Xingwang Liu, Daoyang Yuan, Qi Su, Bo Zhang. Late Quaternary Tectonic Activity and Slip Rates of Active Faults in the Western Hexi Corridor, NW China. Journal of Earth Science, 2020, 31(5): 968-977. doi: 10.1007/s12583-020-1287-9
Citation:
Xingwang Liu, Daoyang Yuan, Qi Su, Bo Zhang. Late Quaternary Tectonic Activity and Slip Rates of Active Faults in the Western Hexi Corridor, NW China. Journal of Earth Science, 2020, 31(5): 968-977. doi: 10.1007/s12583-020-1287-9
Xingwang Liu, Daoyang Yuan, Qi Su, Bo Zhang. Late Quaternary Tectonic Activity and Slip Rates of Active Faults in the Western Hexi Corridor, NW China. Journal of Earth Science, 2020, 31(5): 968-977. doi: 10.1007/s12583-020-1287-9
Citation:
Xingwang Liu, Daoyang Yuan, Qi Su, Bo Zhang. Late Quaternary Tectonic Activity and Slip Rates of Active Faults in the Western Hexi Corridor, NW China. Journal of Earth Science, 2020, 31(5): 968-977. doi: 10.1007/s12583-020-1287-9
The Yinwashan and Xinminpu faults are located in the Jiuxi Basin in the western end of the Hexi Corridor. The determination of their activity and slip rates is of great significance for understanding the eastward extension of the Altyn Tagh fault. Based on geological and geomorphologic field survey, trench excavation, optically stimulated luminescence dating, we define the fault geometry and kinematic properties of the two faults. Based on fault scarps measurement using differential GPS and 10Be surface exposure dating, we determined vertical slip rate of 0.09±0.01 mm/yr for the Yinwashan fault and 0.1±0.02 mm/yr for the Xinminpu fault. Using the dips observed in trenches and natural sections, we estimated horizontal shortening rates of 0.05±0.03 and 0.23±0.06 mm/yr, respectively. No significant strike slip motion is observed on these two faults, and we infer that this region was dominated by horizontal shortening in the Late Quaternary. Although the shortening rate is quite low on each individual fault, together with other faults in this area, these two faults have an essential role in transferring slip from the eastern end of the Altyn Tagh fault and in accommodating the northeastward growth of Tibetan Plateau.
The Qilian Shan situated at the northeastern boundary of the Tibet Plateau (Fig. 1a), has been subjected to rapid uplift and expansion from approximately the Miocene due to the collision between Indian and Eurasian plates (Wang et al., 2016; Yuan et al., 2013; Zheng et al., 2010; George et al., 2001; Tapponnier et al., 2001). Deformation within the Qilian Shan is linked to two large sinistral strike-slip faults, the Altyn Tagh and the Haiyuan faults (Fig. 1a). A large number of active folds and faults have developed to absorb the sinistral displacement between the two faults (Peng et al., 2018; Zhang et al., 2018; Liu et al., 2017; Zheng et al., 2013a, b; Champagnac et al., 2010; Hetzel et al., 2004, 2002; van der Woerd et al., 2001; Meyer et al., 1998). Located at the northwestern boundary of the Qilian Shan, the Altyn Tagh fault has a significant role in transferring and absorbing the convergence between the Indian and Eurasian plates (Xu et al., 2005; Tapponnier et al., 2001). Total slip, slip rate, and eastward growth of the Altyn Tagh fault constitute important parameters for elucidating the expansion and geodynamics of the Tibetan Plateau. However, it remains controversial on the eastern end of the Altyn Tagh fault because of poor understanding of the activity and kinematics of faults in the western Hexi Corridor. Some researchers have suggested that the Altyn Tagh fault runs eastward alongside the southern boundary of the Gobi-Alashan Block as far as the Badain Jaran desert to the 100 km east of Heli Shan (Chen and Xu, 2006; Darby et al., 2005; Tapponnier et al., 2001). Conversely, based on the decreasing slip rate, other studies have reported that the Altyn Tagh fault terminates at the western end of the Hexi Corridor based on the observation of decreasing slip rate (Zheng et al., 2013a; Zhang et al., 2007).
Figure
1.
