Yanxiu Shao
email: shaoyx@tju.edu.cn

Jerome Van Der Woerd
email: jerome.vanderwoerd@unistra.fr

Jing Liu-Zeng

Daoyang Yuan

Yunsheng Yao

Xiaobo Zou

Pengtao Wang

Nan Shan

Tibet Yanxiu

Shortening Rates and Recurrence of Large Earthquakes From Folded and Uplifted Terraces in the Western Danghe Nan Shan Foreland, North Tibet

des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

The repetitive nature of earthquake occurrence is a characteristic of faulting behavior, allowing for seismic hazard assessment. While crustal strain can be concentrated and localized along a single fault, some fault systems are highly segmented and distributed, making it more difficult to constrain each fault's behavior.

For instance, it has been shown that the Carrizo plain segment of the San Andreas fault concentrates most of the tectonic plate displacement and with 15 recurrent events in the last 1500 years (e.g., [START_REF] Sieh | Holocene activity of the San Andreas fault at Wallace creek, California[END_REF][START_REF] Weldon | Wrightwood and the earthquake cycle: What a long recurrence record tells us about how faults work[END_REF][START_REF] Liu-Zeng | Serial ruptures of the San Andreas fault, Carrizo Plain, California, revealed by three-dimensional excavations[END_REF]. Strike-slip faults produce long single-trace ruptures, like, for example, along the Kunlun fault, with the Manyi (Mw7.6 1997, [START_REF] Peltzer | Evidence of nonlinear elasticity of the crust from the Mw7. 6 Manyi (Tibet) earthquake[END_REF] and Kokoxili (Mw7.9 14/11/2001, [START_REF] Klinger | High-resolution satellite imagery mapping of the surface rupture and slip distribution of the M w∼ 7.8, 14 November 2001 Kokoxili earthquake, Kunlun fault, northern Tibet, China[END_REF] events, or along the North Anatolian fault [START_REF] Stein | Progressive failure on the North Anatolian fault since 1939 by earthquake stress triggering[END_REF]. Thrust or combined strike-slip/thrust events are sometimes more complex, like for instance, the Wenchuan earthquake (Mw7.9 2008; [START_REF] Xu | Coseismic reverse-and oblique-slip surface faulting generated by the 2008 Mw 7.9 Wenchuan earthquake, China[END_REF][START_REF] Hubbard | Structural Setting of the 2008 Mw 7[END_REF] with two obliquely slipping major thrusts, or the Kaikoura earthquake (Mw7.8 2016; [START_REF] Shi | Triple junction kinematics accounts for the 2016 Mw 7.8 Kaikoura earthquake rupture complexity[END_REF] with a complex combination of thrusts and strike-slip faults or may even stay blind as the recent Himalaya Gorkha earthquake (Mw7.8 2015; [START_REF] Hubbard | Structural segmentation controlled the 2015 Mw 7.8 Gorkha earthquake rupture in Nepal[END_REF]. It thus requires quantification of fault geometry, activity, and rupture history along adjacent and intersecting fault networks to understand how such complex rupture occurs.

The Qilian Shan region has long been recognized as a crustal-scale restraining step-over between the two northernmost major strike-slip faults at the margin of the Tibetan plateau [START_REF] Tapponnier | Active thrusting and folding in the Qilian Shan, and decoupling between upper crust and mantle in northeastern Tibet[END_REF][START_REF] Métivier | Northeastward growth of the Tibet plateau deduced from balanced reconstruction of two depositional areas: The Qaidam and Hexi Corridor basins, China[END_REF]Zuza et al., 2018).

