Wang Shifeng a, Wang Erchie a, Fang Xiaomin a & Fu Bihong a a Chinese Academy of Sciences, Beijing, China

Transcription

1 This article was downloaded by: [Institute of Tibetan Plateau Research] On: 17 April 2013, At: 19:52 Publisher: Taylor & Francis Informa Ltd Registered in England and Wales Registered Number: Registered office: Mortimer House, Mortimer Street, London W1T 3JH, UK International Geology Review Publication details, including instructions for authors and subscription information: Late Cenozoic Systematic Left-Lateral Stream Deflections along the Ganzi- Yushu Fault, Xianshuihe Fault System, Eastern Tibet Wang Shifeng a, Wang Erchie a, Fang Xiaomin a & Fu Bihong a a Chinese Academy of Sciences, Beijing, China Version of record first published: 06 Aug To cite this article: Wang Shifeng, Wang Erchie, Fang Xiaomin & Fu Bihong (2008): Late Cenozoic Systematic Left-Lateral Stream Deflections along the Ganzi-Yushu Fault, Xianshuihe Fault System, Eastern Tibet, International Geology Review, 50:7, To link to this article: PLEASE SCROLL DOWN FOR ARTICLE Full terms and conditions of use: This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. The publisher does not give any warranty express or implied or make any representation that the contents will be complete or accurate or up to date. The accuracy of any instructions, formulae, and drug doses should be independently verified with primary sources. The publisher shall not be liable for any loss, actions, claims, proceedings, demand, or costs or damages whatsoever or howsoever caused arising directly or indirectly in connection with or arising out of the use of this material.

2 International Geology Review, Vol. 50, 2008, p DOI: / Copyright 2008 by Bellwether Publishing, Ltd. All rights reserved. Late Cenozoic Systematic Left-Lateral Stream Deflections along the Ganzi-Yushu Fault, Xianshuihe Fault System, Eastern Tibet WANG SHIFENG, 1 Institute of Tibetan Plateau Research, Chinese Academy of Sciences, P.O. Box 2871, Shuangqing Road 18, Beijing , China WANG ERCHIE, Institute of Tibetan Plateau Research, Chinese Academy of Sciences, P.O. Box 2871, Shuangqing Road 18, Beijing , China and Institute of Geology and Geophysics, Chinese Academy of Sciences, P.O. Box 9825, Beijing, , China FANG XIAOMIN, China and Institute of Tibetan Plateau Research, Chinese Academy of Sciences, P.O. Box 2871, Shuangqing Road 18, Beijing , China AND FU BIHONG Institute of Geology and Geophysics, Chinese Academy of Sciences, P.O. Box 9825, Beijing, , China Abstract The Xianshuihe fault system (XSF) is one of the most active faults in eastern Tibet, China. In this study, we document the distribution of stream deflections along the Ganzi-Yushu segment of the XSF at scales of 2.5 m to 80 km. The total offset along this fault suggests that left-lateral movement began at ~8 5.6 Ma if the Holocene slip rate has remained steady over this time interval. From Dengke in the southeast to Dangjiang in the northwest along the Ganzi Yushu fault, the documented total geological offset decreases from ~80 to ~39 km, whereas the deflections of the Yalong, Batang, Jielong, and Dangjiang rivers, almost all of which initiated around early Quaternary time, show gradually decreasing offsets from 30 ± 2 km to 16 ± 1 km in the same direction. This implies that about 41 km and 14 km offset are accommodated by transtensional and transpresional structures such as pullapart basins and thrust faults along different parts of the fault at different times; this is also consistent with a northwestward decrease in the Holocene slip rate along the XSF, from 10 to 14 mm/yr around Dengke to ~7 mm/yr around Dangjiang. Thus the spatial and temporal evolution of stream patterns along the XSF supports the hypothesis that the tectonic evolution of eastern Tibet is largely characterized by distributed deformation. Introduction SEVERAL OF the major young, active faults in eastern Tibet, such as the Xianshuihe fault system (XSF) or the Red River fault, have variably been interpreted to accomodate either large-scale eastward extrusion of lithospheric fragments that extend from Tibet to the South China Sea (e.g., Tapponnier et al., 1986; Briais et al., 