(a) Topographic map indicating the northeastern margin of the Tibetan Plateau, the maps shows the main active faults and the study locality, including the western Qilian Shan, the west end of the Hexi Corridor, and adjacent areas. Active faults are modified from Zheng et al.(2013a, b) and Yuan et al. (2013), dashed rectangle shows the location of Fig. b. (b) Color shaded digital elevation model of the Jiuxi Basin derived from 90 m Shuttle Radar Topography Mission (SRTM) data. Abbreviations of names of mountains and faults: HS. Hei Shan; WS. Wenshu Shan; WS. Yinwa Shan; BF. Baiyanghe fault; CF. Changma fault. HF. Heishan fault; HDF. Hanxia-Dahuanggou fault; JF. Jiayuguan fault; KF. Kuangtanshan fault; XF. Xinminpu fault; YBF. Yumen-Beidahe fault; YF. Yinwashan fault.
The WNW-ESE-trending Hexi Corridor is a foreland basin between the Qilian Shan to the south and the Gobi-Alashan Block to the north (Fig. 1a). Some WNW-trending uplifted ranges divide the basin into four separated sub-basins. At the western end of the Hexi Corridor, the Jiayuguan-Wenshushan uplift separates the Jiuxi Basin and Jiudong Basin (Xu et al., 2010; Institute of Geology and Lanzhou Institute of Seismology, SSB, 1993) (Fig. 1b). The Jiuxi Basin, the westernmost sub-basin, is bounded by the Altyn Tagh, Kuantanshan and Heishan faults to the north, the Jiayuguan fault to the northeast, and the Hanxia-Dahuanggou and Yumen-Beidahe faults to the southwest (Fig. 1b). A series of southwest dipping active faults skewed by the Altyn Tagh fault have developed in this basin (Fig. 1b). During the past decades, much effort has been dedicated to constraining the slip rates of the faults in the Jiuxi Basin (Liu et al., 2017; Zheng et al., 2013a; Zheng, 2009; Hetzel et al., 2002; Min et al., 2002). Zheng et al. (2013a) summarized the slip rates of the active faults in the western Hexi Corridor in order to discuss the termination of the Altyn Tagh fault. Howerver, some of those studies lacked age control for faulted geomorphic surfaces (Min et al., 2002). Therefore, more accurate estimation of the slip rates of the active faults are still needed to understand how and where the shortening linked to the Altyn Tagh fault is accommodated.
In this paper, we define fault slip rates for two active faults named the Yinwashan and Xinminpu faults in the Jiuxi Basin based on new age controls on deformed fluvial terrace deposits using optically stimulated luminescence (OSL) and surface exposure 10Be dating (Fig. 1b). For the two faults, Min et al. (2002) estimated vertical slip rate of ~0.18 and ~0.24 mm/yr from displacements in trenches and thermo-luminescence (TL) dating. As a result of the uncertainties in the calculated displacements and the dating technique, significant unexpected errors might arise when constraining the slip rate, thus requiring more accurate re-evaluation. Thus, the current analysis aimed to approximate the slip rate of the two faults based on 10Be exposure dating and topographic surveys. More precise estimations of slip rates can offer additional rate constraints of horizontal shortening at the northeastern margin of the Tibetan Plateau.
1.
GEOLOGICAL SETTING AND TECTONIC DEFORMATION OF THE JIUXI BASIN
The Qilian Shan, with an average elevation of ~3 500 m, is the youngest part of the Tibetan Plateau (Tapponnier et al., 2001). It extends for approximately 1 000 km long from east to west and approximately 200–400 km wide from north to south. Active NW-trending thrusts divide its western part into a series of subparallel mountain ranges, such as the Tanghe Nan Shan, Daxue Shan, and Qilian Shan from southwest to northeast (Fig. 1a). The Altyn Tagh fault lies at the northwestern margin of the Qilian Shan. Previous studies pointed out eastward, along-strike decrease in fault slip rates at the eastern end of the Altyn Tagh fault (Zhang et al., 2007; Mériaux et al., 2005; Xu et al., 2005). Using a variety of methods for dating and measuring terrace offsets, Xu et al. (2005) derived slip rates of ~11 mm/yr at a site near Subei, ~5.5 mm/yr near Shibaocheng, ~4.2 mm/yr near Changma and ~2.2 mm/yr near Hongliuxia (Fig. 1a). Based on an analysis of left-lateral strike-slip offsets and evolution of strath terraces, Zhang et al. (2007) obtained revised slip rates of ~10 mm/yr at the Subei site, ~3 mm/yr at Shibaocheng, and ~1.4 mm/yr at Changma (Fig. 1a). The reduction in the left-lateral slip rates is due to crustal shortening and strong uplift of several WNW-trending mountain ranges bounded by thrust faults (Zheng et al., 2013a; Zhang et al., 2007; Xu et al., 2005). Further to the east, the slip rate on the Altyn Tagh fault is estimated to ~2 mm/yr in the Jiuxi Basin (Zhang et al., 2007; Xu et al., 2005), gradually approaching to zero near the Kuantan Shan. Active thrust faults and folds in the western Hexi Corridor absorb the remaining sliprate of the Altyn Tagh fault (Zheng et al., 2013a).
