Late Quaternary Slip Rate of the Aksay Segment and Its Rapidly Decreasing Gradient Along the Altyn Tagh Fault GEOSPHERE

Research Paper

GEOSPHERE Late Quaternary slip rate of the Aksay segment and its rapidly decreasing gradient along the Altyn Tagh fault

1,2 1,2 3,4 2 3,4 5 GEOSPHERE, v. 16, no. 6 Jinrui Liu , Zhikun Ren , Wenjun Zheng , Wei Min , Zhigang Li , and Gang Zheng 1State Key Laboratory of Earthquake Dynamics, Institute of Geology, China Earthquake Administration, Beijing 100029, China 2Key Laboratory of Seismic and Volcanic Hazards, China Earthquake Administration, Beijing 100029, China https://doi.org/10.1130/GES02250.1 3Guangdong Provincial Key Laboratory of Geodynamics and Geohazards, School of Earth Sciences and Engineering, Sun Yat-sen University, Guangzhou 510275, China 4Southern Marine Science and Engineering Guangdong Laboratory (Zhuhai), Zhuhai 519000, China 15 figures; 4 tables; 1 supplemental file 5GNSS Research Center, Wuhan University, Wuhan 430079, China

CORRESPONDENCE: [email protected]; 2007; Molnar and Dayem, 2010; Royden et al., 2008; and deformation pattern are critical to understand- [email protected] ABSTRACT Tapponnier et al., 2001). Meanwhile, the fault slip ing the mechanics of intracontinental deformation. CITATION: Liu, J., Ren, Z., Zheng, W., Min, W., Li, Z., Constraining the fault slip rate on a fault can rate pattern along strike is also helpful for under- Several researchers have suggested that a decreas- and Zheng, G., 2020, Late Quaternary slip rate of the reveal the strain accumulation and partitioning standing the strain accumulation pattern, fault slip ing trend in the slip rate can be roughly obtained Aksay segment and its rapidly decreasing gradient pattern. The Aksay segment, the eastern segment history, and the strain adjustment and its influence in the east segment of the Altyn Tagh fault from along the Altyn Tagh fault: Geosphere, v. 16, no. 6, p. 1538–1557, https://doi.org/10.1130/GES02250.1. of the Altyn Tagh fault, as the starting area where on adjacent faults or structures (Resor et al., 2018; these constraints (Zhang et al., 2007; Zheng et al., the slip rate of the Altyn Tagh fault decreases, is a Zechar and Frankel, 2009). A decreasing pattern of 2013). Although previous studies have shown that Science Editor: Andrea Hampel strain partitioning zone. The spatial and temporal slip rates has been widely observed along the typ- the slip rate begins to decrease from the Aksay Associate Editor: Francesco Mazzarini distribution of its fault slip rate is of great signif- ical strike-slip faults around the world, such as the segment (Xu et al., 2005; Zhang et al., 2007), it is icance to clarify the strain-partitioning pattern of Altyn Tagh (Gold et al., 2009; Zhang et al., 2007; still unclear whether the slip rate decreases with a Received 28 February 2020 the eastern Altyn Tagh fault. In this study, we deter- Zheng et al., 2013), Kunlun (Kirby et al., 2007; Lin uniform gradient. Revision received 25 July 2020 Accepted 27 August 2020 mined the slip rates at four sites along the Aksay and Guo, 2008), and North Anatolian fault zones Therefore, we focused on the ~60-km-long Aksay segment. The results demonstrated that the slip (Walters et al., 2014). For instance, a rapid decrease segment in order to systematically constrain the Published online 15 October 2020 rate decreases dramatically, with an overwhelm- in the slip rate of the Haiyuan fault occurs at ~106°E late Quaternary slip rates on the eastern segment ingly high slip gradient of ~9.8 mm/yr/100 km (a in the Madongshan-Liupanshan region, where the of the Altyn Tagh fault based on multilevel displaced 9.8 mm/yr reduction of slip rate occurs over a dis- deformation is dominated by reverse faulting, fold- river terraces at four sites. The initial decrease in the tance of 100 km) within a distance of ~50 km. The ing, and the uplift of regional structures (Li et al., slip rate on the eastern Altyn Tagh fault and multi slip rate gradient along strike at the Aksay seg- 2009; Zheng et al., 2013). Similarly, the uplift of level offset geomorphic markers could be used ment is four times that of the Subei segment to the the Anyemaqen Shan and the clockwise rotation to constrain more reliable slip rates on the Aksay eastward termination of the Altyn Tagh fault. Our of eastern Kunlun faults accommodated the slip segment. Along this segment, the West of Aksay, results indicate that the slip rate gradient along the rate along the Kunlun fault during structural thick- Old Aksay, Jiaerwuzongcun, and Yandantu sites Altyn Tagh fault is not uniform and decreases east- ening (Kirby and Harkins, 2013). (Fig. 2), from west to east, were investigated both ward with variable slip rate gradients on different The Altyn Tagh fault, as a large-scale boundary in the field and by applying a Monte Carlo analy- segments, resulting in the uplift of the mountains fault between the Tarim block to the north and the sis method. Combined with local geodetic results, oblique to the Altyn Tagh fault. Tibetan Plateau to the south, plays an important we determined the kinematic model of the Aksay role in affecting, and even controlling, the evolution segment, and we discuss its role in the strain par- of the Tibetan Plateau during its NE growth (Molnar titioning of the eastern Altyn Tagh fault here. ■■ INTRODUCTION and Rajagopalan, 2012; Tapponnier et al., 2001; Yuan et al., 2013). Studies on temporal and spatial distri- Large active faults in the interior and boundary butions of the slip rate along the Altyn Tagh fault are ■■ GEOLOGICAL BACKGROUND of a plateau can provide kinematic constraints for essential for understanding the tectonic evolution fault interactions and rupture dynamics, which is of of the northern Tibetan Plateau. With a series of Qilian Shan great significance to understanding the strain par- mountains oblique to the fault (Fig. 1), the eastern titioning and seismic hazard assessment (Meade, segment of the Altyn Tagh fault is a very important The Qilian Shan (Shan = Mountains), located This paper is published under the terms of the strain partitioning zone on the northeastern Tibetan on the northeastern margin of the Tibetan Plateau CC‑BY-NC license. Zhikun Ren https://orcid.org/0000-0002-6391-9503 Plateau. The strike-slip rate gradients along the fault (Fig. 1), has experienced intensive deformation

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90°E 95°E 100°E Gobi-Alashan 9 Tarim 8 1 6 Jiuxi Basin Figure 2 1 Hexi Corridor 3 4 7 6 1 Daxue Shan Qilian Shan 39°N 5 Danghe Nan Shan 4 5 3 2 3 4 2 1 Haiyuan Fault Qaidam Figure 1. Tectonic setting of study area. 1 Altyn Tagh Fault (A) Tectonic map of the Tibetan Plateau

85°E 90°E 95°E 100°E 105°E (from Tapponnier et al., 2001). (B) Ge- 40°N ometry and locations of slip rates from previous studies along Altyn Tagh fault 36°N Figure 1B 35°N (ATF), based on the Shuttle Radar Topog- Kunlun Fault Tibetan Plateau raphy Mission data with 90 m resolution. 30°N The locations of the active fault traces 0 100 200 0 500 1000 follow Deng et al. (2003). Seismic data 85°E km 90°E 95°E 40-50mm/yr km 25°N are from the China Earthquake Infor- 7 Geological Study Chen et al., 2012; InSAR Study Historical Earthquakes mation Network (2020). GPS—global Chen et al., 2013 1 Zhang et al., 2007 Ms=8.0-8.9 positioning system; InSAR—interfero- 8 Kang et al, 2019 Jolivet et al., 2008 2 Cowgill, 2007; Ms=7.0-7.9 metric synthetic aperture radar. 9 Zhang, 2016 Cowgill et al., 2009 Liu et al., 2018a Ms=6.0-6.9 3 Gold et al, 2009; GPS Study Bendick et al., 2000 Ms=5.0-5.9 Gold and Cowgill, 2011 Altyn Tagh Fault 4 Meriaux et al., 2004; Shen et al., 2001 Elevation (m) Meriaux et al., 2005; 7000 Wallace et al., 2004 Main Strike-slip Faults Meriaux et al., 2012 Zhang et al., 2007 5000 5 Washburn et al., 2001; He et al., 2013 Thrust Faults Washburn et al., 2003 2000 Zheng et al., 2017 Other Faults 6 Xu et al., 2005 Li et al., 2018 0

