Cenozoic Tilting History of the South Slope of the Altyn Tagh As Revealed by Seismic Profiling: Implications for the Kinematics GEOSPHERE; V

Research Paper

GEOSPHERE Cenozoic tilting history of the south slope of the Altyn Tagh as revealed by seismic profiling: Implications for the kinematics GEOSPHERE; v. 12, no. 3 of the Altyn Tagh fault bounding the northern margin of the doi:10.1130/GES01269.1 Tibetan Plateau 10 figures; 1 table Haifeng Zhao1, Yanyan Wei1, Ya Shen2, Ancheng Xiao1, Liguang Mao1, Liqun Wang3, Junya Guan2, and Lei Wu1 CORRESPONDENCE: Xiao: xiaoanch@zju.edu.cn; 1School of Earth Sciences, Zhejiang University, Hangzhou 310027, China Wu: leiwu@zju.edu.cn 2Bureau of Geophysical Prospecting Inc., China National Petroleum Corporation, Zhuozhou 072750, China 3Research Institute of Oil and Gas Exploration and Exploitation of Qinghai Oilfield Company, PetroChina, Dunhuang 736202, China CITATION: Zhao, H., Wei, Y., Shen, Y., Xiao, A., Mao, L., Wang, L., Guan, J., and Wu, L., 2016, Ceno- zoic tilting history of the south slope of the Altyn Tagh ABSTRACT is important for understanding the growth of the Tibetan Plateau during the as revealed by seismic profiling: Implications for the Cenozoic era (Tapponnier et al., 2001; Yin et al., 2002; Cowgill et al., 2003; kinematics of the Altyn Tagh fault bounding the northern margin of the Tibetan Plateau: Geosphere, v. 12, The Altyn Tagh fault (ATF) plays a significant role in the northward growth Meng et al., 2012), as well as understanding the development of adjacent no. 3, p. 884–899, doi:10.1130/GES01269.1. of the Tibetan Plateau, but its Cenozoic kinematics and related structural re- basins (Chen et al., 2004; Ritts et al., 2004; Yue et al., 2001; Wu et al., 2012a, sponse in adjacent basins remain debated. In this study, we identified a tran- 2012b; Xiao et al., 2013). Since its identification on remote sensing images Received 19 September 2015 sition zone between the ATF and the Qaidam Basin interior and termed it the four decades ago (Molnar and Tapponnier, 1975), an increasing number of Revision received 25 January 2016 Altyn Slope, based on a dense network of two- and three-dimensional seis- studies have been carried out on this large-scale fault system, detailing its Accepted 28 March 2016 Published online 11 May 2016 mic reflection profiles and isopach maps. Tilted by a series of E-W–trending geometry (Cowgill et al., 2000; Wittlinger et al., 1998), kinematics (Bendick transpressional faults that constitute the positive flower structure of the ATF, et al., 2000; Mériaux et al., 2005; Cowgill et al., 2009; Elliott et al., 2015), and the present Altyn Slope is characterized by a southeast-dipping slope with evolution (Yue and Liou, 1999; Yin et al., 2002; Cowgill et al., 2003; Liu et al., its undulating southeastern boundary with peaks coincidentally located at 2007; Wu et al., 2012b). Some studies also examined possible source-to-sink the major anticlinal belts in the basin. We propose a method for restoring relationships between the Altyn Tagh and the sediments exposed along spe- the Cenozoic tilting history of the Altyn Slope during different time periods cific sections (e.g., Chen et al., 2004; Wu et al., 2012b; Li et al., 2014; J.M. Sun by identifying growth-strata geometry from the recent isopach maps. The re- et al., 2005; Cheng et al., 2015). But few studies concentrated on the structural sults show that the Altyn Slope began to form in the late Eocene (ca. 40 Ma) responses to the activity of the ATF in the adjacent basins, which is also of and continued to expand until the mid-Miocene (ca. 15 Ma) albeit with a crucial importance for acquiring a detailed knowledge of the specific evolu- temporally developing shape. However, the Altyn Slope shrank toward the tion of the ATF. ATF and underwent significant NE-SW–directed folding since the mid-Mio- The Qaidam Basin is the largest sedimentary basin within the Tibetan cene (ca. 15 Ma), resulting in formation of undulations of its southeastern Plateau and is bounded by the ATF to the northwest (Fig. 1A). Thick and boundary. We thus infer that the left-slip motion on the ATF is divided into nearly continuous Cenozoic sediments (Wu et al., 2014) have been well pre- two distinct stages: during the first stage, ca. 40–15 Ma, the ATF was activated served within the basin, containing the most integrated information record- with slow slip rate, and most transpressional stress was converted to vertical ing the Cenozoic history of the ATF. Much work has been done to establish strain, raising the Altyn Slope instead of producing strike-slip motion. During a tectonic-sedimentary link between the ATF and the Qaidam Basin (e.g., the second stage, since ca. 15 Ma, faster sinistral strike-slip motion on the ATF Bally et al., 1986; Chen et al., 2002; Zhuang et al., 2011; Wu et al., 2012b; took place, releasing the stress beneath the Altyn Slope and inducing intense Wang et al., 2006; Wang et al., 2010). Bally et al. (1986) proposed that the ATF NE-SW–directed shortening within the Northern Tibetan Plateau. and the structures along the western edge of the Qaidam Basin were active since the middle Eocene. Wang et al. (2006) inferred that most of the sedi- INTRODUCTION ments within the depocenters of the Qaidam Basin were sourced from the Pamir folded belt in western Tibet and transported by a lengthy longitudinal As the northern boundary of the Tibetan Plateau, the ~1600-km-long left- river that developed along with the left-lateral motion of the ATF since the For permission to copy, contact Copyright slip Altyn Tagh fault (ATF) links all the major thrust belts and basins in and Oligocene, and that disappeared at ca. 2–4 Ma. Yin et al. (2002) suggested Permissions, GSA, or [email protected]. around the northern plateau (Yin et al., 2002). Knowledge of the ATF therefore that the Qaidam Basin was offset from the Tarim Basin by the left strikeslip

38.5 E 39 E E Xorkol Basin Altyn Tagh E Lapeiquan AltynTagh Fault E 90 ( 92 94 ±

39.5 N ( 38 N Mang'ai

( Huatugou ( Qima Serten 39 N n T Lenghu 37. E ag E E 5 N h Huahaizi( 90 92 38 g 94 B 38 E .5 E 90 E 100 E A Legend 40 N Upper Pleisto- Middle Miocene Proterozoic- cene-Holocene Paleozoic 40 N ATF Lower-Middle Paleocene- ( Town Pleistocene Early Miocene EKF Late Miocene- Mesozoic Pliocene 30 N

30 N Elevation 050100 km 7859m 180m

90 E 100 E

Figure 1. (A) Digital elevation model of Tibetan Plateau and adjacent areas superimposed with main traces of the Altyn Tagh fault (ATF) and the East Kunlun Fault (EKF); red rectangle marks the location of our study area. (B) Simplified geological map of the western Qaidam Basin. EK—East Kunlun Mountains; QB—Qaidam Basin; TB—Tarim Basin; TS—Tien Shan; WK—West Kunlun Mountains.

of the ATF and developed internal drainage since the Oligocene or possibly mainly on newly obtained high-resolution seismic reflection data, borehole late Eocene due to the relative motion of the Altyn Tagh, i.e., the sliding logs, and isopach maps, which place new constraints on the Cenozoic move- door mechanism. These studies, however, lack a systematic understanding ment of the ATF. of the structural coupling between the ATF and the Qaidam Basin, possibly because they are mainly based on field investigations, although most areas of the western Qaidam Basin are now covered by late Quaternary sediments GEOLOGICAL SETTING (Fig. 1B). Fortunately, natural gas exploration during the past decade has yielded a growing number of two-dimensional (2-D) and three-dimensional The Qaidam basin, with an area of ~120,000 km2 and an average elevation (3-D) seismic surveys conducted in the northwestern Qaidam Basin adjacent of >3000 m, is a rhomb-shaped intermontane basin bounded by the Altyn Tagh to the ATF, providing a good opportunity to unravel its structural features to the northwest, the Qilian Shan to the northeast, and the East Kunlun Shan and to detail its structural coupling with the ATF. In this study, we identified to the south (Yin and Harrison, 2000) (Fig. 1A). It developed on Proterozoic– a structural transition zone between the ATF and the Qaidam Basin interior, Paleozoic basement and accumulated thick Mesozoic and Cenozoic terrestrial which we termed the “Altyn Slope” because its formation and evolution sediments with an obvious regional unconformity between the Paleogene and were closely related to the ATF. The tilting history of the Altyn Slope was re- the lower Cretaceous (Fig. 1B; GMBQP, 1991). The present-day Qaidam Basin is constructed through the analysis of the 3-D patterns of growth strata, based generally accepted to have formed in the Cenozoic in response to the ongoing