1993; Leloup et al., 1995), or clockwise rotation of crustal blocks around the eastern Himalayan syntaxis (e.g., Burchfiel and Royden, 1991; Ratschbacher et al., 1996; Wang et al., 1998; Zhang et al., 2004). The geometry and evolution of stream patterns, which are sensitive to fault activity, can provide valuable information that allows distinction between 1 Corresponding author; wsf@itpcas.ac.cn these deformational styles (e.g., Brookfield, 1998; Hallet and Molnar., 2001; Clark et al., 2004). But until now, whether the largest stream deflections along such faults provide a minimum estimate for the total fault offset or not is still debated, and there are no studies describing the spatial and temporal evolution of stream deflection on both short and long time scales i.e., how streams accumulate deflections from meters to tens of kilometers during progressive fault movement. Accordingly, this limits our understanding of the geometric and kinematic evolution of such major strike-slip faults. The XSF, one of the most important strike-slip faults in eastern Tibet, is the locus of many geomorphic features of strike-slip faulting, such as stream deflections, push-up and shutter ridges, and sag ponds (Allen et al., 1991; Wang and Burchfiel, /08/1008/ $

3 LATE CENOZOIC STREAM DEFLECTION 625 FIG. 1. Major active faults and large rivers in eastern Tibet. Box indicates the study area. Abbreviations: HF = Haiyuan fault; KF = Kunlun fault; LMS = Longmen Shan thrust fault; XSF = Xianshuihe fault system; RRF = Red River fault; WF = Wanding fault; GyF= Ganzi-Yushu segment of the Xianshuihe fault system; XsF= Xianshuihe segment of the Xianshuihe fault system; AzF=Anninghe-Zemuhe segment of the Xianshuihe fault system; XjF= Xiaojiang segment of the Xianshuihe fault system. 2000). The Ganzi Yushu fault in our study area is located in the northwestern segment of the XSF (Fig. 1), where the fault cuts through the drainage basin of the Jinsha River, deflecting its course and that of several of its branches (Fig. 2). It is an ideal place to constrain the relationship between fault activity and the evolution of stream deflections. After a description of the accumulation of stream deflections with time, we will emphasize two issues. The first is the timing of the initiation of the XSF, which is poorly constrained; King et al. (1997) inferred fault initiation at 8 6 Ma, extrapolating GPS-derived modern slip rates back in time to obtain a total fault offset estimated as 60 km, whereas Wang et al. (1998)

4 WANG SHIFENG ET AL. 626 FIG. 2. Drainage basin of the Jinsha River and active fault trace of the Ganzi Yushu segment of the XSF. Channel deflections of some Jinsha River tributaries are also shown. inferred fault initiation at 4 2 Ma, based on the age of sediments in fault-controlled basins near the southern end of the XSF. The second main issue is the role of the XSF in eastern Tibetan crustal deformation. The fault was originally interpreted as a boundary fault of a southeastward-extruding crustal block (Tapponnier et al., 1986; Leloup et al., 1995), but more recent work suggests that the XSF

5 LATE CENOZOIC STREAM DEFLECTION 627 FIG. 3. Schematic illustration of stream deflection geometries and methods used to quantify their magnitudes. A C. Left-laterally deflected channel, right-laterally deflected channel, undeflected channel. D G. Methods of stream deflection measurement: D is the true displacement of a stream and D' is the apparent displacement. contributes to more distributed deformation as a part of a large-scale fault system (e.g., Ratschbacher et al., 1996; Wang et al., 1998). Geological Setting The XSF is ~1400 km long, extending from the Dangjiang area of Qinghai Province through Sichuan to the Xiaojiang area in Yunnan Province. The fault system consists of four primary segments from southeast to northwest the Xiaojiang (XjF), the Anninghe (AzF), the Xianshuihe (XsF), and the Ganzi Yushu (GyF) faults (Wang et al., 1998) (Fig. 1). The total amount of left-lateral offset is about 80 km, which is absorbed by an transtensional basin and range system at the southern end of the fault (Wang et al., 1998), and by both transpressional and transtensional