since the late Cenozoic related to the northeast- Shan and the Altyn Tagh fault. Previous research lithosphere scale in the Eurasian plate (Fig. 1; Mol- ward growth of the plateau (Meyer et al., 1998; proposed that the subparallel Cenozoic thrust nar and Tapponnier, 1975; Tapponnier et al., 1982, Zhang et al., 2004; Duvall et al., 2013; Yuan et al., branches southeastward from the Altyn Tagh fault 2001; Yin et al., 2002; Xu et al., 2005; Zhang et al., 2013; Allen et al., 2017). Due to the NE-SW regional absorbed the crustal shortening along these struc- 2007). Studying its temporal and spatial distribu- stress induced by plate motion, the strain in the tures (Meyer et al., 1998; Van der Woerd et al., 2001). tions of slip rates is essential to reveal the pattern of Qilian area is partitioned into dip-slip and strike-slip However, Allen et al. (2017) suggested that the Qil- northeastward growth of the Tibetan Plateau (Meyer components (Allen et al., 2017). The strike-slip struc- ian thrust was not a secondary feature caused by et al., 1998; Cunningham et al., 2016). tures are mainly large-scale strike-slip faults, such the activity of Altyn Tagh fault, but a major part of The long-term average slip rates over millions as the Altyn Tagh fault and Haiyuan fault. In contrast, the whole convergence of India-Eurasia. In any case, of years are constrained by offset geological mark- the dip-slip structures are dominated by thrusts the Qilian Shan is an ideal region to study strain ers and the time of initiation. The total left-lateral in the Qilian Shan, causing series of NW-trending partitioning. displacement of the Altyn Tagh fault was thought mountain ranges, e.g., the Qilian Shan, Daxue Shan, to be up to 350–400 km (Ritts et al., 2004; Yin et al., Danghe Nan Shan, and Qaidam Shan from NE to 2002; Yue et al., 2001, 2004). Yin and Harrison SW (Meyer et al., 1998; Xu et al., 2005; Allen et al., Altyn Tagh Fault (2000) estimated the displacement of 280 ± 30 km 2017), and late Cenozoic uplift parallel to the Altyn since the late Pleistocene (ca. 30 Ma) and calcu- Tagh fault, observed in the Sanwei Shan and Nan- The Altyn Tagh fault, marking the north- lated an average slip rate of 7–9 mm/yr. Yin et al. jie Shan to the north (Yang et al., 2020). There is a ern boundary of the Tibetan Plateau for nearly (2002) suggested that the slip rate was 9 ± 2 mm/yr close interaction between the thrusts in the Qilian ~2000 km, is a large left-lateral strike-slip fault at a since 49 Ma. However, Yue et al. (2001) determined

94°10'E 94°20'E 94°30'E 94°40'E 94°50'E methods, such as radiocarbon dating (14C), in situ 10 26 Q4 Holocene J Jurassic γ Granite plutons Q4 D cosmogenic dating ( Be, Al), and thermolumi- a Q4 n Q3 L-Pleistocene S Silurian Altyn Tagh Fault g nescence dating (TL), and found that the slip rates

Q1-2 E-Pleistocene € Cambrian Thrust Faults R of the central and western segments were up to iv N Neogene Z Sinian e 17.5 ± 2 mm/yr. These high rates largely support the r

eastward extrusion model.

However, the results of geological mapping Q1-2 Subei Q3 Yandantu and geomorphic studies carried out by the “Altyn 39°30'N N Tagh Active Fault” research group of the Chinese Q1-2 State Bureau of Seismology along the Altyn Tagh A Q1-2 Jiaerwuzongcun fault suggested that the minimum Quaternary slip Old Aksay Z Dan ghe Nan Shan rate of the Altyn Tagh fault was ~5 mm/yr (Chinese West of Aksay State Bureau of Seismology, 1992). Xiang et al. (2000) classified the systemic displacement of the Houtang fault

39°20'N Q3 S 0 5 10km river system in the eastern Altyn Tagh fault and A’ constrained the slip rate of the Altyn Tagh fault in Dangjinshankou Changcaogou this area at 4.7–6.7 mm/yr. Wang et al. (2003, 2004) obtained a Holocene slip rate on the Altyn Tagh Elevation (m) fault of 11.4 ± 2.5 mm/yr by studying river terraces Southern fault Q3 A’ and alluvial fans. Cowgill (2007) and Zhang et al. Elevation (m) Northern fault 3500 3000 (2007) reinterpreted the results of Xu et al. (2005) S A Q3 Z 2500 and Mériaux et al. (2005) and suggested that the 2500 γ 345°∠65° 2000 slip rate on the main segment of the Altyn Tagh fault 2000 Z 167°∠77° Schist Gneiss Granite plutons Quaternary deposit Faults 1500 1500 was 10 ± 2 mm/yr. Gold et al. (2009) and Cowgill et al. (2009) constrained the Quaternary slip rates Figure 2. Geological map of the Aksay segment within the Altyn Tagh fault, modified from 1:200,000 scale Geological Map of Subei (J-46-V) (Geological Bureau of Gansu Province, 1976). at 8–17 mm/yr and 9–14 mm/yr at 86.7°E to 88.5°E, respectively. Mériaux et al. (2012) obtained a slip rate of 13.9 ± 1.1 mm/yr in the Pingdingshan area on the displacement of the eastern and central seg- (England and Houseman, 1986; Houseman and the western segment of the Altyn Tagh fault using ments of the Altyn Tagh fault to be 375 ± 25 km and England, 1993). the in situ cosmogenic dating methods. showed that the slip rate was 12–16 mm/yr since Study of the late Quaternary slip rate of the Relatively low slip rates obtained along the Altyn the late Oligocene, according to a field geologi- Altyn Tagh fault began in the 1970s (Molnar and Tagh fault based on global positioning system (GPS) cal survey. They further obtained displacement of Tapponnier, 1975; Tapponnier et al., 1982; Table 1). and interferometric synthetic aperture radar (InSAR) 165 km since 16.4 Ma and suggested that the slip Molnar et al. (1987) and Peltzer et al. (1989) inter- measurements have been reported during the past rate of Altyn Tagh fault was at least 10 mm/yr since preted 100–400 m of displacement of the river decades. For instance, Bendick et al. (2000) used GPS the late Miocene (Yue et al., 2004). system along the Altyn Tagh fault using SPOT sat- data to obtain a left-lateral slip rate of 9 ± 5 mm/yr For a long time, the activity of the Altyn Tagh ellite images and estimated a high Holocene slip at 89°E–91°E. A similar result was also reported fault has been regarded as a key index to test rate of 20–30 mm/yr. Tapponnier et al. (2001) sum- by Wallace et al. (2004). The present-day average deformation models of the Tibetan Plateau. The marized the reported Quaternary slip rates of the geodetic slip rate between 85°E and 90°E was esti- “eastward extrusion model” is supported by the whole Altyn Tagh fault and obtained the slip rate mated to be 9 ± 2 mm/yr using GPS data (Shen et al., very high slip rates on the main boundary faults in the central segment of Altyn Tagh fault between 2001). Zhang et al. (2007) showed a slip rate of 11.9 of the rigid blocks (Avouac and Tapponnier, 1993; 83°E and 94°E reaching 20–30 mm/yr. Mériaux ± 3.3 mm/yr at 89°E–91°E, 4 mm/yr at 94°E–96°E, and Peltzer and Tapponnier, 1988; Peltzer et al., 1989), et al. (2004, 2005) determined an average slip rate 3.9 ± 2.3 mm/yr at ~96°E, respectively. However, at and the “crust thickening model” is characterized of ~20.3 ± 1.1 mm/yr by dating the alluvial fans and ~86.2°E, He et al. (2013) got a current slip rate of 9.0 by continuous deformation distributed within river terraces of the Altyn Tagh fault. Combining the ± 4 mm/yr based on a GPS array. From west to east, the blocks as well as on the boundary faults, i.e., high-resolution SPOT satellite images with field Zheng et al. (2017) estimated a left-lateral rate of 8.1 a relatively low slip rate on the Altyn Tagh fault investigation, Xu et al. (2005) used various dating ± 0.7 mm/yr at ~86°E, 8.6 ± 1.5 mm/yr at ~90.4°E, and

TABLE 1. SMMARY OF SLIP RATES ALONG THE ALTYN TAGH FALT Type of slip rate Time scale Slip rate Reference mm/yr Long‑term average slip rate 30 m.y. 7– Yin and Harrison 2000 4 m.y. 2 Yin et al. 2002 Late Oligocene 12–16 Yue et al. 2001 16.4 Ma/late Miocene 10 Yue et al. 2004 Late Quaternary slip rate Holocene 20–30 Peltzer et al. 18 Quaternary 20–30 Tapponnier et al. 2001 Late Pleistocene 20.3 1.1 Mriau et al. 2004, 2005 Late Pleistocene 17.5 2 u et al. 2005 Holocene 5 iniang Seismological Bureau of China 12 Late Pleistocene 4.6–6.7 iang et al. 2000 Holocene 11.4 2.5 Wang et al. 2003, 2004 Late Pleistocene 10 2 Cogill 2007, Zhang et al. 2007 Quaternary 8–17/ –14 Gold et al. 200 Late Pleistocene 13. 1.1 Mriau et al. 2012 Geodetic rate 8–1E: 5 Bendick et al. 2000, Wallace et al. 2004 85–0E: 2 Shen et al. 2001 8–1E: 11. 3.3; 4–6E: 4; 6E: 3. 2.3 Zhang et al. 2007 86E: 8.1 0.7; 0.4E: 8.6 1.; 4.6E: 4.5 0.8 Zheng et al. 2017 4E: 8–10 Jolivet et al. 2008 1.5–5E: 6.4 Liu et al. 2018a

