding author. E-mail: [email protected]

© 2018 Geological Society of China Apr. 2018 ACTA GEOLOGICA SINICA (English Edition) Vol. 92 No. 2 537 http://www.geojournals.cn/dzxben/ch/index.aspx http://mc.manuscriptcentral.com/ags extended eastwards into South Mongolia through the Alxa out the apatite fission track (AFT) dating and field work in Block of Inner Mongolia during the Oligocene. However, the Langshan region, to discuss the Cenozoic denudation one recent research argued that the Altyn Tagh fault zone and deformation of the Langshan. spread into the Alxa Block during the Quaternary (Yu et al., 2016), and entered the peripheral graben systems 2 Geological Backgrounds around the Ordos Block from the southern area of Langshan in the northeastern Alxa Block. In recent years, The Langshan is located at the northeastern Alxa Block many researches have focused on the growth pattern and and to the northwest of the Yinshan-Hetao rift basin. This influence scope of the Qinghai-Tibetan Plateau (Meyer et area is considered to have become a part of the North al., 1998; Fang et al., 2003, 2005, 2007; Dai et al., 2006; China Block since the Paleoproterozoic (Zhao et al., Zheng et al., 2006; Dupont-Nivet et al., 2008; Liu et al., 2005), and its basement is the high-graded metamorphic 2013; Zheng et al., 2013a, 2013b; Zhang Kexin et al., Diebusigegroup complex (granulite facies), which is 2010, 2013; Duvall et al., 2011; Wang et al., 2013; Yu et composed of magnetite quartzite, quartzite, marble and al, 2016), and different understandings are obtained. The TTG of the same era. The study region experienced rift system developed around the Ordos Block during the extension during the Neoproterozoic, the Langshan Group same time with the India-Eurasia collision and the mainly consisting of thicker continental clastics subsequent compression, most scholars attributed the interbedded with marble and thinner layers of volcanic formation of this graben system to the India-Eurasia rocks was deposited in rifting basins. Recent studies have collision (Molnar and Tapponnier, 1975; Ordos active indicated that, the age of volcanic rocks in the Langshan fault research group of China Earthquake Administration, Group is around 820–800 Ma (Peng Ruimin et al., 2010; 1988). There are a series of high mountains along the Wang et al., 2016). As the Paleo-Asian Ocean had closed eastern Alxa Block distributed from north to south: the in the north side of the study region since the end of the Langshan-Bayan-Ul, the Helan Mountain, Large and Little late Paleozoic, the Langshan Group was deformed Luoshan Mountains, the Xiangshan-Tianjingshan, the intensively, forming a series of east-west-trending, Liupan Mountain, etc. The Langshan-Bayan-Ul and Helan isoclinally overturned folds verging south. During the Mountain in the north are controlled by the extension, Triassic, the eastern Alxa Block experienced a strong there are Cenozoic rift basins on the east side of these sinistral ductile shearing and the Carboniferous granite in mountains, with the thickest Cenozoic sediments the Dabashan was involved in it (Zhang et al., 2013), almost exceeding 10 km occurring in the Hetao Basin (Ordos all of which became grantic mylonite after deformation. In active fault research group of China Earthquake the Late Jurassic, a series of northeast-southwest trending Administration, 1988). However, the Large and Little top-to-the-southeast thrust developed in the northern study Luoshan Mountains, Xiangshan-Tianjingshan and the region (Darby and Ritts, 2002; 2007). During the Early Liupan Mountain in the south developed in an oblique Cretaceous, a red basin developed and was controlled by a compression condition. Although the stress fields are low-angle detachment fault in the study region (Zhang et al., different, some studies has shown that mountains such as 2014; Tian et al., 2017). In the Early Cenozoic, the early the Helan Mountain and the Liupan Mountain began their deformation was the top-to-the-northwest thrusting that rapid denudation between 8–12 Ma, and most researchers developed along eastern side of Carboniferous granite of believed that this rapid denudation relates to the the Dabashan (Zhang et al., 2014), and then a series of northeastward growth of Qinghai-Tibetian Plateau (Zheng NNE-trending high angle normal faults dipping to SSE et al.,2006; Liu Jianhui et al, 2010). Previous studies have developed in the east side of Carboniferous granite of the shown that the Langshan, located to the north of the Helan Dabashan, and control the development of the Hetao rift Mountain and South Mongolia to the north of Langshan, basin (Zhang et al., 1998). The Dabashan was cut by a set possibly experienced an early deformation event in the of NNE-trending strike-slip faults; however, these brittle Cenozoic (Webb et al., 2006; Zhang et al., 2014). faults did not cut through the red Oligocene deposits (i.e., However, this opinion was not based on irrefutable the Wulanbulage Formation) overlapping on the mountain chronological evidence. Therefore, questions, such as body. Therefore, these strike-slip faults have been inactive whether there is any Cenozoic multi-stage deformation in since the Oligocene. the Langshan, the ages of deformation and whether the Cenozoic deformation in the Langshan is similar to the 3 Samples and Dating Method Helan Mountain, will be important evidence to answer the far filed effect of India-Eurasia collision and the growth 3.1 Sample description time of the northern plateau. For this purpose, we carried The samples of this study were collected from the 538 Vol. 92 No. 2 ACTA GEOLOGICA SINICA (English Edition) Apr. 2018 http://www.geojournals.cn/dzxben/ch/index.aspx http://mc.manuscriptcentral.com/ags

Dabashan which is the southern part of the Langshan, in Topographically, the Dabashan is steep in the southeast the followings the Dabashan is used instead of the side, and gentle in the northwest side, which forms a Langshan for conveniently. Most samples were collected northwestwardly tilted mountain (Fig. 3). The surface of from the monzonitic granite in the Langshan (i.e., the eastern Dabashan is a Cenozoic high angle normal fault Dabashan, Fig. 3), which is the grantic mylonite after the (Figs. 2 and 3). The maximum altitude of the Dabashan is Triassic shear deformation, and two samples were about 1700 m, and the altitude of the eastern foot of the collected from the Oligocene red conglomerates on the top Dabashan is around 1100 m. In this study, two profiles of the Dabashan (Figs. 1 and 2). Granitic samples contain (profile A-B and profile I-J) (Figs. 2 and 3) crossing the minerals of plagioclase 30%–35%, potash feldspar 30%– Dabashan in north and south parts were collected 40% and quartz 20%–30% with a small amount of biotite. respectively. A majority of granite samples experienced The zircon U-Pb age of the granite is 337±5 Ma (Dan et the Mesozoic ductile shearing. Sampling interval is about al., 2016) and 329±1 Ma (Zhang et al., 2014). 100 m; the elevation was measured using a handheld GPS

