Late Quaternary slip rate of the Batang Fault and its strain partitioning role in Yushu area, central Tibet

Tectonophysics 653 (2015) 52–67

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Late Quaternary slip rate of the Batang Fault and its strain partitioning role in Yushu area, central Tibet Xuemeng Huang a,b,c,⁎, Yi Du a, Zhongtai He a, Baoqi Ma a, Furen Xie a a b c

Key Laboratory of Crustal Dynamics, Institute of Crustal Dynamics, China Earthquake Administration, Beijing 100085, China State Key Laboratory of Continental Tectonics and Dynamics, Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China The Key Laboratory of Orogenic Belts and Crustal Evolution, Department of Geology, Peking University, Beijing 100871, China

a r t i c l e

i n f o

Article history: Received 4 August 2014 Received in revised form 21 March 2015 Accepted 29 March 2015 Available online 18 April 2015 Keywords: Batang fault Ganzi–Yushu Fault Late Pleistocene slip rate Ms 7.1 Yushu earthquake

a b s t r a c t The late Quaternary activity of Yushu segment is poorly understood compared with other segments within Ganzi–Yushu Fault system. We focused on the Batang Fault, a major branch fault of the Yushu segment. Interpretation of remote sensing images and field investigations reveals that this fault has a clear geomorphic expression which is characterized by prominent fault escarpment and systematically offset gullies, fluvial terraces and alluvial fans along strike. Morphotectonic mapping, combined with optically stimulated luminescence (OSL) and radiocarbon (14C) data, suggest that the Batang Fault is a late Holocene active left-lateral strike-slip fault, along with some reverse component. The average left-lateral slip rate of this fault is 2–4 mm/yr and vertical slip rate is 0.2–0.6 mm/yr since Late Pleistocene. Comparison with the slip rates of other faults within the Ganzi–Yushu Fault system demonstrates that the Batang Fault partitioned nearly a third of the strike slip deformation within Yushu segment. This study provides insights into the reasons why the Yushu Fault is relatively less active when compared with other segments within Ganzi–Yushu Fault system and is crucial to the seismic hazard assessment in Yushu area especially after the occurrence of 2010 Ms 7.1 Yushu earthquake. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Studying the slip rate associated with major boundary faults of active blocks in the Tibetan Plateau is important to verify the deformation models, such as microplate model (Peltzer and Tapponnier, 1988; Tapponnier and Molnar, 1976; Tapponnier et al., 2001b; Thatcher, 2007) and continuum model (England and Houseman, 1986; Houseman and England, 1993), and provide fundamental basis for the analysis of strain accommodation between active blocks. These two end-member models bear different implications on the slip rates of major strike slip faults within Tibetan Plateau, with high slip rate up to 30 mm/yr on the localized strike slip fault of microplate model and low slip rate of distributed deformation model. The Ganzi–Yushu Fault system, a prominent strike-slip fault in Tibet Plateau, is the northwest continuation of the Xianshuihe Fault, which jointly accommodate the relative motion between Bayan Har block and Qiangtang–Chuandian block (Fig. 1). Based on the geometry and historical earthquakes, the Ganzi–Yushu Fault has been divided into Ganzi, Manigange, Dengke, Yushu and ⁎ Corresponding author at: Key Laboratory of Crustal Dynamics, Institute of Crustal Dynamics, China Earthquake Administration, Beijing 100085, China. Tel.: + 86 10 62846756. E-mail address: [email protected] (X. Huang).

http://dx.doi.org/10.1016/j.tecto.2015.03.026 0040-1951/© 2015 Elsevier B.V. All rights reserved.

Dangjiang segments (Zhou et al., 1996). Compared with other segments, the Yushu segment is geometrically more complex with some branches, among which the Batang Fault is a major one. The Yushu segment mainly consists of the Yushu Fault to the north and the Batang Fault to the south (Fig. 2). The 2010 Ms 7.1 and Ms 6.3 Yushu earthquakes occurred on the Yushu Fault, causing huge losses of people's lives and widespread damages in the Yushu area, central Tibet. Great attention has been paid to the rupture history (Xu et al., 2010; Zhang et al., 2010), surface ruptures (Chen et al., 2010; Lin et al., 2011a,b) and coseismic deformation (Qu et al., 2013; Tobita et al., 2011; Zha et al., 2011). Some researchers reported that co-seismic surface rupture is 31 km-long along with left-lateral offset of ~1.8 m (Chen et al., 2010), or surface rupture of 33 km and left-lateral offset of 0.3–3.2 m (Lin et al., 2011b). However, other researchers found that the surface ruptures have two segments with a seismic gap in between (Li et al., 2012; Pan et al., 2011). Interferometric Synthetic Aperture Radar (InSAR) body wave and surface rupture analyses revealed that the eastern segment of the surface rupture could be related with the Ms 7.1 mainshock and the western segment of the surface rupture with Ms 6.3 aftershock (Li et al., 2011, 2012). Trench analysis suggested that the 1738 historical earthquake probably occurred on the Yushu Fault and the recurrence interval of large earthquakes along this fault should be 450–680 yr (Lin et al., 2011a). The Yushu segment has long been regarded as not obviously active since late Pleistocene, and the

