Initial Rupture and Displacement on the Altyn Tagh Fault, Northern Tibetan Plateau: Constraints Based on Residual Mesozoic To

Research Paper

GEOSPHERE Initial rupture and displacement on the Altyn Tagh fault, northern Tibetan Plateau: Constraints based on residual Mesozoic to

GEOSPHERE; v. 11, no. 3 Cenozoic strata in the western Qaidam Basin

doi:10.1130/GES01070.1 Feng Cheng1,2, Zhaojie Guo1, Hillary S. Jenkins3, Suotang Fu4, and Xiang Cheng1 1Key Laboratory of Orogenic Belts and Crustal Evolution, Ministry of Education, School of Earth and Space Sciences, Peking University, Beijing, 100871, China 11 figures; 1 table; 1 supplemental file 2Laboratoire Géosciences Rennes, CNRS-UMR6118, Université Rennes 1, Observatoire des Sciences de l’Univers, 35042 Rennes, France 3Department of Environmental Studies, University of Redlands, Lewis Hall, 1200 E. Colton Avenue, Redlands, California 92373, USA 4Qinghai Oilfield Company, PetroChina, Dunhuang, Gansu, 736202, China CORRESPONDENCE: [email protected]

CITATION: Cheng, F., Guo, Z., Jenkins, H.S., Fu, S., ABSTRACT son, 2000; Tapponnier et al., 2001; Yin et al., 2002; Wang et al., 2014). Recent and Cheng, X., 2015, Initial rupture and displacement on the Altyn Tagh fault, northern Tibetan Plateau: studies have revealed much about the basic geology of the Altyn Tagh Range Constraints based on residual Mesozoic to Cenozoic The Altyn Tagh fault, located in the northern Tibetan Plateau, is a large and the surrounding region (Wang, 1997; Cowgill et al., 2000, 2003; Yue et al., strata in the western Qaidam Basin: Geosphere, v. 11, left-lateral strike-slip fault heavily responsible for the growth and formation 2001, 2004a; Yin et al., 2002; Chen et al., 2003; Dupont-Nivet et al., 2003, 2004; no. 3, p. 921–942, doi:10.1130/GES01070.1. of the plateau during Cenozoic time. Despite its significance, the initial tim- Ritts et al., 2004; Wu et al., 2012a, 2012b; Cheng et al., 2014; Lu et al., 2014;

Received 7 May 2014 ing and kinematic patterns of movement along the Altyn Tagh fault remain Zhang et al., 2014); however, the immense size and extent of the Altyn Tagh Revision received 19 March 2015 highly debated. Here, we present a detailed analysis of the stratigraphy and Range make it difficult to locate ideal piercing points with which to estimate Accepted 20 April 2015 geochronology of three key lithologic sections (Tula, Anxi, and Caishiling) the initial timing of left-slip movement and total displacement along the Altyn Published online 13 May 2015 along the Altyn Tagh fault to better understand this kinematic history. By Tagh fault. As a result, both the timing and amount of slip along the fault are correlating stratigraphic contacts and lithology with the U-Pb age spectra vigorously debated (e.g., Yin et al., 2002; Cowgill et al., 2003; Gehrels et al., of Mesozoic samples within the western Qaidam Basin, we find the Altyn 2003a, 2003b; Ritts et al., 2004; Wu et al., 2012a, 2012b). Tagh fault has experienced a total of ~360 km of displacement during the Although some studies reference Mesozoic shearing in the Altyn Tagh Cenozoic. By combining seismic profile data with geologic observations, Range, the growth of the Tibetan Plateau is largely related to Cenozoic faulting we divide the activity along this fault into two distinct stages of motion: along the Altyn Tagh fault rather than any pre-Cenozoic shearing in the Altyn (1) an initial stage, which occurred between early Eocene (ca. 49 Ma) and Tagh Range (Tapponnier et al., 1986, 2001; Arnaud et al., 2003; Wang et al., mid-Miocene time (ca. 15 Ma) and resulted in ~170 km of offset, and (2) an 2005; Li et al., 2006; L. Liu et al., 2007). Various approaches have been used early stage, which began in the late Miocene Epoch and continues into the to constrain the initial timing of left-slip movement along the Altyn Tagh fault, present, resulting in ~190 km of offset along the fault. We identify the Tula and the estimates vary greatly. Initial movement along the Altyn Tagh fault is and Anxi sections as piercing points along the western segment of the Altyn estimated to have occurred broadly between the Eocene and Miocene Epochs Tagh fault and define these regions as residual parts of the original Qaidam (Chen et al., 2001; Meng et al., 2001; Wan et al., 2001; Yue et al., 2001; Yin et al., Basin. These estimates suggest that motion along the Altyn Tagh fault has 2002; Robinson et al., 2003; Wu et al., 2012a, 2012b), and estimates of the total accelerated from an average left-lateral strike-slip rate of ~5.0 mm/yr during displacement along the Altyn Tagh fault vary anywhere from ~1200 km to less initial stage faulting to a rate of ~12.6 mm/yr between the late Miocene Ep- than 90 km (Tapponnier et al., 1986; CSBS, 1992; Wang, 1997; Yin and Harrison, och and present day. 2000; Yang et al., 2001; Yin et al., 2002; Cowgill et al., 2003; Gehrels et al., 2003a, 2003b; Ritts et al., 2004). The wide variety of estimates regarding the initial timing and offset of the Altyn Tagh fault highlight the incomplete understanding of INTRODUCTION this remote region. In particular, Jurassic strata in both the southeast Tarim and northwest Qa- As a major strike-slip fault on the Tibetan Plateau, the Altyn Tagh fault plays idam Basins across the Altyn Tagh fault have been used as the geologic pierc- a significant role in the Cenozoic deformation of northern Tibet (Fig. 1). Under- ing point to constrain the total displacement along the Altyn Tagh fault (e.g., standing the Cenozoic kinematic patterns of the Altyn Tagh fault holds import- Ritts and Biffi, 2000; Meng et al., 2001). However, Jurassic strata in western ant implications for unraveling the evolution of northern Tibet, deciphering China are widespread and well developed in the foothills of several mountain For permission to copy, contact Copyright the growth history of the entire Tibetan Plateau, and contributing to oil and belts (e.g., the Tian Shan, Altun Shan, Qilian Shan, and western Kunlun Moun- Permissions, GSA, or [email protected]. gas exploration in the surrounding region (Yue and Liou, 1999; Yin and Harri tains). These Jurassic strata are regionally similar, mainly characterized by a

8°6E 88°E 90°E 92°E 94°E 80 90 100 A Shan Tian Fig.1B Tarim 40

) N basin Tarim basin 35 8 Tibetan Plateau Himalay h R ange 4 30 Alty n T ag 2 a 0200 400km n Elevation(km India l basi 1 Xorko Lenghu Caishiling ault 38° agh F 38° Alty n T Anxi Qaidam N Tula N basin

Gansen Tula subbasin Kumukol basin 0 100km E ast Kunlu Town n R ange Major 36° Fault 36° 1 2 3 4 5 6 Kunlu n fault N Basin N Range Elevation (km) B

Figure 1. Digital topographic map of the center segment of the Altyn Tagh fault and surrounding area. Topography is from Shuttle Radar Topography Mission (SRTM) data.

succession of terrestrial detrital rocks with coal-bearing deposits, locally inter- REGIONAL GEOLOGY calated with relatively few volcanic rocks (Hendrix et al., 1992; Hendrix, 2000; Li et al., 2004; Yue et al., 2004b; Yang et al., 2013; D.D. Liu et al., 2013). While lith- Altyn Tagh Range ologically similar, these sedimentary features do not provide a distinct linear feature that can be matched across the Altyn Tagh fault; thus, Jurassic strata The Altyn Tagh Range is located at the northern edge of the Tibetan Pla- alone on either side of the Altyn Tagh fault are inadequate and insufficient to teau, separating the Tarim Basin to the northwest from the Qaidam Basin to document the total offset of the Altyn Tagh fault. the southeast (Figs. 1 and 2). The bedrock of the Altyn Tagh Range is dominated In this paper, we present a detailed analysis of stratigraphy, sediment by Precambrian igneous and metamorphic rocks and Paleozoic sedimentary types, and detrital zircon U-Pb ages, which enables us to establish strati- rocks, whereas Mesozoic and Cenozoic strata are only sporadically repre- graphic correlations to source terranes and to constrain the amount of tec- sented (Wang, 1997; Sobel et al., 2001; Yin et al., 2002; Chen et al., 2003). Within tonic transport along the Altyn Tagh fault (e.g., Gehrels, 2014). We present a the Altyn Tagh Range, the ~1600-km-long ENE-trending Altyn Tagh fault starts synthesis of the stratigraphic and lithologic data along three sections (Tula, in the western Kunlun Range in the southwest and terminates in the Qilian Anxi, and Caishiling sections) of the western segment of the Altyn Tagh fault Shan Range in the northeast, linking the Kunlun and Qilian Shan thrust belts and the detrital zircon U-Pb geochronology for the Tula and Caishiling sec- (Burchfiel et al., 1989; Wang, 1997; Yue and Liou, 1999; Yin and Harrison, 2000; tions (Figs. 1 and 2). We use this information to constrain the timing of tec- Yin et al., 2002). The origin of the Altyn Tagh fault (Meyer et al., 1998; Chen et al., tonic activity on the northern Tibetan Plateau, to establish a more accurate 2001; Meng et al., 2001; Wan et al., 2001; Yue et al., 2001; Yin et al., 2002; Ritts estimate for the timing of initial movement along the Altyn Tagh fault, and to et al., 2004; Wu et al., 2012a, 2012b) and the total displacement along the Altyn provide a more accurate estimate of the total length of offset along the Altyn Tagh fault remain heavily debated (CSBS, 1992; Ritts and Biffi, 2000; Yang et al., Tagh fault during Cenozoic time. 2001; Cowgill et al., 2003; Gehrels et al., 2003a, 2003b). Quaternary slip rates