Fig. 1. Geologic map of the Langshan region. Apr. 2018 ACTA GEOLOGICA SINICA (English Edition) Vol. 92 No. 2 539 http://www.geojournals.cn/dzxben/ch/index.aspx http://mc.manuscriptcentral.com/ags

Fig. 2. Sample locations and geomorphological sections of the Langshan (see Fig. 1 for location).

Fig. 3. Geomorphological profiles and sample profiles (see Fig. 2 for the profile locations). Topographic data is based on SRT90m DEM data from the CGIAR consortium for Spatial Information (CGIAR-CSI) (http:// srtm.csi.cgiar.org/ SELECTION/inputCoord.asp). 540 Vol. 92 No. 2 ACTA GEOLOGICA SINICA (English Edition) Apr. 2018 http://www.geojournals.cn/dzxben/ch/index.aspx http://mc.manuscriptcentral.com/ags receiver and 1: 25000 scale topographical map (Fig. 2). Dpar of the vast majority of samples from the Dabashan is There were totally 18 samples collected, however, no ≥1.75μm, suggesting that the fluorine and chlorine content enough apatite minerlas for dating were found in five of in the apatite of the samples are normal, the annealing rate them. The geographic locations, lithology, and altitudes of is slower, and the temperature for full annealing is about all samples are given in Table 1 and Figure 2, Figure 3. 110°C. AFT ages of samples from the Dabashan (i.e., the 3.2 Dating Langshan), along with the path length and fission track

The domestic apatite fission track (AFT) chronology etch pit diameter (Dpar), were measured at the China State still mainly relies on an external detector in the form of Key Laboratory of Earthquake Dynamics. The main radiation, and while this method is already more mature, it analysis steps are as follows: (1) Apatite was separated is still restricted by the test process, sample irradiation, using standard magnetic and density methods and then and other factors. Currently, analysis based on laser mounted on glass slides with araldite epoxy. (2) After ablation (Laser Ablation-Inductive Coupled Plasma-Mass grinding, polishing and exposing to the internal surface, Spectrometry) (LA-ICP-MS) (Hasebe et al., 2004) has place the apatite in 5.5 mol/L nitric acid with temperature been well developed and widely used internationally. The of 21°C to etch for 20s, to show naturally spontaneous analysis technology has been becoming mature, and fission track. (3) Use the Zeiss polarizing microscope at relevant equipment has also been introduced to and similar 10×1000 magnification on the AutoSan platform, carrying datings have been carried out in some labs in China. out fission track counting and measuring the track length

The method aims at obtaining uranium content from and diameter (Dpar) of fission track etch pit. When apatite samples by using laser ablation technique possible, at least 1000 tracks distributed between accompanied by mass spectrometry. Hasebe et al. (2004) minimum 20 grains were counted per sample, but this proposed an absolute calibration method to calculate LA- target was not always attained. (4) Use an Agilent 7900s ICP-MS AFT age that is based on 238U total decay mass spectrometer and a Resonetics M-50 with laser constant and 238U spontaneous fission decay constant. The system together (see Table 2 for the detailed analysis) for detail information about the dating methods can be found on-site analysis to determine the uranium concentration in in Hasebe et al. (2004, 2013). each counting grain. In every single measurement, signal To define the chemical composition of apatite affecting strength of 238U is calibrated according to 43Ca (internal the characteristics of annealing and get further effective standard). Both of these two elements appear in standard constraints in the thermal history analysis, we also samples and unknown apatite in the form of matrix measured the etch pit diameter (Dpar) of the path length elements. By using NIST 610, NIST 612 glass and and fission track of apatite grains in this study. Dpar, an Durango apatite standard as the main standard to check for important indicator to quantify the solubility of apatite every 10 unknown apatite grains in a measure sequence. (Donelick et al., 2005), is the maximum diameter of (5) LA-ICP-MS AFT ages were calculated using the Zeta fission track etching parallel with crystallographic axis c (ζ) method which was proposed by Hasebe et al. (2013). and intersecting with polished surface. Its value relates to The sample test results by applying an international the content of crystal fluoride and chlorine. In general, the standard sample (Durango apatite) to measure ζ value in smaller the Dpar is, the higher the fluorine and chlorine Table 2, and refer to Figure 4 for radial plots of age content is, and the faster the track annealing rate is. Often, distribution of single grain AFT ages and track the annealing rate of apatite is relatively faster when Dpar distribution of samples from the Dabasha. ≤1.75μm, and relatively slower when Dpar ≥1.75μm. The

Table 1 Geographic location, elevation and lithology of the samples from the Langshan Sample no. Lab no. Latitude (N) Longitude (E) Altitude (m) Lithology LSA-01 A01 40°33′20.6′′ 106°18′40.7′′ 1295 Monzonitic granite LSA-02 A02 40°33′26.5′′ 106°18′32.9′′ 1398 Monzonitic granite LSA-03 A03 40°33′29′′ 106°18′28.4′′ 1520 Monzonitic granite LSA-04 A04 40°33′28.8′′ 106°18′25.7′′ 1582 Monzonitic granite LSA-05 A05 40°33′34.4′′ 106°18′9.5′′ 1660 Sandstone (Oligocene) LSA-07 A06 40°33′40.0′′ 106°17′35.2′′ 1462 Monzonitic granite LSA-08 A07 40°33′31.4′′ 106°17′16.9′′ 1338 Monzonitic granite LSA-09 A08 40°34′14.5′′ 106°18′41.5′′ 1208 Monzonitic granite LSB-01 A09 40°31′56.4′′ 106°14′25.8′′ 1517 Monzonitic granite LSB-06 A10 40°30′38.9′′ 106°15′27.2′′ 1410 Sandstone (Oligocene) LSB-07 A11 40°30′31.0′′ 106°15′45.22′′ 1429 Monzonitic granite LSB-08 A12 40°30′32.0′′ 106°15′49.6′′ 1345 Monzonitic granite LSB-09 A13 40°30′33.1′′ 106°15′52.4′′ 1286 Monzonitic granite