The Jiuxi Basin, which is dominated by a number of active faults and folds around its margins and in its interior, has undergone horizontal shortening during the Neogene and Quaternary (Liu et al., 2017; Wang et al., 2016; Zheng et al., 2013a; Chen et al., 2005; Fang, 2005; Chen, 2003; Hetzel et al., 2002; Min et al., 2002; Chen et al., 1996). The southern boundary of the Jiuxi Basin is composed of the Hanxia-Dahuanggou and Yumen-Beidahe faults (Fig. 1b). The Hanxi-Dahuanggou fault has been inactive since the Pleistocene (Institute of Geology and Lanzhou Institute of Seismology, SSB, 1993). The Yumen-Beidahe fault offsets alluvial fans showing apparent fault scarps. The vertical component of fault slip rate has been estimated to be 0.73±0.09 mm/yr based on displacement of the fault scarps and 10Be dating (Liu et al., 2017). Assuming a dip of 25°, the horizontal shortening rate is ~1.3 mm/yr (Liu et al., 2017). Hetzel et al. (2002) obtained a vertical slip rate of 0.35±0.05 mm/yr for the Baiyanghe fault (named Yumen fault in their paper), resulting in a shortening rate of 0.7±0.1 mm/yr for a fault dipping ~25° to the north. The Heishan fault forms a part of the northern margin of the Jiuxi Basin (Fig. 1b). Zheng (2009) reported a vertical slip rate and horizontal shortening rate of 0.2–0.3 and 0.07–0.11 mm/yr, respectively. Located at the eastern margin of the Jiuxi Basin (Fig. 1b), the Jiayuguan fault, which possesses scarps exceeding 20 m in height, is clearly visible on satellite images. Its vertical slip rate was estimated to be 0.22±0.03 mm/yr based on dating of the offset alluvial fans and the heights of fault scarps (Zheng et al., 2013a), indicating horizontal shortening rate of 0.03–0.04 mm/yr for a dip angle of ~80° (Zheng et al., 2013a).
2.
LATE QUATERNARY ACTIVITIES OF THE YINWASHAN AND XINMINPU FAULTS
2.1
Dating Method for Faulted Strata and Landform Surfaces
To determine tectonic activity and constrain the slip rates of the faults, the offset strata and landform surfaces must be dated using suitable methods. It is difficult to apply radiocarbon 14C dating in the Hexi Corridor because of the lack of organic material. OSL dating is a reliable method to date fine-grained sand, silt and loess deposits if they have been completely bleached. However, the fine-grained sediments often accumulate on the footwall adjacent to the fault scarp. In contrast, there are usually no loess deposits on the hanging wall along the Yinwashan and Xinminpu faults. Quartz-rich pebbles and gravels on its surface provide materials that allow use of 10Be surface exposure dating. Hence we choose OSL dating to constrain the age of fine-grained sediments at the footwall, and 10Be exposure dating to determine the age of faulted landform surfaces.
In this study, all the OSL samples were collected in stainless steel circular tubes with length of 20 cm, hammered into the fine-grained sand or loess. The samples were processed in the State Key Laboratory of Earthquake Dynamics, Institute of Geology, China Earthquake Administration. Extraction and purification of the quartz grains followed the laboratory protocols for Chinese loess under subdued red light (Wang et al., 2005; Forman, 1991; Lu et al., 1988). The samples were processed with 30% HCl and 37% H2O2 to remove carbonates and organic material. Then the 4–11 μm poly-mineral size fraction was separated by hydrostatic precipitation and 30% H2SiF6 was used repeatedly to remove feldspar and isolate the quartz component. The equivalent dose was determined by following the simplified multiple aliquot regenerative-dose (SMAR) method (Wang et al., 2005). The environmental dose rate was also calculated based on the concentrations of U, Th, and K with the contribution of cosmic rays and water content (Aitken, 1998). The results of the OSL dating of samples are shown in Table 1.