4.5 ± 0.8 mm/yr at ~94.6°E, respectively. Jolivet et al. focused on this area. Based on the exposure ages ■■ DATA AND METHODS (2008) used European Remote Sensing (ERS) radar of terraces and the displacement of terrace scarps, data and Envisat radar data to determine a slip rate Mériaux et al. (2005) obtained a slip rate of 17.8 The fault slip rate represents the average velocity of 8–10 mm/yr at 94°E. For the segment between ± 3.6 mm/yr along the Aksay segment. Based on the of fault movement in a certain period of time and 91.5°E and 95°E, the slip rate derived from InSAR 14C age and displacements of the river system, Xu reflects the rate of strain accumulation on a fault was 6.4 mm/yr (Liu et al., 2018a). et al. (2005) determined that the slip rate was 16.4 zone, so it is widely used in tectonic reconstruc- Recently, the widely accepted slip rate of the ± 2 mm/yr near the Old Aksay town. By reanalyzing tions and seismic hazard assessments. At present, Altyn Tagh fault is ~10 mm/yr, which implies that the research results of Mériaux et al. (2005) and the Holocene fault slip rate is mainly determined the previous estimation of ~20–30 mm/yr may be Xu et al. (2005), Zhang et al. (2007) considered the by dividing the total displacement of the geomor- overestimated. The results of recent research with abandoned age of the upper terrace as the accumu- phic indicators (such as terrace risers of rivers and more viable analysis, consistent with GPS and InSAR lated age of fault displacement, and the result was gullies, etc.) by cumulative time (the formation age measurements, as the important proofs of the “crust a slip rate of 8–12 mm/yr. Chen et al. (2012, 2013) of the geomorphic surface). In recent years, with thickening model,” imply that the Altyn Tagh fault is studied the slip rate near the Old Aksay town by light detection and ranging (LiDAR), stereo pairs not the main fault allowing the northeastward extru- using the method of luminescence dating and the of remote-sensing images, and aerial photographs, sion of the Tibetan Plateau. The large-scale strike-slip scarp erosion model. Through the scarp evolution, photo-based three-dimensional (3-D) reconstruction movement is mainly confined into the interior of the the age closest to the actual displacement of the techniques have been applied to active tectonics, plateau, which is manifested by the crustal thicken- gully was selected, and the slip rate of the Aksay and it is more and more convenient to acquire ing of the Qilian Shan (Zhang et al., 2007). segment since 6 ka was estimated at 11 ± 2 mm/yr, high-precision 3-D topographic data (Ren et al., The Aksay segment is located at the eastern which was basically consistent with the results of 2018). In this study, in order to constrain the slip rate part of the Altyn Tagh fault (Fig. 1). The surface Zhang et al. (2007) and Cowgill (2007). How is the in the study area, high-resolution terrain data at spe- elevation of the alluvial fans varies from 2700– strain partitioned on the eastern segment of the cific sites were obtained using aerial photographs 2800 m to 1650 m in the range of ~25 km (Fig. 2). Altyn Tagh fault? In this study, we focused on the collected by an unmanned aerial vehicle (UAV), The Altyn Tagh fault cuts through most of the fans Aksay segment to obtain the fault slip rates and the according to the high-resolution structure-from-mo- (Mériaux et al., 2005), and many studies have strain partitioning pattern of the Altyn Tagh fault. tion models (Westoby et al., 2012). The offsets were

measured from the digital elevation model (DEM) or compress the topographic profiles to find the Dynamics, Institute of Geology, China Earthquake using the LaDiCaoz_v2 code (Zielke et al., 2010, best-fitting results. Meanwhile, the minimum and Administration and the French National Centre for 2015), which is a widely used tool in strike-slip maximum offsets are also provided regarding the Scientific Research, respectively. fault offset measurements. The basis for offset mea- uncertainties, as well as the goodness-of-fit (GOF) For slip rate calculation, the Monte Carlo anal- surement is fitting the topographic profiles across values. Furthermore, the corresponding back-slip ysis method proposed by Gold and Cowgill (2011) linear markers such as gullies, terrace risers, and maps are provided in the results to check the reliabil- was applied in order to obtain a paleoslip history. mountain ridges, etc. However, due to the natural ity of the offsets. All the collected samples from the In this study, we tried to obtain a reliable fault slip evolution of channels or terrace risers, both sides of four sites were dated by optically stimulated lumi- rate with a synthetic data set in this way (Liu et al., the fault might not fit with each other. The solution nescence (OSL) and situ cosmogenic nuclide (10Be) 2018b). Each pair of offsets and dating results in this code is to horizontally or vertically stretch methods at the State Key Laboratory of Earthquake (including the uncertainties) was used to con- struct the input envelope, the edges of which were bounded by the minimum and maximum limits of 39°23'20"N displacement and the corresponding accumulation time for a given marker. The offsets of the younger terraces should not exceed those of the older terraces, and the ages should follow a similar rule. Hence, there was no negative slope for the gradient of the fitting curve from the results of offsets and dating, i.e., the slip rate curve. Furthermore, the average slip rates were constrained only by the rest of the displacement and age data. By defining the method points of each slip history, we could 39°23'10"N perform a linear regression to yield an average slip rate with its associated uncertainty. The fault 94°10'0"E slip rate was calculated repeatedly for 1000 times T4 2755m until testing showed that the solution did not sig- T3 T3 Figure 3. Hillshade map of the nificantly change for larger numbers of repetitions 2750m R1 R2 94°9'40"E R3 high-resolution digital ele- (Gold and Cowgill, 2011). We calculated the median 0 100 200 T1 m A A’ vation model (DEM) and the 500m value and nonparametric 68% and 95% confidence 125m 250m 375m 667m corresponding geomorphic envelopes, which included the uncertainties in both 39°23'20"N interpretation of the West of Aksay site overlain by 2 m con- the displacement and age. A tour lines.

■■ RESULTS R1 R2 R3 A’ Offsets Measurement Results T3 LED17-63 West of Aksay Site

T3 The West of Aksay site was the westernmost 39°23'10"N LED17-171, 172 site in the study area. The terraces could be classi- 94°9'40"E fied into three levels, T1, T3, and T4 from young to Legend T1 Altyn Tagh fault T4 old (Fig. 3). According to the regional landforms, Stream the T2 terrace was not developed at this site. There Sample site Bedrock hill were three different levels of channels developed T4 T2 0 100 200 on T4, i.e., R1, R2, and R3, respectively (Fig. 3). As T3 T1 m 94°10'0"E channels are inherently younger than the landform

they incise, this common practice may cause under- only got one offset value to be used in fault-slip- on satellite images. Combining the relationships estimation of slip rates, as well as a mismatch in rate determination. between active faults, offset channels, and loess on slip rates between channels that are offset by dif- banks, Chen et al. (2013) determined the slip rate ferent magnitude but incise in the same surface of the Altyn Tagh fault within 6 k.y. Based on the (Shelef et al., 2019). To obtain the accumulated Old Aksay Site summary, it was found that only the lower terrace displacement closest to the formation age of the was studied (Chen et al., 2013; Mériaux et al., 2005; T4 terrace, we measured R3 with the largest dis- The Old Aksay site was located in the east of Zhang et al., 2007). Through the field investigation, placement, which can also be evidenced by the Old Aksay county, where a linear structure could three different levels of old terraces of Changcao- deepest erosion downward on the terrace profile be observed clearly from the satellite image gou (Ta, Tb, and Tc) were found in the Old Aksay of R3. The offset was estimated to be ~98 ± 4 m (Fig. 2). According to previous studies, Mériaux et al. site, which provided an opportunity to determine (Fig. 4). Consequently, at the West of Aksay site, we (2005) identified three groups of terraces based the slip rate on a larger time scale (Fig. 5). These

98m

100m 100m a Elevation (m) Displacement along profile (m) Misfit Misfit

Horizontal displacement (m) Vertical displacement (m) Elevation (m) Displacement along profile (m)

Figure 4. Application of the LaDiCaoz_v2 code for measuring offset of channel R3 at the West of Aksay site. (A) Digital elevation model (DEM) topography delineates the general position of the fault (black), fault-parallel profile lines (red and blue), and longitudinal tracking (yellow). (B) LaDiCaoz_v2 reconstruction of the geomorphic offset based on DEM topography. (C) Fault-parallel profiles projected onto the fault plane according to the feature slope and shifted by the optimal lateral offset. (D) Evaluation of the reconstruction of the geomorphic offset.