collision between India and Asia (Tapponnier et al., 2001; Yin and Harrison, beds on the Altyn Slope that are examined in this paper. We use these fea- 2000; Yin et al., 2008a, 2008b; Wang et al., 2006) and is characterized by a large tures to infer the geological evolution of the ATF and its relationship to the NW-trending syn-depositional Tertiary synclinorium with the depocenters that Qaidam Basin. persist along the central axis but that have migrated eastward throughout the Cenozoic (Bally et al., 1986; Wang et al., 2006; Yin et al., 2008a). Thus, the Ceno zoic Qaidam Basin developed as a synclinal depression (Meng and Fang, 2008) DEFINITION AND GEOMETRY OF THE CURRENT ALTYN SLOPE rather than a foreland basin (Zhu et al., 2006). Yin et al. (2002) suggested that the Qaidam Basin began to receive sediments from both the north and the The Altyn Slope is defined as the transition zone between the ATF and the south as the corresponding thrust belts were initiated during or immediately Qaidam Basin Interior. It extends in the ENE direction from Mang’ai to Lenghu, after the onset of the Indo-Asian collision (ca. 50–55 Ma). This timing is con- approximately parallel to the ATF (Fig. 3), and is characterized by the following sistent with the results of balanced cross-section restoration indicating that two features (Fig. 4): (1) its pre-Jurassic basement dips generally toward the the Qaidam Basin has undergone continuous shortening since the Paleocene southeast; and (2) the thicknesses of the Cenozoic sequences decrease toward or early Eocene (Zhou et al., 2006; Yin et al., 2008a). Recent study focusing on the northwest. The present geometry of the Altyn Slope is accurately outlined the activities of the fault system in the southwestern part of the Qaidam Basin from the dense network of 2-D and 3-D seismic reflection sections calibrated based on up-to-date 3-D seismic data suggests that the Qaidam Basin started by drilling data (Fig. 3) because most areas of the Altyn Slope are now covered to experience compressional deformation as early as the Paleocene (Wu et al., by Quaternary sediments (Fig. 1B). Tens of NW-, NNW-, and NE-striking seis- 2014). Deformation in the northwestern Qaidam Basin is considered to have mic reflection sections (in time domain) oriented orthogonally across the Altyn occurred since at least the early Oligocene in response to the activation of the Slope and the Qaidam Basin interior have been interpreted based on borehole ATF, according to growth strata and abruptly altered sedimentary facies (Meng logging and surface geology (gray dotted lines in Fig. 3). We identified the and Fang, 2008; Yue and Liou, 1999; Yue et al., 2005; Wu et al., 2012a, 2012b). hinge points between the basement of the Altyn Slope and the basin interior in Intense NE-SW–directed intrabasinal folding and faulting, which formed the each section and projected these points onto the surface (Fig. 4). The present present structural framework within the basin, decreasing eastward (Yin et al., southeastern boundary of the Altyn Slope is depicted by the best-fit line of 2007; Zhang et al., 2013), mainly commenced as late as Pliocene (Meng and these projected hinge points, which may represent the maximum extent into Fang, 2008; Zhou et al., 2006; Xiao et al., 2013). the northwestern Qaidam Basin of influence by the ATF. The hinge line undu- Cenozoic deposits within the Qaidam Basin consist generally of nonmarine lates, with peaks and valleys, and the peaks are coincidentally located at the clastic rocks with a maximum thickness of >15 km (Wang et al., 2006), and can anticlinal belts within the basin (Fig. 3). be assigned to the following eight formations from oldest to youngest (Fig. 2): Through interpretation of these subsurface data, we detailed the Cenozoic Lulehe Formation (LLH, 53.5–43.8 Ma), Lower Xiaganchaigou Formation (LXG, fault system within the identified Altyn Slope and adjacent QaidamBasin in- 43.8–40 Ma), Upper Xiaganchaigou Formation (UXG, 40–35.5 Ma), Shanggan- terior (Fig. 3). In addition to the dominant NW- to NNW-trending faults (e.g., chaigou Formation (35.5–22 Ma), Xiayoushashan Formation (XY, 22–14.9 Ma), labeled Chainan, Pingdong, and E’dong faults in Fig. 3) constituting the over Shangyoushashan Formation (SY, 14.9–8.2 Ma), Shizigou Formation (SZG, arching structural framework of the Qaidam Basin, previously well docu- 8.2–2.5 Ma), and Qigequan Formation (QGQ, 2.5–0.8 Ma). Ages of these for- mented (Zhou et al., 2006; Yin et al., 2008a, 2008b; Wu et al., 2014), we identify mations have been well constrained by magnetostratigraphy, fossil records, a set of E-W–trending faults that are en echelon distributed and slightly oblique and apatite fission-track ages (Fig. 2; Gao et al., 2009; Z.M. Sun et al., 2005; Lu to the main trace of the ATF. To better demonstrate the geometry and deforma- and Xiong, 2009; Fang et al., 2007). Cenozoic successions in the northwestern tion history of the Altyn Slope and their coupling with the ATF and the Qaidam Qaidam Basin were predominantly deposited in alluvial, fluvial, and lacustrine Basin interior, we constructed three geologic sections based on surface geol environments with significant spatial and temporal facies variations (Meng ogy, topography, and seismic and borehole data (Fig. 4). These sections all and Fang, 2008; Wu et al., 2012a), and are composed primarily of conglom- show that the Altyn Slope is tilted by those E-W–trending, north-dipping erate and sandstone intercalated with siltstone and mudstone, as well local basement-involved reverse faults that probably link with the ATF at depth to marlstone and gypsum (Fig. 2). form typical positive flower structures, e.g., the Niubei, Yuebei, and Caishiling Provenance analysis from paleo-current directions, clast composition, and faults. These observations allow us to conclude that the E-W–trending faults heavy mineral composition indicates that the Cenozoic sediments in the north- are flanking structures of the ATF, manifesting the tilting of the Altyn Slope that western Qaidam Basin were derived from the Altyn Tagh to the north (Rieser is dominated by the left-slip motion of the ATF. et al., 2005, 2006; Wu et al., 2012a; Zhuang et al., 2011) and consequently record Northwestward-thinning growth-strata sequences, from the Upper Xiagan plentiful information on the Cenozoic evolution of the ATF. Cenozoic sedimen- chaigou formation to the Qigequan formation, are widely preserved along tary strata were deformed by faulting and folding generating syn-deforma- the Altyn Slope (Fig. 4), manifesting late Eocene–early Oligocene initiation tional successions (growth strata) and unconformities between the individual of those E-W–trending faults that attach to the Altyn Tagh fault system.

Age Geomagnetic Polarity Epoch Formation Ma Lithology (Ma)

Symbol ABC D 0 Quatenary Qigequan QGQ 2.5 2.5 Pliocene 5 Shizigou SZG 8.1 8.2 8.5 L 8.2 10 Shangyou- e shashan SY M 13 15 14.9 14.9 15.3

iocen

M Xiayousha- XY 20 E shan 22 22 20-22 25 L 26.5 Figure 2. Magnetostratigraphy and lithol- Shanggan- SG 30 chaigou ogy of the western Qaidam Basin, China. ligocene 31.5 Black and white stripes indicate normal

O E and reversed polarities, respectively. 34 Numbers denote boundary ages (in Ma) 35 L 35.5 35.5 between lithological formations and top U.Xiagan- and bottom ages. The geomagnetic polar- UXG ity columns are referred to: A—Gao et al. chaigou 40 (2009); B—Z.M. Sun et al. (2005); C—Lu and 40 40 L.Xiagan- Xiong (2009); D—Fang et al. (2007). In the LXG chaigou geologic time scale: E—early; M—middle; M 43.8 L—late. In the formation names: U.—Upper; 45 L.—Lower. AFT—apatite fission-track.

Eocene Lulehe LLH 50 E 53.5 55 L 60 M Lacuna

Paleocene E 65

Mudstone Sandstone Conglomerate Gypsum Fossil Siltstone Pebbly Marlstone Sandstone Constraints

Legend Conformity Disconformity Unconformity AFT Constraints

90 E 92 E 94 E

A N ±

39 B Figure 3. Cenozoic tectonic map of the ED Lenghu YB NB western Qaidam Basin, with shaded relief. C YN Seismic lines are used to determine the boundary of the Altyn Slope. Green lines CSL indicate geologic sections shown in Fig- PD ure 4. Major E-W–striking transpressional QD faults are indicated as: (1) Caishiling fault M (CSL), (2) Niubei fault (NB), (3) Qidong fault (QD), (4) Yuebei fault (YB), (5) and

N CN Yuenan fault (YN). Major NW-striking fault system includes: (1) Chainan fault (CN), (2) E’dong fault (ED), (3) and Pingdong

38 O Location fault (PD). ATF—Altyn Tagh fault. Hinge point Current slope boundary Geologic section Seismic lines Thrust faults