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4 Results the dispersity of most samples exceeded 40%, and 1–3 age peaks appeared (Fig. 4). Therefore, the median ages 4.1 AFT ages obtained from these samples were mixed ages, whose According to AFT ages from the Dabashan, most of geological meaning is ambiguous. In the samples, the samples underwent a complicated tectono-thermal history. median ages are between 112–44 Ma (Table 2), in which The ages of all single grains in samples were dispersive, the apparent ages of A5 (91±4.6 Ma) and A10 (112±11

542 Vol. 92 No. 2 ACTA GEOLOGICA SINICA (English Edition) Apr. 2018 http://www.geojournals.cn/dzxben/ch/index.aspx http://mc.manuscriptcentral.com/ags

Fig. 4. Radial plots and length distribution of mean confined track lengths for samples from the Dabashan, Langshan. MTL: mean confined track length; SD: standard deviation; N: number of the fission tracks; Dpar: etch pit diameter of fission track.

Table 2 LA-ICP-MS AFT dating results Sample in No. Area Central age Dispersion MTL SD Dpar Sample N Σ(m) 1σΣ(m) ζ 1σζ N Lab grain s (cm2) MS MS (Ma) (%) (μm) (μm) (μm) LSA-01 A01 20 160 1.64E-03 3.69E-02 6.06E-04 0.9161 0.0121 53.4±6.5 37 13.7±0.1 9 0.64 1.65 LSA-02 A02 31 722 2.18E-03 1.44E-01 2.94E-03 0.9177 0.0061 72.3±6.6 46 12.6±0.2 33 1.29 2.1 LSA-03 A03 10 251 7.55E-04 6.54E-02 1.08E-03 0.9280 0.0128 47.2±5.6 30 12.3±0.2 24 1.61 2 LSA-04 A04 16 379 1.18E-03 1.09E-01 1.67E-03 0.9066 0.0074 44.2±5.8 47 12.3±0.2 21 1.17 1.76 LSA-05 A05 4 403 3.60E-04 4.95E-02 5.33E-04 0.8965 0.0121 91±4.6 0 13.1±0.2 38 1.85 2.39 LSA-07 A06 20 755 1.41E-03 1.60E-01 2.39E-03 0.9024 0.0148 60.7±6.2 41 12.5±0.1 48 1.57 2.1 LSA-08 A07 12 291 7.05E-04 1.00E-01 2.99E-03 0.9000 0.0208 53.4±9.5 57 12.0±0.3 18 1.46 1.74 LSA-09 A08 18 1012 1.58E-03 1.87E-01 2.68E-03 0.9105 0.0100 53.1±4.4 31 13.6±0.1 9 0.64 2.08 LSB-01 A09 13 675 1.30E-03 1.13E-01 9.07E-04 0.9334 0.0124 57.4±7.3 43 13.1±0.1 13 1.17 2.36 LSB-06 A10 10 688 9.50E-04 6.53E-02 8.42E-04 0.9249 0.0091 112±11 27 12.6±0.2 34 1.88 1.78 LSB-07 A11 18 943 1.63E-03 1.66E-01 2.01E-03 0.9277 0.0049 61.7±6.1 39 13.1±0.2 31 1.41 2.36 LSB-08 A12 12 534 1.08E-03 6.28E-02 6.81E-04 0.9191 0.0091 81.6±5.5 16 12.5±0.1 19 1.13 1.84 LSB-09 A13 12 386 8.60E-04 7.09E-02 1.31E-03 0.8867 0.0414 77.3±4.8 11 12.3±0.2 12 1.19 1.8 238 43 Ns, number of counted spontaneous tracks; Area, grain area analyzed; Σ(m), area weighted intensity signal of U/ Ca, summed over all the grains per sample; σ, standard error; ζMS, zeta calibration factor; MTL, mean track length; N, number of the fission tracks; SD, standard deviation of fission tracks; Dpar, fission track etch pit diameter. Ma) are older than the sedimentary age (Oligocene) of the stratum. Hence, the single grain ages in the two samples Apr. 2018 ACTA GEOLOGICA SINICA (English Edition) Vol. 92 No. 2 543 http://www.geojournals.cn/dzxben/ch/index.aspx http://mc.manuscriptcentral.com/ags represent the ages of detrital apatite fission track ages in A03 and A04 samples located in the central part of the source regions, which also shows that these two samples Dabashan, the ages of samples are gradually older as they have not experienced an obvious annealing effect. are far away from Fault F3. A03 and A04 samples have a Furthermore, there were few apatite grains in these two high elevation, but the ages are very young, 47.2±5.6 Ma samples for analysis purpose (≤10), and their apparent and 44.2±5.8 Ma, respectively (Table 2); these two ages didn’t have statistical significance. The crystallized samples were mainly collected from the regions near a ages of other samples (monzonitic granite) are 337±5 Ma large dextral strike-slip fault and its secondary fault in (Dan et al., 2016) or 329±1 Ma (Zhang et al., 2014), central part of the Dabashan (Fig. 2), whose age may be whose AFT ages (81–44 Ma) are far less than their affected by the fault activity. Except A03 and A04, all crystallized ages; in addition, the confined track lengths in other samples show that ages have a positive correlation samples are mostly between 12–14 μm (Table 2), which with the distance from the F3 transversely, which may means all samples stayed in partial annealing zone (PAZ) indicate that F3 is an important boundary fault to control for a long time. Therefore, their APT ages represent that the deformation of the Dabashan, though the piedmont these samples have experienced different annealing normal slip fault F1 in the east side of the Dabashan is the processes most active at present. In the Dabashan, Langshan region , more samples in The diagrams of the relationship between the ages of profile A-B provide were dated, while four samples from AFT of samples from the Dabashan and the horizontal profile G-H, we couldn’t find enough apatites for further mean confined track length, and the standard deviation of dating, only five samples from profile G-H were dated, track length indicated a U-shaped (Fig. 6a) and an inverse four samples of these five samples are located on the east U-shaped pattern (Fig. 6b), respectively (Fig. 6). Usually, side of the Dabashan (Fig. 3). Except for A5 and A10 this U-shape pattern between the horizontal mean confined samples, there was no significant correlation between the track length and ages is called the “boomerang plot.” In median age of the remaining 11 samples and their this plot, the timing at which the geologic body altitudes. On the contrary, the higher the altitudes are, the experienced an important tectonic thermal evolution younger the ages (e.g. A03 and A04) are. There wasn’t history can be determined through comparing among large any inflection point found on the age-elevation map, so it amount of ages, horizontal mean confined track lengths is not possible to determine the timing of the occurrence of and the standard deviation of track lengths. However, in rapid denudation crossing the bottom of PAZ (Fig. 5a). principle, there are tracks with a horizontal mean confined The relationship between the distance from the western track length of more than 14 μm or less than 13 μm boundary of Langshan Fault F3 and the ages (Fig. 2) and (Gallagher and Brown, 1997). The horizontal mean the age distribution diagram (Fig. 5b) show that except confined track lengths of samples from the Dabashan are