structures at the northwestern end of the fault (Wang et al., 2008). Historical and instrumental records attest to pronounced seismic activity along the XSF: there have been 23 earthquakes above magnitude 6.5 since 1735 A.D. (Wen, 2000). As shown in Figure 2, the Gazi Yushu fault in the study area consists of three primary segments, from the southeast to northwest: the Ganzi, Yushu, and Dangjiang faults. Methods In this study we document stream deflections at different scales along the Ganzi Yushu fault, based on: (1) the geomorphic analysis of 8 30 m ground resolution satellite images; (2) field observations; and (3) 14 C dating of alluvial fan displacements. As documented in similar studies such as those by Wallace (1968), Sieh and Jahn (1984), Gaudemer (1989), Fu et al (2005), we observed three types of stream channel deflection along the fault trace: left laterally deflected, right laterally deflected, and undeflected channels (Figs. 3A 3C). The statistically valid stream deflection provides the sense of fault movement. To provide a quantitative estimate for the magnitude of stream channel deflections, we classified the geometry of measured deflections and determined magnitude as shown in Figures 3D 3G, and assigned uncertainties to these estimates based on the way each individual measurement was taken. Lateral Stream Deflections Consistently left-lateral stream deflections along the Yushu Ganzi segment of the XSF are clear both in the field and in images, and the magnitudes of the documented stream deflections vary from as little as several meters to as much as tens of kilometers; the deflections generally increase with the size of the streams drainage areas and stream lengths upstream from the fault. Stream deflections of <1 km are usually developed on the slopes of alluvial fans, whereas stream deflections >1 km are mostly observed for channels cut into basement rocks. In the following, we describe the observed stream

6 628 WANG SHIFENG ET AL. FIG. 4. Photographs of stream channel deflections observed in the field. A. 2.7 m stream deflection attributed to a historically documented earthquake in B. 25 m stream deflection dated at 3700 ± 30 yr. The location of the samples used for 14 C dating is marked by the white dot in Figure 4B. For the location, see the star symbols in Figure 2A. deflections subdivided into 4 scales: <1 km; 3 8 km; km; and ~80 km. The age of these displacements ranges from Holocene to late or even middle Miocene time. Holocene stream deflections The stream deflections of <1 km magnitude demonstrate the most recent fault activity, and some can be directly linked to historically documented seismic events. For example, a ~7.0-magnitude earthquake occurred in 1735, with an 80 km long surface break; its was just south of Dangjiang village. The rupture zone is marked by horizontal slikensides, and is associated with a left-lateral stream deflection of ~2.7 m; a m high fault scarp is indicative of an extensional component of fault movement (Fig. 4A). At a site nearby where the fault trace is associated with a 25 ± 2 m stream deflection, the 14 C age of a charcoal sample from the bottom of a ruptured humus layer displaced by the fault is 300 ± 20 years, whereas snail shells from 1.2 m below the fan surface yielded a 14 C age of 3700 ± 30 years. The location of the samples are shown in Figure 4, and the 14 C AMS ages were measured at the Radiocarbon Dating Laboratory of Peking University. If the 25 m stream deflection here accumulated over 3700 years, then we estimate the fault slip-rate to be ~7 mm/yr, which is almost same as the 7.3 mm/yr Quaternary slip rate along the Dangjiang fault based on a larger geological offset and a longer period (Zhou et al., 1996). A longer-term record of systematic stream deflection is preserved south of Dangjiang village. Here a linear fault trace, clearly visible both on the ground and on satellite images, offsets a series of gullies with apparent displacements ranging from 200 to 300 m. These apparent offsets are minima, however, as mis-matches in channel size across the fault strongly suggest recent stream capture events, as commonly documented elsewhere (e.g., Wen et al., 2003). Reconstruction