Table
1.
Result of OSL analysis for the layers in the trench
For 10Be exposure dating, we used the amalgamation method proposed by Anderson et al. (1996). At every site, a large number (> 50) of cm-sized, quartz-rich pebbles on the terrace surface were obtained. Additionally, a sample (YWS-1) in the active channel was collected to calculate the quantity of inherited 10Be concentration before the deposition the pebbles on the surface of the terraces. We think this sample to represent an inherited 10Be concentration similar to the induced inheritance in the profile (Brown et al., 1998; Anderson et al., 1996). The samples collected in Xinminpu fault were from terraces of the Baiyang River. To calculate exposure age, we used the published value of (3.32±0.57)×105 atmos/g as the inherited 10Be concentration (Liu et al., 2019). Using standard chemical procedures, quartz extraction and BeO preparation were completed at the State Key Laboratory of Earthquake Dynamics, China Earthquake Administration (Zheng et al., 2013a; Hetzel et al., 2002; Kohl and Nishiizumi, 1992). The AMS (Accelerator Mass Spectrometry) analysis to determinate the 10Be/9Be ratios were carried out at CEREGE (Centre Européen de Recherche et D'Enseignement des Géosciences de L'Environnement) in France. We used a low-elevation, high-latitude production rate of 4.3 atoms a-1 g-1 SiO2, which had been adjusted for latitude and elevation (Lal, 1991), to ultimately estimate the surface ages based on the concentrations (Granger et al., 2013). The calculations were all executed with the Microsoft Excel calculator Cosmocalc (Vermeesch, 2007). Table 2 depicts the 10Be dating results.
Table
2.10Be and exposure age data used in this study
The Yinwa Shan is an uplifted mountain in the Jiuxi Basin with its highest elevation ~2 195 m, and surrounded by alluvial fans with elevation of ~1 800–1 900 m, and mainly consists of Ordovician greyish-green metamorphic rock (Bureau of Geology and Mineral Resources of Gansu Province, 1989). The NNW-trending Yinwashan fault, with a length of ~25 km, lies along the eastern side of the Yinwa Shan (Fig. 2). At its northwestern end, the fault has developed in the eroded Cretaceous platform. It is difficult to determine its activity here because of the absence of Quaternary sediments. In its middle portion, the fault forms a topographic boundary between the Yinwa Shan and alluvial fans (Fig. 3a). A reverse fault outcrop with dip angle of ~53° was found in this segment (Fig. 3b). The hanging wall is weathered Ordovician greyish-green metamorphic detritus, while the footwall is Late Pleistocene light-grey loose alluvium, whose age is estimated to be about 22.4±2.6 ka by TL dating (Chen et al., 2005). The fault offsets this stratum and is covered by Holocene alluvium which is approximately 8.6±1.6 ka (Chen et al., 2005). To the southeast, the fault is developed in the alluvial fans derived from the Hei Shan. A series of north facing scarps can be found in satellite images and field observations (Fig. 3c). A trench, dug across the scarp, revealed a reverse fault with a dip angle of ~65°, with Cretaceous fuchsia mudstones and sandstones are thrust over Pleistocene alluvial gravels (Fig. 3d). The fault offset and colluvial wedge are overlain with loess and surface soil. One optically stimulated luminescence (OSL) sample collected at the base of the colluvial wedge in the footwall yielded a depositional age of 16.3±0.7 ka. An OSL sample from the loess cover yields an age of 6.2±0.6 ka, suggesting the latest earthquake occurred sometime before this time.
Figure
3.
Photographs of features along the Yinwashan fault and sections across it. (a) Fault troughs along the Yinwashan fault where it neighbors the Yinwa Shan; (b) natural fault section near the Yinwa Shan revealing a reverse fault dipping at ~53°; (c) fault scarps developed on the alluvial fan; (d) trench across the fault scarp on the alluvial fan reveals a reverse fault with dip angle of 65°, black solid circles denote the location of OSL samples.