Fig. 5. Relationship between AFT ages of the Dabashan and sample elevations. (a), and their distances from F3 at western margin of the Dabanshan; (b), PAZ: partial annealing zone (the black spots are samples from profile A-B, and the yellow spots are samples from profile I-J). 544 Vol. 92 No. 2 ACTA GEOLOGICA SINICA (English Edition) Apr. 2018 http://www.geojournals.cn/dzxben/ch/index.aspx http://mc.manuscriptcentral.com/ags

Fig. 6. Relationship between AFT ages and horizontal confined track lengths/deviations of the track length. (a), Relationship between AFT ages and horizontal confined track lengths; (b), relationship between AFT ages and standard deviations of lengths; The gray column represents the rapid cooling time. mainly between 12–14 μm, suggesting that they were from the Oligocene conglomerate of the Wulanbulag principally distributed inside the PAZs. Although there is Formation on top of the Dabashan, the apparent ages of its a relatively obvious U-shaped pattern (Fig. 6a), and apatite are older than the sedimentary age of the stratum. because the ages are highly dispersed, the timing of Furthermore, the dated apatite grains are few, which could tectonic thermal event determined by boomerang plot may not meet the general conditions required for thermal be inaccurate (55–50Ma) (Fig. 6). modeling. Therefore, additional thermal modeling is not performed on samples A05 and A10. See Figure7 for the 4.2 Thermal modeling thermal modeling results for other samples. Because most samples from the Dabashan didn’t obtain Good thermal modeling results (GOF>0.5) were ideal AFT ages and the dispersity was larger, we obtained for most modeled samples from the Dabashan. It speculated that they had experienced a more complex was also revealed that the majority of samples began thermal history. To further study the thermal history entering into the partial annealing zone (PAZ) in the Late process that samples from the Dabashan experienced, the Cretaceous (~90–70 Ma). However, different samples authors performed the thermal modeling of most samples were inconsistent in their process in the PAZ. Samples in this study. During thermal modeling, HeFty (Version. A02, A04, A06 and A07 had stayed a long time in the 1.7.5, 2012; Ketcham, 2005) was adopted, and the PAZ, and did not leave until the late Cenozoic, while multicomponent annealing model proposed by Ketcham samples A08, A09 and A11 re-entered into the upper (2007) was applied to all samples. The constraint region of the PAZ in the late Cenozoic after leaving the conditions are: (1) Full annealing state is set as the time PAZ in the early Cenozoic, then left rapidly (Fig. 7). limit larger than the maximum age of single grains Although above samples have different paths of thermal (Ketcham, 2005); (2) The surface temperature range is 0– history, it is still apparent that most samples left the PAZ 20°C; (3) The geothermal gradient was 30–40 °C/km and rapidly denuded in the late Cenozoic (~10 Ma). (Research Group of Peripheral Fault System around the Although there are still many different opinions about Ordos, Institute of Geology, State Bureau of Seismology, whether the low-temperature parts have geological 1988); (4) The range of partial annealing zones of AFT is meaning, it will be discussed in detail in the following 60–110°C (Gleadow et al., 1986); (5) The Dpar value is section. The leaving of ~10 Ma from PAZ indicates that given in Table 2; (6) Evaluation functions K-S and GOF these samples may experience important tectonic events in are used to check the modeling results. When K-S the late Cenozoic. Several samples (A12, A13) left the evaluation values are 0.5 and 0.05, preferable and PAZ rapidly and entered into a steady and slowly rising acceptable results would be obtained respectively, and stage in the late Mesozoic (Fig. 7), which indicates that there are 10000 paths for the statistics of each sample. The sample A12, A13 were basically stabilized after deviation of all thermal history modeling in this study was experiencing a tectonic thermal event in the Late ~5%; (7) The initial track length is 16.3μm (Gleadow et Cretaceous. al., 1986). Because samples A05 and A10 were collected Apr. 2018 ACTA GEOLOGICA SINICA (English Edition) Vol. 92 No. 2 545 http://www.geojournals.cn/dzxben/ch/index.aspx http://mc.manuscriptcentral.com/ags

Fig. 7. Thermal history modeling of samples from the Dabashan. The purple area represents good fits (Ketcham et al., 2007), the green area represents acceptable fits, the blue line represents the weighted mean path, and PAZ represents the apatite partial annealing zone (60–110°C).

