Tectonic geomorphology of the Qilian Shan in the northeastern Tibetan Plateau: Insights into the plateau formation processes

Tectonophysics 706–707 (2017) 103–115

Contents lists available at ScienceDirect

Tectonophysics journal homepage: www.elsevier.com/locate/tecto

Tectonic geomorphology of the Qilian Shan in the northeastern Tibetan Plateau: Insights into the plateau formation processes Huiping Zhang a,⁎, Peizhen Zhang a,b, Veronica Prush c, Dewen Zheng a, Wenjun Zheng b, Weitao Wang a, Caicai Liu a, Zhikun Ren a a b c

State Key Laboratory of Earthquake Dynamics, Institute of Geology, China Earthquake Administration, Beijing, China School of Earth Science and Geological Engineering, Sun Yat-Sen University, Guangzhou 510275, China Department of Earth and Planetary Sciences, University of California, Davis, California 95616, USA

a r t i c l e

i n f o

Article history: Received 12 May 2016 Received in revised form 5 April 2017 Accepted 9 April 2017 Available online 13 April 2017 Keywords: Tectonic geomorphology Qilian Shan plateau Tibetan Plateau Low-relief landscape Plateau formation

a b s t r a c t We examined the regional scale topography of the Qilian Shan on the northeastern margin of the Tibetan plateau. Longitudinal profiles and geomorphometric indices, such as slope, local relief and channel steepness reveal that the Qilian Shan was developed as a local plateau with high-relief steep marginal ranges, and interior low-relief topography. Landscape mapping across this local plateau revealed spatially varied origins of the low-relief landscape in the Qilian Shan. North of the Haiyuan fault, universal massive intermontane aggradation was identified. However, the low-relief plateau to the south of the Haiyuan fault was dominated by relict erosional surfaces, even though accompany basin-filling still contributes. The geomorphologic contrasts led us to integrate both the basin-filling processes and antecedent low-relief erosional surfaces into a new insight into the development of the Qilian Shan plateau in the northeastern Tibet. Our present study highlights that explanation for the low-relief plateau formation needs to consider the complex geological evolution of the Qilian Shan, and the Tibetan Plateau itself. © 2017 Elsevier B.V. All rights reserved.

1. Introduction Flanked by highly-dissected steep margins, the Tibetan Plateau is dominated by low-relief, internally-drained basins in the interior regions (Fielding et al., 1994; Fig. 1). The topographic transition from a high-elevation low-relief interior to moderate-elevation high-relief margins is a characteristic feature of the Tibetan Plateau, and is also descriptive for the other orogenic plateaus around the globe. Understanding how low-relief plateau is formed and maintained is crucial to revealing the link between landscape evolution and geodynamic processes, since the low-relief surfaces were often used as markers for reconstructing the deformation history of the plateau (Clark et al., 2005; Hetzel et al., 2011; Giletycz et al., 2015). Until present, the formation of these low-relief plateau landscapes is still the subject of debate. One hypothesis suggests that partially melted lower crust beneath portions of the Tibetan Plateau allows for gravitational gradients to drive outward crustal flow, smoothing lateral variations caused by upper crustal thickening and flattening the plateau's surface over time (Fielding et al., 1994; McKenzie and Jackson, 2002; Bendick et al., 2008). This gravitational forcing may further contribute to expansion of the lower crust to the east, driving late-Miocene surface uplift of ⁎ Corresponding author. E-mail address: [email protected] (H. Zhang).

http://dx.doi.org/10.1016/j.tecto.2017.04.016 0040-1951/© 2017 Elsevier B.V. All rights reserved.

the SE Tibet (Clark and Royden, 2000; Clark et al., 2005). Alternatively, the low-relief interior morphology of Tibet and its close association with internal drainages has been interpreted as a consequence of the so-called “bathtub” basin-filling process (Métivier et al., 1998; Meyer et al., 1998; Liu-Zeng et al., 2008). This basin-filling process is thought to be the dominant way to shape the Qilian Shan plateau in NE Tibet, and further was viewed as present-day analogs for the long-term relief-smoothing processes of the Tibetan Plateau itself (Métivier et al., 1998; Meyer et al., 1998; Tapponnier et al., 2001). As a local plateau on the northeastern margin of the Tibetan plateau, with a present size of ~250,000 km2 and average elevation of ~4 km, the Qilian Shan thus provides an ideal, actively growing, small-scale model of the Tibetan plateau. But the high altitude and relief of the Qilian Shan plateau makes it as inaccessible as topographically similar portions of the Tibetan Plateau itself, hindering a thorough understanding of the evolution of this uplifting low-relief plateau. Detailed topographic analyses of the Qilian Shan plateau are rare, but have significant potential to understand how the plateau landscape developed. Previous studies have focused primarily on the Hexi Corridor north of the Qilian Shan. Uplift and erosion rates (Hetzel et al., 2002, 2004; Champagnac et al., 2010; Palumbo et al., 2011), in conjunction with the observed steep profiles of river channels (Hu et al., 2010), have led to the conclusion that local ranges along the frontal rim of the northern Qilian Shan are approaching topographic steady-state (Palumbo et al., 2010, 2011). In a

104

H. Zhang et al. / Tectonophysics 706–707 (2017) 103–115

recent study focused on the large-scale landscape of the Qilian Shan, Liu-Zeng et al. (2008) suggested that “bathtub” basin-filling played the dominant role in smoothing out the tectonically-generated structural relief across the Qilian Shan plateau and, in an analogous sense, the Tibetan Plateau. However, recent geologic and topographic studies around the Qinghai Nan Shan and Gonghe Nan Shan, two ranges in the SE Qilian Shan, revealed locally gently-dipping relict erosional surfaces along both ranges (Zhang et al., 2012; Craddock et al., 2014). The distribution of such relict erosional surfaces may have extended more broadly, critical to the development of the low-relief plateau landscape across the Qilian Shan plateau. Moreover, if testified, the contribution of the basin-filling process seems to be overestimated, at least for portions of the plateau. The Qilian Shan's moderate size, relatively young age, and prominence as a growing structure along the northeastern Tibetan Plateau make it a prime location for studying the formation and development of nascent plateaus (Figs. 1 and 2). Our study will thus focus and examine the regional scale topography of the Qilian Shan plateau. Along with landscape mapping, longitudinal profiles and geomorphometric indices, such as slope, local relief and channel steepness will be used for analyzing the details of the tectonic geomorphology of the Qilian Shan. This analysis will help to an understanding of the origins and processes related to the development of the low-relief plateau of the Qilian Shan, the Tibetan Plateau, and similar orogenic plateaus worldwide. 2. Geological background The Qilian Shan plateau, a fold-thrust belt located north of the Kunlun Range and Qaidam Basin and south of the Hexi corridor and the Altyn Tagh fault, is characterized by Cenozoic folding, thrust, and strike-slip faulting (Figs. 1 and 2a; Métivier et al., 1998). Its formation has been attributed to the development of early-middle Paleozoic orogenic suture belts, which are composed of numerous thrust slices of deformed lower Paleozoic metasedimentary and metavolcanic strata (Yin and Harrison, 2000; Xiao et al., 2009). Different portions of the Qilian Shan fold-thrust belt are thought to have been sutured to the Qaidam block during the middle Paleozoic at different intervals (Yin and Harrison, 2000). The Qilian Shan fold-thrust belt was reactivated during the Cenozoic, by thrusting and folding following the initiation of the India-Eurasia collision (Yin and Harrison, 2000; Clark, 2012), and it has been mostly viewed as the northeastward growing front, and formed during Plio-Pleistocene (Tapponnier et al., 2001).