of these gullies to their original positions implies total offsets on the order of 1 km. Similarly, stream deflections of hundreds of meters are also developed along the southeastern segment of the Ganzi Yushu fault. Historical records show that a ~7.0 earthquake occurred west of Ganzi in 1854, and this ground rupture is associated with a 2.5 m left-lateral offset. Furthermore, historical records show there was a magnitude ~7.5 earthquake at Dengke (midway between Yushu and Ganzi) in 1896, which destroyed many houses and temples along the fault, and even dammed the Jinsha River for tens of days due to a landslide. Moreover, the alluvial fans, shutter ridges, and terraces along the fault are systematically offset m in a left-lateral sense, from which Wen et al (2003) estimated a mm/yr slip rate based on thermoluminescence dating of quartz. In addition to the small offsets described above, numerous landforms record offsets >300 m. The best-preserved examples are in the vicinity of Longshida, west of Yushu (Figs. 2B and 5). Figure 5 shows a series of channels and interfluves with

7 LATE CENOZOIC STREAM DEFLECTION 629 FIG. 5. ETM remote sensing image showing systematic stream deflections of <1 km along the fault trace at Longshida. For location, see Figure 2. consistent left-lateral offsets along the linear fault trace (white arrows in Fig. 5). Observed offsets vary from 250 ± 20 m to 500 ± 30 m. There is a significant mismatch in channel size across the fault at point D, suggesting that stream capture may place a limit on the size of offsets that can be preserved in this stream setting. The most plausible stream path reconstruction is to connect with the downstream channel segment at point A, which is like the case B B' shown in Figure 3C. If this correlation is correct, it implies a ~1.3 km total recorded dip. An offset of this magnitude associated with such a clear topographic expression suggests substantial movement of the fault during Holocene and earlier Quaternary time. Early Quaternary stream deflections The streams characterized by 3 8 km channel deflections along the XSF in the study area that typically extend km upstream from the fault, are much longer than the streams with <1 km channel offsets, and their drainage areas are also much bigger than those of streams displaying smaller channel offsets. These streams are far less common than those associated with smaller channel displacements, but they imply the same sense of fault displacement as other streams of different sizes. Seven examples, based on field observation and remote sensing imagery, are described below. The Batang River rises south of the Batang Basin, crosses the Batang normal fault, and then flows northward through Yushu where it is joined by another tributary from Longshida (Fig. 2). At this junction, the Yushu fault cuts through behind an isolated hill, and triangle faces and scarps on the northern slope of relatively high mountains to the south of the valley imply that the fault is located between the isolated hill and the mountains to its south. We have found fluvial sediment in the wind gap, from which we conclude that the gap represents a fossil channel that was abandoned as the result of capture due to the left-lateral fault movement. The offset of the abandoned channel is ~3 ± 0.5 km. In the Longshida area, all the streams described above are tributaries to the river labeled E in Figure 5, which extends for ~20 km upstream of the Yushu fault. As shown in Figure 6A, the fault comprises two separate branches at this location, both predominantly strike-slip with a normal fault component that down-drops the block between the fault strands. Between them the river channel is deflected leftlaterally by ~3 ± 0.5 km. The bigger stream (river E in Fig. 5) shows a greater offset than its tributaries, described in the previous section, due to its greater age. In the southwestern part of the Jielong Basin, the size of gullies and corresponding shutter ridges

8 630 WANG SHIFENG ET AL. FIG. 6. Stream deflections of about 1 4 km magnitude and corresponding shutter ridges at several localities along the Ganzi Yushu fault. A. Longshida. B. Jielong. C. Dengke. D. Ganzi.