The Xinminpu fault, striking at ~300° in the middle part of the Jiuxi Basin, extends for ~20 km from Qingquan in the west to the Xinminpu Village in the east (Fig. 2). A natural outcrop near the Xinminpu Village reveals a low angle (~25°) thrust dips SW (Fig. 4a). The hanging wall block is comprised by Neogene mudstones and sandstones, while the footwall is comprised by Late Pleistocene light-grey loose gravels, showing oriented rearrangements and drag deformation. The geomorphic surface is displaced by about 2–3 m in vertical (Fig. 4a). To the west, a series of springs occur along the north facing fault scarps. The new fault scarps are often developed at the base of the old scarps (Fig. 4b). Surface rupture is still preserved along the new scarps (Fig. 4c), which is associated with a historical earthquake in AD 1 785 with a magnitude of 6.8 (He et al., 2010). A trench across the scarp reveals Neogene brick-red mudstones and sandstones are thrust over Holocene alluvial gravels (Fig. 4d). Three thrusts are found on the west trench wall. The thrusts dip SW with a dip of ~28°. Two paleoseismic events can be distinguished in the trench. The lower two thrusts offset the silt soil layer, and is covered by the loose soil at the surface. Two OSL samples obtained from the silt soil layer suggested the earlier event occurred after ~3.8–4.6 ka (Fig. 4d). The latest earthquake event is the top thrust offsets the loose soil near the surface and connects to the surface rupture produced by the latest earthquake (Fig. 4d).
Figure
4.
Photographs of feature along the Xinminpu fault and sections across it. (a) Natural fault section on the Xinminpu fault to the east of Xinminpu village; (b) fault scarps developed on the alluvial fan; (c) surface ruptures are still preserved along the new fault scarp; (d) trench across the new fault scarp at the foot of old scarp reveals a thrust fault with a dip of 28°. Black solid circles denote the location of OSL samples.
The study site for the Yinwashan fault is situated at the alluvial fan at the south of the Hei Shan (Fig. 2). The alluvial fans have been incised by ephemeral streams. The hanging wall of fault continues to rise since the activity of fault, resulting in a NE-facing fault scarp with several fluvial terraces of different heights on the hanging wall (Fig. 5a). On the footwall, it was covered by the most wide-spread active alluvial fans. Combining satellite image interpretations with field investigations, three level terraces can be identified at this site, marked as T1, T2, and T3 going from the youngest to the oldest (Fig. 5b). The fault scarp of the T3 terrace is the most obvious (Fig. 5c), Cretaceous sandstones and mudstones having been exposed because of the erosion of its surface. To constrain the vertical displacement on the three terrace surfaces, three topographic profiles (P1 to P3) were obtained using differential GPS (Fig. 5b). We projected the measured profiles onto a profile perpendicular to the fault (Directions are 46°, 51° and 26° from P1 to P3, respectively), and the vertical offsets on every profile were calculated by fitting straight trend lines to the hanging wall as well as the footwall. The displacements for T1, T2 and T3 are 2.1±0.1, 3.0±0.2, and 5.2±0.3 m, respectively (Fig. 5d). As the footwall has been buried by the younger alluvial fan deposits, these vertical displacements are minimum values, and therefore, the estimated slip rates will also be minimum values. We use the vertical offset of the terrace surfaces from P1 to P3 (2.1±0.1, 3.0±0.2, and 5.2±0.3 m) and corresponding abandonment ages (24.6±2.2, 51±2.9, and 54±3.5 ka) to determine the vertical component of the slip rate. Vertical slip rates show mostly two values 0.1–0.09 and 0.06 mm/yr. The smaller rate is about a half of bigger one. The bigger slip rate could be more reasonable than the smaller slip rate since the height of fault scarp is a minimum value. Therefore, we obtain a vertical rate of 0.09±0.01 mm/yr for the fault (Fig. 6). Trenches and natural exposures show that reverse fault dips 50°–65° to the southwest (Figs. 3b, 3d). Considering uncertainties of dip angle, we use 60°±10° to estimate a horizontal shortening rate of 0.05±0.03 mm/yr.
Figure
5.
Maps and topographic profiles for the Yinwashan fault study site. (a) Detail of satellite imagery (data from Google Earth), a clear fault scarp cuts the alluvial fan and terraces formed by downcutting streams; (b) geomorphological interpretation of the imagery showing abandoned alluvial surfaces, fault trace, sample locations (stars), locations of topographic profiles, and the location and orientation of the photograph in panel (c); (c) field photograph of the fault scarp for the T3 terrace; (d) topographic profiles across the fault scarp at the locations shown in panel (b) determined using differential GPS.