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Fig. 1. Active blocks, segmentation and rupture history of Ganzi–Yushu Fault system within central-eastern Tibet Plateau. Background is Shuttle Radar Topographic Mission (STRM, resolution of 90 m) shaded relief map.

unexpected occurrence of this catastrophic earthquake along the Yushu segment uncovered our lack of knowledge of the late Quaternary activity of this segment.

The Batang Fault, a major branch fault within Yushu segment and about 10 km south of Yushu, did not record any surface rupture in the 2010 Ms 7.1 Yushu earthquake. Moderate historical earthquakes have

Fig. 2. Geological interpretation of remote sensing images of Yushu segment within Ganzi–Yushu Fault system, see location in Fig. 1.

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occurred on the west segment of this fault, such as the earthquakes of 2006 M 5.0, M 5.4 and M 5.6 in Balongda (Dou et al., 2006, 2007). Seen from Google earth images, this fault has a clear geomorphic expression, however, geometry and late Quaternary slip-rate are still unknown. In this study, we combined interpretation of Google earth images, morphotectonic mapping, structural section analysis, optically stimulated luminescence (OSL) and radiocarbon (14C) dating to study the Late Quaternary slip rate of the Batang Fault, and explored the strain partitioning role this fault has played within the Ganzi–Yushu Fault system. 2. Geological setting The study area is located at the boundary between Bayan Har block to the north and Qiangtang–Chuandian block to the south (Fig. 1). Global Positioning System (GPS) data show that Qiangtang block is moving eastwardly much faster than the Bayan Har block at a rate of 22–28 mm/yr, and the Byan Har block moving eastwardly at a rate of 15–19 mm/yr (Shen et al., 2005; Wang et al., 2001; Zhang et al.,

2004). A GPS velocity profile sampled perpendicularly to the Ganzi– Yushu Fault system reveals that the slip rate on this fault is 6.6 ± 1.5 mm/yr (Wang et al., 2013). In recent years, a series of destructive earthquakes occurred around the Bayan Har block, such as the 1997 Mani earthquake, 2001 Kunlun earthquake, 2008 Wenchuan earthquake, 2010 Yutian earthquake, 2010 Yushu earthquake and 2013 Lushan earthquake. Great attention has been paid to the kinematics and seismicity of these boundary faults, such as Kunlun Fault (Lin et al., 2002; Xu et al., 2002), Longmen shan Fault (He et al., 2012; Shen et al., 2009; Xu et al., 2009), and Yulong Kashgar Fault (Elliott et al., 2010; Shan et al., 2011; Xu et al., 2013). The Ganzi–Yushu Fault has a clear expression in the geomorphology, and great attention has been paid to its late Quaternary activity (Li et al., 1995; Peng et al., 2006; Wen et al, 2003; Zhang et al., 1996; Zhou et al., 1996, 1997). The late Quaternary slip rate along the Yushu segment is about 2–5 mm/yr (Lin et al., 2011a), while the slip rate along other segments of the Ganzi–Yushu Fault system is 7 mm/yr (Li et al., 1995; Zhou et al., 1996) or 12 ± 2 mm/yr (Wen et al., 2003). A series of historical earthquakes occurred on this fault, with the 1738 ~ M 7.5 earthquake

Fig. 3. Google earth image (a) and interpretation (b) of terraces and fault trace at site 1.

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Fig. 4. Offset landforms at Xiabatang site (G1); (a) topographic map using Differential GPS, see location in Fig. 3b; (b) topographic profiles across the fault escarpment. See location in Fig. 3.

on the Dangjiang segment, 1896 ~ M 7.0 earthquake on the Dengke segment, (1320 ± 65) ~ Ms 8.0 earthquake on the Manigange segment, and 1845 ~ M 7.5 earthquake on Ganzi segment. Conversely, the Yushu segment has long been regarded as not active or a seismic gap prior the occurrence of 2010 Yushu Ms 7.1 earthquake (Zhou et al., 1997).

3. Material and method The two main parameters for the calculation of fault slip rate are the offset of geomorphic piercing line and the inception time of the offset. Terrace riser and fluvial fans are two widely-used geomorphic markers

Fig. 5. Sketch fault structural section at Xiabatang site (G1). (a) Structural section across terrace T3; (b) field photograph of oriented gravels under the escarpment of terrace T2; (c) black line is the projection of a–b face of gravels; red line is the projection of fault surface; lower hemisphere stereonet projection; (d) field photograph of rock intruding into gravels at the southern branch; (e) striations on the bed rock fault surface.