Intense faulting took place at the end of the deposition of the Xiayoushashan Methodology Formation (ca. 14.9 Ma), as demonstrated by the obvious unconformity between the Shangyoushashan and Shizigou Formations and underlying strata, Deposited over the top or against the flanks of growing structures, growth as previously shown by Wang et al. (2010). A typical structure that developed strata are often used to decipher kinematic histories of fault-related folds during this period is the notable pop-up bounded by the Niubei fault and its (Suppe et al., 1997). Much work has been conducted to decipher the relation- back-thrust; this structure is strongly eroded and unconformably covered by ship between fault-induced tilting of strata and the deposition of syntectonic the Shangyoushashan Formation (Fig. 4A). Activity of these E-W–striking faults sediments (e.g., Suppe et al., 1992; Storti and Poblet, 1997; Ford et al., 1997; has diminished since then, only slightly bending the late Cenozoic layers. In Zapata and Allmendinger, 1996). Salvini and Storti (2002) provided a method contrast, the NW- to NNW-trending reverse faults within the basin were ac- of forward numerical modeling to determine 3-D patterns of growth strata tive and accompanied by significant folding in their hanging walls since ca geometries associated with fault-related folds, particularly concerning the 14.9 Ma (Zhou et al., 2006; Xiao et al., 2013), as revealed by the younger growth thickness variations that clearly reflect the shape of anticline surfaces. Inspired sequences (GS-2 in Fig. 4A) ages spanning the mid-Miocene to Pliocene by this work, we developed a way to semiquantitatively restore the tilting his- (Shangyoushashan to Qigequan Formations). tory of the Altyn Slope by extracting the extent of the syntectonic sequences affected by the uplift of the Altyn Slope from newly obtained isopach maps of the Cenozoic stratigraphic units. TEMPORAL DEVELOPMENT OF THE ALTYN SLOPE We assumed that in a compressional depression such as the Qaidam Basin, strata thickness should remain constant or decrease linearly and gently from The present geometry of the Altyn Slope is to be a composite outcome the basin interior to the margin, given a sedimentation process without tec- after long-term evolution. To further constrain how the Altyn Slope evolved tonic perturbation (Fig. 5A). Once this process is influenced by tectonic ac- over time, we applied an approach to delineate its shape during different time tivities (e.g., uplift in the hanging walls of reverse faults along the margin), periods whereby we identified the extent of syntectonic sequences from pre- thickness variation no longer maintains this trend but develops steeper cise isopach maps of the corresponding Cenozoic stratigraphic units. gradients toward the margin (Suppe et al., 1992; Salvini and Storti, 2002).

B TWTT(S) 5 4 3 2 1 0 10km 10km AT SE well1 F SS E 5k m unconformity ). Geologic sections orthogonal across the Altyn Slope (See Fig. 3 for locations and fault abbreviations, and Fig. 2 for formation YB unconformity Section(2) LX G G AT s-1 F SY LLH UX G SG XY Mz 5k Section(1) NB m Alty well2 Alty wel l nS nS unconformit y 1 lope lope growt h strata Gs-1 Gs-2 ED well well LX QGQ SZ UX LL H SG XY SY Mz 2 hing e YN G G G growth poin Gs-2 t strata well hinge LL H Mz UX G LX QGQ SZ SG XY SY G point G SSE well SE 3 B′ A′

GEOSPHERE | Volume 12 | Number 3 Zhao et al. | Cenozoic tilting history of the Altyn Tagh fault 889 Research Paper

unconformity well growth strata SE QGQ XY SY SZG 1

TWTT(S) SZG SY SG SY XY UXG 2 SG XY UXG LXG SG LLH 3 UXG Mz LLH LXG Mz

4 5km Figure 4 (continued).

C SE Elevation(km) Section(3) unconformity well growth strata 4 2 CSL 0 –2 –4 QD –6 ATF hinge point CN –8 5km

C C′ AltynSlope

QGQ SZGSSY XY G UXGLXG LLH Mz Pz

Thus, corresponding hinge points occur in the plot of thickness (T) versus dis- tion between basement uplift and sedimentation, and erosion rate (Storti and tance (D) (Fig. 5B), which can be obtained from isopach maps. This assump- Poblet, 1997). tion is verified by forward kinematic modeling of limb-rotation folding process To precisely identify the extent of the Altyn Slope in each period, we first (Storti and Poblet, 1997), which demonstrates clear hinge points on the T-D plot interpolated scattered thickness data into the isopach data for gridding based regardless of whether or not the uplift rate is larger than the syntectonic sedi- on the Delaunay triangulation method (Field, 1991), and then measured the mentary rate (Fig. 5C). As the hinge point locations are usually consistent with thickness variations with a series of NW-trending cross-section rulers (approxi the growth axial surface marking the transition between folded and unfolded mately orthogonal to the ATF) to detect the hinge points, and then projected strata (Fig. 5), the area between the Qaidam basin margin (the ATF) and the them onto the corresponding gridded isopach maps manually (Fig. 6). Note hinge points is then regarded as the extent of the Altyn Slope at each depo that the erosion lines are also presented on each isopach map, and none of sitional period. This method is logically simple and reasonable, as attested the NW-trending cross-section rulers exceeds the erosion lines northward, in by forward kinematic modeling (Suppe et al., 1997; Salvini and Storti, 2002), order to avoid the impact of denudation on the determination of hinge points. although it lacks accurate quantification because of the variable threshold A best-fit line is drawn to link these hinge points, depicting the southeastern value of thickness gradient change, influenced by hinge migration, competi- boundary of the Altyn Slope during each period (Figs. 6C–6G). Generally, the

current Altyn Slope boundary must be constituted by envelope curves of all A Natural sedimentation R R′ these boundaries. section ruler T Following this method, we delineated the extents and geometries of the R′ Altyn Slope of seven periods: 53.5–43.8 Ma (Lulehe Formation), 43.8–40 Ma R (Lower Xiaganchaigou Formation), 40–35.5 Ma (Upper Xiaganchaigou Forma- tion), 35.5–22 Ma (Shangganchaigou Formation), 22–14.9 Ma (Xiayoushashan D Formation), 14.9–8.2 Ma (Shangyoushashan Formation), and 8.2–2.5 Ma (Shizigou Formation). That of the Qigequan Formation (2.5–0.8 Ma) was not analyzed because this formation suffered intense erosion and could not be B Syntectonic sedimentation accurately restored. The isopach maps are modified from Yin et al. (2008b) and R R′ section ruler T further rectified with newly obtained seismic reflection data and borehole data provided by Qinghai Oilfield Company, China National Petroleum Corporation. R′ We also present four typical cross-sections at different locations showing Hinge point R the T-D plots of individual periods in detail (Fig. 7; see Fig. 6A for locations). In D order to highlight the effect of late Cenozoic NE-SW–directed shortening within the basin on the Altyn Slope, we placed three of them (Figs. 6A, 6B, and 6D) on the major anticline belts within the northwestern Qaidam Basin. The values C Forward models of thickness on these T-D curves are normalized to range between 0 and 1 for U/S>1 comparison. Step 2 hinge line This work is preliminary and resolution is limited by the density of bore- holes and resolution of seismic reflection profiles. However, it can provide an T S2 Growth overall view of how the Altyn Slope evolved over time, which can in turn be Step 1 Strata used as a proxy to study the movement of the ATF. S1 D Results

During the deposition of the Lulehe and Lower Xiaganchaigou formations, U/S<1 hinge line the western Qaidam Basin was divided into two NW-striking sub-basins (Figs. Growth 6A and 6B). The northeastern part was mainly occupied by the larger Yiliping S3 Strata T depression (Y) and a much smaller Niubei depression (N) close to the ATF, while the southwestern part contained several minor depocenters named the S2 Mang’ai (M), OldMang’ai (O), and Jinhongshan (J) depressions. The Mang’ai D and Jinhongshan depressions lay against the Altyn Tagh, as inferred by Meng S1 and Fang (2008), and perhaps connect to the Tarim Basin northwestward. The Niubei depression, with a maximum remnant thickness of 600 m, correlates with the thickness in the basin interior. Hinge points were detected locally near Figure 5. Diagram showing the method for identifying the extent of Altyn Slope based on syn- the E’boliang and Lenghu areas on the T-D curves of the Lulehe formation (Fig. tectonic sedimentation with thickness gradient change. (A) In the case of natural sedimentation 6A) but were difficult to resolve on the T-D curves of the subsequent Lower without influence of tectonic activities, the strata thickness increases linearly and gently toward the basin interior, resulting in a nearly straight line on a plot of thickness versus distance from Xiaganchaigou formation (Fig. 6B). the margin (T-D). (B) In the case of syntectonic sedimentation, fault activities increase the topo- Isopach maps of the Upper Xiaganchaigou formation to the Xiayou graphic slope, leading to a steeper thickness gradient close to the margin and the occurrence of shashan formation exhibit similar characteristics to that of the Lower Xiagan a slope break (hinge point) on the T-D plot. (C) Balanced forward models of limb rotation folding chaigou formation, except for the gradual disappearance of the depressions with different ratios between the uplift and syntectonic sedimentary rates (U/S), which are used to verify the inferred method showing obvious thickness gradient change of every single layer mentioned above in the slope area. Hinge points are evident on all of the T-D of the growth strata (modified from Storti and Poblet, 1997). It is clear that, in both the models, curves derived from these maps (Figs. 6C–6E and 7). The southeastern bound- the hinge points on the T-D plots indicate well the outer extent of the uplift area, testifying to aries of the Altyn Slope, as depicted by the best-fit lines of hinge points, have the applicability of our proposed method. S1, S2, and S3 are syntectonic sequences; while Step 1 and Step 2 refer to the formation process of these syntectonic sequences on the condition that similar undulatory geometry, but with much smaller amplitude. The extent tectonic uplift rate is higher than the sedimentation rate (U/S > 1). of the Altyn Slope during these periods was smaller than the modern range.