Figure
6.
Average vertical slip rates on the Yinwashan and Xinminpu faults, which were estimated from vertical offsets and their corresponding abandonment ages.
The study site for the Xinminpu fault was situated at the west of Qingquan on an alluvial fan (Fig. 2). The faulted landform features are similar to those of the Yinwashan fault. The terraces in the hanging wall are separated from a younger alluvial fan in the footwall by a NE-facing fault scarp (Figs. 7a, 7b). Three level terraces are preserved on the hanging wall, denoted as T1 to T3 from youngest to oldest (Fig. 7b). Surface ruptures are found developed at the base of scarps in T3 (Fig. 7c). Fresh faces have been preserved, suggesting that it formed relatively recently. The T1 terrace is covered by loess with thickness of ~1 m. As previous study indicates that the loess accumulation in the western Qilian was initiated in Holocene (Küster et al., 2006; Stokes et al., 2003), the age at the bottom of the loess is much younger than the abandonment age of the fluvial terraces. We did not collect OSL or 10Be sample for this terrace. Based on the fault scarp heights of T2 and T3 (Fig. 7d; 12.6±0.6, 17.7±0.6 m, directions are 44° and 56° for P2 and P3) and the corresponding abandonment ages (111.8±5.9, 181.3±6.9 ka) obtained from 10Be dating, we estimate the vertical slip rate of the Xinminpu fault to be 0.1±0.02 mm/yr (Fig. 6). Using the estimated dip of 25°±5° revealed by trenching and natural exposures (Figs. 4a, 4b), the horizontal shortening rate is determined to 0.23±0.06 mm/yr.
Figure
7.
Maps and topographic profiles for the Huoshaogou study area of the Xinminpu fault. (a) Satellite image near Huoshaoggou Village (data from Google Earth); (b) geopmorphological interpretation showing abandoned alluvial surfaces, fault trace, sample locations (stars), the location of topographic profiles, and the location and orinetation of the photograph in panel (c); (c) field photograph of the fault scarp for the T3 terrace. New scarp developed in the front of the old scarp. Fresh fault scarps are preserved along the new trace, suggesting that it formed relatively recently; (d) topographic profiles across the fault scarp at the locations shown in panel (b) determined using differential GPS.
Along the northern Qilian Shan and Hexi Corridor, the active deformation by thrust faults are predominant. Substantial efforts have been made towards constraining the slip rate for these active thrusts by previous studies. The northern frontal thrust of the Qilian Shan commonly accommodate, the vertical slip rate ~0.5–1.0 mm/yr along its whole length (Liu et al., 2017; Xiong et al., 2017; Palumbo et al., 2009; Hetzel et al., 2004; Chen, 2003). Meanwhile, the rates for the active faults in the central and northern margin of the Hexi Corridor are ~0.1–0.5 mm/yr (Zheng et al., 2013a, b; Zheng, 2009; Hetzel et al., 2002). The estimates suggest that fault slip rates are slightly higher for the faults alongside the southern boundary of the Hexi Corridor (northern margin of the Qilian Shan) in comparison to those in the central and northern margins. Our results are consistent with those previous results. The low mountain ranges bordering the northern edge of the Hexi Corridor, including the Jintanan Shan and Heli Shan (Fig. 1b), started to uplift approximately ~2 Ma ago (Zheng et al., 2013a, b). This uplift occurred after the Yumu Shan and Qilian Shan to the south (Zheng et al., 2010; Palumbo et al., 2009; Métivier, et al., 1998), suggesting that the Tibetan Plateau has experienced continued northeastward expansion across the Hexi Corridor (Zheng et al., 2013a, b). In the central part of the Hexi Corridor, the oldest geomorphic surfaces controlled by active fault are quite young. The oldest terraces along the Yumen and Xinminpu faults have similar ages of ~170–180 ka (Hetzel et al., 2002; this study). It seems that the onset of faulting in the central parts of the Hexi Corridor is later than these at the northern border, which suggests the center of the basin is the location of the youngest deformation.
