Late Cenozoic deformation along the northwestern continuation of the Xianshuihe fault system, Eastern Tibetan Plateau Shifeng Wang* Key Lab of Lithosphere Tectonic Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Beijing 100085, China Chun Fan Gang Wang Key Lab of Lithosphere Tectonic Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China Erchie Wang Key Lab of Lithosphere Tectonic Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Beijing 100085, China ABSTRACT absorbed by NE-SW shortening along the the Indian plate has moved northward for rugged North Yushu block in late Neogene ~2500 km since its collision with the Eurasian The Xianshuihe fault system is a highly time. This is interpreted to have formed as a plate, 45 ± 5 Ma ago (Rowley, 1996), and has active, left-lateral slip fault that has played rhomb-shaped restraining bend, along which undergone a counterclockwise rotation of ~30° an important role in accommodating late the left-lateral movement along the north- (Dewey et al., 1989; Molnar et al., 1993). Con- Cenozoic crustal deformation of the Eastern western end of the fault system was partially tinuing convergence between the Indian plate Tibetan Plateau. The left-lateral displace- transferred into crustal shortening. and Eurasia has mostly been absorbed by intra- ment on the western portion of the fault sys- The westward continuation of the Dangji- continental deformation primarily within the tem, the Ganzi fault, is ~80 km, based on geo- ang fault is marked by an E-W–trending fault, Eurasian plate (Tapponnier et al., 1982; England logic and geomorphic markers. Left-lateral which can be traced westward for 150 km and Houseman, 1986; Houseman and England, movement along the fault may have initiated to central Tibet, where it joins the Fenghuo 1993; Avouac and Tapponnier, 1993; Leloup et in middle Miocene time (12 Ma), followed by Shan thrust belt. This thrust belt is inter- al., 1995; Royden et al., 1997; Yin and Harrison, the intensive, brittle, left-lateral movement preted to represent the northwestern contin- 2000). Intra-continental deformation appears to in late Cenozoic time (2–4 Ma). Lying within uation of the Xianshuihe fault system, along have occurred spatially and heterogeneously, the headwater regions of the Jinsha (Yangtze) which 39 km of the left-lateral movement was and was concentrated along several large fault River in the Yushu region, the northwestern entirely transferred into the N-S shortening. zones, including the major strike-slip systems of end of the Xianshuihe fault system consists It indicates that the left-lateral movement the Altyn Tagh, Kunlun, Karakorum, Haiyuan, of four branches. These are, from north to within the southeastern part of the Tibetan Xianshuihe, and Red River-Ailao Shan fault south—the Dangjiang fault, the Yushu fault, Plateau along the fault system, either caused zones (Molnar and Tapponnier, 1975; Tappon- the Batang fault, and the Xialaxiu fault. Both by lateral extrusion or by clockwise rotation, nier and Molnar, 1977), all of which are pres- the Yushu and Dangjiang faults display clear is largely restricted to the northwestern end ently active (e.g., Cowgill et al., 2003; Armijo et geologic and geomorphic evidence for active, of the fault system. Quaternary extension al., 1989; Lacassin et al., 2004; Allen et al., 1984, left-lateral slip movement. Several offset occurred along the northwestern continua- 1991; Replumaz et al., 2001). However, the spa- markers of Triassic age along the Dangjiang tion of the Xianshuihe-Xiaojiang fault was tial and temporal evolution of the strike-slip fault suggest ~39 km of left-lateral slip, but coeval with extension along its southern end, movement along most of these faults remains the geomorphic evidence along the Dangji- indicating that the southeastern margin of either unclear or still under debate. For example, ang and Yushu faults suggests only ~16 km the plateau underwent clockwise rotation. the Quaternary left-lateral slip on the Haiyuan and ~25 km of left-lateral slip, respectively. fault is interpreted to have been absorbed by Both the Batang and Xialaxiu faults dem- Keywords: Xianshuihe fault system, stream transpressional deformation along the Liupan onstrate clear evidence for normal dip slip, offset, late Cenozoic time, restraining bend, Shan thrust belt (Burchfi el et al. 1989; Zhang by an associated series of E-W–trending clockwise rotation. et al., 1991). Other studies (e.g., Gaudemer et grabens and horsts that absorb ~9 km of left- al., 1995; Zhang et al., 1998), however, argue lateral movement in early to middle Pleisto- INTRODUCTION that the Liupan Shan thrust belt serves as a left- cene time. About 32 km of left-lateral move- lateral transfer fault, extending eastward into ment on the fault system was estimated to be Plate reconstructions (e.g., Molnar and Tap- the Qinling belt. Moreover, the left-lateral East pon nier, 1975; Tapponnier and Molnar, 1977; Kunlun fault, known as one of the most active *[email protected]; [email protected] Patriat and Achache, 1984) indicate that structures within the plateau (Van der Woerd et GSA Bulletin; March/April 2008; v. 120; no. 3/4; p. 312–327; doi: 10.1130/B25833.1; 16 fi gures; 1 table. 