GEOSPHERE | Volume 12 | Number 3 Zhao et al. | Cenozoic tilting history of the Altyn Tagh fault Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/12/3/884/3337902/884.pdf 891 by guest on 26 September 2021 on 26 September 2021 by guest Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/12/3/884/3337902/884.pdf Research Paper J—Jinhongshan depression; N—Niubei depression. in different depositional stages, while the black solid line is current boundary. Y—Yiliping depression; M—Mang’ai depression; O—Old Mang’ai depression; for decompaction. Red solid lines indicate the maximum erosional extents. Blue dots are hinge points. Blue solid lines represent boundaries of the Altyn Slope in the insets correspond to the northwestern ends of the numbered lines shown in the corresponding panel. Note that these isopach maps are not corrected sponding isopach maps, with hinge points highlighted by small rectangles and thickness increasing to the right. The upper ends of the numbered T - D curves formation (LXG). (C) Upper Xiaganchaigou formation (UXG). ATF—Altyn Tagh fault. Insets show the typical thickness-distance plot extracted from the corre - Figure 6 ( on this and following two pages 38° N 39° N 38° N 39° N 38° N 39° N 1500 2200 1000 Thickness(m ) Thickness(m) Thickness(m) 1 1 T T T hickness 1 hickness hickness 2 2 2 0 0 curv e

0 curv e curv e 3 3 90° M M 90° M 90° X Xorkol Xork 3 90° ang’ai ang’ai

ang’ai ork 90° 90° E 4 4 E E 4 E o Fitted E Fitted E Fitted o

l l Basi 5 " 5

" " Basi 5 Basi

curv e

curv e curv e 6 6 6

n n 7 Qima Q Qima

7 7 H

Hinge H ima inge inge ). Gridded isopach maps of the Cenozoic stratigraphic units. (A) Lulehe formation (LLH). (B) Lower Xiaganchaigou p 8 p p

8 ENE

EN 8 ENE

oint

M oint oint T n 1 T n T n E 1

M agh ag agh

Fig.7 M h

Alty

Alty Alty 2 D 2 J 2 J O OldMang’ai T n

T n T n O Fig.7 ldMang’ai ldMang’ai 3 LLH Fm. (53.5–43.8 Ma LXG Fm. (43.8–40 Ma )

agh

agh agh C O 3 UXG Fm. (40–35.5 Ma ) 3 " " " Dongpin 4 Dongpin g O

92° 92 °E 92 °E Q 92° 92 °E 92 °E

Q 4 aidam E

E aida Q g " " " O

N N aidam Fig.7B

5 m 5

5 Basi Basi E E’boliang E’bolian ) ’boliang

6 6

" Basi 6 " "

n n Fig.7A Y 7

g n Y 7 7 8 8

AT AT AT Y " " 8 " Lenghu Lenghu Lenghu F F F 02 02

S Serten Serteng

94 °E 94 °E 94° 94 °E erten 94 °E 94° 55 55 55 E

g g 0k 0k ) " ) " ± 0k ± B ± C A m m m

38° N 39° N 38° N 39° N 38° N 39° N

GEOSPHERE | Volume 12 | Number 3 Zhao et al. | Cenozoic tilting history of the Altyn Tagh fault 892 on 26 September 2021 by guest Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/12/3/884/3337902/884.pdf Research Paper

38° N 39° N 38° N 39° N 38° N 39° N 2500 1800 2400 Thickness(m ) Thickness(m ) T hickness(m) 1 1 1 T Thickness Thickness T T hicknes Figure 6 ( continued hicknes hicknes 2 2 2

s curve curv curv s curve s curve 0 0 0 3 3 3 90 °E 90 °E 90 °E e e Mang’a

Mang’a Xorko Xorkol

Mang’ai Xorko 90 °E 90 °E 90 °E 4 4 4 Fitte Fitted Fitte Fitte Fitte d curv

l B l Basi i i curv 5 Basi " " 5 " 5 d d curve d ). (D) Shangganchaigou formation (SG). (E) Xiayoushashan formation (XY). (F) Shangyoushashan formation (SY). curve

asi curve e e 6 6 6

n n H n Qima Qima Qima inge inge Hinge Hinge Hinge 7 7 point point 7 p p p

8 EN oint M EN oint T n T n oint T n 8 8 E E E

agh agh ag

1 h 1

Alty Alty Alty 2 2 2 O OldMang’ai

T n T n T n OldMang’a i ldMang’ai 3 3

ag ag ag SY Fm. (14.9–8.2 Ma ) SG Fm. (35.5–22 Ma ) XY Fm. (22–14.9 Ma ) 3

h h h " " " 4 Dongping Dongpin O 92° 92 °E 92 °E 4 92° 92 °E 92 °E O

Q E 4

Q O "

" " Q

aida aidam

g aidam 5 B m 5 5

E E’bolian E

6 Basi ’boliang

’boliang

" "

6 Basi as

6 i

7 n

n g n 7 7 Y Y 8 Y 8 8

AT AT AT " " " Lenghu Lenghu Lenghu

F F F

02 02 02

Serten S Serten

94° 94 °E 94 °E erteng 94° 94 °E 94 °E 55 55 55 E

g g 0k ) " 0k ) " 0k ) " ± ± ± D E F m m m

38° N 39° N 38° N 39° N 38° N 39° N

GEOSPHERE | Volume 12 | Number 3 Zhao et al. | Cenozoic tilting history of the Altyn Tagh fault 893 Research Paper

90° E 92°E 94°E

ENE SZG Fm. (8.2–2.5 Ma) 1 2 3 4 5 6 7 8 ATF )"G

Alty n T agh N

39°

± 39° E’boliang Thickness curve Fitted curve Hinge point " Lenghu n " Basi Dongping Xorkol " Serten Figure 6 (continued). (G) Shizigou 8 g formation (SZG). Mang’ai" 3 6 " 7 4 5 N 2

° N

38° 1 Qa ida m 38 " Basi n Thickness(m) Qima OldMang’ai n T ag 02550km 1500 0 h 90°E 92°E 94°E

TN N A T B Figure 7. Comparison diagram of the basinward basinward thickness-distance (T-D) plots and cor- 0.8 0.8 responding hinge points of each period (see Fig. 6A for locations). The vertical and horizontal axes indicate the normalized 0.6 0.6 thickness (T 3 6 5 N) and distances away from 1 the basin margin (ATF), respectively. TN 5 0.4 0.4 6 is obtained following the equation TN = 1 3 (T–T )/(T –T ), where T, T , and T 4 4 Min Max Min Max Min 0.2 0.2 represent the true, the maximum, and the minimum strata thicknesses in a cer- 2 2 Distance(km) Distance(km) tain period, respectively. Eroded portions 0 0 are marked as dotted lines. The shaded 01020304050 01020304050 blue and gray areas are used to highlight the tendencies of the T-D curves during LLH-LXG UXG-XY SY-SZG the stages of the Upper Xiaganchaigou– Xiayoushashan formations and the TN C TN D Shangyoushashan–Shizigou formations, basinward basinward respectively. 1—Lulehe Formation (LLH); 2—Lower Xiaganchaigou Formation (LXG); 0.8 0.8 3 3—Upper Xiaganchaigou Formation (UXG); 4—Shangganchaigou Formation (SG); 5— 0.6 0.6 Xiayoushashan Formation (XY); 6—Shang 1 youshashan Formation (SY); 7—Shizigou 5 4 5 6 Formation (SZG). It is evident that the 0.4 1 3 6 0.4 4 hinge points are located relatively farther 2 away (by >10 km) from the Altyn Tagh fault 0.2 0.2 2 on the major anticline belts after ca. 15 Ma compared to the previous stage (panels A, Distance(km) Distance(km) 0 0 B, and D). 01020304050 01020304050