old terraces are relatively high, and they represent the measurement error and obtained the displace- middle layer composed of river gravel accumula- residual terraces. Unfortunately, the corresponding ment sequences, which were ~1360 ± 110 m, ~1790 tion, and the upper part covered with thick loess. No geomorphic markers on the other side of the fault ± 130 m, and ~2290 ± 220 m, respectively (Fig. 6). detailed work has been carried out on the slip rate have not been found for displacement restoration. there. There are also multilevel terraces displaced Therefore, the displacements were obtained by by the Altyn Tagh fault at the Jiaerwuzongcun site matching the terrace risers upstream of Changcao Jiaerwuzongcun Site (Fig. 7), providing an ideal place to study the fault gou (Fig. 5), which were closer to the accumulated slip rates. As before, we used a UAV with a differ- offset since the formation of these geomorphic The Jiaerwuzongcun site was located in the ential GPS (DGPS) system to obtain high-resolution markers. The displacements of these old terraces middle part of the Aksay segment. There are four DEM data. T2 on the western bank is eroded and are very large, and the corresponding errors will not levels of terraces and multilevel secondary terraces cut, which makes the adjacent T2′ only partially exceed the distance between the two terraces. We (Fig. 7). The terraces are mainly base terraces, with remain. According to the high-resolution DEM data, took the distances between these old terraces as the bottom composed of Tertiary red bedrock, the the scarp riser was identified, and the measured displacement was found to be ~25.4 +1.6/–1.4 m (Fig. S11). The scarp of T3′ was relatively obvious, and the offset was estimated to be ~46.8 +1.2/–1.3 m (Fig. S2). The T3 riser was eroded by temporary flow, especially at the fault position. According to the

94°16'0"E 94°17'0"E 94°18'0"E obvious scarp away from the fault position, the displacement was estimated to be ~49.3 +1.7/–1.8 m (Fig. S3). The T2′ terrace on the eastern bank was also eroded near the gully bank, and the measured 39°25'0"N result was ~18.7 +1.3/–1.2 m using the marker at the back of T2′ terrace (Fig. S4). The processes of Supplementary data for displacement measurement are given in the Sup- Late Quaternary Slip Rate of the Aksay Segment and its fast decreasing gradient along the Altyn Tagh Fault plemental Material. At the Jiaerwuzongcun site Jinrui Liu1,2, Zhikun Ren1,2*, Wenjun Zheng3,4, Wei Min2, Zhigang Li3,4, Gang Zheng5 (Fig. 7), we finally got four offset values that could 1State Key Laboratory of Earthquake Dynamics, Institute of Geology, China

Earthquake Administration, Beijing, China be used in fault slip rate determination (Table 2).

2Key Laboratory of Active Tectonics and Volcano, Institute of Geology, China Earthquake Administration, Beijing, China 0 300 600 39°24'30"N 3Guangdong Provincial Key Lab of Geodynamics and Geohazards, School of Earth m Sciences and Engineering, Sun Yat-sen University, Guangzhou, China Yandantu Site

4Southern Marine Science and Engineering Guangdong Laboratory (Zhuhai), Zhuhai, China The Yandantu site was the easternmost site in this 5GNSS Research Center, Wuhan University, Wuhan, China study, lying ~15 km east of the Jiaerwuzongcun site. T2 Changcaogou 94°16'0"E 94°17'0"E 94°18'0"E For the displacement measurement at Yandantu, we Aksay old county also obtained high-resolution DEM data by photo- T3 T2 AKS01~06 grammetry technology (Fig. 8). The T4 terrace, with Contents of this file T2 Figures S1 to S5 Tb obvious offset, is well preserved on the northern side Tc T4 Ta 39°25'0"N Introduction of the fault. Several gullies have developed on the This supporting information provides the processes of displacements measurement at AKS16 AKS15 the Jiaerwuzongcun site and the Yandantu site with the LaDiCaoz_v2 code AKS07 T4 terrace, while it is partly eroded on the southern AKS14 T1 AKS08~13 T4 side of the fault. The measured displacement of ~36.2 T2 ± 1.5 m is the lower limit of T4 (Fig. S5). Legend Altyn Tagh fault 1 Supplemental Material. Processes of displacements Stream measurement at the Jiaerwuzongcun site and the Yan- Sample site T4 T3 Dating Results dantu site with the LaDiCaoz_v2 code. Please visit 0 300 600 39°24'30"N https://doi.org/10.1130/GEOS.S.12885419 to access Bedrock hill T2 T1 m the supplemental material, and contact editing@ Along the Aksay fault, most of the terraces are geosociety.org with any questions. Figure 5. High-resolution satellite image and the corresponding geomorphic interpretation of the Old Aksay site. covered by eolian loess. The eolian loess has a

Changcaogou

1360±110m

Figure 6. Displacement recovery on different levels of terraces at the Old Aksay site. (A) Original geomorphic interpretation image (as in Fig. 5). (B) Displacement recovery of Ta with an offset of ~1360 ± 110 m. (C) Displace- ment recovery of Tb with an offset of Changcaogou ~1790 ± 130 m. (D) Displacement recovery of Tc with an offset of ~2290 ± 220 m.

1790±130m

Changcaogou

2290±220m

39°27'20"N 39°27'30"N

T4 T3 2770m T3’ T2 T2‘ 2740m T3’ 150 300 0 2710m Figure 7. Hillshade map of the high-resolu- m C River Bed C’ 94°29'0"E 94°29'30"E 250m 750m 1250m tion digital elevation model (DEM) and the corresponding geomorphic interpretation 39°27'20"N 39°27'30"N of Jiaerwuzongcun site overlain by 2 m C’ contour lines. T1 C T2’ T3’ T4 LED17-53 LED17-56 LED17-55 LED17-50 LED17-58 T3’ T2 Legend T3 T2’ Altyn Tagh fault T4 T3 Stream T3 T3’ T2 LED17-49 Sample site 0 150 300 Bedrock hill T2’ T1 m LED17-57 94°29'30"E 94°30'0"E

fine grain size, which could ensure the reset of the 5–8 ka, and 8–10 ka for the T1–T4 terraces, respec- West of Aksay Site OSL signal in quartz due to the exposure to light tively. Among them, the ages of the T2 terrace and for enough time during its transport by the wind. T3 terrace are close to the ages of the correspond- Three OSL samples were collected at the West Such eolian loess samples are suitable for OSL dating secondary terraces, which are consistent with of Aksay site on T3 and T4. The age of T3 is ca. 7.6 ing (Küster et al., 2006). The terraces were formed the results interpreted from satellite images. ± 0.7 ka, from a depth of ~60 cm under the ground by incision, fluvial erosion, and uplift due to the motions of active faults. The terraces were uplifted above the fluvial channel and provided ideal flat TABLE 2. RESLTS OF OFFSET MEASREMENTS ON THE ASAY SEGMENT surfaces for the deposition of eolian loess. Hence, Site Latitude Longitude Terrace Displacement the age at the bottom of the loess represents the N E m lower limit to the age of the terrace. To constrain West of Aksay 3.386 4.1656 T4/T3 8 4 the ages of geomorphic markers, we collected 13 OSL samples and 16 situ cosmogenic nuclide 3.4557 4.41 T2′/T1 18.7 1.3/–1.2 (10Be) samples. Jiaeruzongcun 3.4562 4.443 T2/T2′ 25.4 1.6/–1.4 Through a comprehensive analysis of regional 3.4555 4.40 T3′/T2 46.8 1.2/–1.3 3.4548 4.482 T3/T3′ 4.3 1.7/–1.8 landforms mainly covered by loess along the Yandantu 3.44 4.6642 T4/T3 36.2 1.5 Aksay segment, four levels of terraces were identified, accompanied by some secondary terraces. 3.411 4.2853 Ta 1360 110 The results of OSL age dating in different research Old Aksay 3.4116 4.2810 Tb 170 130 3.4102 4.2753 Tc 220 220 sites are consistent, with ages of ca. 1–2 ka, 2–4 ka,

surface. We infer that the offset terrace riser has been seriously eroded, so there is great uncertainty in displacement, which is far less than the accumulated offset since the formation of the terrace. This age was not used in the calculation of the slip rate. The ages of the other two samples from T4 are ca. 8.3 ± 1.1 ka and ca. 9.0 ± 0.6 ka, respectively (Table 3). The oldest age of the geomorphic

94°40'0"E marker is interpreted to be closer to the formation of the geomorphic surface, so the age of T4 is ca. 9.0 ± 0.6 ka.

Old Aksay Site

39°29'40"N Due to the high terrain and long retention time T4 2525m of these old landforms, the remaining terrace sur- 2515m faces are mainly covered by gravels, so the age 2505m T3 constraint depends on the in situ cosmogenic 94°39'40"E 2495m T2 nuclide (10Be) dating method at the Old Aksay site. River Bed 2485m For the Tc terrace, the age was determined by age 0 100 200 D D’ m 50m 150m 250m 350m 450m fitting based on the cosmic nuclide concentration in different depth layers in the profile, using the calculation tool developed by Hidy et al. (2010). For the D’ Aksay area, the offset terraces are almost uncov- ered by cosmic-ray irradiation, so the shading effect was assumed to be 1. The arid or semiarid climate T4 conditions lead to a very small degree of erosion, so the erosion rate can be 0. The fitting age of Tc

94°40'0"E LED17-62 was 226.7 +21.9/–34.6 ka, and the inherited concentration was ~12.99 × 104 atoms·g-1 (Fig. 9). For the LED17-61 calculation of the exposure ages of the Ta and Tb D terraces, the inherited concentration obtained from 10 26 T3 Tc needs to be eliminated based on the Be- Al T2 exposure age calculator, version 2.3 (http://hess .ess.washington.edu/). The exposure age of Ta is ca.