9 LATE CENOZOIC STREAM DEFLECTION 631 corresponds well with apparent stream deflections. Figure 2 shows the north and south sides of the Jielong Basin, both controlled by branches of the Yushu fault. The south branch of the fault cuts through the ridges south of the Jielong Basin, where wine cup like ridges indicate a normal component of displacement in addition to the predominant leftlateral strike-slip movement. Stream deflections are on the order of 3 8 km where they pass the faults: the offset of channel A is 4 ± 0.5 km (Figs. 2A and 6B), that of channel B is 3 ± 0.5 km, and that of the Jielong River is 8 ± 0.5 km (Fig. 2A). Of these channels, A and B are small tributaries to the Jielong River. After passing through the Jielong Basin, the Jielong River flows northward through the rightoverstep area between the Dangjiang and Yushu faults. The right-overstep is a restraining bend along the Ganzi Yushu fault that may have absorbed ~32 km of fault offset (Wang et al., 2008). Accordingly, the slip rate along the Dangjiang fault must be smaller than that along the Yushu fault; this is proved by the fact that the Jielong River flows over the Dangjiang fault with ~5 km of deflection (Fig. 2A). So the life span of a stream with 5 km deflection appears to be at least 700 k.y., based on the 7 mm/yr slip rate on the middle Dangjiang fault segment. Triangular facets are developed on both sides of the Dangjiangyong Valley, and the rupture zone of an earthquake in 1783 along the valley indicates recent activity of the Dangjiang fault. The Zhiduo River flows northward through this area, and is deflected by ~6 ± 0.5 km according to the sense of the Dangjiang fault (Fig. 2A). The Dangjiang fault splits into several branches in this area; the slip rate at one branch must be slower than that on the middle segment of the fault, thus we can infer a minimum age of 860 ka for the initiation of the Zhiduo River. About 10 km east of the Dengke, the branches of the Dengke River are systemically deflected in leftlateral strike slip sense as indicated by shutter ridges, and the magnitude of stream deflections can be estimated to be between 0.8 ± 0.2 km and 4.1 ± 0.5 km (Fig. 6C). Additionally, the rupture zone of modern fault activity in the shutter ridge is very clear, indicating continued movement of the Ganzi Yushu fault from ka, based on the mm/yr slip rate (Wen et al., 2003). At the eastern end of the Yushu Ganzi fault, near Ganzi, tributaries of the Yalong River associated with pronounced shutter ridges show left-lateral deflections of 1.2 km to about 3.7 ± 0.5 km (Fig. 6D). This implies that the fault has been active since Pleistocene time. Late Pleistocene to Pliocene stream deflections The Ganzi Yushu fault passes through the drainage basin of the Jinsha River northwestward, deflecting the Jinsha River as well as several of its tributaries, including the Zhiduo, Dangjiang, Jielong, Batang, Deke, and Yalong rivers (Fig. 2). The magnitude of these deflections are 30 ± 2 km for the Yalong River, 22 ± 6 km for the Batang River, 16 ± 1 km for the Dangjiang River, and 15 ± l km for the Dengke River. However, deflections of the Jielong and Zhiduo rivers are only 5 8 km and 6 km, respectively. This scale of stream deflection does not necessarily result in the formation of shutter ridges, and we cannot exclude the possibility that rivers simply exploit the presence of weak rocks found near fault zones and thus maintain a relatively straight course. But there are two possible reasons to regard large stream deflections as the result of continuous fault activity: (1) where large stream deflections occur, there is generally not only evidence for active faulting but also for a positive correlation between stream size and the magnitude of stream channel deflection; and (2) geological offsets also are consistent with the magnitudes of the observed stream deflections. For example, the Dangjiang River is left-laterally deflected by 16 ± 1 km, whereas a Triassic volcanic body on both sides of it is leftlaterally offset by ~39 km (Fig. 7; Wang et al., 2008). This indicates that the deflection of the Dangjiang River records a significant period of the Ganzi Yushu fault activity. Another example is the Yalong River, which shows a ~30 ± 2 km left-lateral bend across the fault, while a regional geological map (Fig. 8; Sichuan BGMR) suggests a ~30 km offset for the same fault; this implies that the observed stream channel deflection represents the entire amount of displacement along this fault segment. An interesting phenomenon about the streams showing these relatively large deflections is that most of the drainage basins are structurally controlled by the Ganzi Yushu fault. For example, the Yalong River passes through the Ganzi Basin, the Batang River passes through the Batang Basin, the Jielong River passes through the Jielong Basin, and the Zhiduo and Dangjiang rivers pass through small grabens. So, we speculate that the stream deflections accompany fault movements and the

10 632 WANG SHIFENG ET AL. FIG. 7. Geological map showing the 39 km offset of Mesozoic volcanic rocks, a 16 km stream deflection of the Dangjiang River, and a 6 km stream deflection of the Zhiduo River along the Dangjiang fault segment. development of pull-apart basins. The relationship between stream deflection and basin development may be as follows: early stages of stream deflections by <1 km may accompany development of sag ponds; during later stages and continued strike-slip movement, sag ponds become pull-apart basins and stream deflections increase with time; ultimately, deposition in the basins ceases due to capture and stream deflections are preserved. If this is the case, the age of river deflections can be inferred from dating the basin sediments. Because Pleistocene strata are recorded in the Batang Basin, and Pliocene strata in the Ganzi Basin (Qinghai BGMR, 1991; Sichuan BGMR, 1991), this would imply that the Batang River may have formed in the late Pleistocene, and the Yalong River in the early Pliocene. If the mm/yr Holocene slip rate around the Ganzi (Wen et al., 2003) is relatively constant through time, the Yalong River deflection would have occurred over the past 2.5 ± 0.4 Myr. This estimate is consistent with the Pliocene age deduced from the sedimentary record of the Ganzi basin. Late Miocene stream deflections The fault trace between Shango and Ganzi comprises two overstep branches of the XSF, where a 40 km long pull-apart basin developed (Fig. 2). The Jinsha River flows southeastward across the Ganzi Yushu fault around Shango, then follows the fault trace for about 80 ± 5 km eastward, and finally flows southeastward again. This implies that the Jinsha River has been left-laterally deflected by ~80 ± 5 km along the Ganzi Yushu fault. This is similar to the offset of basement rocks (Triassic granitic rocks) farther east (Fig. 8; Wang and Burchfiel, 2000), suggesting that the Jinsha River may be coeval with the Yushu Ganzi fault. Additionally, there are consistent stream deflections on the scale of meters to tens of kilometers for example, the 15 km stream deflection of the Dengke River and km deflection of its tributaries (Figs. 2B and 6C). It thus seems possible that the 80 km stream deflection of the Jinsha River occurred since Ma, if the mm/yr Holocene slip rate is representative of the fault s longer-term history. Discussion Due to the brittle character of the Xianshuihe fault zone, the initiation age of the Ganzi Yushu fault has never been studied in detail. There are a few studies about the time of initiation of the fault on the Xianshuihe and the Xiaojiang segments (Roger et al., 1995; King et al., 1997; Wang et al., 1998), but the results are somewhat different. The stream deflections over widely different time scales documented in the present study provide important new information on the timing of fault activity. The Holocene rate of left-lateral slip along the Dangjiang segment is about 7 mm/yr, based on 14 C-dated stream channel deflections, and if this slip rate is representative of the long-term slip behavior of the Dangjiang segment, then we can infer that the ~16 km Dangjiang River deflection commenced at ~2.2