However, the boundaries obtained from isopach maps of the Shangyou- and the Slope geometry is characterized by an undulatory boundary with shashan and Shizigou formations become much more undulatory, with ampli peaks located on the major anticlinal belts within the basin (Figs. 8D and 8E). tudes as large as that of present day (Figs. 6F and 6G). The hinge points are The above analysis shows that the southeastern boundary of the Altyn located >10 km farther away from the ATF on the major anticline belts within Slope was significantly transformed into the undulatory shape of the current the northwestern Qaidam Basin than those during previous stages (Figs. 7A, geometry at ca. 15 Ma (Figs. 6F, 6G, and 8) and that the area of the Altyn Slope 7B, and 7D), but show subtle disparity elsewhere (Fig. 7C). increased gradually until ca. 15 Ma, but has remained nearly constant since then (Table 1 and Fig. 9). Two possible interpretations could account for these phenomena: first deposition, uplift, and erosion on the Altyn Slope reached a DISCUSSION steady state in the early Miocene (22–15 Ma), continuing to present. However, this is incompatible with the fact that despite a similar area, the shapes of Geometrical Evolution of the Altyn Slope the Altyn Slope varied during the deposition periods of the Xiayoushashan, Shangyoushashan, and Shizigou formations (Figs. 6 and 8). The alternative Our study on the current geometry of the Altyn Slope shows that it is a and acceptable explanation is that the area of the Altyn Slope was enlarged by NE-trending, SE-dipping slope corrugated with ridges on the surface correlat- late Cenozoic NE-directed shortening within the Qaidam Basin, as indicated: ing with fold axes trending perpendicular to the general strike of the slope. (1) by the increasing undulation of the southeastern boundary of the Altyn Its southeastern boundary is undulatory, with the peaks coincidentally located Slope since ca. 15 Ma, with its valleys shrinking toward the ATF and peaks ex- at the major anticlinal belts within the basin (Fig. 3). As revealed by the sub- panding into the basin interior; (2) the peaks coincide well with major anticlinal surface data, the Altyn Slope was primarily tilted by motion on a series of belts inside the basin (Fig. 8); and (3) previous studies proposed that these anti- E-W–trending, north-dipping transpressional faults that constitute the positive clines mainly formed since ca. 15 Ma with southeastward-decreasing intensity flower structure of the ATF (Fig. 4), indicating that the formation of the Altyn of deformation (Yin et al., 2007; Xiao et al., 2013), leading to southeastward tilt Slope is closely related to the activity of the ATF. of the anticlines (Fig. 4A). If the areas generated by this later NE-directed con- Given these analyses of our isopach maps, the Altyn Slope did not form traction were removed, the southeastern boundary of the Altyn Slope would during the deposition of the Lulehe and Lower Xiaganchaigou formations (ca. systematically shrink toward the ATF (Figs. 8D and 8E), and the slope area 53.5–40 Ma), because (1) several depocenters (e.g., the Mang’ai depression) per- shows a sharp decrease since ca. 15 Ma (the red dotted line in Fig. 9), indi- sisted in the western margin of the Qaidam Basin, where the strata thickened cating that tilting of the Altyn Slope caused by the E-W–trending faults has toward the ATF, reflecting a low relief (Figs. 4B, 4C, 6A, and 6B); and (2) the hinge weakened since then. points can only be detected locally near the E’boliang and Lenghu areas on the T-D plots of the Lulehe formation (Fig. 6A), but are absent on those of the sub- Implications for the Cenozoic Kinematic Pattern sequent Lower Xiaganchaigou formation (Fig. 6B), which we interpreted as the on the Altyn Tagh Fault result of paleo-topography forming in the late Mesozoic when the Altyn Tagh uplifted and suffered erosion (Jolivet et al., 2001). As the hinge points are widely Our result shows that the Altyn Slope experienced a dramatic structural detected on T-D plots of the growth strata spanning the Upper Xiaganchaigou change at ca. 15 Ma. Because the above analysis demonstrates that the for- and Shizigou formations (ca. 40–2.5 Ma; Figs. 6C–6G), we infer that the Altyn mation of the Altyn Slope is closely related to the ATF, it suggests a kinematic Slope began to form at ca. 40 Ma. This agrees well with the occurrence of the change of the ATF at that time. We therefore propose a simple model to ex- NW-thinning growth strata in the slope area (GS-1 in Fig. 4), and with previous plain this change and its structural coupling with the Altyn Slope as well as studies of sedimentary features adjacent to the ATF and thermochronologic dat- deformation inside the Qaidam Basin in the Cenozoic era (Fig. 10). ing in the Altyn Tagh showing that the ATF initiated its activity in the late Eocene– The ATF was not active prior to ca. 40 Ma (Fig. 10A). The southwestern early Oligocene (Bally et al., 1986; Yue et al., 2001; Wang, 1997; Meng et al., 2001; Qaidam Basin was connected to the Tarim Basin (Meng and Fang, 2008), Yin et al., 2002; Chen et al., 2004, 2006; Wu et al., 2012b; Cheng et al., 2015). where a large sedimentary depocenter persisted to receive sediments through To quantitatively unravel the geometrical evolution of the Altyn Slope since westward-flowing drainages in the Qaidam Basin (Yin et al., 2002), while the its initiation at ca. 40 Ma, we plotted width of the Altyn Slope during different northern part developed as a paleohigh inherited from the late Mesozoic periods versus the location along the ATF (Fig. 8), and the area of the Altyn paleotopography (Jolivet et al., 2001; Wu et al., 2015). Slope versus time (black solid line in Fig. 9). It is obvious that: (1) during ca. During ca. 40–15 Ma (Fig. 10B), the ATF was activated, but with a relatively 40–15 Ma, the Altyn Slope continued to enlarge southeastward with increasing slower slip rate than at present (Cheng et al., 2015). It behaved more like a areas (Figs. 8A–8C and 9), and that the slope shapes apparently differed from basal shear zone and was characterized by a series of en echelon distributed that of the current surface (Figs. 8A–8C); and (2) since ca. 15 Ma, the area and E-W–trending transpressional faults (Wu et al., 2012a, 2012b), forming typi- shape of the Altyn Slope have been nearly constant (black solid line in Fig. 9), cal positive flower structures convergent downward toward the current ATF.

E’boliang Hongsanhan Yingxiongling Figure 8. Superimposed diagram of the geometries of the Altyn Slope during different periods in anticlinal belt anticlinal belt anticlinal belt the Cenozoic; each subsequent slope geometry is shown superimposed on the previous one(s) UXG Fm (40–35.5 Ma) (see Fig. 2 for formation abbreviations). The horizontal axis of each panel is the location along 80 A SW

km) the Altyn Tagh fault from Lenghu to Mang’ai (Fig. 6), while the vertical axis shows the width

h ( of the Altyn Slope of the corresponding location extracted from Fig. 6. It is clear that the Altyn 60 Current slope Slope continued to enlarge after 40 Ma, with its shape differing from that of the current slope. boundary

Widt However, it abruptly began to be corrugated with folding-generated ridges (the anticlinal belts)

40 and has gradually formed the current geometry since ca. 15 Ma. Note that the red dotted lines in Panels d and e indicate the width of the Altyn Slope after eliminating the influence of NE-

Slope 20 directed folding since ca. 15 Ma. See text for detail.

G SG Fm (35.5–22 Ma) SZG SY XY SG UXG LLH

LX 80 B SW QG

(km) 60 idth Ns

W 0.8 40 Ns*

a Trendline of Ns Slope 20 Are Trendline of Ns*

e 0.6

0 lop

XY Fm (22–14.9 Ma) 0.4 80 C SW

km)

h ( 60

Normalized 0.2

Widt 40

lope

S 20 0.0 Time(Ma)

0 0 10 20 30 40 50 60

Figure 9. Plots of the normalized areas of the Altyn Slope over time. (See Fig. 2 SY Fm (14.9–8.2 Ma) 80 D SW for formation abbreviations.) The red dotted line is a trendline fitting the areas

km) eliminating the effect of NE-directed folding since ca. 15 Ma, while the black

h ( solid line represents the measured area-variation trend of the Altyn Slope ex- 60

idt tracted from Fig. 6 (corresponding data are shown in Table 1). The plot shows a

W significant increasing trend in slope area prior to ca. 15 Ma but a decreasing one 40 since then, suggesting a kinematic change of the Altyn Tagh fault at that time.

lope

S 20 TABLE 1. SUMMARY OF AREA (S) AND MEAN WIDTH 0 (w) OF THE ALTYN SLOPE AT EACH PERIOD UXG SG XY SY SZG Present SZG Fm (8.2–2.5 Ma) 80 E SW S (km2)10750.7 11246.6 13005.612861.9 12927.814253.9

km) w (km) 39.8 41.6 48.1 47.6 47.8 52.7

h ( 60 Ns 00.140.640.6 0.62 1

idt 2

W S*( km ) –––12133.411540.6 – 40 Ns* –––0.390.23–

lope

S 20 Note: Ns—normalized slope area following the calculation of Ns=(S−SUXG)/SUXG. S*— Lenghu E’boliang Dongping Mang’ai area of the Altyn Slope excluding the portions generated by late Cenozoic shortening of the basin. Ns*—normalized slope area derived from S*. See Figure 2 for formation 0 0 50 100 150 200 250 km abbreviations.