The research results on vertical slip rate and horizontal shortening rate for the active thrust or reverse faults in the Jiuxi Basin discussed above are summarized in Fig. 8. Based on the analysis of left-lateral strike-slip offsets and evolution of terraces (Zhang et al., 2007), the slip rate of the Altyn Tagh fault near the Jiuxi Basin was revised from ~4.2 to ~1.4 mm/yr at a site near Changma (Fig. 1b). The slip rate of ~2.2 mm/yr at the Hongliuxia site (Fig. 1b) in the Jiuxi Basin was considered as a maximum value for the fault (Zhang et al., 2007; Xu et al., 2005). Using displacement measurements from small unmanned aerial vehicles and 10Be exposure dating, the slip rates at these two sites has been re-estimated to be ~1.6 and ~1 mm/yr, respectively (Zhang, 2016). Hence we think that a slip rate of 1.0–1.4 mm/yr near the two sites is reliable. If the Altyn Tagh fault terminates in the Jiuxi Basin, the amount of left-slip rate will be absorbed by crustal shortening in the direction parallel to the Altyn Tagh fault. In the Jinxi Basin, there are four active faults, Yumen-Beidahe, Baiyanghe, Xinminpu, and Yinwashan faults, which have oblique angle against the Altyn Tagh fault. A total shortening rate of them is approximately 2.1–2.5 mm/yr perpendicular to the Qilian Shan (N20°E). The total shortening rate, and a rotation of the shortening direction with respect to the strike of the Altyn Tagh fault (the intersection angle is ~60°) yield a horizontal shortening rate parallel to the fault of 1.0–1.3 mm/yr. This rate agrees with the left-lateral strike-slip rate of the fault near the basin. So our results support the idea that the Altyn Tagh fault ends at the Jiuxi Basin with thrusting in accommodating its termination (Zheng et al., 2013a; Zhang et al., 2007).
Figure
8.
Map providing a collection of the slip rates (white boxes) in faults in the vicinity of and inside the Jiuxi Basin located in the anterior of the western Qilian Shan. Along with our findings (blue boxes), we have also provided slip rate estimates from additional current research (Zhang, 2016; Zheng et al., 2013a; Zheng, 2009; Zhang et al., 2007; Xu et al., 2005; Hetzel et al., 2002). H. Horizontal slip rate; V. vertical slip rate; S. shortening rate.
We also got a shortening rate across the whole of the Jiuxi Basin perpendicular to the Qilian Shan (N20°E) of 2.2–2.6 mm/yr (Fig. 8). The combined shortening rate may be underestimated because seismic reflection profiles explored in the Jiuxi Basin revealed that deformation within the basin is not only accommodated by these active thrusts but also significant WNW trending folding in Quaternary sedimentary rocks (Yang et al., 2007; Petroleum Geology of Yumen, 1989). This implies that the active shortening in the Jiuxi Basin is associated with both the strike slip on the Altyn Tagh fault and the northward expansion of the Qilian Shan. GPS observations and a balanced cross section indicate NNE-SSW shortening at a rate of 4–10 mm/yr from the Qaidam Basin in the south to the Hexi Corridor in the north (Zheng et al., 2013c; Hetzel et al., 2004; Zhang et al., 2004; Métivier, et al., 1998; Meyer et al., 1998). At the western front of the Qilian Shan and Jiuxi basins, active faults only absorb a part of the shortening. The remaining shortening rate has been distributed among Quaternary thrust faults across this region (Zheng et al., 2013c; van der Woerd et al., 2001; Meyer et al., 1998). Although the fault slip rates derived at the western front of Qilian Shan and Hexi Corridor is quite low, usually < 1 mm/yr, together they play an important role in absorbing the horizontal shortening of the northeastward growth of the Qilian Shan.
4.
CONCLUSIONS
The Yinwashan and Xinminpu faults in the westernmost Hexi Corridor have been continuously active since the Late Quaternary. Using topographic profiling and 10Be exposure dating, we obtained the slip rate to be 0.09±0.01 and 0.11±0.02 mm/yr for the two faults, respectively, yielding corresponding horizontal shortening rates of 0.05±0.03 and 0.23±0.06 mm/yr. These rates are consistent with those results for similar active faults in the middle or on the northern margin of the Hexi Corridor (Zheng et al., 2013a; Zheng, 2009; Hetzel et al., 2002). Combining our results with other slip rates in the area, we get a shortening rate parallel to the Altyn Tagh fault of 1.0–1.3 mm/yr, and perpendicular to the Qilian Shan of 2.2–2.6 mm/yr for the whole basin. The tectonic deformation of the Jiuxi Basin has been mainly distributed over several active faults within it. Despite the slip rate being relatively low for each fault, in combination they play a significant part in transferring the slip from the eastern end of the Altyn Tagh fault as well as in the northeastern expansion of the Qilian Shan.