Left-lateral slip along the N70°E-striking Altyn Tagh fault (ATF) is transferred to the N100°E-striking Haiyuan fault through a set of thrusts and strike-slip faults distributed within the Qilian Shan region. The complex links between strike-slip and thrust faults have been highlighted in some historical earthquakes, such as the Changma event of 1932 [START_REF] Meyer | Mécanismes des grands tremblements de terre et du raccourcissement crustal oblique au bord nord-est du Tibet[END_REF][START_REF] Xu | Five Short Historical Earthquake Surface Ruptures near the Silk Road, Gansu Province, China[END_REF] or the Gulang earthquake of 1927 [START_REF] Gaudemer | Partitioning of crustal slip between linked, active faults in the eastern Qilian Shan, and evidence for a major seismic gap, the 'Tianzhu gap', on the western Haiyuan Fault, Gansu (China)[END_REF][START_REF] Guo | A new tectonic model for the 1927 M8. 0 Gulang earthquake on the NE Tibetan Plateau[END_REF]. How thrust faults connect at depth to accommodate strain responsible for folding and mountain building may allow us to assess the rupture area at depth better and thus the magnitude of past and future earthquakes (e.g., [START_REF] Stockmeyer | Seismic hazards of multisegment thrust fault ruptures: Insights from the 1906 Mw 7.4-8.2 Manas, China, earthquake[END_REF][START_REF] Shi | Triple junction kinematics accounts for the 2016 Mw 7.8 Kaikoura earthquake rupture complexity[END_REF]. Figure 1. Active faults map of northwestern Qilian Shan region (modified from [START_REF] Tapponnier | Active thrusting and folding in the Qilian Shan, and decoupling between upper crust and mantle in northeastern Tibet[END_REF][START_REF] Métivier | Northeastward growth of the Tibet plateau deduced from balanced reconstruction of two depositional areas: The Qaidam and Hexi Corridor basins, China[END_REF][START_REF] Van Der Woerd | Rapid active thrusting along the northwestern range front of the Tanghe Nan Shan (western Gansu, China)[END_REF]. Inset shows location of area in simplified tectonic map of Tibet. Seismicity (magnitude >4) from China Earthquake Networks Center (CENC) catalog (1970 -2021). Focal mechanisms from GCMT (1970GCMT ( -2021)). Yellow shaded fault traces with dates are historical and most recent paleo-earthquake rupture extensions from [START_REF] Meyer | Mécanismes des grands tremblements de terre et du raccourcissement crustal oblique au bord nord-est du Tibet[END_REF], [START_REF] Xu | Five Short Historical Earthquake Surface Ruptures near the Silk Road, Gansu Province, China[END_REF], [START_REF] Kang | Paleoseismic events in banguoba trench along aksay segment of the Altyn Tagh fault zone[END_REF], [START_REF] Shao | Historical (Yuan Dynasty) Earthquake on the North Danghe Nanshan Thrust, Western Qilian Shan, ChinaHistorical (Yuan Dynasty) Earthquake on the North Danghe Nanshan Thrust, Western Qilian Shan, China[END_REF], [START_REF] Wu | Investigation of Late Quaternary Activity along Subei Fault[END_REF], He et al. (2018) and [START_REF] Luo | Spatial and temporal distribution of earthquake ruptures in the eastern segment of the Altyn Tagh fault, China[END_REF]. Gray shaded area with black line is position of swath profile in Figure 12. Black circles are major triple junctions along Altyn Tagh fault, Subei, Shibao Cheng/Changma, and Qilian junctions, from west to east.