86° 88°90° 92° 39° 4 2 443, 485 Ma 482 Ma 2 00′ 39° 1 479 Ma 0 100km 440 Ma 00′ 2 2 413 Ma 1 1 444 Ma N e 922 Ma 398 Ma ng n CSL3 A’ a 2 38° h R basi 270, 272 Ma 38° n Tag 1 30′ tyn 475 Ma Xorkol CSL2 30′ asi Al 1 6 969 Ma CSL1 m b 906-938 Ma 1 6 1 518 Ma D’ Caishiling ari 910,923 Ma 457 Ma E T Fig. 10 Fig. 3 4 D E’ 38° 1 Anxi 439, 485 Ma 4 38° 448 Ma 262 Ma G2 00′ Fig.5 4 00′ 1 424 Ma Qaidam Tula 404 Ma Qi 4 m A 1 ult 271 Ma e 290 Ma n T basin lt 4 ag 37° fau 251 Ma h 4 37° gh R 429, 462 Ma Ta BaiganhuFa an 30′ tyn 4 g 30′ Al 4 446 Ma e Fig.4 G1 284 Ma 204, 242 Ma3 Kumukol basin 1 3 3 25 0 Ma 186 Ma 253 Ma 4 37° 219, 235 Ma 37° Pluton Pluton Quaternary Neogene Paleogene Cretaceous Jurassic 00′ Age<270 Ma Age>270 Ma 00′ Altyn Tagh Kunlun Strike- slip Inferred Sampling Seismic Thrust A basement basement fault fault site A’ profile

86° 88°90° 92° Figure 2. Geologic map of the center segment of the Altyn Tagh fault and the surrounding area, adapted from the Geologic Map of the Tibetan Plateau and Adjacent Areas compiled by Chengdu Institute of Geology and Mineral Resources and Chinese Geological Survey (map scale 1:1,500,000). Ages (zircon U-Pb) of plutons are mainly compiled from: 1—Cowgill et al. (2003); 2—Gehrels et al. (2003a); 3—HGSI (2003); 4—Chen et al. (2004); 5—Li et al. (2013); 6—Wang et al. (2013). See provenance analysis of Jurassic to Cretaceous strata sections for details.

of ~10 mm/yr on the Altyn Tagh fault have been determined using field data, though several hypotheses have been proposed (Métivier et al., 1998; Mock geodetic observations, and global positioning system (GPS) measurements et al., 1999; Xia et al., 2001; E. Wang et al., 2006; Meng and Fang, 2008; Yin et (Bendick et al., 2000; Shen et al., 2001; Yin et al., 2002; Zhang et al., 2004, 2007; al., 2007, 2008a, 2008b; C.S. Wang et al., 2011). One hypothesis suggests that Cowgill et al., 2009; Yin, 2010). the western Qaidam Basin experienced two Cenozoic structural phases: the first between the Paleocene and Eocene Epochs and the second between the middle Miocene and Pleistocene Epochs (Y.D. Wang et al., 2010). Growth strata Qaidam Basin and a regional angular unconformity between the early Miocene and late Mio- cene successions in the western Qaidam Basin are identified as depositional The triangular-shaped Qaidam Basin, which lies on the southeastern side of responses to regional tectonic events in both the Altyn Tagh Range and the the Altyn Tagh Range, is the largest petroliferous basin within the northern Ti- Eastern Kunlun Range during middle Miocene time (Song and Wang, 1993; Yin betan Plateau (Figs. 1 and 2). The western Qaidam Basin is bound by the Altyn et al., 2007, 2008b; L. Wang et al., 2010; Y.D. Wang et al., 2010; Wu et al., 2012a). Tagh Range to the northwest and the Qimen Tagh Range (western segment of Geologic mapping and petroleum exploration have revealed that the Qa- the Eastern Kunlun Range) to the south. The westernmost part of the Qaidam idam Basin consists mainly of Cenozoic to Mesozoic nonmarine strata uncon- Basin is characterized by a unique NE-trending elongate landform known as formably overlying an uncertain basement (Xia et al., 2001; Meng and Fang, the Tula trough (E. Wang et al., 2006) or Tula subbasin (Guo et al., 1998; Figs. 1 2008). The basement rock is too deep to be reached via drilling, and its lithol- and 2). High-quality subsurface data have revealed that the southeastern Qa- ogy therefore remains uncertain (Xia et al., 2001; Fang et al., 2007; Yin et al., idam Basin is governed by a series of high-angle NW-trending reverse faults 2008b). The Mesozoic strata are mainly located along the southern flank of the with considerable strike-slip component (Cheng et al., 2014, 2015), resulting in Altyn Tagh and the Qilian Shan Ranges (Yin et al., 2008a; Wu et al., 2011). A stratigraphic throw on either side of these faults in seismic profile (Fig. 3). The succession of depocenters of Cenozoic strata is present along the long axis of tectonic-sedimentary evolution of the Qaidam Basin remains largely unknown, the basin and has gradually migrated eastward since the Eocene Epoch (Song

Period Epoch Age(Ma) Formation(Fm.) Thickness(m) Symbol Lithological description

Holocene 0.01 Dabuxun-Yanqiao 0-171 Q2-4 Dominated by gray and yellow-graysand gravels. Dominated by gray, yellow-gray conglomerates and pebble sandstones interbed Pleistocene Qigequan 30-2,500 Qq 1.8 1 with few argillaceous sandstones; unconformablewith the underlying Shizigou Fm. 2.5 Characterized by gray-white conglomerates, pebble sandstones intercalated with Pliocene 3 Shizigou 370-1,800 sandstones and argillaceous siltstones; separated with underlying Shangyoushashan Fm. 5.3 Ns2 by color changing from brown in Shangyoushashan Fm. to gray white in Shizigou Fm.

eogene 8.1 2 N Shangyoushashan 350-2,230 Nsy Dominated by gray-brown mudstones, intercalated with dark-gray carbonaceous 2 mudstones; unconformable with the underlying lower Xiayoushashan Fm. and Miocene 15.3 conformable or locally unconformable with overlying Shizigou Fm. Xiayoushashan 300-2,900 Nx1 y 2 Characterized by gray, brownish-red thick bedded mudstones and siltstones,intercalated 22 23.0 with marlstones and limestones; separated with the overlying Shangyoushashan Fm. by a local unconformity.

Oligocene Shangganchaigou 600-1,000 Ns1 g Dominated by grayish-green, yellowish-green mudstones andsandstones intercalated with mudstones; separated with the overlying upper Xiayoushashan Fm. by appearance of 33.9 green gray sandstones; conformable contactwiththe underlyingShangganchaigou Fm. 35.5 2 Upper Ex3 g Dominated by successions of gray, dark-gray mudstones andcarbonaceous siltstones, 37.8 Xiaganchaigou intercalated with argillaceous siltstones and marlstones; conformable contactwiththe

180-1,230 1 underlying lower Xiaganchaigou Fm. Eocene Ex3 g Lower Characterized by successions of brownish-red sandstones interbed with mudstones; Paleogene 43.8 separated with the overlying upper Xiaganchaigou Fm. by the appearance of gray mudstones; concordant or locally discordant with the underlying Lulehe Fm. El 55.8 Lulehe 267-1,400 1+2 Dominated by successions of purple-red conglomerates, pebble sandstones, interbed Paleocene with sandstones, siltstones and mudstones; separated with the overlying lower 65.5 >53.5 ? Xiaganchaigou Fm. by the disappearing of the conglomerates; uncomformable contact with Quanyagou Kq the underlying Quanyagou Fm.

Mesozoic Caishiling Jc3 Mainly characterized by successions of terrestrial detrital rocks with coal-bearing Dameigou Jd deposition, dominated by brownish and purple red conglomerates interbed with dark 201 1+2 brown mudstones; unconformable overlying on the basement of surrounding mountains. A A′ 0.0

) 1.0 Growth Growth Growth

e (s strata strata strata 2.0

3.0 raveltim 4.0 yT 3 2 1 wa 5.0 Q N2 N2 N2 N1 o- 20 km 2 1 Basement Tw Mesozoic 6.0 E3 E3 E1+2

Figure 3. Stratigraphy and lithologic unit descriptions of the Qaidam Basin. The Cenozoic stratigraphy of the basin has been defined and dated in detail using magnetostratigraphy, palynology, and paleontology studies within the entire basin (Fang et al., 2007; Gao et al., 2009; Huang et al., 1996; Huo, 1990; Lu and Xiong, 2009; QBGMR, 1991; Qiu, 2002; Z.M. Sun et al., 2005; Xia et al., 2001; 1 Yang et al., 1992; Zhao et al., 2006). See Figure 2 for seismic profile location. Note the growth strata above the Xiayoushashan Formation (N2 xy).

and Wang, 1993; Xia et al., 2001; Qiu, 2002; J.M. Sun et al., 2005; E. Wang et Dameigou Formation (J1+2d) is composed of fining-upward gray-brown con- al., 2006; Yin et al., 2008b). Based on magnetostratigraphy, palynology, and glomerate beds interbedded with greenish-gray sandstone and siltstone beds.