As the slip rate of the ATF was fairly slow at the time (Cheng et al., 2015), 53.5–40 Ma A stresses concentrated on the ATF system were perhaps accommodated by NE vertical shear rather than strike-slip motion, which tilted the basement of the Yiliping depression northwestern margin of the Qaidam Basin, leading to the formation of the Mang’ai depression Altyn Slope between the ATF and the Qaidam Basin interior. This process persisted throughout this period, and the extent of the Altyn Slope continued to increase because of sustaining stress build-up from the ongoing indentation sin Ba of the Indian plate into the Eurasian (Dayem et al., 2009). At this time, except am aid Basement Q for the slight southeastward migration of the depocenters within the Qaidam Jurassicm- iddle Rock Basin (Fig. 10B), only weak deformation was detected (Zhou et al., 2006; Wu s source direction Eocene deposits et al., 2014), even in other areas within the northern Tibetan Plateau (Ritts et al., 2008; Lin et al., 2011). Faster slip on the ATF developed since ca. 15 Ma (Cheng et al., 2015), pos- 40–15 Ma B sibly due to formation of a through-going fault system allowing pure strike- ATF slip motion that localized the strain on the ATF (Le Guerroué and Cobbold, Medium NE slopeextent 2006; Fig. 10C). A regional unconformity was therefore formed and recorded Incorporating in the northwestern margin of the Qaidam Basin because of this change in depocenter kinematics of the ATF (Fig. 4; Wang et al., 2010; Wu et al., 2012a). This process released the strain accumulated beneath the Altyn Slope, causing its southeastern boundary to shrink toward the ATF (Fig. 8) and its area to decrease sin Ba (red dotted line in Fig. 9). To absorb the fast slip on the ATF, intense NE-directed am uplift aid Basement Q crustal shortening occurred within the northern Tibetan Plateau (Yue and Liou, 1999; Turner et al., 1993; Ding et al., 2004; Fang et al., 2007; Royden et al., 2008; Rock s Early Miocene Bovet et al., 2009; Lease et al., 2011; Wang et al., 2012), and many NW-trending deposits faults and folds formed within the Qaidam basin at the time (Zhou et al., 2006; C 15 Ma–now Wu et al., 2014; Xiao et al., 2013; Fig. 4 in this paper), folding the Altyn Slope to En-echelon shape its current geometry (Fig. 10C). In response to this kinematic change on paleo-upheavals NE the ATF, sedimentary facies in the western Qaidam Basin changed from fluvial- Current slope extent Synclinal lacustrine facies prior to ca. 15 Ma to alluvial fan and fan delta facies since trough NW-striking then, with acceleration of sedimentary rates, as revealed by new well-logging ATF folds data and field investigations (Wu et al., 2012a, 2012b; Chang et al., 2015).

sin Ba m ida Qa CONCLUSIONS

Basement Roc As a transition zone between the Altyn Tagh and the Qaidam Basin, the ks Middle Miocene- Incorporating Altyn Slope preserved critical geological evidence for the strike-slip history of depocenter Quatenr ary deposits the ATF. The current geometry of the Altyn Slope was identified by a dense network of NW- to NWN-directed seismic profiles and is characterized by a south- Figure 10. Cartoons illustrating development of the Altyn Slope and kinematic process east-dipping slope tilted by a set of E-W–trending, north-dipping reverse faults of the Altyn Tagh fault (ATF). (A) The ATF was not initiated prior to ca. 15 Ma, and most forming typical flower structures of the ATF. The Altyn Slope has an undu part of the northwestern Qaidam Basin developed as a depression connecting to the latory southeastern boundary with peaks coincidentally located at the major Tarim Basin. (B) During ca. 40–15 Ma, the ATF was activated as a basal shear zone with fairly slow left-slip rate and characterized by a set of E-W–trending, north-dipping faults, anticlinal belts in the Qaidam Basin. An outline of the temporal evolution of the and a large amount of the transpressional stress was converted to vertical strain, raising Altyn Slope geometry was obtained by analyzing the syntectonic sequences the Altyn Slope instead of causing strike-slip motion. (C) Since ca. 15 Ma, faster left- from isopach maps of adjacent stratigraphic units, and seismic reflection pro- lateral strike-slip motion on the ATF has taken place due to the formation of a through- going trace, releasing the stress beneath the Altyn Slope and inducing intense NE-SW– files. The result shows that the Altyn Slope began to form at ca. 40 Ma, con- directed shortening within the Northern Tibetan Plateau that folded the Altyn Slope. tinued to expand southeastward over time with a relatively straight boundary

prior to ca. 15 Ma, but started to shrink toward the ATF with decreasing area Dayem, K.E., Molnar, P., Clark, M.K., and Houseman, G.A., 2009, Far-field lithospheric deformation since ca. 15 Ma. This implies a kinematic change of the ATF at ca. 15 Ma, and in Tibet during continental collision: Tectonics, v. 28, TC6005, doi:10.1029 /2008TC002344 . Ding, G., Chen, J., Tian, Q., Shen, X., Xing, C., Wei, K., and Abbink, A.O., 2004, Active faults and two distinct stages of the left-slip motion on the ATF are therefore inferred for magnitudes of left-lateral displacement along the northern margin of the Tibetan Plateau: the Cenozoic era. During the first stage of ca. 40–15 Ma, the ATF was activated Tectonophysics, v. 380, p. 243–260, doi:10 .1016 /j .tecto .2003 .09 .022 . as a basal shear zone with fairly slow left-slip rate, and large transpressional Elliott, A.J., Oskin, M.E., Liu-Zeng, J., and Shao, Y., 2015, Rupture termination at restraining bends: The last great earthquake on the Altyn Tagh Fault: Geophysical Research Letters, v. 42, stresses were converted to vertical strain, raising the Altyn Slope, instead of p. 2164–2170, doi:10 .1002 /2015GL063107 . producing strike-slip motion. However, during the second stage, since ca. Fang, X., Zhang, W., Meng, Q., Gao, J., Wang, X., King, J., Song, C., Dai, S., Miao, Y., Sun, Z., Yang, 15 Ma, faster left-lateral strike-slip motion on the ATF took place due to the for- Z., Pei, J., Yang, T., Wang, X., Lu, H., and Xiong, S., 2007, High-resolution magnetostratigraphy of the Neogene Huaitoutala section in the eastern Qaidam Basin on the NE Tibetan Plateau, mation of a through-going fault system, releasing the stress beneath the Altyn Qinghai Province, China and its implication on tectonic uplift of the NE Tibetan Plateau: Earth Slope and inducing intense NE-SW–directed shortening within the Northern and Planetary Science Letters, v. 258, p. 293–306, doi:10 .1016 /j .epsl .2007 .03 .042 . Tibetan Plateau that folded the Altyn Slope. Field, D.A., 1991, A generic Delaunay triangulation algorithm for finite element meshes: Ad- vances in Engineering Software and Workstations, v. 13, p. 263–272, doi:10 .1016 /0961 -3552 (91)90031-X . ACKNOWLEDGMENTS Ford, M., Williams, E.A., Artoni, A., Vergés, J., and Hardy, S., 1997, Progressive evolution of a We are grateful to Dr. Raymond Russo, Dr. An Yin, and Dr. Walter Mooney for their construc- fault-related fold pair from growth strata geometries, Sant Llorenç de Morunys, SE Pyrenees: tive reviews. This study is funded by National Natural Science Foundation of China (Grant nos. Journal of Structural Geology, v. 19, p. 413–441, doi:10.1016 /S0191 -8141(96)00116 -2 . 