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to constrain the late Quaternary slip rate of Tibetan strike slip faults (Cowgill, 2007; Cowgill et al., 2009; Li et al., 2005; Mériaux et al., 2004, 2005; Weldon and Sieh, 1985; Zhang et al., 2007). Terrace riser offset is relatively easy to constrain especially when the fault cuts nearly orthogonal to the terrace risers, however, the time the risers beginning to accumulate offset is not easy to be bracketed because the formation of terrace riser is a complex erosional and depositional process. In this paper, terrace surfaces, terrace risers and fluvial fans are used as geomorphic markers. We initially measured the offset from Google Earth images and a measuring tape in the field. As for the typical investigation sites, the detailed topographic maps and profiles were measured by Differential GPS, which is a frequently used instrument and the accuracy of this method is about 0.5 m. The offsets of geomorphic markers are deduced from the measured topographic map and profiles. Accelerator mass spectrometry (AMS) radiocarbon dating and optically stimulated luminescence (OSL) dating methods are both

employed in this research. Seven radiocarbon (14C) and eleven optically stimulated luminescence (OSL) dates were collected along this fault. The materials used for OSL dating are mainly intercalated fine-grained sand within fluvial and alluvial sediments. The materials used for 14C are mainly organic sediment and charcoal. OSL dating was carried out at the laboratory in the Institute of Crustal Dynamics, China Earthquake Administration (CEA). 14C samples were analyzed at the Beta Analytic Inc.

4. Fault geomorphology of the Batang Fault The Batang Fault, which trends NWW and extends from south of Batang Basin to Nianjicuo lake, is a major branch fault of the Yushu segment (Fig. 2). This fault offsets a series of gullies, mountain ridges, fluvial terraces and alluvial fans. Fault escarpment, pressure ridges, sag ponds

Fig. 6. Field photographs of fault escarpment at Xiabatang site (G1). (a) Fault escarpment on T1′, T1, and T2 (seeing to W); (b) fault escarpment on T1′, T1, and T3 (seeing to E); (c) offset of a small gully on T1′ (seeing to E); (d) offset of T1′/T1 riser (seeing to E); (e) folded gravels of T2; (f) pressure ridge on T3 (seeing to S).

X. Huang et al. / Tectonophysics 653 (2015) 52–67 Table 1 OSL dating Results. Sample No

Laboratory IDa

Dose Rate (Gy/ka)

Equivalent dose (Gy)

Age (ka BP)

OSL-01 OSL-03 OSL-07 OSL-08 OSL-11 OSL-15 OSL-16 OSL-17 YS-01 YS-02 YS-03 YS-04

11-OSL-124 11-OSL-126 11-OSL-130 11-OSL-131 11-OSL-134 11-OSL-135 11-OSL-136 11-OSL-137 13-OSL-123 13-OSL-124 13-OSL-125 13-OSL-126

5.09 ± 0.49 6.19 ± 0.62 4.08 ± 0.39 5.07 ± 0.43 4.53 ± 0.41 4.29 ± 0.38 4.18 ± 0.37 4.11 ± 0.52 4.20 4.90 4.95 4.62

35.04 ± 0.67 26.43 ± 0.97 22.68 ± 2.660 13.76 ± 0.56 63.72 ± 2.920 103.97 ± 4.22 115.50 ± 10.40 92.62 ± 3.62 85.69 ± 3.80 39.14 ± 1.93 42.38 ± 0.69 40.73 ± 2.63

6.89 ± 0.57 4.27 ± 0.38 5.56 ± 0.79 2.72 ± 0.24 14.08 ± 1.30 24.21 ± 2.17 27.65 ± 3.33 22.55 ± 2.01 20.39 ± 2.23 7.99 ± 0.89 8.56 ± 0.87 8.82 ± 1.05

a OSL samples were analyzed in Key Laboratory of Crustal Dynamics, China Earthquake Administration.

are prominent especially along the southern boundary of the Batang basin. Five selected sites will be discussed in detail below. 4.1. Site 1 (Xiabatang Gequ River, G1) The Xiabatang site is located at the outlet of Gequ River at the southern boundary of the Batang Basin (Fig. 2). The river swings left laterally when it flows out of the mountain and develops several obsolete river courses on its west bank (Fig. 3). Unpaired terraces are well-preserved on both banks, with T0, T1′, T1 and T2 on the west bank and T3 on the east bank. The terrace fills are mainly composed of medium-sized gravels with intercalated sandy lens. The Batang Fault nearly cut perpendicular across the terraces, leaving northward facing fault