Several lines of evidence, however, suggest onset of contractional forces shortly after the initiation of the India-Eurasia collision in the vicinity of the Qilian Shan (Yin et al., 2002, 2008a,b, 2010; Dupont-Nivet et al., 2004; Dayem et al., 2009; Clark et al., 2010; Duvall et al., 2011), and recent observations indicate that the Qilian Shan underwent a renewed episode of topographic growth during the late Miocene (e.g., Fang et al., 2005b; Ritts et al., 2008; Bovet et al., 2009; Zheng et al., 2010; Zhuang et al., 2011). Along the northern Qilian Shan, apatite fission-track cooling ages of ~10 to 20 Ma have been interpreted as the result of thrusting-related uplift and exhumation (George et al., 2001). Apatite (U-Th)/He data confirms rapid cooling and topographic growth beginning at ~10 Ma (Zheng et al., 2010). In the SE Qilian Shan, basinbounding faults initiated during the late Miocene between ~ 6 and 10 Ma, as evidenced by growth strata, change of sedimentary clast provenance, and local vertical-axis rotation in the flanking basin sediments (Fang et al., 2005a; Lease et al., 2007; Craddock et al., 2011; Zhang et al., 2012). Late-Miocene increased sedimentation rates and a change of local climate along the SE Qilian Shan also indicate basin isolation by emergence of local ranges (Hough et al., 2011). Cenozoic deformation and landscape development around the Qilian Shan plateau has been driven by the long-term slip along numerous active strike-slip and thrust faults within the Qilian Shan and along the range margins in NE Tibet (Figs.1 and 2a). Along the left-lateral strike-slip Haiyuan fault (Fig. 2a), which cuts through the central Qilian Shan plateau and separates it into its northern and southern portions, high Quaternary slip rates (N10 mm/yr) for the central segment (Lasserre et al., 1999, 2002) were reported and decrease towards the eastern fault tip (4–6 mm/yr, Li et al., 2009). Locally where the Haiyuan fault bends, the ranges are usually 1500–2500 m higher than the adjacent valleys. Along the Elashan (EF) and Riyueshan (RF) faults (Fig. 2a), two north-northwest-trending right-lateral strike-slip faults south of the Haiyuan fault, dating of offset terrace risers by Carbon-14 (14C), optically stimulated luminescence (OSL), and Beryllium-10 (10Be) techniques indicate slip rates of 1.1 ± 0.3 mm/yr and 1.2 ± 0.4 mm/yr along the faults, respectively (Yuan et al., 2011). Although these faults are strike-slip motion dominated, they contribute uplift of the local ranges due to the deformation partitioning near the strike-slip restraining bends or fault terminus (Duvall et al., 2013; Zheng et al., 2013b). At present, most studies on active thrust faults are from regions to the north of the northern Qilian Shan. A series of offset terraces and alluvial fan surfaces reveal late Quaternary dip-slip rates of ~ 0.3–0.9 mm/yr along the range front faults (Hetzel et al., 2002, 2004; Palumbo et al.,

Fig. 1. Landscape feature of the Tibetan Plateau and the Qilian Shan. Geological structures are modified from Taylor and Yin, (2009), MBT, Main Boundary Thrust; IYS, Indus - Yarlung Zangbo (Brahmaputra) Suture; BNS, Bangongco - Nujiang Suture; JS, Jinsha Suture; KrF, Karakoram fault; KF, Kunlun fault; AF, Altyn-Taugh fault; HF, Haiyuan fault; QFF, Qilian Shan frontal fault; XF, Xianshuihe fault; LF, Longmen Shan fault; TB, Tarim Basin; QB, Qaidam Basin; SB, Sichuan Basin. Shaded dark-gray areas in central Tibetan Plateau and Qilian Shan indicate the internally-drained regions. Also shown are the externally-drained Nu River (NR, Salween River), Lancang River (LR, Mekong River) and Jinsha River (JR, Yangtze River) in southeastern Tibetan Plateau.

H. Zhang et al. / Tectonophysics 706–707 (2017) 103–115

105

Fig. 2. Active faults (a) and drainage basins (b) in the Qilian Shan. Red lines represent the active strike-slip faults, and black lines with triangles are dip-slip faults (a). RF, Riyueshan fault; EF, Elashan fault. Ten drainage basins within four different settings are analyzed (b). Four rivers are flowing farther into the interior Qilian Shan, i.e., Dang River (DA), Shule River (SL), Beida River (BD) and Hei River (HE), two are just along the range front, Hongshuiba River (HB) and Liyuan River (LY), and the other two are externally-drained Yellow River (YR) tributaries, i.e., Datong River (DT) and Huangshui River (HS). Two internally-drained drainages, Lake Qinghai (Q) and Lake Hala (H) are also selected. Locations of the swath profiles (Figs. 3a–d), composite geological cross-section (Fig. 3e) and field photos (Fig. 4) are also shown.

2009; Zheng et al., 2013a). Millennial-scale catchment erosion rates from cosmogenic nuclides (Palumbo et al., 2010, 2011) and shortterm sediment budgets from gauging stations (Pan et al., 2010) are consistent with late-Miocene exhumation rate of ~ 0.5 mm/a near the steepest portion of the northern Qilian Shan (Zheng et al., 2010). The agreement between erosion and uplift rates was previously hypothesized to represent a topographic steady-state for those local ranges along the northern frontal rim of the Qilian Shan (Palumbo et al., 2010, 2011). Rates of thrust fault slip and catchment erosion from interior Qilian Shan plateau are still sparse, thus hindering our further understanding of the relief generation/destruction driving forces. 3. Geomorphic pattern of drainage basins within the Qilian Shan The mean elevation of the Qilian Shan is ~4 km, and it is ~2000 km long and ~450 km wide, respectively. The elevation of ranges bounding the margins of the Qilian Shan plateau typically exceeds ~4.5–5 km (Fig. 2b), with a summit elevation of ~ 5.8 km. Between the corridor basin and the ranges along the margins of the plateau, local relief is ~ 3 km, but decreases to b 1 km in the interior of the Qilian Shan plateau (Fig. 3). Most upstream drainage basins within the Qilian Shan plateau are broad and filled with Quaternary basin sediments (Fig. 2a), as the supporting evidence of the ongoing “bathtub” basin-filling process (Métivier et al., 1998; Meyer et al., 1998; Liu-Zeng et al., 2008). These basins are usually 10–30 km wide with mean elevations around ~3.8– 4 km (Fig. 3). Notable examples include the Yanchiwan (YC; Figs. 3a

and 4a), Muli (ML; Figs. 3c and 4b) and Menyuan (MY; Fig. 3d) basins. In addition to these upstream basins, the internally-drained lakes Hala (H; Figs. 3b and 4c) and Qinghai (Q; Fig. 3d) also occupy the interior low-relief landscape of the Qilian Shan plateau. The rivers of the Qilian Shan can be categorized into five drainage settings by their water-divides and base-levels (Fig. 2b): 1) rivers that originate within the Qilian Shan plateau and flow downstream into the Tarim Basin (the Dang (DA) and Shule (SL) rivers); 2) rivers that originate in the interior of the Qilian Shan plateau and flow downstream into the Hexi corridor basin (the Beida (BD) and Heihe (HE) rivers); 3) rivers that drain into the Hexi corridor basin, but whose headwaters are sourced along the frontal Qilian Shan ranges (the Hongshuiba (HB) and Liyuan (LY) rivers); 4) rivers that are externally-drained and flow east to join the Yellow River (the Datong (DT) and Huangshui (HS) rivers); and 5) rivers that terminate in internally-drained systems, such as the drainages into lakes Qinghai (Q) and Hala (H) in the interior of the Qilian Shan plateau. We discuss the patterns of some of these drainages below. Rivers with headwaters within the interior Qilian Shan plateau are usually ~ 200–400 km long, for example, the Dang River (DA), Shule River (SL), Beda River (BD), and Hei River (HE) (Fig. 2b). These rivers flow parallel to the orientation of mountain ranges and drain into the adjacent basins (Fig. 2b). Along the northern Qilian Shan, a few intermediate-length (~ 80–120 km) rivers, such as the Hongshuiba (HB) and Liyuan Rivers (LY)), do not extend far into the plateau. These rivers instead run downstream towards the front of the Qilian Shan (Fig. 2b).

106

H. Zhang et al. / Tectonophysics 706–707 (2017) 103–115

Fig. 3. Topographic swath elevation and basin distribution across the Qilian Shan plateau (a–d), and a composite geological cross-section (e, see Fig. 2a for location) modified after Craddock et al. (2014). Quaternary intermontane basins are highlighted. YC, Yanchiwan basin; CM, Changma basin; H, Lake Hala basin; SL, Shule basin; TL, Tuolai basin; CK, Chaka basin; ML, Muli basin; HE, Heihe basin; GH, Gonghe basin; Q, Lake Qinghai basin; MY, Menyuan basin. ATF, Altyn-Taugh fault; KF, Kunlun fault; HF, Haiyuan fault. Tr strata, internally deformed Triassic strata; Pre Tr, strata that predate Triassic.