39°29'40"N 125.7 ± 11.3 ka, and that of Tb is ca. 172.9 ± 15.7 ka. The formation ages of these three old terraces have Legend a good sequence. Altyn Tagh fault T2 T3 Stream 94°39'40"E Sample site Jiaerwuzongcun Site Bedrock hill T4 T2 T1 39°29'30"N 0 100 200 T3 T1 m The loess covering the terraces was well collected and exposed thoroughly, which is very suitable for Figure 8. Hillshade map of the high-resolution digital elevation model (DEM) and the OSL dating (Fig. 10A). On the western bank of Jia- corresponding geomorphic interpretation of Yandantu site overlain by 2 m contour lines. erwuzongcun, T2′ is only developed along a small

TABLE 3. RESLTS OF OPTICALLY STIMLATED LMINESCENCE part upstream of the fault, but it is well preserved OSL DATING ON THE ASAY SEGMENT in the downstream area. We dug a sampling sec- Site Laboratory Terrace Depth Ambient dose Euivalent dose Age tion of 1 m depth in the upstream T2′ terrace, which number m Gy/k.y. Gy ka was covered by loess with a thickness of 0.93 m, LED17‑63 T3 0.6 3.7 0.1 27.8 1.8 7.6 0.7 and some grassroots were mixed in the top of the West of Aksay LED17‑171 T4 1.5 3.4 0.1 30.8 1.0 .0 0.6 loess. The OSL samples collected at the position of LED17‑172 T4 1.2 3.6 0.1 2.8 3.6 8.3 1.1 0.78 m depth yielded an age of ca. 2.8 ± 0.4 ka. The LED17‑4 T2′ 0.78 3.56 0.13 . 1.2 2.8 0.4 thickness of the loess on T2 is close to that of T2′ on LED17‑50 T2 0.78 3.26 0.12 10.3 0.7 3.2 0.3 the western bank, indicating that its age is close to LED17‑53 T3′ 1.32 3.41 0.12 20.5 1.3 6.0 0.6 that of T2′, with an age of ca. 3.2 ± 0.3 ka. The loess LED17‑54 T3′ 0. 3.63 0.13 17.6 1.1 4. 0.4 Jiaeruzongcun thickness of T3′ was 1.45 m. Two samples were col- LED17‑55 T3 3.47 3.41 0.13 22.5 2.7 6.6 0. LED17‑56 T4 5.18 3.20 0.12 28.7 3.1 .0 1.2 lected at 0.9 m and 1.32 m depth, which were dated LED17‑57 T2 1.23 3.25 0.12 10.5 1.3 3.2 0.5 at ca. 4.9 ± 0.4 ka and ca. 6.0 ± 0.6 ka (Fig. 10C), LED17‑58 T3′ 1.37 3.33 0.13 1.8 1.8 6.0 0.7 respectively. The thickness of the loess deposited LED17‑61 T2 0.45 3.23 0.12 6.7 1.5 2.1 0.5 on T3 can be up to 4.42 m, which is much thicker Yandantu LED17‑62 T4 0.58 3.34 0.12 24.1 1.2 7.2 0.6 than that of T3′. Age samples were collected at the location of 3.47 m depth, with a measured age of ca. 6.6 ± 0.9 ka. T4 terrace is older and eroded more seriously. It is estimated that the loess cover thickness can be up to 5.26 m. Age samples collected at 0 a depth of 5.18 m yielded an age of ca. 9.0 ± 1.2 ka. AKS-13 Mean inheritance On the eastern bank of Jiaerwuzongcun, T2 is cov- 12.99 x104atoms g-1 ered by loess with a thickness of 1.31 m, and the age of samples collected at 1.23 m depth is 3.2 ± 0.5 ka AKS-12 (Fig. 10B). The measured age of T3′ is relatively old, −50 ca. 6.0 ± 0.7 ka. The ages of T2 and T3′ on the eastern AKS-11 bank are consistent with those on the western bank, which further proves the reliability of the ages.

−100 ) m c

( AKS-10

Yandantu Site h p t e

D +21.9 Although the Tertiary red bedrock has appeared Best-fit model age 226.7 -34.6 ka −150 0.04 AKS-09 in Yandantu, the surface of T4 on the north side of the fault is covered by loess, with a thickness of

y 0.03 t

l i 0.58 m. We collected the OSL samples at the lowest b i

a 0.02

b layer of the loess, with an age of ca. 7.2 ± 0.6 ka. r o

P We also collected an age sample at T2, but the T2 is −200 0.01 2σ error AKS-08 covered by thin loess. The age obtained was only 140 160 180 200 220 240 2.1 ± 0.5 ka, so it was not used in the calculation Age (ka) of the slip rate.

−250 0 1 2 3 4 5 6 7 Concentration (106atom g-1) Late Pleistocene Slip Rate Figure 9. Sampling photo of terrace Tc and depth profile fitting result. Blue dots with associated error bars mark 10Be concentrations. Solid blue line shows the best-fit solution within the green space that The formation age of T4 is ca. 9.0 ± 0.6 ka at the defines the full range of 100,000 Monte Carlo solutions. West of Aksay site, and the maximum displacement

± 15.7 ka, and 226.7 +21.9/–34.6 ka, respectively. When calculating the slip rate, we considered the T3 S errors of offset and age comprehensively and got a slip rate of 10.2 +1.2/–1.1 mm/yr since 220 ka at the T3’ Old Aksay site (Fig. 11). Meanwhile, we also summa- T3’ rized the results of previous studies (Table 4). Using T2’ T2’ the Monte Carlo analysis method, we obtained a slip rate of 9.9 ± 1.1 mm/yr since 15 ka at the Old Aksay site and Huermo Bulak site (Fig. 12; Table 4), which shows that the slip rate of the Altyn Tagh fault was stable over different time scales. At the Jiaerwuzongcun site, the displacements of the four terraces are 18.7 +1.3/–1.2 m, 25.4 +1.6/– 5 m 1.4 m, 46.8 +1.2/–1.3 m, and 49.3+1.7/–1.8 m, and the age sequences are 2.8 ± 0.4 ka, 3.2 ± 0.3 ka, 6.0 ± 0.6 ka, and 6.6 ± 0.9 ka. We used the same 0 method—the Monte Carlo analysis method—to fit 0 the slip rate, and the result is 7.5 +1.2/–0.6 mm/yr since 7 ka (Fig. 13). At the Yandantu site, combining the measured displacement of 36.2 ± 1 m and the age of 7.2 ± 0.6 ka, the calculated slip rate is 5.1 ± 0.8 mm/yr. Because T4 is eroded to some extent, )

) LED17-54

m the result is the lower limit of the slip rate. m

c 4.9±0.4ka c ( (

h 90 h t t p p e e D D LED17-53 ■■ DISCUSSION 6.0±0.6ka LED17-57 132 3.2±0.5ka 145 Fault Slip Rate Uncertainties 123 131 Reliability of Slip Rate 145 180 There are many factors that affect the results Loess Gravels Sampling Sites of the fault slip rate. In practice, the measurement error in displacement, the error in dating, and the Figure 10. Field photograph of the east bank of Jiaerwuzongcun site and sampling photos. (A) Geomorphological photo shows multilevel river terraces at the Jiaerwuzongcun site. (B) Field photograph showing the sampling and methods used to match the two parameters will corresponding sketch of T2. (C) Field photograph showing the sampling and corresponding sketch of T3′. affect the final results. In this study, displacements were measured based on the high-resolution DEM collected by photogrammetry technology, which of the gully is ~98 ± 4 m; these were are used to evolution of the Altyn Tagh fault averaged over the was also validated during the field survey. High- calculate the slip rate of 10.9 ± 1.2 mm/yr. Because late Pleistocene, the fault has not reversed its slip resolution DEMs are helpful for us to classify the the formation of the gully is younger than the age sense. Based on these assumptions, the data show geomorphic surfaces more reliably and reduce of T4, the obtained result is the lower limit of the that the reversal of fault motion should not be used the offset errors at the same time. The semiauto- slip rate at this location. in the final slip rate calculation. Based on the above matic measurement code LaDiCaoz_v2 was used In this study, for multilevel offset geomorphic preliminary work, we obtained the displacement in displacement measurements, which could also markers, we calculated the fault slip rate using the and age sequences of Ta, Tb, and Tc at the Old partially reduce the error of human measurement. Monte Carlo analysis method proposed by Gold Aksay site, which are ~1360 ± 150 m, ~1790 ± 150 m, The ages of the geomorphic surfaces were con- and Cowgill (2011). Assuming consistent geological and ~2290 ± 200 m, and ca. 125.7 ± 11.3 ka, 172.9 strained by OSL and 10Be dating methods, combined