paleontology, the Mesozoic–Cenozoic strata of the Qaidam Basin have been The upper part of the Dameigou Formation (J1+2d) is characterized by a set subdivided into 11 units, and the chronostratigraphy of the Cenozoic units has of sandstone beds interbedded with conglomerate, siltstone, and lithic sand- been well constrained (Huo, 1990; QBGMR, 1991; Yang et al., 1992; Huang et stone (Fig. 5A). Grain size increases up section, indicating gradual shallowing al., 1996; Xia et al., 2001; Qiu, 2002; J.M. Sun et al., 2005; Zhao et al., 2006; and subsidence of the basin. In the Caishiling section, thick-bedded lithic sand- Fang et al., 2007; Yin et al., 2007; Gao et al., 2009; Lu and Xiong, 2009; Wu et al., stone and conglomerate (Fig. 5B) are developed at the base of the Dameigou

2011). These units are listed in order from oldest to youngest, followed by the Formation (J1+2d), indicating that denudation of the source area began in Early symbol for each unit, and the age range capture of each unit, if available: (1) Jurassic time (Robinson et al., 2003).

the Dameigou Formation, J1+2d; (2) Caishiling Formation, J3c; (3) Quanyagou The Caishiling Formation (J3c) is composed of yellowish massive sandy

Formation, Kq; (4) the Lulehe Formation, E1+2l, older than 53.5–43.8 Ma (Yang et conglomerate (Fig. 5C) intercalated with quartzolithic sandstone at the base, al., 1992; Zhang, 2006; Ke et al., 2013); (5) the lower Xiaganchaigou Formation, and it is characterized by purple-red, gray-purple siltstone in the lower part 1 E3 xg, 43.8–37.8 Ma (Zhang, 2006; Z.C. Sun et al., 2007; Pei et al., 2009); (6) the of the section. These rocks transition to variegated sandstone and siltstone 2 upper Xiaganchaigou Formation, E3 xg, 37.8–35.5 Ma (Z.M. Sun et al., 2005; intercalated with conglomerate in the middle and upper parts of the section Z.C. Sun et al., 2007; Pei et al., 2009); (7) the Shangganchaigou Formation, (Ritts and Biffi, 2000; Robinson et al., 2003). In the upper part of the Caishiling

N1sg, 35.5–22.0 Ma (Z.M. Sun et al., 2005; Lu and Xiong, 2009); (8) the lower Formation (J3c), cross-bedding is well developed in the sandstone, and imbri- 1 Youshashan Formation, N2 xy, 22.0–15.3 Ma (Fang et al., 2007; Lu and Xiong, cation can be found in the conglomerate (Fig. 5D). The uppermost part of the 2 2009); (9) the upper Youshashan Formation, N2 sy, 15.3–8.1 Ma (Fang et al., Caishiling Formation (J3c) contains thick-bedded oil sands (Fig. 5E) and natural 3 2007); (10) the Shizigou Formation, N2 s, 8.1–2.5 Ma (Fang et al., 2007); and (11) asphalt (Fig. 5F), indicating the Tula unit was once a subsidence area charac-

Quaternary deposits, including the Qigequan Formation (Q1q) and the Dabux- terized by a topographic low different from the narrow valley that exists at

un-Yanqiao Formation (Q2–4), 2.5–0 Ma (Fang et al., 2007; Yin et al., 2008b). The present (Fig. 1). Geochemical fingerprints (e.g., carbon isotopic composition) lithologic features of each unit are summarized in detail Figure 3. in the Jurassic oil-bearing sandstone of the Tula Basin are similar to the geochemical fingerprints in the Jurassic oil-bearing sandstone oil in the Qaidam Basin (Guo et al., 1998). RESIDUAL MESOZOIC–CENOZOIC STRATIGRAPHY ALONG THE More than 2000 m of Cretaceous strata (Quanyagou Formation) uncon- ALTYN TAGH FAULT formably overlie the Late Jurassic strata of the Caishiling Formation (XBGMR, 1993; Robinson et al., 2003). In the Tula section, Cretaceous plutons (ca. 74 Ma) Mesozoic to Eocene Strata in the Tula Section intrude Cretaceous strata, providing a minimum depositional age (XBGMR, 1993; Robinson et al., 2003). The lower part of the Cretaceous strata is domi- The Tula section is bounded by the Altyn Tagh fault to the north and the nated by purple-red conglomerate intercalated with sandstone, siltstone, and Kunlun Precambrian and Paleozoic basement to the south (Figs. 2 and 4). A mudstone interpreted to represent a lacustrine depositional environment. The narrow valley runs through this region at the westernmost part of the Qaidam upper part of Cretaceous strata consists of purple-red siltstone interbedded Basin. Precambrian, Ordovician, and Carboniferous rocks comprise the base- with silty mudstone interpreted to represent a lacustrine deltaic depositional ment rock, which is unconformably overlain by Mesozoic to Cenozoic strata environment (Ritts and Biffi, 2000; Robinson et al., 2003). The lacustrine to la- (Guo et al., 1998; HGSI, 2003; Ritts et al., 2004). Mesozoic strata in the Tula custrine delta deposits in the Tula unit reinforce the idea that the Tula unit was section are dominantly terrestrial clastic rocks. Several faults are present on characterized by a topographic low during the Cretaceous, a setting very differ- the south side of the Tula Basin (Fig. 2). The left-lateral NE-SW–trending strike- ent from the narrow valley that exists at present (Fig. 1). slip Baiganhu fault can be identified by a series of distinctive fault scarps and In the Tula unit, the well-exposed Paleocene to early Eocene Lulehe For- structural lineaments within the mountains, although the fault later becomes mation overlies the Mesozoic strata along a low-angle unconformity (XBGMR, obscured by Quaternary deposits in the Tula Basin (Fig. 2; Cowgill et al., 2003; 1993; HGSI, 2003; Robinson et al., 2003). The Lulehe Formation consists mainly HGSI, 2003). of purple-red conglomerate interbedded with fine-grained sandstone at the Jurassic clastic coal-bearing strata rest unconformably on the Altyn Tagh base, which becomes coarser up section, along with an increase in the abun- basement (Fig. 4). Based on lithologic associations, sequence stratigraphy, and dance of conglomerate toward the top. The formation contains a high met- paleontology, these strata have been divided into two units: the Dameigou amorphic lithic fraction and limestone clasts with coral fossils that suggest

Formation (J1+2d), located in the lower part of the section, and the Caishiling a proximal provenance and syntectonic deposits (Zuffa, 1980; Mack and Ras-

Formation (J3c), located in the upper part of the section (XBGMR, 1993; HGSI, mussen, 1984; HGSI, 2003; Robinson et al., 2003). The distinctive purple-red 2003; Robinson et al., 2003; Dupont-Nivet et al., 2004). The lower part of the conglomerates found in the Lulehe Formation are regionally similar to those

86°15′E886°30′E 86°45′E 7°00′E

Q N 1 O 37° 37° ault J1+2 45′ gh f 45′ n Ta Tula N Alty N Tula subbasin J1+2 Q2-4 B’ G2 50 60 C1 J3 23 J 40 40 28 1+2 Q 38 2-4 K 37 Q2-4 47 61 40 J 39 3 K 20 20 7 Western half 21 Easter38n half C 70 80 42 1 50 E 40 37° K 1+2 G1 20 37° Figure 4. Geologic map (after Qie- B mo J45C003002 geologic map, scale 30′ Asphalt and oil 30′ 1:250,000) and geologic cross sec- J3 sand described tion of the study area, compiled from N 253Ma N C1 in Fig.5 HGSI (2003), Robinson et al. (2003), and XBGMR (1993). Red stars denote 0 10km locations of U-Pb detrital zircon sam- 204, 242Ma ples. Ages (zircon U-Pb) of plutons are mainly compiled from HGSI (2003). 86°15′E886°30′E86°45′E 7°00′E The western half and eastern half of N the Eocene strata are based on the B 04km B’ division of Dupont-Nivet et al. (2004). 5000 Elevation(m) C 1 E 4000 1+2 K J3 Q2-4 3000

2000 Deformed

Conglo- Pebbly Siltstone merate sandstone Sandstone Mudstone

Andesite Rhyolite Volcanic Tuff Slate breccia Quaternary Quaternary Paleogene Cretaceous Jurassic Dameigou Q2-4 Sediments Q1 Qigequan E1+2 Lulehe K Quanyagou J1+2 Formation Formation Formation Formation Jurassic Carboniferous Ordovician J3 Caishiling C1 O Granite Precambrian Formation Sediments Sediments Basement Attitude of Fault 20 Unconformity Sampling A A’ Section Stratum Point

A B Q Q Q

Q Sch Pl Sch Q Bt Q Sch Kp 500μm 500μm

C D Figure 5. Typical photomicrographs and field photographs. (A) Lithic sandstone

in early Jurassic strata (J1+2d) of the Tula section, rich in lithic fragments, under plane polarized light. (B) Lith- ic sandstone in Early Jurassic strata

(J1+2d) of the Caishiling section, rich in various lithic fragments, under crossed polarized light. (C) Conglomerate and

(D) cross-bedding in Late Jurassic (J3c) strata of the Tula section. (E) Oil sand and (F) asphalt developed in Late Ju-

rassic (J3c) strata. Abbreviations: Sch— schist, Pl—plagioclase, Kp—potash Conglomerates Cross-bedding 10 cm feldspar, Q—quartz, Bt—biotite.