11 LATE CENOZOIC STREAM DEFLECTION 633 FIG. 8. Sketch map showing the 80 km stream deflection of the Jinsha River channel, as well as the same magnitude offset of Triassic granite bodies. Note that in the east the fault offset is distributed onto two splays of the fault. Simplified from Sichuan BGMR (1991). Ma. This initiation age is also consistent with formation ages of the Batang and Yalong rivers. Leftlateral faulting is not only recorded by stream deflections but also by geological offsets of ~39 km documented along the Dangjiang fault zone (Fig. 7). If this ~39 km offset represents the maximum displacement along this fault segment, then we can infer that the Dangjiang segment of the Ganzi Yushu fault formed at ~5.6 Ma. The Holocene slip rate along Ganzi segment of the Ganzi-Yushu fault is about mm/yr (Wen et al., 2003), and the largest documented geological offset along the Ganzi segment of the Ganzi Yushu fault is ~80 (Fig. 8; Wang and Burchfiel, 2000). Therefore, we infer that the Ganzi fault segment began left-lateral movement at Ma. This estimate for the initiation of left-lateral faulting along the Ganzi Yushu fault is similar to a series of important strike-slip faults in eastern Tibet. For example, the initiation age of the east Kunlun fault is ~10 8 Ma (Fu and Awata, 2007), the rightlateral Red River fault is 4.7 Ma (Leloup et al., 1995), and the Haiyuan fault is about Ma (Zheng et al., 2005). The timing of lateral displacement on these faults is also consistent with the 11 5 Ma uplift of easternmost Tibet (Kirby et al., 2002) and ~8 Ma deformation in the Qilian Shan in north Tibet (Tapponnier et al., 2001). The initiation of this strike-slip deformation may be the result of lateral transfer of crust during regional shortening in a southwest-northeast direction. Interpretation of the manner of crust moving eastward from the central part of Tibet is controversial, with contrasting hypotheses including: (1) the large-scale eastward extrusion of a lithospheric block toward the South China Sea (Tapponnier et al., 1986; Briais et al., 1993; Leloup et al., 1995); (2) lateral spreading of a thickened, viscous lithosphere with little eastward extrusion (Houseman and England, 1993); and (3) a clockwise rotation around the eastern syntaxis accompanying distributed deformation (e.g., Ratschbacher et al., 1996; Wang et al., 1998; Zhang et al., 2004). The stream deflections described in the present paper provide the opportunity to better constrain the evolution of faulting in eastern Tibet. From Dengke in the southeast to the Dangjiang in the northwest, the documented total geological offset decreases from ~80 to ~39 km, whereas deflections of the Yalong, Batang, Jielong, and Dangjiang rivers, almost all of which

12 634 WANG SHIFENG ET AL. were initiated in late Pleistocene time, show gradually decreasing deflections from 30 ± 2 km to 16 ± 1 km in the same direction along the Ganzi Yushu fault. This implies offsets of about 41 km to 14 km are accommodated by transtensional and transpresional structures such as pull-apart basins and thrust faults along different part of the fault at different times. This is also consistent with a northward decrease in the Holocene slip rate along the Ganzi Yushu fault, from 10 to 14 mm/yr around Dengke to ~7 mm/yr around Dangjiang. Conclusions This study has described and analyzed systematic left-lateral stream deflections along the Ganzi Yushu segment of the XSF. In general, stream deflections are positively correlated with size of streams and their catchment areas; however, similarly sized streams can display very different magnitudes of stream channel deflection, attesting to a