312 For permission to copy, contact [email protected] © 2007 Geological Society of America Late Cenozoic deformation of the Xianshuihe Fault System 950 E 0 1050 Fig.2 100 N Yalong R. 4000 2000 40 00 Da Gz du 50 F Xianshuihe I M 400 00 R. i 0 nR. 80 km 4000 Figure 1. Regional tectonic map shows the trace of Xianshuihe III fault system within eastern 50 0 Tibet. The major tectonic units 00 0 Lanc 0 3 Xs of the Eastern Tibetan Plateau LMS ang F are: I—Songpan-Ganzi Flysch Jinsh 0 0 30 belt; II—Yidun Volcanic Arc; 100 III—Three River Fold belt; M=6.5-7.4 Historical R a IV—Kungdian High. Abbrevia- . M > 7.4 seismicity R. tions: ATF—Altyn Tagh Fault; { } Fa Strike-slip fault HF—Haiyuan Fault; KLF— ult Kunlun Fault; KF—Karako- Thrust fault 30 II Yangtze R. 00 rum Fault; LMS—Longmen Normal fault Shan thrust fault; RRF—Red JhF River Fault; FF—Fenghuoshan Jinsha Suture AzF thrust fault; SF—Sagaing Stream fault; GzF—Ganzi Fault; 4000 Contour line (in meters) XsF—Xianshuihe Fault; AzF— o o Anninghe-Zemuhe Fault; 80 100 South China ATF S XjF—Xiaojiang Fault; JhF— HF y Jinghe thrust fault. The box KLF s IV tem 00 0 shows the location of Figure 2. Fenghuoshan 2 KF Kunming 250N FF X Ganzi S o Yushu 30 j X F Tibet Plateau SFS LM R.R. F. Him alaya Lhasa Indi subcontia S F nent R Fig.1 RF 100km 1050 al., 1998, 2002), loses its clear trace around the Anninghe-Zemuhe, and the Xiaojiang faults system consists of four branching faults separat- Yellow River to the east due to a decrease of the (Allen et al., 1991; Wang and Burchfi el, 2000). ing horst and graben structures and other rugged left-lateral movement. According to Kirby et al. The most prominent geomorphic evidence for high mountains (Fig. 2). Little is known about (2007), this resulted from a horsetail tectonic the left-lateral movement along the fault sys- the young and active geologic and geomor- transformation, whereas Zhang et al. (1995, tem is present at the Ganzi fault, along which phic features of this region. Moreover, the link 1998) propose that the left-lateral movement the Jinsha River is defl ected and a large granite between these structures and the displacement oversteps easterly into the Qinling belt. In most batholith is offset by ~80 km (Wang et al., 1998; along the Ganzi fault is uncertain. previous studies (e.g., Allen et al., 1991; Wang Wang and Burchfi el, 2000). Westward of the This study focuses on the aforementioned and Burchfi el, 1998, 2000), the Xianshuihe fault defl ected Jinsha River, the Xianshuihe fault sys- active and young fault systems. We elaborate on system was drawn to be ending northwestward tem continues for 200 km into the Yushu area. their genesis and attempt to answer the follow- in the southeastern corner of the Tibetan Pla- Presently, no study explains how its 80-km, ing questions: (1) Was the basin development in teau, however, without any explanation of the left-lateral displacement is accommodated far- the study area coeval with left-lateral displace- nature of this termination with respect to other ther west. During our recent fi eld study west of ment along the northwestern end of the Xians- strike-slip faults. the Shango area, where the fl at plateau emerges, huihe fault system? (2) What is the tectonic The Xianshuihe fault system has four seg- we found that the northwestern termination of nature of the rugged high mountains in the area? ments (Fig. 1), which are, from northwest to the fault system is not a single structure as most (3) When did these faults that we have identifi ed southeast—the Ganzi, the Xianshuihe, the studies indicate. Instead, the Xianshuihe fault initiate and how have they evolved with time? Geological Society of America Bulletin, March/April 2008 313 Late Cenozoic deformation of the Xianshuihe Fault System GEOLOGIC SETTING OF THE Jiezha group and volcanic rocks and limestone The eastern segment, termed East Yushu XIANSHUIHE FAULT SYSTEM of the Batang group. fault, stretches between Shango and Longshida These three tectonostratigraphic units are (Fig. 3). The east end of the fault cuts through The Xianshuihe fault system is a prominent, unconformably overlain by a series of Cenozoic a rhomb-shaped divide, separating the Jinsha linear, NW-SE–trending tectonic feature within sedimentary rocks of Paleogene to Quaternary River to the east and the Batang River to the the southeastern margin of the Tibetan Plateau.