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escarpment and offset terrace risers along strike (Figs. 4, 5, 6). A large scale contour map of the offset landforms and topographic profiles across the fault escarpment are measured by the Differential GPS technique. The measured offsets of terrace risers increase with distance from, and elevation above, the present-day river bed. The front line of T1′ and a gully on the T1′ have been offset 3 m left laterally by the fault. T1/T1′ riser is offset 10 ± 2 m and T1/T2 riser is offset 20 ± 2 m by the fault. The offset of the front edge of the T3 is not wellpreserved on the east bank because this bank north of the fault was displaced into the river and much of the displacement has been eroded. Large pressure ridges on T3 imply cumulative surface deformation by several earthquakes (Fig. 6f). Topographic profiles across the fault escarpment yield vertical offsets of about 0.5 m on T1′, about 2 m on T1, about 3 m on T2 and about 6 m on T3 (Fig. 4). Additionally, there are some newly-formed push-ups and sag ponds along the fault trace (Fig. 6d, f). A structural analysis of the section outcropping along the eastern bank of the river reveals three fault strands (Fig. 5). The south branch (F1) in the Triassic limestone is characterized by brittle to ductile deformed fault rocks with striations showing a left-lateral shear sense and some reverse component. The middle branch (F2) in the piedmont area is characterized by the wedging of Triassic bed rocks onto the lower part of T3 fill. The north branch (F3) in the basin is characterized by strongly oriented and steeply dipping gravels under the fault escarpment. These three fault branches suggest a propagation tendency from mountain to basin area. Together, these geomorphic features testify to a fairly long faulting history at least from post Triassic to late Quaternary. The age of T1′ is constrained from the sample of OSL-08 at site 1, with the age of 2.7 ± 0.2 ka BP (Table 1). The sample about 0.5 m below surface is fine-grained sand. The age of T1 is constrained from

Fig. 7. Photos of sampling sites and ages of different topographic surfaces. (a) Sampling location and age of T1 at Xiabatang (site 1); (b) sampling location and age of T2 at Xiabatang (site 1); (c) sampling location and age of T3 at Xiabatang (site 1); (d) sampling location and age of T3 at Zhada and Yingfang (site 2); (e) sampling location and age of the alluvial fan at Shangbatang (site 4).

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the samples of OSL-03 and OSL-07 at Xiabatang site, with ages of 4.3 ± 0.4 ka BP and 5.6 ± 0.8 ka BP respectively (Table 1). The samples about 0.5 m below surface are fine-grained sand (Fig. 7a). The age of T2 is constrained from samples of YS-02, YS-03 and YS-04 at Xiabatang site, with ages of 8 ± 1 ka BP, 9 ± 1 ka BP and 9 ± 1 ka BP respectively (Table 1). The samples about 0.5 m below surface are fine-grained sand (Fig. 7b). The age of T3 is constrained from samples of OSL-16 and OSL-17 at Xiabatang site, with ages of 28 ± 3 ka BP and 23 ± 2 ka BP respectively (Table 1). The samples about 1 m below surface are intercalated sandy lens within gravel sediments (Fig. 7c). 4.2. Site 2 (Zhada, G2) At Zhada site, three terraces are developed at the piedmont area of Zhaqu River. T2, T3, T4 are ~8 m, 20–26 m, ~50 m above the river bed, respectively (Figs. 8, 9). The surface of T2 constitutes the main surface of the Batang Basin. The boundary between the rear edge of T2 and the adjoining alluvial fan is offset by about 80 m, and the main gully at the center of the fan is offset by nearly 60 m (Figs. 8, 9). Topographic map and profiles show prominent fault escarpment across the terraces (Figs. 9, 10). Vertical offset accumulated by T2 and T3 is about 2–3 m and 6 m, respectively. The offset of T3/T2 riser is difficult to distinguish because it was partly destroyed by the construction of a country road. The offset of T2/T1 riser is about 10–15 m. The age of T3 is constrained from samples of OSL-15 at Zhada site (Fig. 7), with ages of 24 ± 2 ka BP (Table 1). The sample about 4 m

below surface is intercalated sandy lens within gravel sediments (Fig. 7d). The age of the older fan is constrained by the sample of YS-01 from Zhada site, with the age of 20 ± 2 ka BP. The sample about 0.5 m below surface is fine-grained sand sediments. 4.3. Site 3 (Yingfang, G3) At Yingfang site, Google earth images and field investigations reveal that fault escarpment and offset terraces risers are prominent along the fault trace. Three terraces are well-preserved along the river. Vertical offset of T3 is about 6–8 m, and horizontal displacement of T3/T2 riser is about 15–20 m. Vertical offset of T2 is about 1–2 m, vertical offset of T1 is 0.3 ± 0.1 m and horizontal offset of the T1/T2 riser is 3 ± 0.5 m (Figs. 11, 12). 4.4. Site 4 (Shangbatang, G4) At Shangbatang site, a linear NWW trending fault cuts across a series of alluvial fans, leaving prominent north facing fault escarpment and faulted gullies and rills on the fan. We choose one of the bestpreserved fans with less landform modification from human and more gullies and rills on the fan to carry out detailed investigation. Employing Differential GPS techniques, we surveyed the contour map of the fan and the topographic profiles across fault escarpment to constrain the horizontal and vertical displacements (Fig. 13). The slope of the fan is about 5° so that its boundary can hardly be used as piercing point to measure the horizontal displacement. Four gullies (g1–g4) on the fan