5 Discussion divided into three regions: the western slope, the central part and the eastern slope (Fig. 5b). 5.1 Denudation stages and characteristics of the Samples from the central part can be divided into two Dabashan types, one of which is the sample of Wulanbulag The ideal apparent ages of apatite were not obtained Formation from the top of mountain, and the other were from two profiles crossing the Dabashan, because some collected from the monzonitic granite. Both APT ages and samples have no enough apatite grains for dating and the median ages of samples (A05 and A10) collected from some even did not contain apatite. Most of the samples the sandstone in the red Oligocene Wulanbulag Formation that can be dated had age dispersity of single grains >40% (Fig. 8a) covering the top of the Dabashan were older than (Table 2, Fig. 4), so these ages were mostly mixed ages, the age of stratum, indicating that they may not undergo whose geological significance needs to be carefully the annealing process after the deposition of the considered and analyzed. However, except for samples Wulanbulag Formation. Moreover, the number of single A05 and A10 (there were few grains in dated apatite that grains with ages in samples A05and A10 is less than 10, did not experience the annealing process, so their ages which is also relatively dispersive, and their median age were mostly source area ages), all other samples basically didn’t have reliable geological significance, so we will not showed similar multi-stage denudation process. Generally, discuss these two samples further in this study. The ages according to the age similarity, the Dabashan can be of two important samples A03and A04 which were 546 Vol. 92 No. 2 ACTA GEOLOGICA SINICA (English Edition) Apr. 2018 http://www.geojournals.cn/dzxben/ch/index.aspx http://mc.manuscriptcentral.com/ags

Fig. 8. Wulanbulag Formation on the top of the Dabashan. (a), covering relation with F2 (b), a fault valley along the central part of the Dabashan (c) and slickenlines on the fault plane indicating a sinistral strike-slip (d) (all photos were taken in the southern Dabashan). collected near the dextral strike-slip fault in the central that the fault was active before the Oligocene. part are 47.2±5.6 Ma and 44.2±5.8 Ma, respectively, but A06, A07. A08 and A09 are the samples collected from the single grain ages of both samples are relatively the western slope of the Dabashan (Figs. 2, 5b), whose dispersive, and both of them have an age peak around 31 ages are generally consistent (60–53Ma); however the Ma. A double-peak pattern presented in the track length ages of single grains of each sample have multiple peaks distribution of sample A03, and the two samples have (Fig. 4). The length distribution is bimodal, the horizontal similar horizontal confined track lengths (Fig. 4), so the confined track length of A06 and A07 is shorter (12.5±0.1 ages of both of them are mixed ages. The locations of –12.0±0.3 μm), in addition, the standard deviation of track samples A03 and A04 are close to each other, and both of length is larger (1.46–1.57μm), and the range of track them have similar track age distributions, implying that length distribution is also wider, indicating that the two they experienced a similar thermal event. The thermal samples may have stayed longer in the PAZ. The thermal modeling also showed that they passed through the PAZ modeling of A06 and A07 also reveals that they entered rapidly in the period of 50–45 Ma (Fig. 7). The sampling into the PAZ in ~70 Ma and ~55 Ma separately, while locations of A03 and A04 are the highest among all they passed through it at ~10 Ma coevally (Fig. 7). samples (Fig. 5a), but their ages are the youngest (Fig. 5a), The horizontal confined track length of A08 and A09 is which is different from the trend that the ages usually longer (13.1±0.1–13.6±0.1μm) (Fig. 4), and they have become older with the increase in altitude. Because two similar results of thermal modeling, that is, they both samples are located near the dextral strike-slip fault which rapidly passed through the PAZ at~70–60 Ma (Fig. 7) and cut the central part of the Dabashan (Figs. 2, 8c, 8d), the re-entered into the upper PAZ at approximately ~10 Ma authors speculated that their ages were related to the fault and then denuded rapidly (Fig. 7). Because both A08 and activity. It is necessary to point out that the dextral strike- A09 are near the boundary fault at the western Dabashan, slip fault is covered by the Oligocene conglomerate of the the fault may experience important activities in the Wulanbulag Formation (Fig. 8b), which also constrains Cenozoic. Therefore, the age of ~10 Ma may be associated Apr. 2018 ACTA GEOLOGICA SINICA (English Edition) Vol. 92 No. 2 547 http://www.geojournals.cn/dzxben/ch/index.aspx http://mc.manuscriptcentral.com/ags with the fault activity. temperature on the top of the PAZ was 60°C, the mean On the profile A-B, there are two samples in the eastern long-term denudation rate of Dabashan is 0.06–0.16 mm/y slope of Dabashan (A01 and A02), there are few since ~10 Ma, and the slip rate is similar with the present measurable tracks (n=9) from A01. Hence, it is unclear slip rate (0.1–0.3 mm/y) of piedmont normal fault of whether the age is significant, which will not be further Dabashan (Ordos active fault research group of China discussed in this study. The age of A02 is 72.3±6.6 Ma, its Earthquake Administration, 1988; Shen Xiaoming et al., single grain ages are largely dispersive, and includes two 2016) age peaks, its horizontal confined track length is 12.6±0.2 There are three samples (A11, A12 and A13) in the μm, and the standard deviation of the length is longer eastern slope of Dabashan, whose apparent ages are the (1.29 μm), indicating that the thermal history of the oldest and all three samples are collected in the hanging sample is more complex. The thermal modeling shows wall of a thrust in the east part of profile G-H (Figs. 2, 3 that the sample entered into the PZA at ~80 Ma and 10). Their ages are between 61.7–81.6 Ma (Figs. 4 and approximately. However, it stayed for a longer time in the 5b). Among them, the age dispersities of single grains of PAZ and finally passed through it at ~10 Ma (Fig. 7). The A12 and A13 are less than 40%, which indicates that their fission track characteristics of A02 are exactly similar to age compositions are comparatively simple and their those of A06 and A07 from the western slope, that is, they median ages are 81.6±5.5 Ma and 77.3±4.8 Ma, basically entered into and left the PAZ at the same time. respectively. The thermal modeling showed that samples Among these samples (A06, A07, A08, A09 and A02), A12 and A13 had passed through the PAZ rapidly during A09 is the highest in the altitude, and its track length 90–70 Ma. While the history of A11 is relatively complex, distribution and thermal modeling are completely different there are two single grain age peaks, the track length from A06, A07 and A02 (Fig. 7). In this study, due to the distribution is wider (Fig. 4). According to the thermal factors that A06, A07and A02 stayed in the PAZ for a modeling, sample A11 entered into the PAZ at ~70 Ma long time and their thermal histories completely different and left at approximately 50 Ma, but it returned to the from A09 above them, the authors suggested that A06, upper PAZ at around ~15 Ma, then left again rapidly (Fig. A07 and A02 represented the upper part of an ancient PAZ 7). that has been denuded to the earth’s surface, the top of the The above data and modeling of samples from the PAZ should be located between A09 and A06 (Fig. 5a), Dabashan indicate that the Dabashan experienced three but the lower-middle part of the PAZ hasn’t been denuded important tectonic-thermal events in the Late Cretaceous to the earth’s surface. The exposure lower-middle part of (~90–70 Ma), the Eocene (~50–45 Ma) and the Late Dabashan currently still belongs to an ancient PAZ (Fig. Cenozoic (since ~10 Ma) respectively. 9), the approximate age of the PAZ is ~80–70 Ma, and the upper PAZ was not denudate to the earth’s surface until 5.2 The tectonic setting of the Late Mesozoic-Cenozoic the Late Cenozoic (~10 Ma). If the geothermal gradient of denudation events the area was 36–40°C/km (Ordos active fault research 5.2.1 The Late Cretaceous (~90–70 Ma) group of China Earthquake Administration, 1988) and the Almost thermal modeling of all samples and the several