41402170 and 41372206) and the Fundamental Research Funds for the Central Universities. We Gao, J., Li, S., Dai, S., Li, A., and Peng, Y., 2009, Constraints of tectonic evolution in provenance from appreciate the seismic data provided by the Qinghai Oil Company of the China National Petroleum detrital zircon fission-track data of Cenozoic strata of Xichagou district in westernQaidam Basin: Corporation. Journal of Lanzhou University: Natural Sciences, v. 45, no. 3, p. 1–7, doi:10.3321 /j .issn: 0455 -2059 .2009.03.001. GMBQP (Geology and Mineral Bureau of the Qinghai Province), 1991, Regional Geology of Qing- REFERENCES CITED hai Province: Beijing, Geological Publishing House, 662 p. Bally, A.W., Chou, I.-M., Clayton, R., Eugster, H.P., Kidwell, S., Meckel, L.D., Ryder, R.T., Watts, A.B., Jolivet, M., Brunel, M., Seward, D., Xu, Z., Yang, J., Roger, F., Tapponnier, P., Malavieille, J., Arnaud, and Wilson, A.A., 1986, Notes on sedimentary basins in China: Report of the American Sedi- N., and Wu, C., 2001, Mesozoic and Cenozoic tectonics of the northern edge of the Tibetan mentary Basins Delegation to the People’s Republic of China: U.S. Geological Survey Open- plateau: fission-track constraints: Tectonophysics, v. 343, p. 111–134, doi:10 .1016 /S0040 -1951 File Report 86-327, 108 p. (01)00196-2 . Bendick, R., Bilham, R., Freymueller, J., Larson, K., and Yin, G., 2000, Geodetic evidence for a low Lease, R.O., Burbank, D.W., Clark, M.K., Farley, K.A., Zheng, D., and Zhang, H., 2011, Middle Mio- slip rate in the Altyn Tagh Fault system: Nature, v. 404, p. 69–72, doi:10 .1038 /35003555 . cene reorganization of deformation along the northeastern Tibetan Plateau: Geology, v. 39, Bovet, P.M., Ritts, B.D., Gehrels, G., Abbink, A.O., Darby, B., and Hourigan, J., 2009, Evidence of p. 359–362, doi:10 .1130 /G31356 .1 . Miocene crustal shortening in the North Qilian Shan from Cenozoic stratigraphy of the west- Le Guerroué, E., and Cobbold, P. R., 2006, Influence of erosion and sedimentation on strike-slip ern Hexi Corridor, Gansu Province, China: American Journal of Science, v. 309, p. 290–329, fault systems: insights from analogue models: Journal of Structural Geology, v. 28, p. 421– doi:10 .2475 /00 .4009 .02 . 430, doi:10 .1016 /j .jsg .2005 .11 .007 . Chang, H., Li, L., Qiang, X., Garzione, C.N., Pullen, A., and An, Z., 2015, Magnetostratigraphy of Li, J., Zhang, Z., Tang, W., Li, K., Luo, Z., and Li, J., 2014, Provenance of Oligocene–Miocene sedi Cenozoic deposits in the western Qaidam Basin and its implication for the surface uplift of ments in the Subei area, eastern Altyn Tagh Fault and its geological implications: Evidence the northeastern margin of the Tibetan Plateau: Earth and Planetary Science Letters, v. 430, from detrital zircons LA-ICP-MS U-Pb chronology: Journal of Asian Earth Sciences, v. 87, p. 271–283, doi:10 .1016 /j .epsl .2015 .08 .029 . p. 130–140, doi:10 .1016 /j .jseaes .2014 .02 .015 . Chen, Y., Gilder, S., Halim, N.O., Cogné, J.P., and Courtillot, V., 2002, New paleomagnetic con- Lin, X., Chen, H., Wyrwoll, K., Batt, G. E., Liao, L., and Xiao, J., 2011, The uplift history of the straints on central Asian kinematics: Displacement along the Altyn Tagh Fault and rotation of Haiyuan-Liupan Shan region northeast of the present Tibetan Plateau: Integrated constraint the Qaidam Basin: Tectonics, v. 21, 1042, doi:10 .1029 /2001TC901030 . from stratigraphy and thermochronology: The Journal of Geology, v. 119, p. 372–393, doi:10 Chen, Z., Wang, X., Yin, A., Chen, B., and Chen, X., 2004, Cenozoic Left-Slip Motion along the Cen- .1086/660190 . tral Altyn Tagh Fault as Inferred from the Sedimentary Record: International Geology Review, Liu, Y., Neubauer, F., Genser, J., Ge, X., Takasu, A., Yuan, S., Chang, L., and Li, W., 2007, Geochro- v. 46, p. 839–856, doi:10 .2747 /0020 -6814 .46 .9 .839 . nology of the initiation and displacement of the Altyn Strike-Slip Fault, western China: Journal Chen, Z., Gong, H., Li, L, Wang, X., Chen, B, and Chen, X., 2006, Cenozoic uplift and exhumation of Asian Earth Sciences, v. 29, p. 243–252, doi:10 .1016 /j .jseaes .2006 .03 .002 . process of the Altyn Tagh mountains: Earth Science Frontiers, v. 13, no. 4, p, 91–102, doi:10 Lu, H., and Xiong, S., 2009, Magnetostratigraphy of the Dahonggou section, northern Qaidam .3321/j .issn: 1005 -2321 .2006 .04 .008 [in Chinese with English abstract]. Basin and its bearing on Cenozoic tectonic evolution of the Qilian Shan and Altyn Tagh Fault: Cheng, F., Guo, Z., Jenkins, H.S., Fu, S., and Cheng, X., 2015, Initial rupture and displacement on the Earth and Planetary Science Letters, v. 288, p. 539–550, doi:10 .1016 /j .epsl .2009 .10.016 . Altyn Tagh Fault, northern Tibetan Plateau: Constraints based on residual Mesozoic to Ceno- Meng, J., Wang, C., Zhao, X., Coe, R., Li, Y., and Finn, D., 2012, India-Asia collision was at 24°N zoic strata in the western Qaidam Basin: Geosphere, v. 11, p. 921–942, doi:10 .1130 /GES01070 .1 . and 50 Ma: Palaeomagnetic proof from southernmost Asia: Scientific Reports, v. 2, doi:10 Cowgill, E., Yin, A., Wang, X., and Zhang, Q., 2000, Is the North Altyn fault part of a strike-slip .1038/srep00925 . duplex along the Altyn Tagh Fault system?: Geology, v. 28, p. 255–258, doi:10 .1130 /0091 -7613 Meng, Q.-R., and Fang, X., 2008, Cenozoic tectonic development of the Qaidam Basin in the north- (2000)28<255: ITNAFP>2 .0 .CO;2 . eastern Tibetan Plateau, in Burchfiel, B.C., and Wang, E., eds., Investigations into the Tectonics Cowgill, E., Yin, A., Harrison, T.M., and Xiao-Feng, W., 2003, Reconstruction of the Altyn Tagh of the Tibetan Plateau: Geological Society of America Special Paper 444, p. 1–24, doi:10.1130 Fault based on U-Pb geochronology: Role of back thrusts, mantle sutures, and heterogeneous /2008.2444 (01) . crustal strength in forming the Tibetan Plateau: Journal of Geophysical Research, v. 108, 2346, Meng, Q.R., Hu, J.M., and Yang, F.Z., 2001, Timing and magnitude of displacement on the Altyn doi:10 .1029 /2002JB002080 . Tagh fault: Constraints from stratigraphic correlation of adjoining Tarim and Qaidam basins, Cowgill, E., Gold, R.D., Xuanhua, C., Xiao-Feng, W., Arrowsmith, J.R., and Southon, J., 2009, Low NW China: Terra Nova, v. 13, p. 86–91, doi:10 .1046 /j .1365 -3121 .2001 .00320 .x . Quaternary slip rate reconciles geodetic and geologic rates along the Altyn Tagh Fault, north- Mériaux, A.S., Tapponnier, P., Ryerson, F.J., Xiwei, X., King, G., Van der Woerd, J., Finkel, R.C., Li, H., western Tibet: Geology, v. 37, p. 647–650, doi:10 .1130 /G25623A .1 . Caffee, M.W., Xu, Z., and Chen, W., 2005, The Aksay segment of the northern Altyn Tagh Fault:

Tectonic geomorphology, landscape evolution, and Holocene slip rate: Journal of Geophysical rising mechanism of the Altyn Mountain during the Cenozoic: Science China Earth Sciences, Research, v. 110, B04404, doi:10 .1029 /2004JB003210 . v. 55, p. 926–939, doi:10 .1007 /s11430 -012 -4402 -7 . Molnar, P., and Tapponnier, P., 1975, Cenozoic tectonics of Asia: Effects of a continental collision: Wu, L., Xiao, A., Yang, S., Wang, L., Mao, L., Wang, L., Dong, Y., and Xu, B., 2012b, Two-stage Science, v. 189, p. 419–426, doi:10 .1126 /science .189 .4201 .419 . evolution of the Altyn Tagh Fault during the Cenozoic: new insight from provenance analysis Rieser, A.B., Neubauer, F., Liu, Y.J., and Ge, X.H., 2005, Sandstone provenance of north-western of a geological section in NW Qaidam Basin, NW China: Terra Nova, v. 24, p. 387–395, doi:10 sectors of the intracontinental Cenozoic Qaidam basin, western China: Tectonic vs. climatic .1111/j .1365 -3121 .2012 .01077 .x . control: Sedimentary Geology, v. 177, p. 1–18, doi:10 .1016 /j .sedgeo .2005 .01 .012 . Wu, L., Xiao, A., Ma, D., Li, H., Xu, B., Shen, Y., and Mao, L., 2014, Cenozoic fault systems in southwest Rieser, A.B., Liu, Y., Genser, J., Neubauer, F., Handler, R., Friedl, G., and Ge, X., 2006, 40Ar/39Ar ages of Qaidam Basin, northeastern Tibetan Plateau: geometry, temporal development and significance detrital white mica constrain the Cenozoic development of the intracontinental Qaidam Basin, for hydrocarbon accumulation: AAPG Bulletin, v. 98, p. 1213–1234, doi:10 .1306 /11131313087 . China: Geological Society of America Bulletin, v. 118, p. 1522–1534, doi:10 .1130 /B25962 .1 . Wu, Z., Barosh, P.J., Ye, P., and Hu, D., 2015, Late Cretaceous tectonic framework of the Tibetan Ritts, B.D., Yue, Y., and Graham, S.A., 2004, Oligocene-Miocene tectonics and sedimentation along Plateau: Journal of Asian Earth Sciences, v. 114, p. 693–703, doi:10 .1016 /j .jseaes .2014 .11 .021. the Altyn Tagh Fault, northern Tibetan Plateau: Analysis of the Xorkol, Subei, and Aksay Ba- Xiao, A., Wu, L., Li, H., and Wang, L., 2013, Tectonic processes of the Cenozoic Altyn Tagh Fault sins: The Journal of Geology, v. 112, p. 207–229, doi:10.1086 /381658 . and its coupling with the Qaidam Basin, NW China: Acta Petrolei Sinica, v. 29, p. 2826–2836 [in Ritts, B.D., Yue, Y., Graham, S.A., Sobel, E.R., Abbink, O.A., and Stockli, D., 2008, From sea level to Chinese with English abstract]. high elevation in 15 million years: Uplift history of the northern Tibetan Plateau margin in the Yin, A., and Harrison, T.M., 2000, Geologic evolution of the Himalayan-Tibetan orogen: Annual Altun Shan: American Journal of Science, v. 308, p. 657–678, doi:10 .2475 /05 .2008 .01. Review of Earth and Planetary Sciences, v. 28, p. 211–280, doi:10.1146 /annurev .earth.28 .1 .211. Royden, L.H., Burchfiel, B.C., and van der Hilst, R.D., 2008, The geological evolution of the Tibetan Yin, A., Rumelhart, P.E., Butler, R., Cowgill, E., Harrison, T.M., Foster, D.A., Ingersoll, R.V., Zhang, Q., Plateau: Science, v. 321, p. 1054–1058, doi:10 .1126 /science .1155371 . Zhou, X.Q., Wang, X.F., Hanson, A., and Raza, A., 2002, Tectonic history of the Altyn Tagh Fault Salvini, F., and Storti, F., 2002, Three-dimensional architecture of growth strata associated to fault- system in northern Tibet inferred from Cenozoic sedimentation: Geological Society of Amer- bend, fault-propagation, and décollement anticlines in non-erosional environments: Sedimen- ica Bulletin, v. 114, p. 1257–1295, doi:10 .1130 /0016 -7606 (2002)114 <1257: THOTAT>2 .0 .CO;2 . tary Geology, v. 146, p. 57–73, doi:10 .1016 /S0037 -0738 (01)00166 -X . Yin, A., Dang, Y., Zhang, M., McRivette, M.W., Burgess, W.P., and Chen, X., 2007, Cenozoic tectonic Storti, F., and Poblet, J., 1997, Growth stratal architectures associated to decollement folds and evolution of Qaidam basin and its surrounding regions (part 2): Wedge tectonics in southern fault-propagation folds: Inferences on fold kinematics: Tectonophysics, v. 282, p. 353–373, doi: Qaidam basin and the Eastern Kunlun Range, in Sears, J.W., Harms, T.A., and Evenchick, C.A., 10.1016 /S0040 -1951 (97)00230 -8 . eds., Whence the Mountains? Inquiries into the Evolution of Orogenic Systems: A Volume in Sun, J.M., Zhu, R.X., and An, Z.S., 2005, Tectonic uplift in the northern Tibetan Plateau since Honor of Raymond A. Price: Geological Society of America Special Paper 433, p. 369–390, doi: 13.7 Ma ago inferred from molasse deposits along the Altyn Tagh Fault: Earth and Planetary 10.1130 /2007 .2433 (18) . Science Letters, v. 235, p. 641–653, doi:10 .1016 /j .epsl .2005 .04 .034 . Yin, A., Dang, Y., Wang, L., Jiang, W., Zhou, S., Chen, X., Gehrels, G. E., and McRivette, M.W., Sun, Z.M., Yang, Z.Y., Pei, J.L., Ge, X.H., Wang, X.S., Yang, T.S., Li, W.M., and Yuan, S.H., 2005, Mag- 2008a, Cenozoic tectonic evolution of Qaidam basin and its surrounding regions (Part 1): The netostratigraphy of Paleogene sediments from northern Qaidam Basin,China: Implications for southern Qilian Shan-Nan Shan thrust belt and northern Qaidam basin: Geological Society of tectonic uplift and block rotation in northern Tibetan plateau: Earth and Planetary Science America Bulletin, v. 120, p. 813–846, doi:10 .1130 /B26180 .1 . Letters, v. 237, p. 635–646, doi:10 .1016 /j .epsl .2005 .07 .007 . Yin, A., Dang, Y., Zhang, M., Chen, X., and McRivette, M.W., 2008b, Cenozoic tectonic evolution of Suppe, J., Chou, G., and Hook, S.C., 1992, Rates of folding and faulting determined from growth the Qaidam basin and its surrounding regions (Part 3): Structural geology, sedimentation, and strata, in McClay, K.R., ed., Thrust Tectonics: Dordrecht, Springer, p. 105–121, doi:10.1007 /978 regional tectonic reconstruction: Geological Society of America Bulletin, v. 120, p. 847–876, -94-011 -3066 -0_9 . doi:10 .1130 /B26232 .1 . Suppe, J., Sàbat, F., Muñoz, J.A., Poblet, J., Roca, E., and Vergés, J., 1997, Bed-by-bed fold growth Yue, Y.J., and Liou, J.G., 1999, Two-stage evolution model for the Altyn Tagh Fault, China: Geology, by kink-band migration: Sant Llorenç de Morunys, eastern Pyrenees: Journal of Structural v. 27, p. 227–230, doi:10 .1130 /0091 -7613 (1999)027 <0227: TSEMFT>2 .3 .CO;2 . Geology, v. 19, p. 443–461, doi:10 .1016 /S0191 -8141 (96)00103 -4 . Yue, Y., Ritts, B.D., and Graham, S.A., 2001, Initiation and Long-Term Slip History of the Altyn Tapponnier, P., Xu, Z.Q., Roger, F., Meyer, B., Arnaud, N.O., Wittlinger, G., and Yang, J.S., 2001, Tagh Fault: International Geology Review, v. 43, p. 1087–1093, doi:10 .1080 /00206810109465062 . Oblique stepwise rise and growth of the Tibet plateau: Science, v. 294, p. 1671–1677, doi:10 Yue, Y., Graham, S.A., Ritts, B.D., and Wooden, J.L., 2005, Detrital zircon provenance evidence for .1126/science .105978 . large-scale extrusion along the Altyn Tagh Fault: Tectonophysics, v. 406, p. 165–178, doi:10 Turner, S., Hawkesworth, C., Liu, J., Rogers, N.O., Kelley, S., and van Calsteren, P., 1993, Timing of .1016/j .tecto .2005 .05 .023 . Tibetan uplift constrained by analysis of volcanic rocks: Nature, v. 364, p. 50–54, doi:10.1038 Zapata, T.R., and Allmendinger, R.W., 1996, Growth stratal records of instantaneous and progres- /364050a0. sive limb rotation in the Precordillera thrust belt and Bermejo basin, Argentina: Tectonics, Wang, E., 1997, Displacement and timing along the northern strand of the Altyn Tagh Fault zone, v. 15, p. 1065–1083, doi:10 .1029 /96TC00431 . Northern Tibet: Earth and Planetary Science Letters, v. 150, p. 55–64, doi:10.1016 /S0012 -821X Zhang, W., Fang, X., Song, C., Appel, E., Yan, M., and Wang, Y., 2013, Late Neogene magneto- (97)00085-X . stratigraphy in the western Qaidam Basin (NE Tibetan Plateau) and its constraints on active Wang, E., Xu, F.Y., Zhou, J.X., Wan, J.L., and Burchfiel, B.C., 2006, Eastward migration of the Qaidam tectonic uplift and progressive evolution of growth strata: Tectonophysics, v. 599, p. 107–116, basin and its implications for Cenozoic evolution of the Altyn Tagh Fault and associated river doi:10 .1016 /j .tecto .2013 .04 .010 . systems: Geological Society of America Bulletin, v. 118, p. 349–365, doi:10.1130 /B25778 .1 . Zhou, J.X., Xu, F.Y., Wang, T.C., Cao, A.F., and Yin, C.M., 2006, Cenozoic deformation history of the Wang, E., Kirby, E., Furlong, K. P., van Soest, M., Xu, G., Shi, X., Kamp, P.J.J., and Hodges, K.V., Qaidam Basin, NW China: Results from cross-section restoration and implications for Qing- 2012, Two-phase growth of high topography in eastern Tibet during the Cenozoic: Nature hai–Tibet Plateau tectonics: Earth and Planetary Science Letters, v. 243, p. 195–210, doi:10 .1016 Geoscience, v. 5, p. 640–645, doi:10 .1038 /NGEO1538 . /j.epsl .2005 .11 .033 . Wang, L., Xiao, A., Gong, Q., Liu, D., Wu, L., Zhou, S., Shen, Z., Lou, Q., and Sun, X., 2010, The Zhu, L., Wang, C., Zheng, H., Xiang, F., Yi, H., and Liu, D., 2006, Tectonic and sedimentary evolution unconformity in Miocene sequence of western Qaidam Basin and its tectonic significance: of basins in the northeast of Qinghai-Tibet Plateau and their implication for the northward Science China Earth Sciences, v. 53, p. 1126–1133, doi:10 .1007 /s11430-010 -4006 -z. growth of the Plateau: Palaeogeography, Palaeoclimatology, Palaeoecology, v. 241, p. 49–60, Wittlinger, G., Tapponnier, P., Poupinet, G., Mei, J., Danian, S., Herquel, G., and Masson, F., 1998, doi:10 .1016 /j .palaeo .2006 .06 .019 . Tomographic evidence for localized lithospheric shear along the Altyn Tagh fault: Science, Zhuang, G., Hourigan, J.K., Ritts, B.D., Kent-Corson, M.L., and Abbink, A.O., 2011, Cenozoic multi v. 282, p. 74–76, doi:10 .1126 /science .282 .5386 .74 . ple-phase tectonic evolution of the northern Tibetan Plateau: Constraints from sedimentary Wu, L., Xiao, A., Wang, L., Mao, L., Wang, L., Dong, Y., and Xu, B., 2012a, EW-trending uplifts along records from Qaidam basin, Hexi Corridor, and Subei basin, northwest China: American Jour- the southern side of the central segment of the Altyn Tagh Fault, NW China: Insight into the nal of Science, v. 311, p. 116–152, doi:10 .2475 /02 .2011 .02.