Fig. 8. Offset landforms at Zhada site (G2), see location in Fig. 2. (a) Google earth image. (b) Interpretations of terraces and fault trace. (c) Typical offset image of back edge of T3 terrace, see location in Fig. 7a. (d) Offset image of a fluvial fan. See location in Fig. 8a.

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Fig. 9. Offset terraces at Zhada site (G2) using Differential GPS technique. (a) Topographic map showing the offset of terraces; (b) transverse profile of Zhada river; (c) longitudinal profile of the river terraces; see location from A–B in Fig. 8; (d) topographic profiles across the fault escarpment.

are used as geomorphic markers to quantify the displacements. Two beheaded gullies are developed on the northern side of the fault escarpment. Topographic reconstruction reveals that the smallest offset is 3 ±

0.5 m and the largest offset is 20 ± 2 m (Fig. 14). Topographic profiles across the fault escarpment show that the vertical offset of the fan surface is about 4.5 ± 0.5 m (Fig. 13).

Fig. 10. Field photos of offset landforms at Zhada site (G2). (a) Fault escarpment on T2 (seeing to W); (b) fault escarpment on the T3 (seeing to W); (c) fault escarpment on the T3 (seeing to E). (d) A offset gully (seeing to S).

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Fig. 11. Offset landforms at Yingfang (G3). (a) Google earth image; (b) interpretation of the images.

The younger fan is constrained from the samples of 14C-01, 14C-02 and OSL-01 at Shangbatang site, with ages of 9450–9540 yr BP, 7500– 7610 yr BP, and 6.9 ± 0.6 ka BP respectively (Tables 1, 2). The radiocarbon samples were collected from organic sediments and the OSL sample was collected from fine-grained sand layer (Fig. 7e).

Based on the faulting and covering relationship, the first event should predate 7750–7940 yr BP and the second event should be in the range between 5910–6000 yr BP and 4040–4070 yr BP.

4.5. Site 5 (G5)

Offset and the inception time of offset are two basic parameters to calculate fault slip rate. The uncertainty associated with offset inception is the main source of slip rate error. One end-member scenario is that terrace riser cannot act as a passive marker until the lower terrace is abandoned due to the lateral incision on the lower terrace (Tapponnier et al., 2001a). The terrace risers that are displaced away from the river course are better preserved than those offset into the river bed. This method has been used to calculate high slip rate of Altyn Tagh Fault of about 18–27 mm/yr (Mériaux et al., 2004, 2005). Conversely, the left-lateral displacement may cause the upstream terrace to shield the terrace riser downstream of the fault from lateral erosion, so that offset of the terrace riser begins to accumulate when the upper terrace forms (Zhang et al., 2007). The abandonment age of the upper terrace not only places an upper bound on the age of the riser, but also approximates the offset age itself (Zhang et al., 2007). Additionally, the width of the river relative to the riser offset and the diachroneity of terrace abandonment may also play important roles in the reconstruction of risers (Cowgill, 2007). In an ideal situation, the interval between the ages of the upper terrace and the lower terrace is so small that they can tightly constrain the age of the riser between them.

Four terraces (T1, T2, T3, and T4) are developed at this site, the Batang Fault nearly cuts perpendicularly through the river and its terraces. Fault escarpment and releasing bend are prominent in the remote sensing images and field view (Fig. 15). The T3/T4 riser was offset by 75 ± 5 m, the T2/T3 riser was offset by 15 ± 1 m, the T1/T3 riser was offset by 10 ± 2 m and the T1/T2 riser was offset by 5 ± 1 m (Fig. 15). The vertical offset of T1 is 0.5 ± 0.1 m, the vertical offset of T2 is 1.5 ± 0.3 m, the vertical offset of T3 is 6 ± 1 m and vertical offset of T4 is 10–15 m (Fig. 16). A fault outcrop is exposed at the front edge of T3 on the eastern side of the river (Fig. 16). Two paleoseismic events can be identified from this outcrop: the first one is characterized by the offset of fluvial gravel (U3) and deposition of sag pond sediment (U2); the second one is featured by the dismemberment of U2 and coverage of U1. The ages of the radiocarbon samples (BT-02, BT-03) from the lower part of U2 are 7750–7940 yr BP and 7230–7310 yr BP respectively. The age of the sample (BT-04) from the upper part of U2 is 5910–6000 yr BP. The age of the sample (BT-05) from the lower part of U1 is 4040–4070 yr BP (Table 2).