Fig. 9. The ancient PAZ (~90–70 Ma) in the Carboniferous granite of the Dabashan. 548 Vol. 92 No. 2 ACTA GEOLOGICA SINICA (English Edition) Apr. 2018 http://www.geojournals.cn/dzxben/ch/index.aspx http://mc.manuscriptcentral.com/ags

Fig. 10. Thrust along the eastern slope of the Dabashan and AFT sample position (photos were taken in the eastern front of the Dabashan). samples (A12 and A13) with concentration of single grain North Qilian Orogenic Belt and eastern Kunlun Orogenic ages from the Dabashan show that they experienced a Belt (George et al., 2001; Jolivet et al., 2001; Yuan et al., significant thermal event in the Late Cretaceous (~90– 2006; Wan et al., 2010). Recently, the late Cretaceous 70Ma). In the Early Cretaceous, extensional basins and tectonic event was found at the northern margin of Hexi many metamorphic core complexes developed in the vast Corridor (Zhang et al., 2017). areas in China (Meng et al., 2003; Wang et al., 2011). The In the Late Cretaceous, the movement speed of the Alxa Block with the Langshan at its eastern margin also Izanagi Plate to the east of Eurasian Plate was about 120– experienced significant extension, namely the 140 mm/y (Northrup et al., 1995). The plate moved in a development of many rift basins and the subsequent basalt NNW direction, acting on the Eurasian Plate by means of eruptions in large areas (Meng et al., 2003). In the Late a dextral transpression (Tian and Du, 1987; Maruyama et Cretaceous, most regions in China experienced a al., 1997). At this time, a left-strike-slipping occurred on significant tectonic event, many early Cretaceous rift the Tancheng-Lujiang Fault. Therefore, under the high- basins stopped developing, and regional uplift started, an speed oblique subduction, the principal compressive stress angular unconformity developed in regions like the Ordos in the eastern part of East Asia was in the NNW-SSE Basin, Songnen Basin, Sichuan Basin, Erlian Basin and direction, and the stress field may be probably the main the Huabei Basin (Ren et al., 2002). In the same period, factor for the tectonic inversion of many early Cretaceous South China also experienced an important change of basins in the eastern China. A recent research also tectonic settings (i.e., from NWW-SEE extension at early suggested that, in the Late Cretaceous, the eastern shore of stage to NWW-SSE compression) (Li et al., 2014). Chinese continent might collide with an unknown However, the cause for this tectonic event is unknown. microcontinental or oceanic plateau (Niu Yaoling et al., This tectonic-thermal event was very common in the 2015). We can see that the ancient stress fields in eastern Qinghai-Tibet Plateau, especially in the period of ~90–70 and western China in the Late Cretaceous were different, Ma. At this period, the Neo-Tethys Ocean subducted the western China was affected by the subduction of Neo- northerly under the Lhasa Block along the Yaluzangbu Tethys Ocean and the compression between the Lhasa and Suture Zone (Wen et al., 2008). At the same time, the Qiangtang blocks, while the eastern China was associated Lhasa Block continued to compress the Qiangtang Block with the movement of the Izanagi Plate and/or the northerly along the Bangong-Nujiang Suture Zone, collision between Chinese continent with an unknown leading to the uplift of the Lhasa Block and the crustal block. shortening in the north of the block by ~50% (England and In the Late Cretaceous, the northeastern China also Searle, 1986; Murphy et al., 1997; Kapp et al., 2003, 2005, experienced a strong tectonic thermal event, while the 2007; DeCelles et al., 2007; He et al., 2007; Leier et al., Great Khingan, Changbai mountains began rapid 2007; Volkmer et al., 2007; Pullen et al., 2008; Haider et denudation (~100–70Ma) (Li et al., 2010, 2011). But the al., 2013) and the formation of compressional basins in the tectonic thermal event hasn’t been recorded by the low- Qinghai-Tibet Plateau (Horton et al., 2002). Rapid cooling temperature thermochronology in the Mandula area at the events between ~115 and 70Ma are also found in the northern margin of Block at the same latitude Apr. 2018 ACTA GEOLOGICA SINICA (English Edition) Vol. 92 No. 2 549 http://www.geojournals.cn/dzxben/ch/index.aspx http://mc.manuscriptcentral.com/ags