E F

Oil sand Asphalt

found in the Qaidam Basin. The Lulehe Formation has been dated via magne- of the valley. The lack of paleomagnetic data in the Anxi section has thus far tostratigraphy to between 65–53.5 and 43.8 Ma (Yang et al., 1992; Zhang, 2006; prevented a further division of these strata into two units as has been done in Ke et al., 2013). These dates further support a Paleocene to early Eocene age for the western Qaidam Basin (Fig. 3). The lower part of the Ganchaigou Formation the Lulehe Formation. is dominated by gray to grayish-green sandstones intercalated with siltstones, The Jurassic–early Eocene strata in the Tula section are heavily deformed, biomicrites, and mudstones. The upper part of the Ganchaigou Formation, forming a major north-verging syncline with a steeply dipping south limb and however, is characterized by grayish-green calcareous shales and mudstones. a shallowly dipping north limb (Fig. 4). The Jurassic, Cretaceous, and Paleo- The Ganchaigou Formation is overlain by the Miocene Youshashan Formation, cene to early Eocene strata in the south limb of the syncline are inverted with which contains brown granule conglomerate at its base (GGSI, 2003). Like a high dip angle (dipping mainly to the south from 180° to 200° at angles of the overlying Ganchaigou Formation, the Youshashan Formation crops out in 65°–80°). The contact between the Cretaceous and Paleocene to early Eocene the northern part of the valley. The Youshashan Formation has not been sub- strata is characterized by a low-angle angular unconformity or a local fault divided into two units as has been done in the western Qaidam Basin (Fig. boundary (XBGMR, 1993; HGSI, 2003; Robinson et al., 2003). 3; GGSI, 2003). The lower and middle sections of the Youshashan Formation strata consist mainly of brown and maroon granule conglomerate interbedded with coarse sandstone, while the upper part is characterized by purple Cenozoic Strata in the Anxi Section and brown sandstone and pebble sandstone interbedded with conglomerate, siltstone, and mudstone. These primarily brown strata distinguish the Yousha- The Anxi section, ~150 km northeast of the Tula section, is located in the Al- shan Formation from the overlying Pliocene strata of the Shizigou Formation, tyn Tagh fault zone and is bounded by the Altyn Tagh basement to the north and which is characterized by earthy yellow and gray pebble sandstone at its base the Kunlun Precambrian and Paleozoic basement to the south (Figs. 2 and 6). (GGSI, 2003). The Pliocene strata are then overlain by Quaternary sand-gravel Precambrian and early Paleozoic metamorphic rocks with igneous intrusions beds, which lie atop an angular unconformity (Fig. 6). The lithology in the lower comprise this basement, which is overlain by Mesozoic and Cenozoic strata part of the Pliocene strata consists of sandstone interbedded with siltstone and (Guo et al., 1998; Ritts and Biffi, 2000; GGSI, 2003). The Cenozoic units in the mudstone, while the middle and upper parts of the Pliocene strata are char- Anxi section have been identified based on detailed field mapping of previous acterized by conglomerate and sandstone intercalated with siltstone (GGSI, studies, and depositional ages are estimated by correlating these units with 2003). These thickly bedded conglomerates are interpreted to represent alluvial the well-constrained Cenozoic strata of the Qaidam Basin (Fig. 7; Huo, 1990; fan to fluvial facies (Ritts and Biffi, 2000). QBGMR, 1991; Yang et al., 1992; Huang et al., 1996; Xia et al., 2001; Qiu, 2002; GGSI, 2003; J.M. Sun et al., 2005; Zhao et al., 2006; Fang et al., 2007; Yin et al., 2007; Gao et al., 2009; Lu and Xiong, 2009; Wu et al., 2011). METHODS Unlike the poorly preserved Cenozoic strata of the Tula section, the Anxi section contains a well-exposed Cenozoic stratigraphic succession that in- Field Investigation and Seismic Profile Interpretation cludes most of the Cenozoic units described already. The Anxi section includes the Paleocene to early Eocene Lulehe Formation, the Eocene to Oligocene We used three sections (Tula, Anxi, and Caishiling) along the western seg- Ganchaigou Formation, the Miocene Youshashan Formation, and the Pliocene ment of the Altyn Tagh fault to study stratigraphic features of the Mesozoic– Shizigou Formation. As visible in cross-sectional profile (Fig. 6), the Lulehe, Cenozoic deposits (Fig. 2). We integrate three high-quality seismic profiles of Ganchaigou, and Youshashan Formations of the Anxi section are in conform- the western Qaidam Basin with our surficial field observations to explain the able contact with one another and have been deformed synchronously after tectonic history of this region. Two of the seismic profiles (A–A′ and D–D′) trend early Miocene time. However, in plan view, these Paleocene to Miocene strata along strike (northeast) of the Altyn Tagh fault, while one of the profiles (E–E′) are cut by the left-lateral strike-slip Altyn Tagh fault and are separated from the trends perpendicular (northwest) to the Altyn Tagh fault (Fig. 2). Seismic data overlying Pliocene strata of the Shizigou Formation by an apparent angular were interpreted using SMT Kingdom software. unconformity (Fig. 6). The Paleocene to early Eocene strata of the Lulehe Formation crop out in the northern part of the valley and trend in a NE direction. These strata are char- Sampling and Analytical Process acterized by brick-red conglomerates and pebble sandstones intercalated with sandstones and siltstones. This portion of the Lulehe Formation is separated Detrital zircon geochronology is rapidly developing into an essential tool from the typically overlying Eocene strata of the Xiaganchaigou Formation by for sediment provenance analysis and geodynamic study (e.g., Fedo et al., the disappearance of the conglomerate layers (GGSI, 2003). The Eocene to Oli- 2003; Thomas, 2011; Gehrels, 2014). U-Pb dating of detrital zircons from sed- gocene strata of the Ganchaigou Formation only crop out in the northern part iments is widely used to reconstruct the provenance of ancient sedimentary

88°10′E88°20′E 88°30′E 88°40′E

Q E1+2

N 60 C’ 1 65 N2 58 50 38° 40 38° E3 13 60 05′ 48 05′ 45 J E E1+2 1+2 N 3 Q 65 N

E1+2 30 15 63 E1+2 26 32 55 Q 38° 3 38° N2 00′ C Q 00′ N N ault gh F n Ta Alty Tula subbasin 37° 37° 55′ 55′ N 0 10km Q N

88°10′E 88°20′E888°30′E 8°40′E C C’ Elevation(m) 4° 02 km 3 Unconformity shown in Fig. 7 5000 N2 2 E1+2 ηγ3 J E1+2

4500 E3 1 N2

4000 Deformed Deformed

Conglo- Pebbly Sandstone Siltstone merate sandstone Mudstone

Granite Ultramafite Fault 50 Attitude of Unconformity stratum

Neogene Quaternary 3 1 Neogene Paleogene Shizigou B Q sediments N2 N2 Youshashan E3 Ganchaigou B’ Section Formation Formation Formation Precambrian Paleogene Jurassic Dameigou ∑- N(Qn-O) Basement E1+2 Lulehe J1+2 Ultramafite Granite Formation Formation

Figure 6. Geologic map (after the Washixia J45C002003 geologic map, scale 1:250,000) and geologic cross section of the study area, compiled from GGSI (2003) and XBGMR (1993).

GEOSPHERE | Volume 11 | Number 3 Cheng et al. | Initial rupture and displacement on the Altyn Tagh fault Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/11/3/921/3338977/921.pdf 929 by guest on 24 September 2021 on 24 September 2021 by guest Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/11/3/921/3338977/921.pdf Research Paper 2000; GGSI,2000; 2003; HGSI, 2003; Robinson et al., 2003). ATF—Altyn Tagh fault. are from Ritts Chronostratigraphic and Biffi (2000). correlation is based on field observation and previous studies (XBGMR, 1993; Guo et al., 1998; Ritts and Biffi, Kent (1995). Observed polarity was compiled from Yin et al. (2002), Z.M. Sun et al. (2005), Zhang (2006), Fang et al. (2007), and Lu Xiong (2009). Paleocurrents Figure 7. Chronostratigraphic correlation of the relic Mesozoic to Cenozoic strata along the Altyn Tagh fault. GPTS—geomagnetic polarity time scale of Cande and (Ma) M. 33. 23. 5. Ag 145 201 66 ~ M 3 e 9 0

udstone Cretaceou E Jurass ic Paleocene - Eocene Oligocene Miocene Pliocene poc s h Fm

Qu ? Dameigo Caish a i ny a Lulehe Xiaganchaigou Shangganchaigou Xiayoushashan Shangyoushashan Shizigou

l . AS. u ing go

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oushasha M nconformit Jurassic

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9 n

4 n 4 e n

g e n s n u e Deformed y

GEOSPHERE | Volume 11 | Number 3 Cheng et al. | Initial rupture and displacement on the Altyn Tagh fault 930 Research Paper

systems. To identify appropriate piercing points along the Altyn Tagh fault, five Jurassic Samples sandstone samples ranging in age from Early Jurassic to Cretaceous were collected from the Tula and Caishiling sections. In the Tula section, we collected The zircons from sample G2 (Dameigou Formation) are ~50–200 mm long two samples: G1 and G2. Sample G1 is gray-yellow pebbly sandstone from and mainly subhedral to subrounded. The grains are generally colorless and the Cretaceous Quanyagou Formation (Kq), and sample G2 is gray-white sand- exhibit prominent oscillatory zoning. Th/U ratios typically clustered between 206 238 stone from the Lower Jurassic Dameigou Formation (J1+2d). In the Caishiling 0.09 and 1.98. Forty-eight out of 99 dated grains yielded a Pb/ U range of section, we took three samples: CSL1, CSL2, and CSL3. Sample CSL1 is a gray- 530–404 Ma, with a single peak in the age spectrum at 447 Ma (Figs. 8 and 9A). white, fine-grained sandstone from the Cretaceous Quanyagou Formation Only one zircon with a Triassic age (250 Ma) was obtained. The ages of all other (Kq), sample CSL2 is a gray-white, medium-grained sandstone from the Upper zircons range from 2781 to 696 Ma.