The externally-drained Yellow River runs through the southeastern portion of the Qilian Shan plateau (Fig. 2b). Its tributary, the Datong River, are sourced within the plateau ~ 500 km upstream of its conjunction with the Yellow River. The other tributary, the Huangshui River, originates ~ 250 km upstream of its confluence with the Yellow River and is separated from lake Qinghai to the west. Lakes Qinghai and Hala are internally-drained system with drainage areas of ~ 30,000 km2 and ~ 4600 km2, respectively (Figs. 1 and 2b). Collectively, the pattern of these Qilian Shan drainages shares many similarities to that of rivers across the Tibetan Plateau itself, with rivers flowing either into flanking basins or internally-drained catchments (Fig. 1). 4. Geomorphic analysis methods In order to address our questions regarding the development of the low-relief landscape of the Qilian Shan plateau, we first undertook a large-scale topographic analysis involving extraction of longitudinal profiles and river steepness measurements of rivers draining throughout the Qilian Shan. These topographically derived measurements

help to characterize the overall landscape features of the Qilian Shan plateau. Guided by this topographic overview, in conjunction with previously recognized morphological expressions of the local low-relief erosional surfaces near lake Qinghai in SE Qilian Shan (Fig. 3e; Craddock et al., 2014), we used landscape mapping principles to characterize the distribution of intermontane aggradation and relict erosional surfaces. 4.1. Topographic indices We used 3-arcsecond (~ 90 m) Shuttle Radar Topography Mission (SRTM) digital elevation models (DEMs) (~ 16 m vertical accuracy, http://srtm.csi.cgiar.org/) (Farr et al., 2007) for topographic analysis. General topographic indices were determined, such as local relief, slope and altitude-slope geometrics, across the Qilian Shan plateau. To visualize the results at a resolution sensitive to topographic information, local topographic relief was averaged over an area of 1 × 1 km (Fig. 5a), consistent with dimensions used in previous studies (Wang et al., 2014).

H. Zhang et al. / Tectonophysics 706–707 (2017) 103–115

107

Fig. 4. Field views of the Quaternary intermontane basins (a–b), erosional surface (c–e) and deeply incised channel in front of the Qilian Shan (f), see Fig. 2 for locations.

Ten drainage basins are selected and analyzed to mimic geomorphic drainage settings for rivers across the Tibetan Plateau (Figs. 1 and 2). To determine the altitude-slope relations of drainage basins for geometric comparison, we first derived an average slope map by calculating the steepest slope for eight neighboring grid nodes of the filled DEM (Fig. 5b). The slope distribution of each drainage basin was determined for comparison with the slope angle (Fig. 6a–d). In order to differentiate slope distribution within the drainage basins, the elevation data within every drainage basin were further reclassified into height intervals of 200 m. The average slope data for each interval were then computed to plot histograms of elevation and slope distribution for each drainage basin (Fig. 7a–d). 4.2. River channel steepness indices The shape of longitudinal profiles of fluvial channels has been utilized to identify patterns and relative rates of rock uplift in actively deforming mountain belts (Snyder et al., 2000; Kirby and Whipple, 2001; Kirby et al., 2003). These methods utilize the empirical scaling between local channel slope (S) and upstream drainage area (A) of channels to describe river profiles in terms of steepness (ks) and concavity (θ) indices (Hack, 1957; Snyder et al., 2000). S ¼ ks A−θ

ð1Þ

To compare segmented channel steepness, correlations between the steepness index and concavity index need to be accounted (e.g., Kirby et al., 2003; Wobus et al., 2006). By employing a fixed reference concavity index (θref = 0.45), we reported the normalized steepness index (ksn) over 1 km reach channel slope (Snyder et al., 2000; Kirby et al., 2003).

Hu et al. (2010) only determined the channel steepness of rivers draining the frontal northern Qilian Shan, rather than on a large-scale analysis across the entire plateau. Here we extend the channel steepness study of Hu et al. (2010) to include the interior Qilian Shan plateau and examine channel steepness along rivers draining out of the range (Fig. 8a). We follow the previously published methods of Hu et al., (2010) for analyzing channel steepness. To easily visualize the sharp contrast of the highest channel steepness, interpolation of rasterized channel steepness values was sampled at 500 m intervals along the channel networks. Readers are referred to the comprehensive reviews of Kirby et al. (2003) and Wobus et al. (2006) for the theory and processing techniques of our methods. Major knickpoints were identified by inspection of the longitudinal profiles as concave-upward segments with relatively high steepness (Fig. 8b). Identifications of major knickpoints/knickzones were mostly verified as narrow river valleys and gorges of such regions during field investigations. 4.3. Landscape mapping of basin fill and relict low-relief surfaces Guided by available high-resolution Google Earth imagery, slope and shaded relief maps derived from the digital topography (Fig. 3), and field investigations, we mapped the distribution of intermontane basin fill and relict erosional surfaces (Fig. 9). Intermontane basin boundaries were inferred based on the distribution of Quaternary sediments, because low-relief basin fill surfaces and alluvial fans can be easily distinguished from the more rugged bedrock ranges surrounding drainage basins (Fig. 9). As presented in Craddock et al. (2014), relict surfaces are typically characterized by low topographic slopes (b15°) over contiguous areas (Figs. 3e and 9). For example, along the northern limb of the

108

H. Zhang et al. / Tectonophysics 706–707 (2017) 103–115

Fig. 5. Local topographic relief (a) and slope distribution (b) of the Qilian Shan. Both local topographic relief and slope were averaged over an area of 1 × 1 km. See Fig. 2 for legends.

Qinghai Nan Shan range, the relict surface dips to the north at a very low angle (generally 3–5°), and topographic relief within 1 km window along this surface is generally b 100 m (Craddock et al., 2014).

To quantify the distribution of the low-relief surfaces outside the Qinghai Nan Shan, we extended our relict surface mapping and identified landscapes with low (~ 15°) local mean slope. Most of these

Fig. 6. Slope frequency diagrams for the drainage basins. a. Slope frequency of drainage basins for four rivers flowing from the interior Qilian Shan into the foreland, i.e., Dang River (DA), Shule River (SL), Beida River (BD) and Hei River (HE). b. Slope frequency of drainage basins for range frontal rivers, Hongshuiba River (HB) and Liyuan River (LY). c. Slope frequency for the two externally-drained Yellow River tributaries, i.e., Datong River (DT) and Huangshui River (HS). d. Two internally-drained drainages, Lakes Qinghai and Lake Hala are also shown. Note the dominance of lower slope regions for rivers originated from the interior Qilian Shan plateau despite their variable physiographic settings.

H. Zhang et al. / Tectonophysics 706–707 (2017) 103–115

109

Fig. 7. Elevation frequency diagram and altitude-slope relation diagrams for the drainage basins. EF with thicker color-coded lines represent the elevation frequency, and SLP with thinner, same color-coded lines (together with error bars) are for averaged slopes with one standard deviation (right vertical coordinates) for individual 200 m height interval. Consistence of lower slope and higher elevation frequency between ~3.5–4.2 km (a, HB in b and DT in c) indicates regions of low-relief plateau.

surfaces lie within an altitude range of 3.8–4.5 km (Figs. 7 and 9). Near the Qinghai Nan Shan, the geometry of this surface has led to the interpretation in previous studies that it is a relict erosional surface representing a paleohorizontal structural marker that predates growth of the ranges in SE Qilian Shan plateau (Zhang et al., 2012; Craddock et al., 2014).

5. Results 5.1. General topographic features Local topographic relief and slope indices across the Qilian Shan plateau show similar distributions (Fig. 5). South of the Hexi corridor, along

Fig. 8. Channel steepness index (Ksn; a) and longitudinal profile (b) of the Qilian Shan rivers. Note the contrast of channel steepness around the northern Qilian Shan and interior plateau. Slight difference due to interpolation of data exists between the vector and raster color scales of the channel steepness.

110

H. Zhang et al. / Tectonophysics 706–707 (2017) 103–115

Fig. 9. Distribution of the Quaternary intermontane basin and erosional surfaces. Antecedent erosional surfaces are mostly distributed to the south of the Haiyuan fault, especially near lakes Qinghai and Hala.

the ranges of the northern Qilian Shan, local relief near ranges is usually 1.5–2 km (Fig. 5a), with N 20° hillslopes (Fig. 5b), and variations of both topographic relief and slope are small. This steep topography tends to merge southeastwards into range and basin regions upstream of the Yellow River that are characterized by less than ~ 1.5 km relief and ~20° slopes (Fig. 5). In contrast to the higher relief and slope of regions north of the Haiyuan fault, the relief of regions south of the Haiyuan fault is usually b1 km and characterized by gentler hillslopes (Fig. 5). Around the regions near Lakes Qinghai and Hala, relief and slope decrease to b0.5 km and 15°, respectively. Ranges of the southwest Qilian Shan are characterized by high relief and steep slopes, but tend to be more subdued than the steeper northern Qilian Shan (Fig. 5). Overall, across the Qilian Shan, the maximum local relief and slope are mostly distributed north of the Haiyuan fault, with relief and slope decreasing towards the interior Qilian Shan plateau from all sides (Fig. 5). 5.2. Drainage basin hypsometry Slope histograms of the four drainage basins originating within the interior of the Qilian Shan plateau show a predominance of slopes b 10°, especially along the Dang River in the west (Fig. 6a). For the two rivers draining the frontal Qilian Shan, the Hongshuiba and Liyuan Rivers, the proportion of high slopes is the greatest compared with that of the other drainages (Fig. 6b). For the externally-drained Datong and Huangshui Rivers, however, hillslopes are similar to the Beida and Hei Rivers (Fig. 6c), with a large proportion characterized by slopes of b 10°. More than 70% of areas of the internally-drained lakes Qinghai and Hala basins are also dominated by slopes less than b10° (Fig. 6d). Analysis of elevation hypsometry across all of the basins confirms our observed slope distributions (Fig. 7). Most of the basins are characterized by regions at the elevation of 3.8–4.2 km, except for the Liyuan River (Fig. 7b). Compared to the predominance of 3.8–4.2 km elevations along basins of the Qilian Shan rivers, the Lake Hala basin is slightly higher at ~4.5 km, with a summit near 5.8 km (Fig. 7d). The Lake Qinghai basin lies at a lower elevation, ~3.2–3.8 km (Fig. 6d). Average slope angles for the drainage basins increase markedly from b10° to ~ 25° from the low land to the upstream territory, except for a decrease to ~15° near 3.8–4.2 km (Figs. 7a–c). These minima of slopes for upstream basins are mostly the regions with 3.8–4.2 km elevation (Fig. 7a–c). 5.3. Channel steepness of the Qilian Shan rivers River channel segments across the Qilian Shan vary in steepness from b50 m0.9 to N500 m0.9 in ten times of difference (Fig. 8a). Along the northern and western margins of the Qilian Shan, rivers tend to be