(Li et al., 2016). The Dabashan at the same latitude is study region, researchers found early Cenozoic north-east located further west to the Mandula area. Therefore, the trending strike-slip faults, which were considered to be effect of Izanagi Plate from the east may be minor. controlled by the activity of the Altyn Tagh Fault at the Although the authors didn’t conduct detailed late northern margin of the Qinghai-Tibet Plateau (Webb and Cretaceous tectonic analysis, we have found the late Johnson, 2006). Unfortunately, such faults lacked precise Cretaceous event in areas such as the Longshou Mountain chronological constraints. Recently, researchers also have at the northern margin of Hexi Corridor recently, which discovered deformation caused by nearly south-north has resulted in the denudation of the Longshou Mountain compression in West Qinling region to the southwest of (Zhang et al., 2017). Furthermore, we have also the Ordos Block (Clark et al., 2010; Duvall et al., 2011). discovered late Cretaceous deformation in the Cretaceous The activity timing of the north-east trending dextral strike basins in the eastern part of Hexi Corridor (the Longzhon -slip faults in Dabashan constrained by samples (A03 and Basin), namely the NNW-SSE folds, showing that at least A04) near the fault (F2) was ~50–45 Ma, which provides a the Hexi Corridor suffered from the NNE-SSW time limit for the formation of the north-east trending compression in the Late Cretaceous, which is consistent dextral strike-slip faults in the study region. At that time, with the tectonic stress from the Qinghai-Tibetan Plateau. the eastern North China Block experienced strong The fault (F3) at the western margin of Dabashan is extension due to the back-arc extension caused by the currently a normal fault (Figs. 2, 11a), but it was a dextral subduction of Pacific Plate (Ma et al., 1987; Ren et al., strike-slip fault at the early stage (Fig. 11b). Also, the 2002). The subduction of Pacific Plate was therefore sample A08 close to the fault also revealed that it may be difficult to provide the driving force of the dextral strike- affected by the fault (F3). Sample A08 passed through the slip in the research region. The authors tended to argue PAZ rapidly in the Late Cretaceous, while samples A06 that the tectonic event which occurred in the study region and A07 nearby show a long-term stay in the PAZ. The in the Eocene may be attributed to the India-Eurasia authors speculate that the major cause of the late collision, while its remote far field effect could reach at Cretaceous tectonic event in the Dabashan is the least the northern area at the northeastern Alxa Block at compression from the Qinghai-Tibetan Plateau at that the early stage of collision. time; however, the far field effect of compression from the southeast due to the collision between the Chinese 5.2.3 The Late Cenozoic (since ~10 Ma) continent and an unknown block (Niu Yaoling et al., In the late Cenozoic (since ~10 Ma), the denudation 2015) can not be excluded. event has been discovered in all margins and many mountains of the Qinghai-Tibetan Plateau, representing a 5.2.2 The Eocene (~50–45 Ma) significant tectonic-thermal event (Molnar et al., 1993; The thermal modeling of samples from central part of Métivier et al., 1998; Wan Jinglin et al., 2001;2010; Chen Dabashan displays that a tectonic thermal event occurred et al., 2002; Yuan et al., 2006; Zheng et al., 2010; in the Eocene (~50–45 Ma) (Fig. 7). It is the time that the Craddock et al., 2011; Duvall et al., 2013; Li et al., 2013). main stage of the India-Eurasia collision occurred Tectonic event occurred in ~10 Ma at the northeastern (Najman et al., 2010), and the thermal event at this stage Qinghai-Tibetan Plateau and the eastern Alxa Block has were found in the vast areas in the Qinghai-Tibet Plateau been also reported recently, such as the Liupan, Lajishan, (Wang Guocan et al., 2010; Rohrmann et al., 2012), such and Jishishan mountains (13~7 Ma, Zheng et al., 2006; as the Xining Basin (Zhang et al., 2015), the large-scale Lease et al., 2007, 2011, 2012; Wang Yannan et al., 2015) uplifting in Gangdise and Qiangtang regions (Xu et al., and the Helan Mountain (12~10 Ma, Zhao Hongge et al., 2013; Ding et al., 2014), crustal thickening in the North 2007; Liu Jianhui et al, 2010). Qilian Mountain (Ritts et al., 2004; Zhuang et al., 2011), Although there wasn’t apparent age around ~10Ma in thrusting in West Qinling (Clark et al., 2010; Duvall et al., samples from Dabashan in this study, the thermal 2011), thrusting in the South Qilian Mountain (Yin et al., modeling of almost all samples shows that there had been 2002), and the formation of a large number of basins in the a stage of rapid cooling since ~10 Ma. Many studies have northeastern plateau (Fang et al., 2003, 2005; Zhang shown that the late Cenozoic rapid cooling in thermal Kexin et al., 2010; Wang et al., 2011; Lu et al., 2012), and modeling is likely to be caused by the selection of initial so on. At present, some researchers believe that track length, but not samples (Gunnell et al., 2003); hence deformation at the northeast margin of the Qinghai-Tibet it is necessary to interpret it with caution when Plateau started from the Late Cenozoic (Zheng et al., encountering cooling events completely occurred outside 2006, 2010) or later than~30 Ma (Wang et al., 2011). In the PAZ, unless there is independent geological or other the adjoining South Mongolia regions to the north of the evidence. Though all thermal modeling of samples from 550 Vol. 92 No. 2 ACTA GEOLOGICA SINICA (English Edition) Apr. 2018 http://www.geojournals.cn/dzxben/ch/index.aspx http://mc.manuscriptcentral.com/ags the Dabashan shows that many samples (A02, A03, A06, 2007). In addition, active tectonics occurred widely across A07, A08, A09 and A11) started cooling from the upper the study region, as a part of the Cenozoic Peri-Ordos PAZ in around ~10 Ma , and the representative of such Graben System (Ordos active fault research group of cooling history has higher possibility (Vassallo et al., China Earthquake Administration, 1988; Deng Qidong,

Fig. 11. Fault (F3) at the western margin of Dabashan. (a), distant view; (b), sinitral strike-slip slickenline (photos were taken on the western front of the Dabashan).