5. Late Quaternary slip rates estimation

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Fig. 12. Field photographs of offset landforms. (a) Fault escarpment on T3 at Yingfang site (seeing to S); (b) fault escarpment and offset of terraces at Yingfang site (seeing to E); (c) overview of the fault escarpment on the shangbatang fan; (d) fault escarpment on shangbatang fan. (e) Fault escarpment and offset gullies on the fan at Shangbatang (seeing to S); (f) fault escarpment and offset gullies (seeing to S).

This method has been successfully used to estimate lower slip rate of central Altyn Tagh Fault of about 9–14 mm/yr (Cowgill et al., 2009). The average slip rate of the Batang Fault is mainly constrained by the geomorphic markers of fluvial terrace risers and alluvial fans and the inception time of offset of terrace risers are both constrained by the ages of the upper terrace and lower terrace (Table 3, Fig. 17). From the above analyses, vertical offset of T3 surface is about 5.5–6.3 m, horizontal offset of the boundary between T3 and contemporary alluvial fan is about 60–80 m and the age of T3 and larger alluvial fan is about 20– 27 ka. Vertical offset of T2 surface is about 2.5–3 m, horizontal offset of T1/T2 riser is about 20 m and the age of T2 is about 8–9 ka. Vertical offset of T1 surface is about 2 m, horizontal offset of T1/T1′ is about 10 m and the age of T1 surface is about 4–5 ka. Vertical offset of T1′ is about 0.5 m and horizontal offset is about 3 m. Vertical offset of the alluvial fan at Shangbatang is about 20 m, horizontal offset is about

4.3 m and the age of the fan surface is about 8–10 ka (Table 3). The offset/age ratios yield left-lateral slip rate of 2–4 mm/yr and vertical slip rate of 0.2–0.6 mm/yr since Late Pleistocene (Fig. 17). Regionally speaking, the ages of geomorphic markers in Yushu segment is generally consistent with other segments, however, the slip rate of Yushu segment is deficit compared with other segments of Ganzi–Yushu Fault system (Table 3). 6. Discussion 6.1. Slip partitioning of the Batang Fault in Yushu segment The Ganzi–Yushu Fault is the northwest continuation of Xianshuihe Fault, making the boundary between Bayan Har block and Qiangtang– Chuandian block. Late Quaternary slip rate of the Ganzi–Yushu Fault

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Fig. 13. (a) Survey map showing fault escarpment and displaced gullies at Shangbatang site (G4); (b) Topographic profiles across fault escarpment. (c) 3D topography and fault structure at Shangbatang site. See location in Fig. 2.

Fig. 14. Geomorphic reconstruction of the cumulative offsets of different gullies at shangbatang site (G4). (a) Shaded topographic map; (b) The smallest offset of 3 m of g2; (c) Offset of 15 m from g2, deduced from beheaded gully; (d) The largest offset of about 20 m from g1 and g3.

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Table 2 Radiocarbon dating results. Sample No

Laboratory IDa

Material

technique

Conventional Age (yr BP)

Calibrated Age (2σ, yr BP)

14C-01 14C-02 BT-01 BT-02 BT-03 BT-04 BT-05

Beta-297002 Beta-297003 Beta-365270 Beta-365271 Beta-365272 Beta-365273 Beta-365274

Organic sediment Organic sediment Organic sediment Organic sediment Organic sediment Organic sediment Organic sediment

AMS AMS AMS AMS AMS AMS AMS

8480 ± 40 6690 ± 40 3600 ± 30 7010 ± 40 6320 ± 30 5210 ± 30 3630 ± 30

9450–9540 7500–7610 3840–3980 7750–7940 7230–7310 5910–6000 4040–4070

a

Samples were analyzed in the Beta Analytic Inc.

system based on geological evidences is about 5–7 mm/yr (Li et al., 1995; Zhang et al., 1996), 3–8 mm/yr (Peng et al., 2006), 3–7 mm/yr (Zhou et al., 1996), 12 mm/yr (Wen et al., 2003) and 14 mm/yr (Xu

et al., 2003), among which the slip rate of 12–14 mm/yr may be an overestimation because they used the ages of the lower terrace as the inception time of the offset of terrace risers (Cowgill, 2007). Global

Fig. 15. Offset landforms at Xiabatang site (G5); (a) Google earth image; (b) Geological interpretation of river terraces and fault trace; (c) Typical observation locations of offset terrace risers.

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Fig. 16. Field photographs of fault escarpment and fault outcrop at site 5. (a) Offset terraces on the west bank of this river (seeing to W); (b) Offset terraces on the east bank of this river (seeing to E); (c) Field picture of the fault (seeing to E); (d) Interpretations of the fault outcrop. U1 surface soil, U2 silt layer, U3 gravel layer. See location in Fig. 16b.