much steeper (usually N200 m0.9) than those flowing across the interior plateau regions. Along the northern Qilian Shan the eastward decrease of channel steepness is consistent with observations from previous local investigations (Hu et al., 2010), validating our present regional analysis across the Qilian Shan. Steep channels are mostly common near the basin/range boundary, for short rivers with headwaters within the Qilian Shan, such as the shorter Hongshuiba (HB) and Liyuan (LY) Rivers with headwaters within the Qilian Shan, and those longer rivers that originate from the interior Qilian Shan. To the northwest, the Dang (DA) and Shule (SL) Rivers also have steeper downstream segments. Along the southern Qilian Shan, channels in the Huangshui and Datong catchments are not as steep as those along the northern margin (Fig. 8a), and channel steepness lies mostly within a range of 100–200 m0.9, with few steep segments. In contrast to steeper channels, the rivers flowing into the lakes and upstream segments of the longer rivers are more gentle, and channel steepness is typically b 100 m0.9. Longitudinal profiles of the studied rivers further confirm the overall channel steepness results (Fig. 8b). Except for the Hongshuiba and Liyuan Rivers, the river profiles show very gentle slopes for the upstream segments above an altitude of ~3.2–3.5 km. All of the rivers have steeper downstream channel segments and are characterized by knickpoints/knickzones (Fig. 4d). These knickzones mark the transition from upstream, gentle segments to the steeper and narrower downstream river valleys. 5.4. Distribution of intermontane filling and relict erosional surfaces Quaternary deposits are widely distributed within the broad, lowrelief, intermontane basins, especially north of the Haiyuan fault (Figs. 3, 4 and 9). Within these basins, sediments are transported onto adjacent alluvial fans, braided channels, and debris cones near the range fronts. In the Yanchiwan basin (Figs. 2a, 3 and 4), basin fill is characterized by shallow lacustrine deposits within confined lakes or meandering channels. Quaternary basins have also developed around upstream regions of the Hei River and Shule River (e.g., the Tuolai (TL), Heihe (HE), Changma (CM), and Shule (SL) basins) (Fig. 2a). The depositional basins, for example, the Menyuan (MY) and Muli (ML) basins, also formed further upstream of the externally-drained Datong River which drains into the Yellow River (Fig. 2). Erosional surfaces that we identified are primarily distributed south of the Haiyuan fault (Figs. 3e, 4c and 9). These relict erosional surfaces are usually dominated by a mean slope of b15°, and most of these regions lie between elevations of 3.8–4.5 km (Fig. 9). Around the internally-drained Lake Hala and Lake Qinghai basins these erosional surfaces are well exposed or partially buried (Figs. 3e, 4c–e and 10; Zhang et al., 2012; Craddock et al., 2014). No matter how the lithology of underlying bedrocks varies, the different patches of erosional surfaces can be

H. Zhang et al. / Tectonophysics 706–707 (2017) 103–115

111

Fig. 10. Simplified geological map around the lake Hala (a) and oblique view of low-relief landscape (b). Dashed lines in (a) highlight the patches of the erosional surfaces in Fig. 9. #1–#5 are correlating points to show the oblique view of the locations in Fig. 10b. The erosional surfaces were developed in different bedrock lithology, and usually covered by very thin, locally fractured sediments as that bedrocks were locally fractured and beddings can be visibly traced (black dashed lines in Fig. 10b).

identified and correlated in dimensions of tens of square kilometers (Figs. 9 and 10a). These surfaces are capped by very thin sediments, supported by evidence that bedrocks were locally fractured and beddings can be visibly traced (Figs. 4c and 10b). 6. Discussion 6.1. Tectonic geomorphology of the Qilian Shan: an overview The morphology of the Qilian Shan plateau is similar to the Tibetan Plateau itself, with a low-relief plateau flanked by steep marginal ranges (Fig. 1). Along the margins of the Qilian Shan, especially the northern Qilian Shan, local topographic relief and slopes are higher (Fig. 5), and channels are much steeper (Fig. 8). Although relief and slope decrease eastward along the strike of the frontal ranges north of the Haiyuan fault, local relief is still ~1.5 km with ~20° hillslopes (Fig. 5). This steep landscape is also characterized by the higher rates of long-term unroofing and millennial/decadal erosion (Palumbo et al., 2010, 2011; Pan et al., 2010; Zheng et al., 2010). Zheng et al. (2010), determined a post ~9.5 Ma exhumation rate of 0.5 mm/a from U-Th/He age-elevation transects near the steepest portion of the northern Qilian Shan. Millennial-scale catchment erosion rate from cosmogenic nuclides (Palumbo et al., 2010, 2011) and short-term sediment budgets from gauging stations (Pan et al., 2010) both provided consistent results around 0.2– 0.9 mm/a. Studies of offset terraces and alluvial fans along the range

front of the northern Qilian Shan indicate that the late Quaternary uplift rates along most of the active faults are ~ 0.3–0.9 mm/a (Hetzel et al., 2002, 2004; Palumbo et al., 2009). The agreement between erosion and uplift rates indicates a topographic steady-state along the northern frontal rim of the Qilian Shan (Palumbo et al., 2010, 2011). Slope histograms for rivers along the range front of the northern Qilian Shan (e.g., the Hongshuiba River and Liyuan River) show much steeper portions (Figs. 5–8). Hillslope angles within the frontal ranges mostly average 32° ± 5° (Figs. 4d and 5b), thus hillslopes are likely at or near the threshold slope angle, coincident with the previous inference that the local landscape along the northern flanks of the Qilian Shan is in topographic steady-state (Palumbo et al., 2010, 2011). Collectively, our results of the local relief, slope, and distribution of channel steepness along the northern Qilian Shan support this inference, but this local topographic steady-state observation may be only indicative for the local uplifting ranges in the northern Qilian Shan, instead of all the Qilian Shan landscapes. South of the Haiyuan fault, ranges of the southwest Qilian Shan are still characterized by relatively high relief and steep slope, but they are subdued than the steepest portions of the northern Qilian Shan (Fig. 5). Near the Xining (XN) and Xunhua (XH) - Guide (GD) basins to the east of Lake Qinghai (Fig. 2a), topographic relief is still higher approaching a steady state, likely developed due to both range growth along the Laji Shan fault (Lease et al., 2007, 2011) and late Quaternary incision by the Yellow River (YR) and its tributaries (the Huangshui and Datong Rivers, Fig. 2; Craddock et

112

H. Zhang et al. / Tectonophysics 706–707 (2017) 103–115

al., 2010; Zhang et al., 2014). However, the relief of the interior plateau region is usually b1 km, and hillslopes are typically gentler (Fig. 5). This is especially noticeable in the vicinity of Lakes Qinghai and Hala, where relief and slope decrease to b 500 m and 15°, respectively (Fig. 5). To the east near the upstream of the Datong Rivers, knickpoints are identified to separate upstream low-slope regions to the downstream steeper landscapes, indicating an overall transient landscape due to the uncompleted propagation of incision along the Datong River (Fig. 6; Craddock et al., 2010; Zhang et al., 2014). In contrast to the steep steady-state landscape along the local ranges of the northern Qilian Shan, the existence of major knickpoints along most of the rivers suggests that the landscape in the other parts of the Qilian Shan plateau is in a transient state (Figs. 5 and 8). Lithology and precipitation contribute minor to the development of these knickpoints and steep segments along the different rivers, as argued previously in a similar study of channel steepness (Hu et al., 2010). We thus suggest that uplift of the northern Qilian Shan and associated catchment erosion explain a local topographic steady-state along the northern frontal rim of the Qilian Shan (Palumbo et al., 2010, 2011), but simultaneously deposition within the intermontane basins, along with the existence of the low-relief relict surfaces upstream of some antecedent drainage basins, as we will discuss below, are the primary contributing factors to development of the large-scale transient landscape for the whole Qilian Shan plateau.