Jurassic Caishiling Formation (J3c), and sample CSL3 is a gray-yellow, coarse- In sample CSL3 (Dameigou Formation), zircon grain morphology ranges

grained sandstone from the Lower Jurassic Dameigou Formation (J3c) (Fig. 2). from euhedral to abraded, with an average size ranging between 50 mm and Detailed descriptions of the zircon separation and laser ablation–inductively 250 mm. CL images show that most of these zircon crystals are characterized coupled plasma–mass spectrometry (LA-ICP-MS) U-Pb dating of zircon are pro- by relatively distinct oscillatory zoning. Th/U ratios vary between 0.25 and 3.82. vided in the Supplemental File.1 The U-Pb ages range from 2480 to 220 Ma (Fig. 9B). The ages can be separated in three main groups with two peak ages at 264 Ma and 433 Ma, and 11 ages between 2480 and 852 Ma. RESULTS In sample CSL2 (Caishiling Formation), zircon crystals display euhedral to abraded shapes for an average size ranging between 50 mm and 250 mm. The U-Pb Geochronology of Detrital Zircons majority of the crystals display distinct oscillatory zoning in CL images, indicating a magmatic origin, which is confirmed by Th/U ratios varying between Isotopic ages with errors and related raw data are listed in full in Table DR1 0.12 and 1.55 (with two exception of 0.03 and 0.06). Among the 100 crystals (see footnote 1). The various zircon age groups and corresponding data for analyzed, all zircon ages with a discordance degree of <10% were used. The each sample are shown in Table 1 and Figure 8. Cathodoluminescence (CL) U-Pb ages range from ca. 2661 to ca. 246 Ma (Fig. 9C). The age distributions are images of typical zircon grains are presented in Figure DR1 (see footnote 1). divided into four groups with two major peaks at ca. 254 Ma and ca. 445 Ma,

Cheng, F., Guo, Z., Jenkins, H.S., Fu, S., and Cheng, X., 2015, Initial rupture and

displacement on the Altyn Tagh fault, northern Tibetan Plateau: Constraints based on

residual Mesozoic to Cenozoic strata in the western Qaidam Basin: Geosphere, v. 11, doi:10.1130/GES01070.1. TABLE 1. SUMMARY OF THE MAJOR CHARACTERISTICS AND CORRESPONDING STATISTICAL DATA FOR EACH OF THE SAMPLES Supplemental File. Description of zircon separation and LA-ICP-MS U-Pb dating of zircon, Figure DR1, and Tables DR1–DR2. doi:10.1130/GES01070.S1. Epoch Formation Sample LithologyDetrital zircon Detrital zircon Number of effective Th/U ratios SUPPLEMENTAL ITEM 1: Description of zircon separation and LA-ICP-MS U–Pb (Fm.) number age range (Ma) peaks (Ma) data points dating of zircon Detrital heavy minerals were separated following standard procedures (e.g. Liu et al., Tula unit 2013). A quantity of zircon grains (generally more than 200) were randomly selected,

enclosed in epoxy resin and polished to yield a smooth flat internal surface (slice). After Cretaceous Quanyagou Fm.G1 Gray-yellow pebbly sandstone218–2532ca. 238 89 0.13–1.51 being photographed under reflected and transmitted light, the samples were prepared for ca. 414 cathodoluminescence (CL) imaging in order to choose potential internal target sites for U-Pb ca. 904 dating (e.g. Liu et al., 2013; Yang et al., 2013). CL imaging was carried out using a Quanta ca. 1737 200 FEG Scanning Electron Microscope at the SEEC lab, in the School of Earth and Space

Science at Peking University. The U-Pb dating was performed on an Agilent 7500a ICP-MS Early–Middle Jurassic Dameigou Fm. G2 Gray-white sandstones 254–2430ca. 250 99 0.09, 0.10–1.98 connected to a 193 nm Excimer laser ablation system of American New Wave UP 193 SS at ca. 447 the China University of Geosciences, Beijing, following the method described by Yang et al. ca. 1435 (2013). The operating parameters were as follows: Ar plasma gas flow rate was 1.13 l/min, ca. 2465 radio frequency (RF) power was 1350 W and elemental integral time was 10 ms for Si and Zr

and 50 ms for other elements. Helium with a flow rate of 0.89 l/min was used as the carrier Caishiling unit Cretaceous Quanyagou Fm. CSL1 Gray-white sandstone228–2915ca. 262 94 0.10–2.05 ca. 456 ca. 897

1Supplemental File. Description of zircon separation Late Jurassic Caishiling Fm. CSL2 Gray-white sandstones 246–2661ca. 254 1000.03, 0.06, and LA-ICP-MS U-Pb dating of zircon, Figure DR1, ca. 445 0.12–1.55 and Tables DR1–DR2. Please visit http://dx.doi .org ca. 926 /10.1130 /GES01070.S1 or the full-text article on www .gsapubs.org to view the Supplemental File. Early–Middle Jurassic Dameigou Fm. CSL3 Gray-yellow sandstones220–2480ca. 264 97 0.25–3.82 ca. 433

0.6 0.10 0.10 G1 600 CSL2 600 Quanyagou Formation 2600 0.6 Caishiling Formation Kq 0.08 500 J3c 0.08 500 n=89 2200 2600 0.4 n=100 U U U U 8 8 38 0.4 2200 23 238 23 1800 400 / / / 400 0.06 1800 0.06 Pb/ Pb Pb Pb 6 6 1400 62 0 20 206 20 0.2 2 300 1400 300 0.2 1000 0.04 1000 0.04 600 200 200 0.0 0.02 0.0 0.02 0246810 12 14 0.00.2 0.40.6 0.8 0481216 0.10.3 0.50.7 2072Pb/U35 2072Pb/U35 207Pb/U235 2072Pb/U35 0.10 G2 600 0.6 CSL3 600 0.6 Dameigou Formation 3000 Dameigou Formation 2600 0.10 J1-2d 500 J d 2600 0.08 1-2 n=99 n=97 2200 500 0.4 0.08 2200 8

38 0.4

23 400 /U /U 1800 U

1800 0.06 8 400 238 23 Pb Pb /U 62 1400 0.06 b/ 20 1400 206 300 0.2 P 0.2 Pb 300

0.04 206 1000 206 1000 0.04 200 600 200

0.0 0.02 0.0 0 4 8121620 0.10.3 0.50.7 0.02 207235 207235 0 2468101214 0.10.3 0.50.7 Pb/U Pb/U 207 235 207235 0.8 0.10 Pb/U Pb/U CSL1 Quanyagou Formation 0.6 Kq 3000 0.08 500 n=94 2600

U 400 38 2200 38 /U 0.4 0.06 Figure 8. U-Pb concordia diagrams for zircon grains from the five sandstone samples. Pb/ Pb 1800 62 62 300 20 1400 20 0.2 0.04 1000 200 600 0.0 0.02 04812 16 20 0.00.2 0.40.6 2072Pb/U35 2072Pb/U35

a distribution spanning from 1073 Ma to 701 Ma with a peak at 926 Ma, and

finally seven ages clustered around 1416 Ma. y East Kunlun Range G (N=195) Relative

Cretaceous Samples probabilit

01200 400 600 800 200 1600 20002400 Age(Ma) The zircons from sample G1 (Quanyagou Formation) are predominantly y colorless, euhedral to subhedral, and with a length of 50–150 mm. Most zircons e Altyn Tagh Range F show oscillatory zoning without inherited cores or overgrowths. All analyses (N=152) have moderate U content, with variable Th/U from 0.13 to 1.51. In this study, 59 Relativ

probabilit out of 89 dated grains yielded a 206Pb/238U range of 521–218 Ma with two age 01200 400 600 800 200 1600 20002400 Age(Ma) peaks at 414 Ma and 238 Ma (Fig. 9D). The age of the remaining zircons range 456

y CSL1 from 2532 to 818 Ma with several subpeaks at 2502, 1737, and 904 Ma. Caishiling Cretaceous section E In sample CSL1 (Quanyagou Formation), zircon grains show euhedral to 262 Kq abraded shapes with an average size between 50 m and 200 m. Most crystals n=94 m m Relative 897

show distinct oscillatory zoning. Th/U ratios vary from 0.10 to 2.05. One-hun- probabilit dred grains were analyzed, and 94 effective data points were obtained. The 01200 400 600 800 200 1600 20002400 Age(Ma) 238 U-Pb ages range from 2915 to 228 Ma. The ages can be separated in four main G1

y Tula 414 groups with three peaks at 897 Ma, 456 Ma, and 262 Ma (Fig. 9E). In addition, section Cretaceous D 15 Proterozoic ages are given older than 1000 Ma. Kq n=89

Relative 904 probabilit

DISCUSSION 01200 400 600 800 200 1600 20002400 Age(Ma) 445 CSL2

y Caishiling Upper Jurassic New Piercing Points 254 section C J3c n=100

Using lithologic and stratigraphic constraints, we conclude that both the Relative 926 Tula and Anxi sections were once regions of extensive subsidence character- probabilit ized by topographic lows (Figs. 1 and 2). The dark mudstone, oil sands (Fig. 5E), 01200 400 600 800 200 1600 20002400 Age(Ma) and asphalt (Fig. 5F) found in the Upper Jurassic strata of the Tula section in- 264 CSL3

y Lower and Middle Jurassic

e B dicate Tula Basin was once a depression during Late Jurassic time. In addition, Caishiling J d section 1+2 matching geochemical fingerprints (e.g., carbon isotopic composition) between 433 n=97

the Jurassic oil-bearing sandstone of the Tula Basin and the Jurassic crude oil of Relativ 228