2500 2500 A. D-t data Tc B. 1000 slip histories ) Tb ) m 2000 m 2000 ( ( t Ta 1500 1500 e m n e m n t a c 1000 a c 1000 D i s p l 500 D i s p l 500

0 0 0 0.5 1 1.5 2 2.5 0 0.5 1 1.5 2 2.5 5 5 Age (year) x 10 Age (year) x 10 Figure 11. Average slip rate at the Old Aksay site based on the Monte Carlo

2500 ) 2500 C. Uniform slip rate solution D. Slip history solution analysis method since 220 ka. (A) Dis- m

( placement-time (D-t) envelopes of Old 2000 2000 Aksay site. (B) 1000 slip history paths through the synthetic data set. (C) Av- 1500 e m n t 1500 erage slip rate of Old Aksay site. (D) Slip a c history of the Old Aksay site from the 1000 1000 Monte Carlo method. D i s p l median rate: 10.16 mm/yr Displacement (m) 500 (+0.65/-0.61 mm/yr @ 68% confidence) 500 (+1.26/-1.15 mm/yr @ 95% confidence) 0 0 0 0.5 1 1.5 2 2.5 0 0.5 1 1.5 2 2.5 Age (year) 5 Age (year) 5 x 10 x 10 linear regression (median) median slip history D-t envelope trimmed portion of slip history path solid line: average slip rate with 68% (blue) & 95% (red) D-t envelope &point dashed line: 68.27% bound confidence envelopes

with the depth profile method. For geomorphic and dating results of multilevel geomorphic mark- comparison with the results of ca. 15 ka obtained surfaces that have been eroded, the age of the geo- ers were used to constrain the slip rates at the Old by previous studies at this site (Zhang et al., 2007; morphic surface obtained by depth profile fitting is Aksay site and the Jiaerwuzongcun site. Chen et al., 2012, 2013), the slip rate of Altyn Tagh more reliable. The inherited concentration obtained fault was determined to be stable over different by using the depth profile approach is more rep- time scales since 220 ka. Wu et al. (2013) recon- resentative of the inherited concentration of the Consistency with the Results of Regional structed the mid-Miocene strike-slip history of regional geomorphic surface. Slip Rate the middle segment of the Altyn Tagh fault and Previously, for calculation of slip rate, the concluded that the Altyn Tagh fault had a stage of results mostly depended on the offset and age of The slip rate since 220 ka was determined to structural adjustment in the Miocene (ca. 15 Ma), a single gully or terrace. However, due to the influ- be 10.2 +1.2/–1.1 mm/yr at the Old Aksay site. By which was also the time of initiation of large-scale ence of special terrain and human factors, the slip rate of a single point may have great uncertainty. The Monte Carlo analysis method was introduced TABLE 4. PREIOS RESLTS AT THE OLD ASAY SITE to comprehensively consider the errors in the two Site Terrace Displacement Age Reference parameters. The method assumes consistent geo- m yr B.P. logical evolution of the Altyn Tagh fault averaged Old Aksay T3/T2′ 135 10 13,00 00 Mriau et al. 2005 over the late Pleistocene; that is to say, the offsets T1/T0 24 2 2080 100 Huermo Bulak Zhang et al. 2007 of the younger terraces should not exceed those T2/T1 36 10 4120 210 of the older terraces, and the ages should follow a T2/T1 78 5 5800 200 similar rule, which can narrow the data range and Chen et al. 2012 Old Aksay T1/T0 20 2 1800 100 help to reduce the uncertainty in the calculation of T2/T1 70 4 630 270 Chen et al. 2013 average slip rate. Therefore, in this study, offsets

150 150 GPS observation rates (Zhang et al., 2007; Zheng A. D-t data B. 1000 slip histories et al., 2017).

100 100 Spatial Distributions of Slip Rates on the Eastern Altyn Tagh Fault

By analyzing the spatial variation in the slip 50 50 rate of the Aksay segment, a slip rate reduction Displacement (m) Displacement (m) has occurred at the middle position of the Aksay segment, from ~10 ± 1 mm/yr at the Old Aksay site to 7.5 +1.2/–0.6 mm/yr at the Jiaerwuzongcun site to 0 0 0 5000 10000 15000 0 5000 10000 15000 5.1 ± 0.8 mm/yr near the Yandantu site. We also syn- Age (yr BP) Age (yr BP) thesized geological rates (Kang et al., 2019; Zhang, 150 150 C. Uniform slip rate solution D. Slip history solution 2016; Zhang et al., 2007), GPS rates (Li et al., 2018; Zhang et al., 2007; Zheng et al., 2017), and InSAR rates (Liu et al., 2018a) to determine the spatial distributions of slip rate on the eastern Altyn Tagh 100 100 fault (Fig. 14). From the Aksay segment to the easternmost extent, the slip rate of the Altyn Tagh fault decreases faster in the Aksay-Subei segment, from ~10 ± 1 mm/yr to 5.1 ± 0.8 mm/yr, which takes up 50 50 ~50% of the slip rate within a distance of ~50 km, Displacement (m) Displacement (m) median rate: 9.9 mm/yr with a slip gradient of ~9.8 mm/yr/100 km. However, (+0.59/-0.60 mm/yr @ 68% confidence) the slip rate decreases slowly from Subei to the (+1.11/-1.10 mm/yr @ 95% confidence) east, where the other 50% of the slip rate decreases 0 0 0 5000 10000 15000 0 5000 10000 15000 to almost zero over a distance of ~200 km, with a Age (yr BP) Age (yr BP) much lower gradient of 2.5 mm/yr/100 km. The slip linear regression (median) median slip history rate gradient at the Aksay segment is ~4 times that D-t envelope trimmed portion of slip history path solid line: average slip rate with 68% (blue) & 95% (red) D-t envelope &point dashed line: 68.27% bound confidence envelopes of Subei to the eastward termination of the Altyn Tagh fault (Fig. 14). Our results indicate that the slip Figure 12. Average slip rate at the Old Aksay site based on the Monte Carlo analysis method since 15 ka. (A) Displace- rate decrease along the Altyn Tagh fault is not uni- ment-time (D-t) envelopes of Old Aksay site. (B) 1000 slip history paths through the synthetic data set. (C) Average slip rate of Old Aksay site. (D) Slip history of the Old Aksay site from the Monte Carlo method. form at its termination segment, and the gradient can be also variable. We think that there are several possible causes sinistral slip on the Altyn Tagh fault. After that, the Zhang et al., 2007; He et al., 2013). Further east- for the phenomenon of fast-decaying slip rates present geometry and kinematics patterns finally ward, i.e., ~95°E, the obtained velocity was 4.5 associated with the Aksay segment in the Altyn formed. Our results of the study on the slip rate ± 0.9 mm/yr, which shows a good agreement Tagh fault, such as the lithological differences at different time scales in the Old Aksay site are with the GPS profile velocity of 3.9 ± 1.8 mm/yr along the fault zone and/or the heterogeneity. Li consistent with those of Wu et al. (2013). of Zheng et al. (2017) at 96°E. Combined with the et al. (2018) estimated the fault-locking depth using Zheng et al. (2017) analyzed the rates of major results of this study, four research sites in the Aksay the screw dislocation model along the Altyn Tagh faults throughout mainland China by using the GPS segment are all located in the range of 94°E–95°E, fault, which showed a heterogeneous distribution. observation results from 1991–2015. The slip rate and the slip rate results are also in the range of The Aksay segment has a deeper fault-locking depth of the main segment of the Altyn Tagh fault was 8–11 mm/yr and 4.5 ± 0.9 mm/yr. The slip rates of 18.9 ± 8.4 km compared to the western and east- ~8–9 mm/yr, which is basically consistent with the gradually decrease starting at the Aksay segment ern segments of the Altyn Tagh fault. The eastward previous GPS observation results of 8–11 mm/yr at from west to east, which has a good consistency rapid decay of the slip rate may be caused by the 84°E–94°E (Bendick et al., 2000; Shen et al., 2001; with the existing eastward-decreasing trend from heterogeneity in the physical properties of the deep

A. D-t data B. 1000 slip histories block extrusion model. In addition, the syngenetic 50 T3 50 crustal thickening and strike-slip faults, which span T3’ almost the whole Qaidam Basin and Qilian Shan, 40 40 confirm the wide-ranging deformation to the south of the Altyn Tagh fault. The decreased rates from