6.2. Tectonic geomorphology of the Qilian Shan: A two-domain low-relief plateau Slope histograms of the drainage basins associated with the Dang, Shule, Beida, and Hei Rivers north of the Haiyuan fault reveal a dominance of slopes of b 10°, especially along the Dang River (Fig. 6a). Longitudinal profiles along the rivers show relatively gentle slopes for the upstream and middle segments between altitudes of ~ 3.5–4.0 km, with much steeper channel segments further downstream (Fig. 8b). The trend of increasing average slope with elevation is interrupted by local slope minima, which we attribute to the existence of the flat intermontane basins (e.g., the Yanchiwan, Yema, Changma, Shule and Yeniugou basins; Figs. 3–5, 7a). These Quaternary basin regions usually correspond to local maxima in the elevation histograms at ~3.8–4.2 km (Fig. 7), which was suggested to form along the upstream segments of these rivers to smoothen topography across the Qilian Shan plateau (Liu-Zeng et al., 2008). Broad intermontane basins are also observed upstream of the Datong River (Figs. 3, 4b and 5), although the Datong River is an externally-drained river. The river still has gentle upstream regions, for example, near the Muli (ML) basin (Fig. 5a), similar to the other rivers in developing the low-relief landscape north of the Haiyuan fault (Fig. 6). The lowrelief regions correspond to the presence of the intermontane basin at ~ 3.5–4.0 km (Fig. 6c). The Datong River extends ~500 km into the interior of the Qilian Shan from the trunk of the Yellow River. Sediment storage dominated within the basin due to deficient fluvial transportation (LiuZeng et al., 2008) and base level rise after piedmont aggradation, consistent with previous analogue model results (Babault et al., 2005). Preservation of the low-relief intermontane basin, upstream of the knickpoints (Fig. 8b), also supports that the transient landscape is still adjusting and upstream migration of the late Quaternary erosion wave is ongoing within the Yellow River catchment (Harkins et al., 2007; Craddock et al., 2010; Zhang et al., 2014). Compared to the Muli basin upstream of the longer and narrower Datong River, we identified the transient knickpoint ~200 km upstream of its confluence with the Yellow River (Fig. 8b). Along the Huangshui and Yellow Rivers, because the late Quaternary incision wave has already bypassed those intermontane basins downstream of the knickpoints, i.e., Xining (XN), Guide (GD)-Xunhua (XH) basins (Craddock et al., 2010; Zhang et al., 2014), the catchments, for example, the Huangshui River, are dominated by elevations lower than 3.5 km (Fig. 5c).

Although Quaternary aggradation around the internally-drained Lake Qinghai and Hala basins has contributed to the formation of gentler landscapes, landscape mapping revealed evidence for regions of low-relief relict erosional surfaces south of the Haiyuan fault (Figs. 9 and 10). These regions are dominated by gentle hillslopes (slope values b15°) over contiguous areas N 20 km2 (Fig. 9). The erosional surface extends further south and can be traced regionally (Fig. 9). For example, south of the Lake Qinghai, the northern limb of the Qinghai Nan Shan range dips to the north at a very low angle (Fig. 4e; generally 3–5°), and topographic relief along this surface is generally b 200 m. Craddock et al. (2014) interpreted this low-relief surface along the northern Qinghai Nan Shan to be a relict erosional surface, predating growth of the south-vergent anticline that cores the range. The low-angle northward dip of the deep strata across the southern part of the basin suggests that the northern limb of the range extends far to the north, beneath the Lake Qinghai basin. This similarity in dip between the strata within the southern Qinghai Lake basin and the gently dipping topographic surface on the northern limb of the Qinghai Nan Shan range supports the conclusion that the northern limb of the Qinghai Nan Shan was once a flat-lying erosional surface, also connecting the southern limb of the Qinghai Nan Shan as an obvious erosional basal contact between the Neogene strata and basement rocks (Zhang et al., 2012). Apatite fission track (AFT) and (U-Th)/He (AHe) samples from the erosional surfaces of the Qinghai Nan Shan date to the Early Cretaceous (AFT: 103.3–113 Ma) and the Cretaceous to the Paleogene (AHe: 45.5– 109.2 Ma) (Craddock et al., 2014). The age-elevation arrays of AHe and AFT data indicate that exhumation of the range has been insufficient to expose rocks that were greater than the AHe closure temperature, suggesting that the erosional surfaces to the south of the Haiyuan fault predate the widespread uplift and deformation of the SE Qilian Shan during the Neogene (Lease et al., 2007; Craddock et al., 2011, 2014; Zhang et al., 2012). Taken together, the geomorphic evidence along the southern Qilian Shan suggests that most of the high-elevation low-relief landscape is antecedent, and has been uplifted regionally along the marginal faults north of the Qaidam basin. Our geomorphic analysis of the drainage basins within the Qilian Shan plateau confirms that the low-relief regional plateau landscape was developed with little influence by the variable physiographic settings of its basins. The evidence suggests that contrasting domains of development of the low-relief surface exist on either side of the Haiyuan fault (Fig. 11). We suggest that a “bath-tub” filling process contributed to plateau formation north of the Haiyuan fault (Fig. 11). Evidence for this process includes the existence of the low-relief intermontane drainage basins, attributed to deposition and deficient fluvial transportation within the basins and incomplete transient knickpoint migration (Babault et al., 2005; Liu-Zeng et al., 2008; Figs. 8 and 11). However, south of the Haiyuan fault, particularly around Lakes Qinghai and Hala, low-relief landscapes were originated from pre-uplift erosional surfaces (Fig. 11). These regions seem to have been preserved for an extensive time period (mostly from early Cenozoic) at a large distance from the marginal ranges of the Qilian Shan plateau (Craddock et al., 2014). Taken together our observation of local steady-state and regional transient landscape features, we suggest that a difference of responsetime for knickpoint propagation may exist along the channels draining out of the Qilian Shan, since rates of knickpoint propagation depend largely on the upstream drainage basin area (Berlin and Anderson, 2007; Harkins et al., 2007). Along the Yellow River and most of its tributaries, also for those rivers around the Qilian Shan, due to the varied response time to the knickpoint propagation, landscape downstream the knickpoint tends to be approaching steady-state, but the overall topography was still featured by transient propagating wave of erosion (Fig. 5). Internally-drained basin system, i.e., Lake Qinghai and Hala drainage basins, was mostly developed around the antecedent low-relief surfaces south of the Haiyuan fault (Fig. 9), indicating a possible control of gentler topography on the presence of endoreic domains. Erosion wave propagation along the Yellow River and its tributary Huangshui River,

H. Zhang et al. / Tectonophysics 706–707 (2017) 103–115

113

Fig. 11. 3D oblique view of the Qilian Shan plateau for illustrating a two domain scheme low-relief plateau development hypothesis. Distribution of the Quaternary intermontane basins and relict erosional surfaces are highlighted on both sides of the Haiyuan fault. Knickpoints are also shown to indicate the transient topographic state of the Qilian Shan plateau.

and those rivers draining into the Qaidam basin (Fig. 2), will be likely to evolve towards a capture domain. But deficient fluvial transportation may still lead to sediment storage within the basin (Liu-Zeng et al., 2008), hampering propagation of the knickpoint into the interior Qilian Shan plateau. 6.3. Implications for the low-relief plateau formation As summarized above, the low-relief interior morphology of the Qilian Shan and Tibetan Plateau and its close association with internal drainage has been previously attributed to a “bath-tub” basin-filling processes (Métivier et al., 1998; Meyer et al., 1998; Tapponnier et al., 2001). As quantified by areal extent, such “bath-tub” intermontane basins, characterized by high elevations, low-relief, and gentle slopes, are the main component of the regional topography. Our present analysis of the drainage basins north of the Haiyuan fault (Fig. 2) suggest that these drainage basins are weakly connected with external drainage towards the Hexi Corridor basin. Mass transport by rivers between the topographic highs of the Hexi Corridor and adjacent basins occurs over only short distances in tens of kilometers (Liu-Zeng et al., 2008; Wang et al., 2014). The morphology of these adjacent basins offers insight into the development of the larger and gentler areas within the interior of the highest part of Tibet: within the internally-drained part of central Tibet and upstream regions of the externally-drained rivers (e.g. the Jinsha Jiang (Yangtze), Lancang Jiang (Mekong), and Nu Jiang (Salween); Fig. 1), amalgamated closed basins form a relatively uniform high elevation base level for both flanking ranges and the piedmonts, and these restricted drainage catchments are effective in smoothing relief at higher elevation. On time scale of mountain denudation and erosion dynamics in mountain catchments, the effect of piedmont deposition as corresponding to the rise in the efficient base level of mountain belts has been stressed by numerical modeling and laboratory experiments (Babault et al., 2005). Cenozoic crustal shortening driven by Qilian Shan thrust fault systems may also help to construct the high-elevation landscape to the north of the Haiyuan fault (George et al., 2001; Bovet et al., 2009; Zheng et al., 2010, 2017). Low-temperature thermochronology and geological mapping indicate that the emergence