Positioning System (GPS) data shows that the Bayan Har block moves eastwardly at a rate of 15–19 mm/yr, the Qiangtang block moves eastwardly at a rate of 22–28 mm/yr (Shen et al., 2005; Zhang et al.,

2004). The relative slip rate of 7–10 mm/yr means that Qiangtang block extrudes eastwardly faster than Bayan Har block along the Ganzi–Yushu Fault. GPS velocity profile revealed that the slip rate on

Table 3 Ages, offsets and slip rates of different segments of the Ganzi–Yushu fault system. Fault segment

Ganzi

Location

Yushu

Dangjiang

Age/ka V

7.5

5 15

Gully Alluvial fan Gully on T3 Gully

34 ± 6 29–42

Cha la

T4/T3

660 ± 50

Er zhong

T2 T1 T5/T3

34 ± 6

T2 T3 T2 T2/T3 T2 T3 T2/T3 T2/T3 Alluvial fan T3/T2

47–53 220 48 95 ± 10 92 ± 6

6 28

92 ± 6

4

3

Zhada Shangbatang

T1′ T1 T1/T1′ T2 T2/T1 T2 T3 T3 T3 Alluvial fan

Dangjiang

Alluvial fan

100

Ri er Cuo er Manigange Ri er Cuo er Ri er

Dengke

Offset H

Road 66 West of Nawa Er'zhong Na wa

Sheng kang Mani-gange

Landform

Tuodang Tuodang Yushu Xiabatang

Note: H represents horizontal; V represents vertical.

0.6 ± 0.2

3.5 ± 0.5

360 ± 60

9±1 4 ± 0.5

80 110 ± 15 0.5 2.2 ± 0.2

10 ± 2 3.0 ± 0.2 20 ± 2 6.3 ± 0.5 60–80 4.3 ± 0.3

5.5 ± 0.2

2.22 ± 0.18 7.15 ± 0.08 2.37 ± 0.19(T2) 4.72 ± 0.07 50.6 ± 4.0(T3) 49.0 ± 4.0(T3) 75.6 ± 5.8(T4) 3.9 ± 0.3 2.36 ± 0.18 46.1 ± 3.5(T5) 16.29 ± 1.27(T3) 7.43 ± 0.57 31.5 ± 2.5 7.43 ± 0.57 7.43 ± 0.57(T2) 6.62 ± 0.52 11.12 ± 0.93 6.62 ± 0.52(T2) 11.12 ± 0.93(T3) 9.76 ± 0.73 9.76 ± 0.73(T2) 2.72 ± 0.24 4.27 ± 0.38 5.56 ± 0.79(T1) 7.99 ± 0.89 8.56 ± 0.87(T2) 8.82 ± 1.05 22.55 ± 2.01 27.65 ± 3.33 24.21 ± 2.17 7.5–9.5 6.89 ± 0.57 13.7 ± 1.1

Dating method

TL C TL 14 C TL TL TL TL TL TL TL TL TL TL TL TL TL TL TL TL TL

Slip rate

References

H

V

3.4 ± 0.3

2. 2 ± 0.1

14

OSL OSL OSL OSL OSL OSL OSL OSL OSL 14 C OSL TL

14.3 ± 3 8.9 ± 1.1 13.3 ± 1.3 10.9 ± 2.0

1.1 ± 0.2

11.5 ± 2.4 7 ± 0.7

12.8 ± 1.7 13.9 ± 1.4

1.2 ± 0.2 0.6 ± 0.1

13.4 ± 2

1.1 ± 0.4

7.2 ± 1.2 11.3 ± 0.7 2–5 2.3–3.7

0.4–0.5 0.3–0.4

2.3–3.6 0.2–0.3 2.6–3.5 2.1–2.9 7.3 ± 0.6

0.2 0.5–0.6

Zhou et al. (1996) Zhou et al. (1996) Xu et al. (2003) Wen et al. (2003) Wen et al. (2003) Wen et al. (2003) Wen et al. (2003) Wen et al. (2003) Wen et al. (2003) Wen et al. (2003) Wen et al. (2003) Zhou et al. (1996) Zhou et al. (1996) Zhou et al. (1996) Wen et al. (2003) Wen et al. (2003) Wen et al. (2003) Xu et al. (2003) Xu et al. (2003) Zhou et al. (1996, 1997) Wen et al. (2003) Lin et al. (2011a, 2011b) This paper This paper This paper This paper This paper This paper This paper This paper This paper This paper Zhou et al. (1996)

X. Huang et al. / Tectonophysics 653 (2015) 52–67

65

Fig. 17. Ages of features versus offsets that constrain late Quaternary slip rate of Yushu Batang Fault. (a) Vertical slip rate; (b) Left-lateral strike slip rate.