30 30 94°E to 97.5°E are accommodated by the thrusts T2 and crustal shortening across the Qilian Shan, which support the “continuous deformation model.” 20 20 T2’ Therefore, the Altyn Tagh fault is not the main fault Displacement (m) Displacement (m) allowing the northeastward extrusion of the Tibetan 10 10 Plateau. Crustal shortening and thrust faults occur in almost the whole extent of the Qilian Shan, which 0 0 0 1000 2000 3000 4000 5000 6000 7000 8000 0 1000 2000 3000 4000 5000 6000 7000 8000 prove the widespread deformation to the south of Age (yr BP) Age (yr BP) Altyn Tagh fault (Meyer et al., 1998). The slip rate of C. Uniform slip rate solution D. Slip history solution Altyn Tagh fault decreases sharply from the Aksay 50 50 segment to zero gradually eastward, accompanied by the uplift of the Danghe Nan Shan and Qilian 40 40 Shan. Therefore, the high decay slip rate on the Aksay segment is related to the uplift of Danghe 30 30 Nan Shan, which indicates that the extrusion of the Tibetan Plateau has been gradually blocked at this 20 20 position, resulting in the uplift of these mountain systems. The slip pattern of eastward-decreasing Displacement (m) Displacement (m) 10 median rate: 7.5 mm/yr 10 values reflects the limited nature of the extrusion (+0.63/-0.40 mm/yr @ 68% confidence) of the northeastern Tibetan Plateau. The large-scale (+1.12/-0.66 mm/yr @ 95% confidence) 0 0 strike-slip movement is mainly confined into the 0 1000 2000 3000 4000 5000 6000 7000 8000 0 1000 2000 3000 4000 5000 6000 7000 8000 interior of the plateau, which is manifested by Age (yr BP) Age (yr BP) crustal thickening and compressional imbricate linear regression (median) median slip history trimmed portion of D-t envelope slip history path solid line: average slip rate with 68% (blue) & 95% (red) fault fans of the Qilian Shan (Zhang et al., 2007). D-t envelope &point dashed line: 68.27% bound confidence envelopes

Figure 13. Average slip rate at Jiaerwuzongcun site based on the Monte Carlo analysis method. (A) Displacement-time (D-t) envelopes of Jiaerwuzongcun site. (B) 1000 slip history paths through the synthetic data set. (C) Average slip rate Role of the Aksay Segment in the Strain of Jiaerwuzongcun site. (D) Slip history of the Jiaerwuzongcun site from the Monte Carlo method. Partitioning of the Eastern Altyn Tagh Fault

The spatial distribution of slip rate along a crust (Liu et al., 2018a). The rapid decrease of slip The recent research results with more viable particular strike-slip fault and the transformation rate means there is a slip rate deficit, which is also analysis, consistent with GPS and InSAR mea- or dissipation of strike-slip displacement near its an important sign of potential seismic hazard. The surements, report a relatively uniform slip rate of termination are of great significance, not only for Altyn Tagh fault zone near Aksay County is located ~10 mm/yr west of 94°E and a decease eastward continental deformation, but also for the behavior at the junction of Cenozoic strata in the north and to only ~0–1 mm/yr near 97.5°E. If “block extru- of strike-slip faults (Zheng et al., 2013). Zhang et al. Sinian metamorphic rocks in the south, with a sion” were occurring, the Altyn Tagh fault would (2007) found that the Altyn Tagh fault did not play fracture width of 1000 m. The rocks on both sides represent a transform fault accommodating the the role of transition fault in crustal strike-slip extru- of the fault are strongly deformed and broken to movement between rigid blocks. Its slip rate sion in the collision between the Indian plate and form a wide mylonitized and fractured zone (Chi- should be fast enough and not decrease to allow the Eurasian plate, but only acted to redistribute nese State Bureau of Seismology, 1992). There is the material on the south side of the fault to extrude crustal thickening. On the basis of geological slip no obvious evidence that the rapid decrease of slip eastward, so the low slip rate and the pattern of rates, Zhang et al. (2007) and Zheng et al. (2013) rate is related to the lithology along the fault zone. eastward decrease are not compatible with the observed that the shortening rate of the whole

Dang River Niuyagou Old Aksay Yandantu

Subei Changma Ms=7.6 39°N 12/25/1932 40°N

50 km

98°E 94°E 96°E

12 Main Segment Aksay Segment Subei to east Segment This study Zhang et al., 2007 Zhang, 2016 10 Kang et al., 2019 y r ) / Slip rate:~10 mm/yr Zheng et al., 2017 8 slip rate gradient Liu et al., 2018a ~9.8 mm/yr/100 km ( m Li et al., 2018 6 a t e R 4 i p

S l slip rate gradient ~2.5 mm/yr/100 km 2

0 94°E 95°E 96°E 97°E Longitude, in degree, along Altyn Tagh Fault (°)

Figure 14. Spatial distribution of slip rates on the eastern Altyn Tagh fault. The big black arrow shows that the Aksay segment has a slip gradient of ~9.8 mm/yr/100 km, and the small black arrow shows that Subei to eastern segment has a slip gradient of ~2.5 mm/yr/100 km.

Qilian Shan is 7.5 ± 2.0 mm/yr. Combined with the thickening (Meyer et al., 1998; Xu et al., 2005). rate of the Altyn Tagh fault is allocated, the eastward shortening amount of the Qilian Shan obtained by Xu et al. (2005) and Shao (2010) inferred that the extent of the Houtang fault will be transformed into Zhang et al. (2014), it is considered that the total decrease of the slip rate mainly starts at the posi- the uplift of mountains and shortening of Cenozoic shortening amount parallel to the Altyn Tagh fault tion of Subei (Fig. 14), and the decrease is mainly basins and thrust faults. Therefore, the uplift of the of the Qilian Shan in the western section is equiva- distributed by shortening of the Danghe Nan Shan, Danghe Nan Shan, the crustal shortening of Ceno- lent to the shortening on thrust faults (Zhang et al., with secondary slip vectors in the direction of N70– zoic basins, and the thrust faults have absorbed 2014; Zheng et al., 2013). For the end of the Altyn 80°E and N280–290°E, which were obtained through the main reduction in the slip rate, which was Tagh fault, Zheng et al. (2013) and Zhang (2016) reanalysis of the shortening rate of Danghe Nan ~3.9–4.1 mm/yr. This absorption is very large in the analyzed the thrust fault and Cenozoic fold in Jiuxi Shan, with results of 1.0–1.2 mm/yr. Based on the distribution of slip rate in the whole eastern Altyn basin at the western end of Hexi Corridor (Fig. 1) results of previous studies on the distribution of Tagh fault, which shows that the Aksay segment and concluded that the shortening rate of thrust slip rates, we find that most of the previous studies plays a crucial role in strain partitioning in the east- faults on the boundary and interior of the basin is focused on the position of Subei (Shao, 2010), while ern part of Altyn Tagh fault (Fig. 15). 0.9–1.43 mm/yr in the direction parallel to the Altyn the reduction of slip rate in the Aksay segment The thrust faults on the western side of Danghe Tagh fault, while the total shortening rate of Ceno- was not considered. According to the results of Nan Shan may be growing as an oblique branch zoic folds is 0.5–1.0 mm/yr in the direction parallel Shao (2010) and Van der Woerd et al. (2001) on the fault from the Altyn Tagh fault. The geometry and to the Altyn Tagh fault. The sum of the shortening shortening rate of the western Danghe Nan Shan, kinematic characteristics of the fault indicate that rates is 1.4–2.4 mm/yr, consistent with 1–2 mm/yr the shortening rate parallel to the direction of the the thrust fault and crustal shortening of Danghe at the end of the Altyn Tagh fault. Altyn Tagh fault is only 0.8–1.0 mm/yr. At the same Nan Shan may be related to the strike-slip tran- The easternmost part of the Altyn Tagh fault, time, Shao et al. (2016) studied the slip rate of the sition of the Altyn Tagh fault (Meyer et al., 1998; including the NW-trending Danghe Nan Shan, southern edge of Danghe Nan Shan (Houtang fault; Van der Woerd et al., 2001; Yin et al., 2002). Based Daxue Shan, and Qilian Shan ranges, is absorb- Fig. 2) and obtained a slip rate of 2.7 ± 0.9 mm/yr. It on the vertical uplift and carbon 14 dating of allu- ing the sinistral displacement through crustal is considered that even after a part of the strike-slip vial fans and terraces, Van der Woerd et al. (2001)

estimated the uplift rate of the Danghe Nan Shan have started at least 4 ± 2 m.y. ago. Yu et al. (2019) uplift of the Danghe Nan Shan is closely related to thrust at 4–7 mm/yr. Considering a thrust ramp dip studied the thermochronological data of the ele- the strike-slip transition of the Altyn Tagh fault, so of 45°, this result can provide a shortening rate vation transects of the western Danghe Nan Shan the age may represent the origin of the transition of 5.5 ± 1.5 mm/yr. Assuming that the shortening and revealed that there has been rapid exhumation from strike-slip movement on Altyn Tagh fault to rate is stable in time, minimum cumulative short- since the middle Miocene (ca. 15 Ma). Generally crustal shortening of the Danghe Nan Shan. At the ening of 10–12 km can imply that thrusting may considered as a branch of the Altyn Tagh fault, the same time, previous studies have shown that the Danghe Nanshan has been active at least since the

~10 mm/yr middle Miocene. Our results also suggest that at 10 Legend the eastern termination of the Altyn Tagh fault, the 8 Altyn Tagh Fault strain is absorbed by shortening of the crust within 6 Thrust Faults the Tibetan Plateau, forming mountains and basins, 4 Rivers consistent with the continuous deformation model 2 Reverse rate (mm/yr) (Molnar et al., 1987; Zhang et al., 2004). This could Slip rate (mm/yr) V 0 ~5 mm/yr H Slip rate (mm/yr) also be the reason why the slip rate of the Altyn