of the central Qilian Shan occurred at 17–14 Ma, that northern Qilian Shan thrusting began at 10–8 Ma, and that deformation propagated to frontal ranges at ~4 Ma (Zheng et al., 2017). Northward growth of the Qilian Shan ranges might have helped to tectonically defeat the rangetraversing channels, and trapped sediments behind the ranges, as dominate means to built the Puna-Altiplano plateau (Sobel et al., 2003; Pingel et al., 2013). In addition to the contribution of “bath-tub” basin-filling processes to the development of the low-relief plateau to the north of the Haiyuan fault in the Qilian Shan plateau, we also presented evidence for development of portions of the plateau from relict erosional surfaces, particularly in the vicinity of the Lake Qinghai and Hala basins (Figs. 9 and 10). These relict erosional surfaces are locally separated but regionally correlated, comprising the majority of the low-relief plateau landscape to the south of the Haiyuan fault. Low-relief erosional surfaces within the Qilian Shan plateau share other characteristics with similar surfaces preserved across the Tibetan Plateau (Clark, 2011; Hetzel et al., 2011). As Clark (2011) highlighted recently, surfaces of late Jurassic to Eocene age have been recognized across Tibet and within the Himalayas, and extend north into Mongolia. In the Tien Shan, an extensive, low-relief surface has been developed on highly deformed bedrock, and is widespread within the high bedrock ranges and ubiquitous wherever the base of the Tertiary stratigraphy is exposed (Abdrakhmatov et al., 2001). This has led to the conclusion, that the entire region was occupied by a gentle, low-relief bedrock surface that has now been uplifted to present elevations. At present, we have no constraints on the origin and timing of formation of these low-relief surfaces in the Qilian Shan plateau, but if dated, they may be used as potential reliable markers for inferring total surface uplift of the northeastern Tibetan plateau, similar to that of the southeastern Tibet (e.g., Clark et al., 2006). Large scale uplift of such low-relief surfaces to the south of the Haiyuan fault would have been contributed by deep crustal or mantle processes, as proposed to drive surface uplift of the SE Tibet (e.g., Clark and Royden, 2000). A low-velocity seismic wave zone at middle – lower crust has been documented beneath the eastern Kunlun – West Qinling region (Yang et al., 2012). Such a zone could facilitate northward crustal flow from the plateau interior and consequent

114

H. Zhang et al. / Tectonophysics 706–707 (2017) 103–115

thickening where it encounters more rigid crust beneath the Qaidam Basin and further south of Kunlun fault (Kirby and Harkins, 2013). However, Cenozoic crustal shortening estimations from eastern Qilian Shan indicate that channel flow in the lower crust was less likely if the preCenozoic crust was thick (e.g., Lease et al., 2012; Craddock et al., 2014). In addition, Cenozoic erosion has been concentrated mostly along of the Qilian Shan margins, erosion-driven isostatic uplift appears to have very limited effect on the overall uplift of erosional surfaces in the interior plateau to their present elevation (Zhang et al., 2014). Tectonic shortening, and sediment aggradation behind the uplift ranges can explain local basins, such as Gonghe, Guide-Xunhua basins (Fig. 2a; Métivier et al., 1998). Regionally to the south of the Haiyuan fault, uplift of such relict low-relief surfaces to their present elevation should have been accomplished through the long-term slip of the thrust faults along the north Qaidam, Chaka and Gonghe basins (Fig. 2; Fang et al., 2005a, 2007; Craddock et al., 2010; Zhang et al., 2012). Our present study suggests that formation and uplift of the low-relief plateau landscape for the Qilian Shan cannot be explained by a single mechanism, including the long-lasting lower crustal flow compensation (Clark and Royden, 2000; McKenzie and Jackson, 2002; Bendick et al., 2008) or “bath-tub” filling models (Métivier et al., 1998; Meyer et al., 1998; Liu-Zeng et al., 2008). A lot of Cenozoic basins exist to the northeast of the Kunlun fault in northern and northeastern Tibet, and some of them must have been developed behind enclosed topography (e.g., Meyer et al., 1998). But, south of the Kunlun fault, Cenozoic basins are sparse (i.e., Hoh Xil, narrow grabens and basins along the suture zones). Since the distribution of Cenozoic basin fill across the Lhasa and Qiangtang terranes is so limited, “bath-tub” filling possibly was not an important plateau building process there. For different portions of the Qilian Shan plateau and, analogously, the Tibetan Plateau, deep crustal flow, surface erosion, and both fluvial and glacial-periglacial depositional processes may all contribute to development of the low-relief plateau. These coupled complex processes likely operate in a similar manner within other regionally comparable orogenic plateaus. 7. Conclusions We have carried out a detailed analysis of the geomorphometric indices for the Qilian Shan plateau, including slope, local relief, and longitudinal profiles. Our study reveals that the Qilian Shan is characterized by high-relief marginal ranges, low-relief intermontane basins, and antecedent erosional surfaces. Our work supports the view that the marginal local ranges of the northern Qilian Shan are at topographic steady-state. But the large-scale landscape of the Qilian Shan is in a transient state, as evidenced by the existence of fluvial knickpoints for rivers across the Qilian Shan plateau. The coexistence of low-relief intermontane basins and antecedent erosional surfaces has led us to propose a two-domain plateau formation history for the Qilian Shan. This history includes both the “bathtub” filling processes within intermontane basins that contribute to the development of the low-relief plateau north of the Haiyuan fault and the amalgamation of antecedent erosional surfaces south of the fault. We further view the Qilian Shan plateau as an analogue of the present topography for the Tien Shan, and the Tibetan Plateau, although contributions from individual processes may vary spatially and/or temporally. Acknowledgements We gratefully acknowledge the editor Philippe Agard, Nicolas Loget and an anonymous reviewer for constructive reviews. We would like to thank Peter Molnar, Martin Stokes and Ian Evans for providing valuable suggestions and revisions on an early draft. This work was jointly supported by State Key Laboratory of Earthquake Dynamics (LED2014A03), and the NSF of China (No. 41622204; 41590861; 41661134011).