the Ganzi–Yushu Fault system is about 6.6 ± 1.5 mm/yr (Wang et al., 2013). Overall, the slip rate of about 7–10 mm/yr on the Ganzi–Yushu Fault system may be relatively more reliable. Compared with other segments, the Yushu segment is geometrically more complex with a few branches, among which the Batang Fault south of Yushu is the major one. Prior to the occurrence of 2010 Ms 7.1 Yushu earthquake, the Yushu segment was regarded as less active or even not active and did not record strong earthquakes (Li et al., 1995; Zhou et al., 1997). According to the recurrence interval of 450– 680 yr and the coseismic offset of 1.5–2.0 m in the 2010 Yushu Ms 7.1 earthquake, Lin et al (2011a) estimated that the average slip rate of the Yushu Fault is about 2–5 mm/yr. The thought that the 1738 M 7.1 earthquake occurred on the Yushu Fault (Lin et al., 2011a) is not in agreement with geological evidences prior to the occurrence of 2010 Yushu Ms 7.1 earthquake (Li et al., 1995; Zhang et al., 1996; Zhou et al., 1996, 1997). If this historical earthquake did not occur on the Yushu Fault, but on the Dangjiang Fault, the recurrence interval of Yushu Fault may be longer than 450–680 yr and resultant slip rate may be smaller than 2–5 mm/yr. The slip rate of the Batang Fault provides insight into slip partitioning of the Yushu segment within Ganzi–Yushu Fault system. Yushu segment (Yushu Fault and Batang Fault) and Dengke segment form a triple junction structure southeast of Yushu. Based on the angle of 20 ± 2° between Yushu and Batang faults, the slip rate of 2–4 mm/yr of the Yushu Fault can be geometrically partitioned to be 1.8–3.6 mm/yr parallel to the Yushu Fault and 0.7–1.4 mm/yr perpendicular to the Yushu Fault. Considering the high dip angle of about 70–80° of the Batang Fault, the horizontal transfer of the vertical slip rate of about 0.2–0.6 mm/yr is negligible. Compared with the slip rate on the Yushu Fault, the Batang

Fault and the Dengke segment, the Batang Fault appears to accommodate nearly a third of the strike slip deformation along the Yushu segment (Fig. 18).

6.2. Tectonic significance of Yushu segment As for the relation between the Batang and the Yushu faults, the Yushu Fault, which is characterized by deep gorges and regional lithological divide, is the main fault, however, the Batang Fault, which only controls the Quaternary deposits, is a branch fault. The west continuation of Batang Fault can be inconsistently connected with the main fault with some reverse branch faults (Wen et al., 2011). The reverse component of the Batang Fault is mainly due to geometrical relationship between NW-trending Ganzi–Yushu Fault and NEE-directed GPS strain field (Zhang et al., 2004). Due the remoteness and high altitude west of Dangjiang, the west continuation of Ganzi–Yushu Fault system is not clear (Tapponnier et al., 2001a). The Ganzi–Yushu Fault was only graphically drawn to be connected with Fenghuoshan Fault (Taylor and An, 2009; Wen et al., 2011; Xu et al., 2013). Structurally and geomorphically, the areas west of Yushu are characterized by more widely developed normal faults in the Qiangtang block and reverse faults in the Bayan Har block, while the areas east of Yushu are featured by large-scale single linear trace of strike slip fault. What the role has the Yushu segment (Yushu Fault and Batang Fault) played in the Fenghuoshan–Yushu– Xianshuihe fault system? May be the area west of Yushu more close to distributed deformation model, while the area east of Yushu more close to the rigid microplate escape model. The Yushu segment is

Fig. 18. Historical earthquakes, fault segments and slip rates along the Ganzi–Yushu Fault.

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located at a transitional structural domain between distributed deformation domain westward and localized deformation domain eastward. 7. Conclusions Based on interpretation of Google earth images, field surveys, structural section analysis, radiocarbon (14C) and OSL dating, we conclude that: (1) Yushu Batang Fault is a left-lateral late Holocene active fault with some reverse component. The latest paleoearthquake occurred after 2.7 ka BP. (2) Average left-lateral slip rate of Yushu Batang Fault is about 2– 4 mm/yr and vertical slip rate is about 0.2–0.6 mm/yr since Late Pleistocene. (3) The study of the slip rate of Yushu Batang Fault provides insights into the reasons of why the slip rate of Yushu segment is relatively lower compared with other segments within Ganzi–Yushu Fault system. Yushu segment (Yushu Fault, Yushu Batang Fault) and Dengke segment form a triple junction structure southeast of Yushu and the strike slip activity of the Yushu Batang Fault partitioned nearly a third of the deformation within Yushu segment. (4) The Yushu segment is located at a transitional structural domain between distributed deformation domain westward and localized deformation domain eastward.

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