ACKNOWLEDGMENTS
This research is supported by the Basic Research Project of Institute of Earthquake Science, CEA (No. 2019IESLZ01) and the National Science Foundation of China (No. 41402186). We thank Jingxing Yu and Barry Parsons for their detailed review on the language. The final publication is available at Springer via https://doi.org/10.1007/s12583-020-1287-9.
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Figure 1. (a) Topographic map indicating the northeastern margin of the Tibetan Plateau, the maps shows the main active faults and the study locality, including the western Qilian Shan, the west end of the Hexi Corridor, and adjacent areas. Active faults are modified from Zheng et al.(2013a, b) and Yuan et al. (2013), dashed rectangle shows the location of Fig. b. (b) Color shaded digital elevation model of the Jiuxi Basin derived from 90 m Shuttle Radar Topography Mission (SRTM) data. Abbreviations of names of mountains and faults: HS. Hei Shan; WS. Wenshu Shan; WS. Yinwa Shan; BF. Baiyanghe fault; CF. Changma fault. HF. Heishan fault; HDF. Hanxia-Dahuanggou fault; JF. Jiayuguan fault; KF. Kuangtanshan fault; XF. Xinminpu fault; YBF. Yumen-Beidahe fault; YF. Yinwashan fault.
Figure 3. Photographs of features along the Yinwashan fault and sections across it. (a) Fault troughs along the Yinwashan fault where it neighbors the Yinwa Shan; (b) natural fault section near the Yinwa Shan revealing a reverse fault dipping at ~53°; (c) fault scarps developed on the alluvial fan; (d) trench across the fault scarp on the alluvial fan reveals a reverse fault with dip angle of 65°, black solid circles denote the location of OSL samples.
Figure 4. Photographs of feature along the Xinminpu fault and sections across it. (a) Natural fault section on the Xinminpu fault to the east of Xinminpu village; (b) fault scarps developed on the alluvial fan; (c) surface ruptures are still preserved along the new fault scarp; (d) trench across the new fault scarp at the foot of old scarp reveals a thrust fault with a dip of 28°. Black solid circles denote the location of OSL samples.
Figure 5. Maps and topographic profiles for the Yinwashan fault study site. (a) Detail of satellite imagery (data from Google Earth), a clear fault scarp cuts the alluvial fan and terraces formed by downcutting streams; (b) geomorphological interpretation of the imagery showing abandoned alluvial surfaces, fault trace, sample locations (stars), locations of topographic profiles, and the location and orientation of the photograph in panel (c); (c) field photograph of the fault scarp for the T3 terrace; (d) topographic profiles across the fault scarp at the locations shown in panel (b) determined using differential GPS.
Figure 6. Average vertical slip rates on the Yinwashan and Xinminpu faults, which were estimated from vertical offsets and their corresponding abandonment ages.
Figure 7. Maps and topographic profiles for the Huoshaogou study area of the Xinminpu fault. (a) Satellite image near Huoshaoggou Village (data from Google Earth); (b) geopmorphological interpretation showing abandoned alluvial surfaces, fault trace, sample locations (stars), the location of topographic profiles, and the location and orinetation of the photograph in panel (c); (c) field photograph of the fault scarp for the T3 terrace. New scarp developed in the front of the old scarp. Fresh fault scarps are preserved along the new trace, suggesting that it formed relatively recently; (d) topographic profiles across the fault scarp at the locations shown in panel (b) determined using differential GPS.
Figure 8. Map providing a collection of the slip rates (white boxes) in faults in the vicinity of and inside the Jiuxi Basin located in the anterior of the western Qilian Shan. Along with our findings (blue boxes), we have also provided slip rate estimates from additional current research (Zhang, 2016; Zheng et al., 2013a; Zheng, 2009; Zhang et al., 2007; Xu et al., 2005; Hetzel et al., 2002). H. Horizontal slip rate; V. vertical slip rate; S. shortening rate.
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