References Abdrakhmatov, K.E., Weldon, R., Thompson, S., Burbank, D., Rubin, Ch., Miller, M., Molnar, P., 2001. Origin, direction, and rate of modern compression in the central Tien Shan, Kyrgyzstan. Russ. Geol. Geophys. 42, 1585–1609. Babault, J., Bonnet, S., Crave, A., Van Den Driessche, J., 2005. Influence of piedmont sedimentation on erosion dynamics of an uplifting landscape: an experimental approach. Geology 33, 301–304. Bendick, R., McKenzie, D., Etienne, J., 2008. Topography associated with crustal flow in continental collisions, with application to Tibet. Geophys. J. Int. 175, 375–385. Berlin, M.M., Anderson, R.S., 2007. Modeling of knickpoint retreat on the Roan Plateau, western Colorado. J. Geophys. Res. 112. Bovet, P.M., Ritts, B.D., Gehrels, G., Abbink, A.O., Darby, B., Hourigan, J., 2009. Evidence of miocene crustal shortening in the North Qilian Shan from Cenozoic stratigraphy of the Western Hexi Corridor, Gansu Province, China. Am. J. Sci. 309, 290–329. Champagnac, J.-D., Yuan, D.-Y., Ge, W.-P., Molnar, P., Zheng, W.-J., 2010. Slip rate at the north-eastern front of the Qilian Shan, China. Terra Nova 22, 180–187. Clark, M.K., 2011. Early Tibetan Plateau uplift history eludes. Geology 39, 991–992. Clark, M.K., 2012. Continental collision slowing due to viscous mantle lithosphere rather than topography. Nature 483, 74–77. Clark, M.K., Royden, L.H., 2000. Topographic ooze: building the eastern margin of Tibet by lower crustal flow. Geology 28, 703–706. Clark, M.K., Bush, J.W.M., Royden, L.H., 2005. Dynamic topography produced by lower crustal flow against rheological strength heterogeneities bordering the Tibetan Plateau. Geophys. J. Int. 162, 575–590. Clark, M.K., Royden, L.H., Whipple, K.X., Burchfiel, B.C., Zhang, X., Tang, W., 2006. Use of a regional, relict landscape to measure vertical deformation of the eastern Tibetan Plateau. J. Geophys. Res. Earth Surf. 111, F03002. http://dx.doi.org/10.1029/2005JF000294. Clark, M.K., Farley, K.A., Zheng, D., Wang, Z.C., Duvall, A., 2010. Early Cenozoic faulting on the northern Tibetan Plateau margin from apatite (U-Th)/He ages. Earth Planet Sc Lett 296, 78–88. Craddock, W.H., Kirby, E., Harkins, N.W., Zhang, H.P., Shi, X.H., Liu, J.H., 2010. Rapid fluvial incision along the Yellow River during headward basin integration. Nat. Geosci. 3, 209–213. Craddock, W., Kirby, E., Zhang, H., 2011. Late Miocene-Pliocene range growth in the interior of the northeastern Tibetan Plateau. Lithosphere 3, 420–438. Craddock, W.H., Kirby, E., Zhang, H., Clark, M.K., Champagnac, J.-D., Yuan, D., 2014. Rates and style of Cenozoic deformation around the Gonghe Basin, northeastern Tibetan plateau. Geosphere 10, 1255–1282. Dayem, K.E., Molnar, P., Clark, M.K., Houseman, G.A., 2009. Far-field lithospheric deformation in Tibet during continental collision. Tectonics 28. Dupont-Nivet, G., Horton, B.K., Zhou, J., Waanders, G.L., Butler, R.F., Wang, J., 2004. Paleogene clockwise tectonic rotation of the Xining-Lanzhou region, northeastern Tibetan Plateau. J. Geophys. Res. 109, B04401. http://dx.doi.org/10.1029/2003JB002620. Duvall, A.R., Clark, M.K., van der Pluijm, B., Li, C.Y., 2011. Direct dating of Eocene reverse faulting in northeastern Tibet using Ar-dating of fault clays and low-temperature thermochronometry. Earth Planet Sc Lett 304, 520–526. Duvall, A.R., Clark, M.K., Kirby, E., Farley, K.A., Craddock, W.H., Li, C., Yuan, D.-Y., 2013. Low-temperature thermochronometry along the Kunlun and Haiyuan Faults, NE Tibetan Plateau: evidence for kinematic change during late-stage orogenesis. Tectonics 32, 1190–1211. Fang, X.M., Yan, M.D., Van der Voo, R., Rea, D.K., Song, C.H., Pares, J.M., Gao, J.P., Nie, J.S., Dai, S., 2005a. Late Cenozoic deformation and uplift of the NE Tibetan plateau: evidence from high-resolution magneto stratigraphy of the Guide Basin, Qinghai Province, China. Geol. Soc. Am. Bull. 117, 1208–1225. Fang, X.M., Zhao, Z.J., Li, J.J., Yan, M.D., Pan, B.T., Song, C.H., Dai, S.A., 2005b. Magnetostratigraphy of the late Cenozoic Laojunmiao anticline in the northern Qilian Mountains and its implications for the northern Tibetan Plateau uplift. Sci. China. Ser. D Earth Sci. 48, 1040–1051. Fang, X.M., Zhang, W.L., Meng, Q.Q., Gao, J.P., Wang, X.M., King, J., Song, C.H., Dai, S., Miao, Y.F., 2007. High-resolution magneto stratigraphy of the Neogene Huaitoutala section in the eastern Qaidam Basin on the NE Tibetan Plateau, Qinghai Province, China and its implication on tectonic uplift of the NE Tibetan Plateau. Earth Planet Sc Lett 258, 293–306. Farr, T.G., Rosen, P.A., Caro, E., Crippen, R., Duren, R., Hensley, S., Kobrick, M., Paller, M., Rodriguez, E., Roth, L., Seal, D., Shaffer, S., Shimada, J., Umland, J., Werner, M., Oskin, M., Burbank, D., Alsdorf, D., 2007. The shuttle radar topography mission. Rev. Geophys. 45, RG2004. Fielding, E., Isacks, B., Barazangi, M., Duncan, C., 1994. How flat is Tibet? Geology 22, 163–167. George, A.D., Marshallsea, S.J., Wyrwoll, K.H., Chen, J., Lu, Y.C., 2001. Miocene cooling in the northern Qilian Shan, northeastern margin of the Tibetan Plateau, revealed by apatite fission-track and vitrinite-reflectance analysis. Geology 29, 939–942. Giletycz, S., Loget, N., Chang, C.P., Mouthereau, F., 2015. Transient fluvial landscape and preservation of low-relief terrains in an emerging orogen: example from Hengchun Peninsula, Taiwan. Geomorphology 231, 169–181. Hack, J.T., 1957. Studies of longitudinal stream profiles in Virginia and Maryland. U.S. Geol. Surv. Prof. Pap. 294-B, 1–97. Harkins, N., Kirby, E., Heimsath, A., Robinson, R., Reiser, U., 2007. Transient fluvial incision in the headwaters of the Yellow River, northeastern Tibet, China. Journal of Geophysical Research-Earth Surface 112. Hetzel, R., Niedermann, S., Tao, M., Kubik, P.W., Ivy-Ochs, S., Gao, B., Strecker, M.R., 2002. Low slip rates and long-term preservation of geomorphic features in Central Asia. Nature 417, 428–432. Hetzel, R., Tao, M.X., Stokes, S., Niedermann, S., Ivy-Ochs, S., Gao, B., Strecker, M.R., Kubik, P.W., 2004. Late Pleistocene/Holocene slip rate of the Zhangye thrust (Qilian Shan,

H. Zhang et al. / Tectonophysics 706–707 (2017) 103–115 China) and implications for the active growth of the northeastern Tibetan Plateau. Tectonics 23. Hetzel, R., Dunkl, I., Haider, V., Strobl, M., von Eynatten, H., Ding, L., Frei, D., 2011. Peneplain formation in southern Tibet predates the India-Asia collision and plateau uplift. Geology 39, 983–986. Hough, B.G., Garzione, C.N., Wang, Z.C., Lease, R.O., Burbank, D.W., Yuan, D.Y., 2011. Stable isotope evidence for topographic growth and basin segmentation: implications for the evolution of the NE Tibetan Plateau. Geol. Soc. Am. Bull. 123, 168–185. Hu, X., Pan, B., Kirby, E., Li, Q., Geng, H., Chen, J., 2010. Spatial differences in rock uplift rates inferred from channel steepness indices along the northern flank of the Qilian Mountain, northeast Tibetan Plateau. Chin. Sci. Bull. 55, 3205–3214. Kirby, E., Whipple, K., 2001. Quantifying differential rock-uplift rates via stream profile analysis. Geology 29, 415–418. Kirby, E., Harkins, N., 2013. Distributed deformation around the eastern tip of the Kunlun fault. Int. J. Earth Sci. 102 (7), 1759–1772. Kirby, E., Whipple, K.X., Tang, W.Q., Chen, Z.L., 2003. Distribution of active rock uplift along the eastern margin of the Tibetan Plateau: inferences from bedrock channel longitudinal profiles. J. Geophys. Res. Solid Earth 108. Lasserre, C., Morel, P.H., Gaudemer, Y., Tapponnier, P., Ryerson, F.J., King, G.C.P., Metivier, F., Kasser, M., Kashgarian, M., Baichi, L., Taiya, L., Daoyang, Y., 1999. Postglacial left slip rate and past occurrence of M ≥ 8 earthquakes on the western Haiyuan fault, Gansu, China. J. Geophys. Res. Solid Earth 104, 17633–17651. Lasserre, C., Gaudemer, Y., Tapponnier, P., Meriaux, A.S., Van der Woerd, J., Yuan, D.Y., Ryerson, F.J., Finkel, R.C., Caffee, M.W., 2002. Fast late Pleistocene slip rate on the Leng Long Ling segment of the Haiyuan fault, Qinghai, China. J. Geophys. Res. Solid Earth 107. Lease, R.O., Burbank, D.W., Gehrels, G., Wang, Z., Yuan, D., 2007. Signatures of mountain building: detrital zircon U/Pb ages from northeastern Tibet. Geology 35, 239–242. Lease, R.O., Burbank, D.W., Clark, M.K., Farley, K.A., Zheng, D., Zhang, H., 2011. Middle Miocene reorganization of deformation along the northeastern Tibetan Plateau. Geology 39, 359–362. Lease, R.O., Burbank, D.W., Zhang, H., Liu, J., Yuan, D., 2012. Cenozoic shortening budget for the northeastern edge of the Tibetan Plateau: is lower crustal flow necessary? Tectonics 31, TC3011. http://dx.doi.org/10.1029/2011TC003066. Li, C., Zhang, P.-z., Yin, J., Min, W., 2009. Late Quaternary left-lateral slip rate of the Haiyuan fault, northeastern margin of the Tibetan Plateau. Tectonics 28. Liu-Zeng, J., Tapponnier, P., Gaudemer, Y., Ding, L., 2008. Quantifying landscape differences across the Tibetan plateau: implications for topographic relief evolution. Journal of Geophysical Research-Earth Surface 113. McKenzie, D., Jackson, J., 2002. Conditions for flow in the continental crust. Tectonics 21, 1055. Métivier, F., Gaudemer, Y., Tapponnier, P., Meyer, B., 1998. Northeastward growth of the Tibet plateau deduced from balanced reconstruction of two depositional areas: the Qaidam and Hexi Corridor basins, China. Tectonics 17, 823–842. Meyer, B., Tapponnier, P., Bourjot, L., Métivier, F., Gaudemer, Y., Peltzer, G., Shunmin, G., Zhitai, C., 1998. Crustal thickening in Gansu-Qinghai, lithospheric mantle subduction, and oblique, strike-slip controlled growth of the Tibet plateau. Geophys. J. Int. 135, 1–47. Palumbo, L., Hetzel, R., Tao, M., Li, X., Guo, J., 2009. Deciphering the rate of mountain growth during topographic presteady state: an example from the NE margin of the Tibetan Plateau. Tectonics 28. Palumbo, L., Hetzel, R., Tao, M., Li, X., 2010. Topographic and lithologic control on catchment-wide denudation rates derived from cosmogenic Be-10 in two mountain ranges at the margin of NE Tibet. Geomorphology 117, 130–142. Palumbo, L., Hetzel, R., Tao, M., Li, X., 2011. Catchment-wide denudation rates at the margin of NE Tibet from in situ-produced cosmogenic 10Be. Terra Nova 23, 42–48. Pan, B.-t., Geng, H.-p., Hu, X.-f., Sun, R.-h., Wang, C., 2010. The topographic controls on the decadal-scale erosion rates in Qilian Shan Mountains, NW China. Earth Planet Sc Lett 292, 148–157. Pingel, H., Strecker, M.R., Alonso, R.N., Schmitt, A.K., 2013. Neotectonic basin and landscape evolution in the Eastern Cordillera of NW Argentina, Humahuaca Basin (~ 24°S). Basin Res. 25, 554–573. Ritts, B.D., Yue, Y., Graham, S.A., Sobel, E.R., Abbink, O.A., Stockli, D., 2008. From sea level to high elevation in 15 million years: uplift history of the northern Tibetan Plateau margin in the Altun Shan. Am. J. Sci. 308, 657–678. Snyder, N.P., Whipple, K.X., Tucker, G.E., Merritts, D.J., 2000. Landscape response to tectonic forcing: digital elevation model analysis of stream profiles in the Mendocino triple junction region, Northern California. Bull. Geol. Soc. Am. 112, 1250–1263.