Aksay Segment S Shortening rate (mm/yr) Tagh fault rapidly decreases in the Aksay segment. H: ~10 The movement and deformation of the Altyn Dang River Subei to east Segment Tagh fault are mainly caused by NE-SW regional H: 2.7±0.9 H: ~5 Dunhuang Anxi compression induced by plate motion. Wu et al. H: ~4.5 Shule River 0 mm/yr H: ~3 (2013) concluded that the Altyn Tagh fault had a P Subei H: 1.4±0.4 stage of structural adjustment in the mid-Miocene, Danghe Nan Shan Shibaocheng H: 0.67~1.2 which may also have been the time of initiation

S: ~1.2 V: ~0.8 V: ~0.1 Daxue ChangmaShan of large-scale sinistral slip on the Altyn Tagh fault. N We mainly targeted the activity of the Altyn Tagh

S: 1~2 S: <1 fault since the late Pleistocene. Field investigations P’ showed that the Altyn Tagh fault inclines southward

Qilian Shan Jiayuguan with a dip angle of 50°–70°. The main movement mode is strike slip, and no obvious vertical compo- nent was found. In addition, there are many thrust 6000 Danghe Nan Shan faults parallel to the Altyn Tagh fault, south of the fault. Even though the Altyn Tagh fault, as a pure strike-slip fault since the Miocene, will only par- 4000 tially contribute to the uplift along the fault, the Daxue Shan thrust faults to the south of Altyn Tagh fault may Elevation (m) also be responsible for the uplift on the south side of the fault. 2000 P 0 Qilian Shan 20 Seismic Potential of the Aksay Segment in 60 the Strain Partitioning of the Eastern Altyn Distance (km)100 Tagh Fault

140 P’ The fault slip rate, which represents the cumula- 180 tive rate of strain, is of great significance for seismic hazard assessment. Such a high decay rate in the Figure 15. Role of Aksay segment in the strain partitioning on the eastern Altyn Tagh fault. The shortening rate parallel slip rate probably indicates that the movement of to the direction of the Altyn Tagh fault was only 0.8–1.0 mm/yr, and uplift of the Danghe Nan Shan, crustal shortening of Cenozoic basins, and thrust faults absorbed the main reduction of the slip rate, which was ~3.9–4.1 mm/yr. This absorp- the Altyn Tagh fault has been sharply decelerated tion shows that the Aksay segment plays a crucial role in the strain partitioning on the eastern part of Altyn Tagh fault. in the Aksay segment. A part of the slip is absorbed

and accommodated by adjacent NW-trending thrust of the West of Aksay segment is 10.9 ± 1.2 mm/yr Avouac, J.P., and Tapponnier, P., 1993, Kinematic model of active deformation in central Asia: Geophysical Research Letters, structures, but the NE extrusion pressure on the since 9 ka. At the Old Aksay site, the slip rate is v. 20, p. 895–898. Altyn Tagh fault still exists, so there is still a high 10.2 +1.2/–1.1 mm/yr since 220 ka, consistent with Bendick, R., Bilham, R., Freymueller, J., Larson, K., and Yin, G., strain accumulation in the Aksay segment. Some a previous study of the Old Aksay site since 15 ka, 2000, Geodetic evidence for a low slip rate in the Altyn Tagh studies have shown that there are many obvious showing that the slip rate of the Altyn Tagh fault fault system: Nature, v. 404, no. 6773, p. 69, https://doi.org /10.1038/35003555. seismic surface rupture zones along with the east- is stable over different time scales. The slip rates Chen, Y., Li, S.H., and Li, B., 2012, Slip rate of the Aksay segment ern Altyn Tagh fault (Shao et al., 2018; Xu et al., 2017). at the Jiaerwuzongcun and Yandantu sites are 7.5 of Altyn Tagh fault revealed by OSL dating of river terraces: A relatively complete seismic catalog began in the +1.2/–0.6 mm/yr and 5.1 ± 0.8 mm/yr since 7 ka. The Quaternary Geochronology, v. 10, p. 291–299, https://doi .org /10.1016/j.quageo.2012.04.012. twentieth century, though some earlier earthquakes reason for and mechanism of the spatial change Chen, Y., Li, S.H., Sun, J., and Fu, B., 2013, OSL dating of off- may be missing in the historical seismic catalog. The in slip rates were discussed herein. The slip rate set streams across the Altyn Tagh fault: Channel deflection, most recent paleo-earthquake near the Old Aksay decreases ~5.0 mm/yr within the 50 km Aksay seg- loess deposition and implication for the slip rate: Tectono- physics, v. 594, p. 182–194, https://doi.org/10.1016/j.tecto town occurred 665 ± 40 yr ago, with a recurrence ment, which is the fastest slip gradient along the .2013.04.002. interval of ~600 yr (Xu et al., 2015), so it is believed Altyn Tagh fault. However, the slip rate decreases to China Earthquake Networks Center, National Earthquake Data that the Aksay segment of the Altyn Tagh fault has almost zero from Subei to the eastern termination Center, 2020, http://data.earthquake.cn/gcywfl/index.html not broken for ~700 yr and may be a seismic gap. of the Altyn Tagh fault, with a much lower gradient (last accessed 26 August 2020). Chinese State Bureau of Seismology, 1992, The Altyn Tagh Active Li et al. (2018) calculated the accumulated seismic of 2.5 mm/yr/100 km over a distance of ~200 km. Fault System: Special Publication, Seismological Bureau moments for different segments, which implied The slip rate gradient on the Aksay segment is ~4 of China, Beijing: Beijing, Seismology Publishing House, that the Aksay bend can be considered as a barrier times that of the section from Subei to the eastward 319 p. [in Chinese]. Cowgill, E., 2007, Impact of riser reconstructions on estimation to earthquake rupture and has a high strain rate termination of the Altyn Tagh fault. It is believed that of secular variation in rates of strike-slip faulting: Revisit- concentration, yet it remains unruptured. By calcu- the remaining 3.9–4.1 mm/yr is absorbed mainly by ing the Cherchen River site along the Altyn Tagh fault, NW lating the moment accumulation, Liu et al. (2018a) uplift of the Danghe Nan Shan and crustal short- China: Earth and Planetary Science Letters, v. 254, no. 3–4, p. 239–255, https://doi.org/10.1016/j.epsl.2006.09.015. suggested that there was a slip deficit at the Aksay ening of the Cenozoic basins. Considering a thrust Cowgill, E., Gold, R.D., Chen, X., Wang, X., Arrowsmith, J.R., segment, which is equivalent to an Mw 7.9 earth- ramp dip of 45°, uplift of the Danghe Nan Shan and Southon, J., 2009, Low Quaternary slip rate reconciles quake based on the elapsed time since the latest M may have been ~10–12 km since 4 ± 2 Ma (Van der geodetic and geologic rates along the Altyn Tagh fault, 7 event. Thus, further field investigations will be nec- Woerd et al., 2001). northwestern Tibet: Geology, v. 37, no. 7, p. 647–650, https:// doi.org/10.1130/G25623A.1. essary to determine future cascade seismic ruptures. Cunningham, D., Zhang, J., and Li, Y., 2016, Late Cenozoic trans- In addition, the observed pattern wherein the pressional mountain building directly north of the Altyn most slip rate is absorbed by the reverse and thrust ACKNOWLEDGMENTS Tagh fault in the Sanweishan and Nanjieshan, north Tibetan foreland, China: Tectonophysics, v. 687, p. 111–128, https:// faults along the eastern Altyn Tagh fault suggests This work was funded by the National Natural Science Foun- dation of China (41590861 and 41761144071), the National Key doi.org/10.1016/j.tecto.2016.09.010. that the NW-trending thrust structures may be R&D Program of China (2017YFC1500401), and the National Deng, Q., Zhang, P., Ran, Y., Yang, X., Min, W., and Chu, Q., 2003, assigned larger strain. For example, the 1932 Ms Nonprofit Fundamental Research Grant of China (IGCEA1901, Basic characteristics of active tectonics of China: Science in China, Series D, Earth Sciences, v. 46, no. 4, p. 356–372. 7.6 Changma earthquake produced a N70°W-strik- IGCEA1803). We are grateful to the editors and the two review- ers, Rodolfo Carosi and Maomao Wang, for their constructive Duvall, A.R., Clark, M.K., Kirby, E., Farley, K.A., Craddock, W.H., ing coseismic surface rupture along the Changma comments and suggestions. We thank Jianguo Xiong, Gan Li, C., and Yuan, D.Y., 2013, Low-temperature thermochro- fault (Peltzer and Tapponnier, 1988). In contrast, the Chen, and Ming Ai for fieldwork, and Huili Yang and Yanwu Lv nometry along the Kunlun and Haiyuan faults, NE Tibetan seismic hazard of the Danghe Nan Shan is also for their assistance with the 10Be and optically stimulated lumi- Plateau: Evidence for kinematic change during late-stage nescence dating. 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