Fig. 12. Piedmont fault of the Dabashan. (a), Piedmont fault surface; (b), Parallel fault scarps along the piedmont fault; (c) and (d) Piedmont normal slip fault cutting the Late Pleistocene Jilantai Formation (all photos were taken on the eastern front of the Dabashan). Apr. 2018 ACTA GEOLOGICA SINICA (English Edition) Vol. 92 No. 2 551 http://www.geojournals.cn/dzxben/ch/index.aspx http://mc.manuscriptcentral.com/ags

1996), Piedmont fault (F1) is a large active normal fault References (Fig. 12) developing at least 4 fault scarps (Fig. 12), which An, Z.S., Kutzbach, J.E., Prell, W.L., and Porter, S.C., 2001. has cut the lacustrine sedimentation of the Jilantai Evolution of Asian monsoons and phased uplift of the Himalaya–Tibetan plateau since late Miocene times. Nature, Formation since the Late Pleistocene (Figs. 12c and 12d). 411(6833): 62–66. A recent study showed that the average earthquake Chen, Z., Wang, X., Feng, X., Wang, C., and Liu, J., 2002. New recurrence interval was about 2500 years (Rao et al., evidence from stable isotope for the uplift of mountains in 2016). Therefore, thermal event around ~10 Ma displayed northern edge of the Qinghai-Tibetan Plateau. Science in in the thermal modeling of samples from the Dabashan is China (Series B), 45(1): 1–10. credible. Clark, M.K., Farley, K.A., Zheng, D., Wang, Z.C., and Duvall, A., 2010. Early Cenozoic faulting on the northern Tibetan Plateau margin from apatite (U-Th)/He ages. Earth and 6 Conclusions Planetary Science Letters, 296(1–2): 78–88. Craddock, W., Kirby, E., and Zhang, H.P., 2011. Late Miocene- The AFT ages of samples from the Dabashan and their Pliocene range growth in the interior of the northeastern thermal modeling indicate that the northeastern Alxa Tibetan Plateau. Lithosphere, 3(6): 420–438. Dai, S., Fang, X.M., Dupont-Nivet, G., Song, C.H, Gao, J.P., Block experienced complicated tectonic thermal events in Krijgsman, W., Langereis, C., and Zhang, W.L., 2006. the late Cretaceous-Cenozoic. The Late Cretaceous Magnetostratigraphy of Cenozoic sediments from the Xining tectonic thermal event (~90–70 Ma) had no relation to the Basin: Tectonic implications for the northeastern Tibetan action of the Izanagi Plate on the eastern Eurasian Plateau. Journal of Geophysical Research, 111(B11): 335– continent, but related to the Neo-Tethys subduction and 360. the compression between the Lhasa and Qiangtang blocks. Dan, W., Li, X.H., Wang, Q., Wang, X.C., Wyman Derek, A., and Liu, Y., 2016. Phanerozoic amalgamation of the Alxa The dextral strike-slip fault activity in around the Eocene Block and North China Craton: Evidence from Paleozoic (~50–45 Ma) shared the same tectonic environment with granitoids, U–Pb geochronology and Sr–Nd–Pb–Hf–O isotope the contemporary deformation in southern Mongolia and geochemistry. Gondwana Research, 32: 105–121. the southwest Ordos Block, which was the remote far-field Darby, B.J., and Ritts, B.D., 2002. Mesozoic contractional response to the India-Eurasia collision. Thermal event in deformation in the middle of the Asian tectonic collage: the intraplate Western Ordos fold thrust belt China. Earth and the late Cenozoic (since ~10 Ma), coeval to the Cenozoic Planetary Science Letters, 205(1): 13–24. rapid denudation of mountains near the Langshan may be Darby, B.J., Ritts, B.D., Yue, Y.J., and Meng, Q.R., 2005. Did associated with the northeastward growth of the Qinghai- the Altyn Tagh fault extend beyond the Tibetan Plateau? Tibetan Plateau. At the same time, it has been also found Earth and Planetary Science Letters, 240(2): 425–435. that main body of Dabashan is an incompletely denuded Darby, B.J., and Ritts, B.D., 2007. Mesozoic structural fossil partial annealing zone whose age is the Late architecture of the Lang Shan, North-Central China: Intraplate contraction, extension, and synorogenic sedimentation. Cretaceous (~70 Ma), and this PAZ suffered the Journal of Structural Geology, 29(12): 2006–2016. destruction of multiple tectonic events with the denudation DeCelles, P.G., Quade, J., Kapp, P., Fan, M., Dettman, D.L., and of Dabashan during the Cenozoic. Ding, L., 2007, High and dry in central Tibet during the Late Oligocene. Earth and Planetary Science Letters, 253(3): 389– Acknowledgements 401. Deng Qidong, 1996. A study on active tectonics in China.

Geological Review, 42(4): 295–299 (in Chinese). The study is funded by Natural Science Foundation of De Grave, J., Buslov, M.M., and Van den haute, P., 2007. China (41572190), National Basic Research Program of Distant effects of India–Eurasia convergence and Mesozoic Ministry of Science and Technology of the People's intracontinental deformation in Central Asia: Constraints from Republic of China (2015CB453002) and China Geological apatite fission-track thermochronology. Journal of Asian Survey (121201102000150009-16, 12120115069601). We Earth Sciences, 29(2–3): 188–204. Ding Lin, Xu Qiang, Yue Yahui, Wang Houqi, Cai Fulong and are grateful to Prof. Zheng Dewen and Gu Yuanzhu of the Li Shun, 2014. The Andean-type Gangdese Mountains: Fission Track Laboratory of the Institute of Geology, the paleoelevation record from the Paleocene–Eocene Linzhou China Earthquake Administration for their kindly help Basin. Earth and Planetary Science Letters, 392: 250–264. during samples’ dating. We also thank Dr. Lu Miaoan of Donelick, R.A., O’Sullivan, P.B., and Ketcham, R.A., 2005. Zhejiang Dongyang Scientific Inquiry Co., Ltd. for Apatite fission-track analysis. Reviews in Mineralogy and Geochemistry. 58(1): 49–94. discussing during the field investigation. Dupont-Nivet, G., Dai, S., Fang, X., Krijgsman, W., Erens, V., Reitsma, M., and Langereis, C., 2008. Timing and distribution Manuscript received Feb. 15, 2017 of tectonic rotations in the northeastern Tibetan Plateau. accepted Aug. 10, 2017 Special Paper of the Geological Society of America, 444: 73– edited by Liu Lian 87. 552 Vol. 92 No. 2 ACTA GEOLOGICA SINICA (English Edition) Apr. 2018 http://www.geojournals.cn/dzxben/ch/index.aspx http://mc.manuscriptcentral.com/ags

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