distributed nature of deformation along the fault. The largest river deflection measured is consistent with displacements of geological units along the fault. Therefore, we infer that the Ganzi Yushu fault began its left-lateral movement at ~8 5.6 Ma, based on the assumption that the Holocene slip rate is representative for the longer-term slip behavior of the fault. It thus seems that the spatial and temporal evolution of stream patterns along the XSF, accompanying the clockwise rotation of crustal blocks around the eastern syntaxis, supports the hypothesis that the tectonic evolution of eastern Tibet was largely characterized by distributed deformation. Acknowledgments This research was funded jointly by the National Natural Science Foundation of China ( ) and China National Key Project (2005CB422000, 2002CB412601). Thanks are given to Dr. Peter Blisniuk for his helpful revising of the manuscript. REFERENCES Allen, C. R., Luo, Z., Qian H., Wen X., Zhou H., and Huang, W., 1991, Field study of a highly active fault zone: The XSF of southwestern China: Geological Society of America Bulletin, v. 103, p Briais, A., Patriat, P. and Tapponnier, P., 1993, Updated interpretation of magnetic anomalies and seafloor spreading stages in the south China: Implications for the Tertiary tectonics of SE Asia: Journal of Geophysical Research, v. 98, p Brookfield, M. E., 1998, the evolution of the great river systems of southern Asia during the Cenozoic India Asia collision: Rivers draining southwards: Geomorphology, v. 22, p Burchfiel, C. and Royden, L., 1991, Tectonics of Asia 50 years after the death of Emile Argand: Eclogae Helveticae, v. 84, p Clark, M. K., Schoenbohm, L. M., Royden, L. H., Whipple, K. X., Burchfiel, B. C., Zhang, X., Tang, W., Wang, E., and Chen, L., 2004, Surface uplift, tectonics, and erosion of eastern Tibet from large-scale drainage pattern: Tectonics, v. 23, TC1006 [doi: / 2002TC001402]. Fu, B., and Awata, Y., 2007, Displacement and timing of the left-lateral faulting in the Kunlun fault zone, northern Tibet inferred from geologic and geomorphic features: Journal of Asian Science, v. 29, p Fu, B., Awata, Y., Du, J., and He, W., 2005, Late Quaternary systematic stream offsets caused by repeated large seismic events along the Kunlun fault, northern Tibet: Geomorphology, v. 71, p Gaudemer, Y., Tapponnier, P., and Turcotte, D. L., 1989, River offset across active strike-slip faults: Annales Tectonics, v. 2, p Hallet, B., and Molnar, P., 2001, Distorted drainage basins as markers of crustal strain east of the Himalaya: Journal of Geophysical Research, v. 106 (B7), p. 13,697 13,709. Houseman, G., and England, P. C., 1993, Crustal thickening verus lateral expulsion in the Indo-Asian continental collision: Journal of Geophysical Research, v. 98, p. 12,233 12,249. King, R. W., Shen, F., Burchfiel, B. C., Chen, Z., Li, Y., Liu, Y., Royden, L. H., Wang, E., Zhang, X., and Zhao, J., 1997, Geodetic measurement of crustal motion in southwest China: Geology, v. 25, p Kirby, E., Reiner, P. W., Krol, M. A., Whipple, K. X., Hodges, K. V., Farley, K. V., Tang, W., and Chen, Z., 2002, Late Cenozoic evolution of the eastern margin of the Tibetan Plateau: Inferences from Ar/Ar and (U-Th)/He thermochronology: Tectonics, v. 21, no. 1 [doi: /2000TC001246]. Leloup, P. H., Lacassin, R. P., Tapponnier, D., Zhong, X., Liu, L., Zhang, S., Ji, P., and Trinh, T., 1995, The Ailao Shan Red River shear zone (Yunnan, China), Tertiary transform boundary of Indochina: Tectonophysics, v. 251, p Qinghai BGMR (Bureau of Geology and Mineral Resource of Qinghai Province), 1991, Regional geology of Qinghai province: Beijing, China, Geological Publishing House, 662 p. Ratschbacher, L., Frisch, W., Chen, C., and Pan, G., 1996, Cenozoic deformation, rotation, and stress patterns in eastern Tibet and western Sichuan, China, in Yin, A., and Harrison, M., eds., The tectonic evolution of Asia:

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