Qaidam Basin suggest that these two regions must have been parts of the same probabilit prototype basin (Guo et al., 1998). Recent studies confirm that the relatively 01200 400 600 800 200 1600 20002400 Age(Ma) rigid Qaidam Basin was transported northeastward along the Altyn Tagh fault 447 G2 Lower and Middle Jurassic e Tula A during the Cenozoic, and it has not undergone obvious basin-scale vertical-axis ility J1+2d b section n=99

rotation with respect to the Eurasian plate since early Eocene time (E. Wang ba

o et al., 2006; Yu et al., 2014). Considering the large-scale, left-lateral, strike-slip Relativ 250 pr displacement of the Altyn Tagh fault during Cenozoic time, we suggest that the 01200 400 600 800 200 1600 20002400 Age(Ma) Tula and Anxi sections were once part of the Qaidam Basin but detached from

the Qaidam during its northeastward migration driven by faulting on the Altyn Figure 9. Relative probability histogram plots of U-Pb ages of detrital zircons from the five Juras- Tagh fault. We propose that the Qaidam Basin may have actually been originally sic and Cretaceous sandstone samples and relative probability histogram plots of pluton ages located much farther southwest along the Altyn Tagh fault than its present posi- in the Altyn Tagh and Eastern Kunlun Ranges. Age data are from Yang et al. (2006), Zhang et al. (2001), Chen et al. (2003), Gehrels et al. (2003a, 2003b), Yue et al. (2005), Yang et al. (2006), L. Liu tion, likely located where the Tula and Anxi sections currently reside. et al. (2007, 2013), Lu et al. (2008), Liu et al. (2009), Zhang et al. (2011), Li et al. (2013), and Wang Stratigraphic successions are generally deposited during basin infilling pro- et al. (2013). More information on dating and ages is provided in Table DR2 (see text footnote 1). cesses, while periods of nondeposition or erosion are likely related to regional

tectonic events (e.g., Wheeler, 1964; Allen and Allen, 1990; Boggs, 2006). In the prominent in the southern foothills of the Altyn Tagh Range, further indicating Tula section, Paleocene strata correlate well with strata in the Caishiling section; intense left-lateral strike-slip faulting along the Altyn Tagh fault (Figs. 3 and 10; however, post-Paleocene strata are missing (Figs. 4 and 7). In the Anxi section, Meng and Fang, 2008; L. Wang et al., 2010; Wu et al., 2012a). Within the western Paleocene to early Miocene strata cross-correlate with comparable strata pre- Qaidam Basin, this unconformity is characterized by growth strata that started served in the Caishiling section, whereas the late Miocene strata are missing to develop after the initial deposition of Miocene strata. In profile D–D′ (Fig. 10), (Figs. 6 and 7). In the Qaidam Basin, and particularly in the Caishiling section, the deposits are cut by a branch fault of the Altyn Tagh fault, and the pre-Mio- the Paleocene to Quaternary strata are well developed (Figs. 2 and 7). If we cene strata are deformed by an anticline, which was later cut and sealed by consider a stepwise northeastward migration of the Qaidam Basin along the Miocene strata. Similarly, in profile E–E′ (Fig. 10), the growth strata are well de- Altyn Tagh fault throughout the Cenozoic, the absence of post-Paleocene strata veloped, and the thickness of the Eocene to Miocene pregrowth strata remains in the Tula section marks the detachment of the Qaidam Basin from this section. constant. The middle Miocene to Quaternary deposits thin northwestward (to- Similarly, the absence of late Miocene strata in the Anxi section marks the de- ward the Altyn Tagh Range). Based on magnetostratigraphy, palynology and tachment of the Qaidam Basin from this section (Fig. 7). We therefore propose paleontology within the basin, the accurate timing of this regional unconfor- that the Paleocene strata in the Tula section and the Paleocene to early Miocene mity has been constrained to ca. 15 Ma (Fang et al., 2007; Gao et al., 2009; strata in the Anxi section represent the two piercing points that can identify the Lu and Xiong, 2009; L. Wang et al., 2010; Wu et al., 2012a). We thus suggest initial position and early stages of Qaidam Basin detachment and movement. that the apparent unconformity between the Shangyoushashan and Xiayou- The Tula section contains consistently southward-dipping strata on both shashan Formations indicates middle Miocene (ca. 15 Ma) motion of the Altyn sides of the low-angle (<10°) Jurassic–Cretaceous and Cretaceous–early Eo- Tagh fault system. Based on this, we propose that the deformed Paleocene cene unconformities (Fig. 4). These Jurassic–early Eocene sequences are heav- to Miocene strata of the Anxi section represent another piercing point with ily deformed, forming a major north-verging syncline with a steeply dipping which to constrain the historical position of the western Qaidam Basin during south limb and a shallowly dipping north limb, indicating that the deformation mid-Miocene time. The initial stage of left-lateral strike-slip faulting along the of these Jurassic–early Eocene sequences occurred during or after early Eo- Altyn Tagh fault led to a gradual separation of the western Qaidam Basin from cene time (Fig. 4). We therefore suggest that the topographic low in the Tula the Anxi section. Tectonic movement precluded new deposition of strata, and area became isolated during early Eocene time, and the folding of the Tula syn- the Anxi section became isolated from the Qaidam Basin during late Miocene cline was related to motion on the fault to the south, which was kinematically time. In addition to the Altyn Tagh fault, we suggest the left-lateral strike-slip linked to the Altyn Tagh fault during this time period. It is therefore likely that faulting of the Baiganhu fault to the south of the basin also accommodated the the Tula section was part of the Qaidam Basin during early Eocene time and northeastward motion of the Qaidam Basin (Cowgill et al., 2003; HGSI, 2003; became isolated when the Qaidam Basin migrated northward. This migration Wang et al., 2014). The Pliocene marginal conglomerate facies of the Anxi sec- was induced by movement along the Altyn Tagh fault, which is consistent with tion formed via proximal accumulation sometime after initial-stage left-lateral the initial timing of faulting along the Altyn Tagh fault (ca. 49 Ma) as proposed strike-slip faulting of the Altyn Tagh fault. by Yin et al. (2002). We therefore conclude that the Tula section is an appropriate piercing point with which to constrain the initial stage (ca. 49 Ma) of faulting along the Altyn Tagh fault. Provenance Analysis of Jurassic to Cretaceous Strata In the Anxi section, the fine-grained deposits in the Oligocene to Miocene strata indicate that the Anxi unit was once a topographic low (HGSI, 2003). The If the Cenozoic strata in the Tula and Anxi sections represent the initial tim- lithofacies of the Pliocene Shizigou Formation of the Anxi section are domi- ing and early stages of motion along the Altyn Tagh fault, and the postulated nated by a succession of proximal fluvial deposits. In the western Qaidam Ba- historical position of the Qaidam Basin is reasonable, the prefaulting deposits sin, the Pliocene Shizigou Formation is characterized by lacustrine facies (Yin (Mesozoic strata) in the Tula section and the western Qaidam Basin (Caishiling et al., 2008b), suggesting that these two regions maintained different deposi- section) should be lithologically similar and their provenance comparable. tional systems during Pliocene time. The continuous sequences of Paleocene Paleocurrents in the Jurassic strata of the Tula, Anxi, and Caishiling sections to early Miocene strata and the angular unconformity between the Youshashan all suggest northward current movement, while Cretaceous strata show south- and Shizigou Formations demonstrate that the Anxi section experienced a tec- ward-moving paleocurrents (Fig. 7; Ritts and Biffi, 2000). In general, there are tonic event between the early Miocene and Pliocene Epochs, and this event relatively few paleocurrent data for the Tula, Anxi, and Caishiling sections (e.g., n in turn resulted in the deformation of the Paleocene to early Miocene strata = 4 in the Tula section); however, the generally consistent orientation of paleocur- (Figs. 6 and 7). Seismic profiles from the western Qaidam Basin reveal that rents across all three sections reflects a similar topography for these regions the Pliocene strata conformably overlie the Miocene strata and illustrate a dis- (Tula, Anxi, and Caishiling) between Jurassic and Cretaceous time. Relative tinctive unconformity between the Shangyoushashan and Xiayoushashan For- probability histogram plots of pluton ages in the Altyn Tagh and Eastern Kun- mations, both of which are Miocene in age. This unconformity becomes most lun ranges are shown in Figure 9. Based on the morphological characteristics,

D SW NE D’ D SW NE D’ 0.0 0.0 0.0

3 Growth N-2 Q strata ) 0.5 0.5 2 0.5 N2

(s Growth

strata 1 N2

time 1.0 1.0 N1 1.0

2 E3

travel 1.5 1.5 1.5 1

y y E3

E1+2 2.0 2.0 2.0

wo-wa U

T Mes 2.5 2.5 2.5 0 5km A Basement B

E NW SE E’ E NW SE E’ 00U 00U 00U

05U 05U 05U

)

(s U 10 10U 3 10U

time N-2 Q

Growth 2 strata N2 U 15 15U 1 15U

travel N2

2.0 2.0 2 2.0 1 E3 E3

Two-way

2.5 2.5 E12+ 2.5

U 0 5km Mes 3.0 C 3.0 Basement D 3.0

Figure 10. Seismic profiles in the southwestern Qaidam Basin; see Figure 2 for location. (A) Uninterpreted and (B) interpreted seismic profiles in the SW–NE direction; (C) uninter- 1 preted and (D) interpreted seismic profiles in NW–SE direction. Note that the growth strata since the Xiayoushashan Formation (N2 xy) indicate the intense tectonic movement of the Altyn Tagh fault during middle Miocene time.