115

Sobel, E.R., Hilley, G.E., Strecker, M.R., 2003. Formation of internally drained contractional basins by aridity-limited bedrock incision. J. Geophys. Res. Solid Earth 108, 2344. Tapponnier, P., Xu, Z.Q., Roger, F., Meyer, B., Arnaud, N., Wittlinger, G., Yang, J.S., 2001. Oblique stepwise rise and growth of the Tibet plateau. Science 294, 1671–1677. Taylor, M., Yin, A., 2009. Active structures of the Himalayan-Tibetan orogen and their relationships to earthquake distribution, contemporary strain field, and Cenozoic volcanism. Geosphere 5, 199–214. Wang, Y., Zhang, H., Zheng, D., Zheng, W., Zhang, Z., Wang, W., Yu, J., 2014. Controls on decadal erosion rates in Qilian Shan: re-evaluation and new insights into landscape evolution in north-east Tibet. Geomorphology 223, 117–128. Wobus, C., Whipple, K.X., Kirby, E., Snyder, N., Johnson, J., Spyropolou, K., Crosby, B., Sheehan, D., 2006. Tectonics from topography: procedures, promise, and pitfalls. Geol. Soc. Am. Spec. Pap. 398, 55–74. Xiao, W., Windley, B.F., Yong, Y., Yan, Z., Yuan, C., Liu, C., Li, J., 2009. Early Paleozoic to Devonian multiple-accretionary model for the Qilian Shan, NW China. J. Asian Earth Sci. 35, 323–333. Yang, Y., Ritzwoller, M.H., Zheng, Y., Shen, W., Levshin, A.L., Xie, Z., 2012. A synoptic view of the distribution and connectivity of the mid-crustal low velocity zone beneath Tibet. J. Geophys. Res. Solid Earth 117. Yin, A., Harrison, T.M., 2000. Geologic evolution of the Himalayan-Tibetan orogen. Annu. Rev. Earth Planet. Sci. 28, 211–280. Yin, A., Rumelhart, P.E., Butler, R., Cowgill, E., Harrison, T.M., Foster, D.A., Ingersoll, R.V., Zhang, Q., Zhou, X.Q., Wang, X.F., Hanson, A., Raza, A., 2002. Tectonic history of the Altyn Tagh fault system in northern Tibet inferred from Cenozoic sedimentation. Geol. Soc. Am. Bull. 114, 1257–1295. Yin, A., Dang, Y.Q., Wang, L.C., Jiang, W.M., Zhou, S.P., Chen, X.H., Gehrels, G.E., McRivette, M.W., 2008a. Cenozoic tectonic evolution of Qaidam basin and its surrounding regions (part 1): the southern Qilian Shan-Nan Shan thrust belt and northern Qaidam basin. Geol. Soc. Am. Bull. 120, 813–846. Yin, A., Dang, Y.Q., Zhang, M., Chen, X.H., McRivette, M.W., 2008b. Cenozoic tectonic evolution of the Qaidam basin and its surrounding regions (part 3): structural geology, sedimentation, and regional tectonic reconstruction. Geol. Soc. Am. Bull. 120, 847–876. Yin, A., Dubey, C.S., Kelty, T.K., Webb, A.A.G., Harrison, T.M., Chou, C.Y., Celerier, J., 2010. Geologic correlation of the Himalayan orogen and Indian craton: part 2. Structural geology, geochronology, and tectonic evolution of the Eastern Himalaya. Geol. Soc. Am. Bull. 122, 360–395. Yuan, D.-Y., Champagnac, J.-D., Ge, W.-P., Molnar, P., Zhang, P.-Z., Zheng, W.-J., Zhang, H.P., Liu, X.-W., 2011. Late Quaternary right-lateral slip rates of faults adjacent to the lake Qinghai, northeastern margin of the Tibetan Plateau. Geol. Soc. Am. Bull. 123, 2016–2030. Zhang, H.-P., Craddock, W.H., Lease, R.O., Wang, W.-t., Yuan, D.-Y., Zhang, P.-Z., Molnar, P., Zheng, D.-W., Zheng, W.-J., 2012. Magnetostratigraphy of the Neogene Chaka basin and its implications for mountain building processes in the north-eastern Tibetan Plateau. Basin Res. 24, 31–50. Zhang, H., Zhang, P., Champagnac, J.-D., Molnar, P., Anderson, R.S., Kirby, E., Craddock, W.H., Liu, S., 2014. Pleistocene drainage reorganization driven by the isostatic response to deep incision into the northeastern Tibetan Plateau. Geology 42, 303–306. Zheng, D., Clark, M.K., Zhang, P., Zheng, W., Farley, K.A., 2010. Erosion, fault initiation and topographic growth of the North Qilian Shan (northern Tibetan Plateau). Geosphere 6, 937–941. Zheng, W.-J., Zhang, H.-P., Zhang, P.-Z., Molnar, P., Liu, X.-W., Yuan, D.-Y., 2013a. Late Quaternary slip rates of the thrust faults in western Hexi Corridor (Northern Qilian Shan, China) and their implications for northeastward growth of the Tibetan Plateau. Geosphere 9, 342–354. Zheng, W.-j., Zhang, P.-z., He, W.-g., Yuan, D.-y., Shao, Y.-x., Zheng, D.-w., Ge, W.-p., Min, W., 2013b. Transformation of displacement between strike-slip and crustal shortening in the northern margin of the Tibetan Plateau: evidence from decadal GPS measurements and late Quaternary slip rates on faults. Tectonophysics 584, 267–280. Zheng, D., Wang, W., Wan, J., Yuan, D., Liu, C., Zheng, W., Zhang, H., Pang, J., Zhang, P., 2017. Progressive northward growth of the northern Qilian Shan–Hexi Corridor (northeastern Tibet) during the Cenozoic. Lithosphere http://dx.doi.org/10.1130/ L587.1. Zhuang, G., Hourigan, J.K., Ritts, B.D., Kent-Corson, M.L., 2011. Cenozoic multiple-phase tectonic evolution of the northern Tibetan Plateau: constraints from sedimentary records from Qaidam basin, Hexi Corridor, and Subei basin, northwest China. Am. J. Sci. 311, 116–152.