internal textures, and Th/U ratios of zircons, magmatic zircons (~80%) appear to zircons from the Jurassic–Cretaceous samples. If this is the case, the U-Pb dominate the section, while metamorphic zircons (~20%) comprise the remain- age spectrum of these samples should display a Paleozoic and Precambrian der (Corfu et al., 2003; Hanchar and Rudnick, 1995; Hoskin and Black, 2000). age peak (>270 Ma). In samples CSL1, CSL2, and CSL3, the detrital zircon age In the Altyn Tagh Range, Archean and Proterozoic basement rocks are spectra are similar and characterized by two main groups of Permian–Trias- widely exposed with Archean zircon U-Pb ages ranging from ca. 3.6 Ga (Lu et sic and Paleozoic ages along with a small number of Precambrian ages. The al., 2008) to ca. 2.6 Ga (Lu and Yuan, 2003) and Proterozoic zircon U-Pb ages widespread distribution of the Paleozoic and Precambrian plutons in both the ranging from ca. 2.4 Ga to ca. 650 Ma (Gehrels et al., 2003a, 2003b; C. Wang Altyn Tagh and Eastern Kunlun Ranges prevents the use of zircon age spectra et al., 2006; Zhang et al., 2011; Wang et al., 2013). Paleozoic intrusions within to estimate the source area of these Cretaceous and Jurassic samples. Never- this range show zircon U-Pb ages ranging from ca. 550 Ma to ca. 400 Ma (Fig. theless, all three samples contain a host of Late Permian to Triassic zircon ages 2; Sobel and Arnaud, 1999; Zhang et al., 2001; Cowgill et al., 2003; Gehrels et (<270 Ma), which are found only in the plutons of the Eastern Kunlun Range, al., 2003a; Chen et al., 2004; Yue et al., 2004a, 2005; Yang et al., 2006; Wang et particularly those in the south Tula section (Figs. 2 and 9). In addition, the anal., 2008; Liu et al., 2009). Only a few isolated Permian igneous rocks are dis- gular grains in the Jurassic samples show poor sphericity and poor sorting, tributed along the center of this range, with zircon U-Pb ages ranging between indicating deposition from a proximal source terrain (Figs. 5A and 5B). Based ca. 300 Ma and ca. 270 Ma (Cowgill et al., 2003; Gehrels et al., 2003a, 2003b). on the proximal source feature and the particular age spectrum (containing a With regard to the Eastern Kunlun Range, the Proterozoic basement rocks host of Late Permian to Triassic zircons) of these three samples, we suggest are isolated and show zircon U-Pb ages ranging from 1900 to 800 Ma (e.g., the Caishiling section was situated far closer to the Tula section in the past as HGSI, 2003; Wang et al., 2013). The Paleozoic to late Proterozoic intrusions compared with its current position. Meanwhile, plutons in the Eastern Kunlun show a zircon U-Pb age peak around 430 Ma, which is based on early Paleozoic Range had already been unroofed during Early Jurassic time, representing a

metamorphic rocks that represent the Ordovician–Silurian docking of the Kun- main source to the Dameigou (J1+2d), Caishiling (J3c), and Quanyagou (Kq) For- lun and Tarim blocks (e.g., Matte et al., 1996; Cowgill et al., 2003; Li et al., 2013). mations since that time. The Kunlun basement age distribution is characterized by the occurrence of In the Tula section, two Jurassic and Cretaceous samples (G1 and G2) were late Paleozoic–early Mesozoic ages (ca. 300 to ca. 200 Ma), which correspond selected from the narrow Tula subbasin. The G1 sample was selected near the to the Permian–Triassic closure of the Paleotethys Ocean and the docking of Eastern Kunlun Mountains, while the G2 sample was selected near the Altyn the Qiangtang block (Roger et al., 2003, 2008, 2010; Dai et al., 2013; Li et al., Tagh Range (Figs. 2 and 4). The age cluster of sample G1 features two main 2013). Finally, a few Mesozoic ages (e.g., ca. 74 Ma) have been obtained from groups of Permian–Triassic and Paleozoic ages along with minor Precambrian the Tula section (Robinson et al., 2003). ages, a pattern similar to that found in the Cretaceous sample from the Caishil- In order to better constrain the provenance of the five sandstone samples, ing section (Figs. 9D and 9E). We thus suggest that the Late Permian to Triassic the plutons of the Altyn Tagh and Eastern Kunlun ranges are simply divided strata of the Eastern Kunlun Range, and the Paleozoic and Precambrian plutons into two groups: those plutons with an age older than 270 Ma, and those with in both Eastern Kunlun and the Altyn Tagh Ranges are the source rocks for the an age younger than 270 Ma. Older plutons are widespread in both the Altyn Cretaceous strata (Figs. 2 and 4). In particular, the age spectrum in the Early Ju- Tagh and Eastern Kunlun Ranges, whereas younger plutons are confined to rassic sample (G2) is characterized by an exceptionally unimodal distribution the Eastern Kunlun Range just south of our sample sites (Fig. 2). This distinc- of detrital zircon (age ranging from 404 to 493 Ma, peak at 447 Ma) with only tive pluton distribution can be used as a provenance signature to identify the one Triassic age (250 Ma), whereas all of the other samples (G1, CSL1, CSL2, source of detrital zircon for our Mesozoic samples. In general, detrital zircon and CSL3) are characterized by bimodal distributions (Permian–Triassic and age clusters in all five samples are basically similar and characterized by two Paleozoic groups), suggesting that G2 is a distinct sample. The site location for main groups of Permian–Triassic and Paleozoic ages along with a small num- G2 is extremely close to the Altyn Tagh Range, and the older plutons within the ber of Precambrian ages (Figs. 8 and 9). The appearance of the Permian–Trias- Altyn Tagh Range serve as a major source of debris for sample G2 (Figs. 2 and sic zircon ages (<270 Ma) in all five samples strongly suggests an East Kunlun 4). Older plutons from the Altyn Tagh Range represent the dominant source Range provenance. We consider that the slightly different peak ages of sample material, resulting in relatively little contribution from the East Kunlun Range, CSL1, CSL2, CSL3, G1, and G2 result from contributions of plutonic debris with as evidenced by very few Permian to Triassic zircons in the age distribution of a variety of ages (Fig. 9; Table 1). sample G2 (Fig. 9A). However, the one zircon grain with a Triassic age found In the Caishiling section, the sampling sites of the three Jurassic–Creta- in sample G2 and the northward paleocurrents do suggest some small contri- ceous samples (CSL1, CSL2, and CSL3) are located at the southern piedmont bution may have come from the Permian–Triassic rocks of the Eastern Kunlun of the Altyn Tagh Range, where the basement is mainly dominated by Paleozoic Range (Figs. 7 and 9A; Ritts and Biffi, 2000). Therefore, we suggest the most and Precambrian plutons with ages older than 270 Ma (Fig. 9). If the Caishiling plausible explanation for the anomalous detrital zircon ages found in the G2 section has been situated in roughly the same region that it occupies today sample is a mixture of source material from both the Altyn Tagh and the East (Fig. 2), the Altyn Tagh Range must serve as the ultimate provenance for the Kunlun Ranges. If the plutons from both locations (Paleozoic and Precambrian

in the Altyn Tagh Range and Permian to Triassic in the Eastern Kunlun Range) On the other hand, magnetostratigraphy and sedimentological analysis were exhumed during Jurassic time, they would represent a mixed source for have led some to suggest that the Altyn Tagh fault has been active since ca. 49 samples G2 and CSL3 during that time. Ma rather than simply 24 Ma (Yin et al., 2002). Thermochronological data from By reconciling the detrital zircon ages of the sediments from the Altyn Tagh the Altyn Tagh Range in conjunction with sedimentological evidence suggest and Eastern Kunlun Ranges and combining these data with the large-scale that left-lateral strike-slip faulting occurred from late Eocene to early Oligocene left-lateral strike-slip displacements measured along the Altyn Tagh fault, we time (Chen et al., 2001; Meng et al., 2001; Yue et al., 2001; Ritts et al., 2004). Yet deduce that the Caishiling section must have been located much closer to the still others point to geologic evidence within the northern Tibetan Plateau that Eastern Kunlun Range between Jurassic and Cretaceous time. The Tula unit, shows the onset of this left-lateral faulting occurred sometime during Miocene on the other hand, likely has not moved since Jurassic time. The lithologic and Epoch (Wan et al., 2001; Wu et al., 2012a, 2012b). stratigraphic contact features of the residual Mesozoic to Cenozoic strata along As is apparent, the tremendous discrepancies both in total displacement the Altyn Tagh fault combined with U-Pb dating of the Jurassic and Cretaceous and the timing of initial left-lateral movement along the fault may be attributed samples of the Tula and Caishiling sections enable us to establish the timing of partially to an incomplete understanding of the lithology and tectonic history movement along the Altyn Tagh fault. We resolve the motion along the Altyn of the Altyn Tagh Range. This also makes it particularly challenging to find an Tagh fault into two distinct stages: initial movement occurring at ca. 49 Ma and ideal piercing point. Some estimates on the displacement of the Altyn Tagh a subsequent early stage of movement occurring at ca. 15 Ma. Both stages fault use plutonic belts or suture zones as the offset markers, but these large promoted strike-slip motion along the fault. The Tula and Anxi sections can be tectonic features have poor spatial definition because of high uncertainties in used as piercing points along the western segment of Altyn Tagh fault, and we their shape and the extent of their geologic margins (Yin and Harrison, 2000; identify these regions as residual components of the original Qaidam Basin. Yin et al., 2002). The intense deformation and shortening of the eastern and western Kunlun Ranges throughout geologic history as well as the deformation along the Qilian Shan have served to profoundly complicate any measure- Implications for the Initial Timing and Total Offset of the Altyn Tagh Fault ment of the actual displacement of the Altyn Tagh fault.

General Review of Previous Estimates Reassessing Cenozoic Kinematic Patterns on the Altyn Tagh Fault In the past few decades, several approaches have been used to estimate Using New Piercing Points the overall displacement along the Altyn Tagh fault. By comparing Paleozoic suture zones in the western Kunlun Range with those in the Qilian Shan, CSBS The Qaidam and Tarim Basins are both relatively rigid blocks when com- (1992) argued for ~1200 km of offset along the fault. By matching similar geo- pared with the Altyn Tagh Range, which gives them relatively higher crustal logical features, particularly late Paleozoic magmatic belts in the western and strength (E. Wang et al., 2006; Zhou et al., 2006). Paleomagnetic studies have eastern Kunlun Shan, ranges of 300–500 km, 475 ± 70 km, and ~550 km of dis- revealed a 13.3° ± 8.8° difference in paleomagnetic declination between the placement have all been estimated (Tapponnier et al., 1986; Peltzer and Tappon- two halves of the arcuate structure outlined in the uplifted region of the Tula nier, 1988; Cowgill et al., 2003). Comprehensive correlations of the geologic section (Fig. 4; see also fig. 3in Dupont-Nivet et al., 2004) and suggest north- features with the magmatic histories in the Altyn Tagh Range, Qilian Shan, and ward transport along the Altyn Tagh fault of this section, the western segment Nan Shan enabled Gehrels et al. (2003a) to suggest offsets of ~370 km, 375 of the Eastern Kunlun Range (Qimen Tagh Range), and the Qaidam Basin (Du- km, and ~400 km along the Altyn Tagh fault. By reconstructing the shortening pont-Nivet et al., 2004; Yu et al., 2014). The absence of a large vertical axis of of the Nan Shan thrust belt, Yin and Harrison (2000) inferred 360 ± 50 km of rotation in the area adjacent to the Altyn Tagh fault indicates that the left-lat- displacement. A comprehensive analysis of the geologic features along the eral shear strain between the Tarim Basin and the northern Tibetan Plateau is north margin of the Tibetan Plateau has yielded estimates of ~400 km of offset concentrated on the relative weak Altyn Tagh fault (Dupont-Nivet et al., 2002, (Molnar and Tapponnier, 1975). Using the measured Pliocene–Quaternary slip 2003, 2004; J.M. Sun et al., 2005). The relative movement of the Qaidam and rate over the inferred duration of faulting along the Altyn Tagh fault, Burchfiel Tarim Basins is therefore representative of the actual displacement along the and Royden (1991) estimated ~200 km of fault displacement. By matching the Altyn Tagh fault. The distinctive sediment features that manifest along the Altyn Tertiary rocks along the northern and the central segments of the Altyn Tagh Tagh fault and identify this displacement history can be used to locate appro- fault, Wang (1997) ascertained 69–90 km of displacement. Chen et al. (2002) priate piercing points, which can further be used to restrict the timing of this suggested an offset of 500 ± 130 km between 24 Ma and present based on pa- displacement (Yin and Harrison, 2000; Chen et al., 2003; Cowgill et al., 2003; E. leomagnetic data from the Qaidam Basin. Notably, 400 ± 60 km has also been Wang et al., 2006; Yin et al., 2002, 2008b). In this paper, the residual Mesozoic suggested based on the reconstruction of a Jurassic facies boundary across to Cenozoic strata within the western Qaidam Basin are documented as the the Altyn Tagh fault (Ritts and Biffi, 2000). piercing points that identify the Cenozoic kinematic pattern of the Altyn Tagh

fault. In order to account for the fact that the Qaidam Basin and the relic Tula the early Eocene. Faulting on the Altyn Tagh fault induced the NE-SW–trending and Anxi units are situated south of the currently active Altyn Tagh fault trace, branch fault (F1) in this region, now covered by Quaternary alluvium, deform- two faults or major unconformities are required to act as boundaries between ing the Jurassic to early Eocene strata in the Tula section as it did so (Figs. 2 and the detached Qaidam Basin and relic Tula subbasin (Fig. 11). 4). The initial strike-slip movement of the Altyn Tagh fault also transported the Between the Jurassic and early Eocene, the westernmost part of the origi- original Qaidam Basin northeastwards and gradually separated the Qaidam nally rigid Qaidam Basin was located where the Tula section is situated today Basin from the Tula section. The depression in the Tula section then uplifted (Fig. 11A). Strike-slip movement along the Altyn Tagh fault began shortly after and was gradually isolated, leading to a pause in deposition during the late Paleogene to Neogene. This left-lateral faulting also lead to compression in the northern Qaidam Basin, which began in the Paleocene to early Eocene as revealed by seismic profile data (Yin et al., 2008a; Fig. 11A). Systematic anal- Present ysis of the Cenozoic stratigraphic sections along the Altyn Tagh fault suggests gh Fault North n Ta N that strike-slip movement along the western and central Altyn Tagh fault was Alty 90km Qaidam ~1 initiated at ca. 49 Ma (Yin et al., 2002), roughly consistent with the speculated Caishiling 70km ~1 sediment termination age of the Tula section. This onset age for large strike- Fa Qaidam ult slip faulting in the northern Tibetan Plateau is also synchronous with the colli- Baiganhu Belt Anxi Fault F1 Basin sion of India and Asia as revealed by newly acquired paleomagnetic evidence Tula Easter n Kunl un Range in southern Tibet (Dupont-Nivet et al., 2010; Huang et al., 2013; Lippert et al., C 2014). This indicates that the deformation in northern Tibet may have begun as India and Eurasia began colliding. It is possible that the dominant cause of the N ort large-scale strike-slip faults in the Asian interior (especially the Altyn Tagh fault) ~15Ma N t Anxi h Qa was the India-Asia collision itself, the effects of which may still be felt on the gh Faulkm Ta 170 id Altyn ~ am northern Tibetan Plateau today. Qaidam Fa Between early Eocene and mid-Miocene time, movement of the Altyn Tagh Basin ult F1 Baiganhu Fault B fault transported the westernmost part of the original Qaidam Basin northeast- Tula el Easter n Kunlun Range t ward to the position where the Anxi unit is at present. During the mid-Miocene, B an intense left-lateral strike-slip motion occurred along the Altyn Tagh fault, and the originally rigid Qaidam separated from the Anxi unit and migrated N ~49Ma lt o northeastward continuously (Fig. 11B). This fault motion tilted the late Paleo- u rt h N Tagh Fa Qai cene to early Miocene strata of the Anxi section and created a regional un- Altyn dam conformity between the Shangyoushashan and Xiayoushashan Formations. Tula Qaidam Fa This unconformity matches the widespread unconformity within the western F1 ul Basin t Belt Qaidam Basin (Figs. 3 and 7; L. Wang et al., 2010; Y.D. Wang et al., 2010; Wu et Easter n Kunlu n Range al., 2012a, 2012b). The growth strata above this unconformity reveal a rapid A tectonic event occurred just after the mid-Miocene Epoch (ca. 15 Ma). Based on the widespread potassic volcanism in northern Tibet, Yue and Liou (1999) also inferred intense fault movement along the Altyn Tagh fault ca. 13–16 Ma. This ActiveGfault InactiveGfault Thrust fault Migration direction Source direction Piercing point mid-Miocene faulting is recorded by the fission track present in both the Altyn Figure 11. Proposed accelerated motion model for the Cenozoic Altyn Tagh fault (ATF). Note the Tagh Range and the north Qaidam Basin (Jolivet et al., 2001; Wan et al., 2001). piercing points along the Altyn Tagh fault. (A) Before early Eocene time, the Caishiling section Overall, the tectonic events of the mid-Miocene are widespread and well devel- was originally located where the Tula section is situated today. During early Eocene time (ca. 49 Ma), the left-lateral strike-slip faulting along the Altyn Tagh fault initiated, transporting the Qaid- oped in the Tibetan Plateau (e.g., Yue and Liou, 1999; Kirby et al., 2002; Clark et am Basin northeastward. Since then, the Tula unit remained stationary and was separated from al., 2004; Ding et al., 2004; J.M. Sun et al., 2005; Wang et al., 2012), which may the Qaidam Basin (B) during middle Miocene time (ca. 15 Ma), when the Caishiling section mi- reflect a critical period in the process of Tibetan Plateau growth. Following this, grated northeastward to where the Anxi section is situated today. During this period, an intense left-lateral strike-slip faulting along the Altyn Tagh fault occurred and deformed the pre-Miocene as seen in the Anxi section, the depression in the Anxi section became folded, strata in the Anxi section. Since then, the Anxi unit was left and separated from the Qaidam Ba- uplifted, and isolated, gradually leading to a pause in deposition during the sin (C) during continuous left-lateral strike-slip faulting along the Altyn Tagh fault that transport- late Miocene (Figs. 6 and 7). ed the Caishiling section to its present position. Consequently, ~360 km total Cenozoic offset is constrained along the Altyn Tagh fault, which is divided into ~170 km of offset between early Since the late Miocene Epoch, continuous left-lateral strike-slip faulting Eocene and middle Miocene time and ~190 km of offset from the late Miocene until present. along the Altyn Tagh fault has gradually transported the Qaidam Basin to its

cathodoluminescence (CL) imaging. We are particularly grateful to Dupont-Nivet Guillaume for present position (Fig. 11C). Finally, recent left-lateral strike-slip faulting oc- his helpful suggestions. Communications with Marc Jolivet, Jianxun Zhou, Dewen Zheng, and curred along the entire Altyn Tagh fault, reshaping the original faulting style. Jinjiang Zhang sharpened the discussion on the appropriate piercing points. We would like to The Tula and Anxi units can be selected as piercing points to constrain the thank An Yin and an anonymous reviewer for reviewing the earlier version of this manuscript. We also would like to thank Science Editor Shanaka de Silva, Associate Editor Todd LaMaskin, and two Cenozoic kinematic patterns of the Altyn Tagh fault. 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