Along the northwestern Qilian Shan region margin, splays from the Altyn Tagh fault veer eastward and connect with the ranges of Danghe Nan Shan (DHNS), Daxue Shan, and north Qilian Shan, in strike-slip/thrust fault triple junctions (e.g., [START_REF] Métivier | Northeastward growth of the Tibet plateau deduced from balanced reconstruction of two depositional areas: The Qaidam and Hexi Corridor basins, China[END_REF][START_REF] Van Der Woerd | Rapid active thrusting along the northwestern range front of the Tanghe Nan Shan (western Gansu, China)[END_REF][START_REF] Xu | Late Quaternary sinistral slip rate along the Altyn Tagh fault and its structural transformation model[END_REF]; Figure 1). The 250 km-long and 20 km-wide DHNS with peaks reaching 5500 m asl, i.e., 4000 m above the Tarim basin to the west and 2000 m above the average elevation of the Qaidam basin, is one of the largest ranges of the Qilian mountains. Its western junction with the ATF, west of the town of Subei, is particularly complex and involves the north and south strands of ATF and several active thrust faults along the northern piedmont of DHNS (Figure 2). Figure 2. Active fault map at Subei junction from field observations, satellite image interpretation, and topographic data (modified from [START_REF] Van Der Woerd | Rapid active thrusting along the northwestern range front of the Tanghe Nan Shan (western Gansu, China)[END_REF]; see Figure S1). Background is simplified geology from Gansu Bureau of Geology and Mineral Resources of Gansu Province (1989). Main thrusts F0', F0, F1, and F2 are responsible for western Danghenan Shan growth southwest of Subei basin. Open rectangles locate study sites 1 and 2 (Figures 4 and8). Dashed line rectangle is site 3 of van der [START_REF] Van Der Woerd | Rapid active thrusting along the northwestern range front of the Tanghe Nan Shan (western Gansu, China)[END_REF]. Grey rectangular box locates palaeomagnetic section in [START_REF] Sun | Tectonic uplift in the northern Tibetan Plateau since 13.7 Ma ago inferred from molasse deposits along the Altyn Tagh Fault[END_REF] and [START_REF] Lin | Detrital apatite fission track evidence for provenance change in the Subei Basin and implications for the tectonic uplift of the Danghe Nan Shan (NW China) since the mid-Miocene[END_REF].

In this study, we investigate the geometry and the kinematics of active faults at 2 sites along the ~20 km-long western front of the DHNS (sites 1 and 2, Figure 2).

Based on topographic profiles of deformed and uplifted terraces, we constrain the sub-surface geometry of the thrust faults and estimate the amounts of uplift and shortening using 2D area balance. We combine several geochronological tools, such as 14 C, optically stimulated luminescence (OSL), and terrestrial cosmogenic nuclide (TCN) to date the various terrace levels. Finally, seismic scarp heights and co-seismic slip in trenches, combined with the inferred thrust geometry at depth make it possible to estimate the magnitude of surface rupturing earthquakes generated by the thrust faults.

Tectonic setting

The Qilian Shan region at the northern margin of the Tibetan Plateau (Figure 1). is characterized by NW-SE trending thrust and fold belts. These ranges accommodate shortening and confine intermontane basins leading to mountain growth and elevation increase [START_REF] Burchfiel | Intracrustal detachment within zones of continental deformation[END_REF][START_REF] Métivier | Northeastward growth of the Tibet plateau deduced from balanced reconstruction of two depositional areas: The Qaidam and Hexi Corridor basins, China[END_REF][START_REF] Métivier | Northeastward growth of the Tibet plateau deduced from balanced reconstruction of two depositional areas: The Qaidam and Hexi Corridor basins, China[END_REF][START_REF] Tapponnier | Active thrusting and folding in the Qilian Shan, and decoupling between upper crust and mantle in northeastern Tibet[END_REF]. Deformation in the Qilian Shan in the Cenozoic results from the collision between Eurasia and India, according to geological structures, palaeomagnetic data and sedimentary records in the Qaidam-Qilian area (e.g., [START_REF] Dai | Magnetostratigraphy of Cenozoic sediments from the Xining Basin: Tectonic implications for the northeastern Tibetan Plateau[END_REF][START_REF] Dupont-Nivet | Paleogene clockwise tectonic rotation of the Xining-Lanzhou region, northeastern Tibetan Plateau[END_REF][START_REF] Dupont-Nivet | Paleogene clockwise tectonic rotation of the Xining-Lanzhou region, northeastern Tibetan Plateau[END_REF][START_REF] Yin | Tectonic history of the Altyn Tagh fault system in northern Tibet inferred from Cenozoic sedimentation[END_REF][START_REF] Yin | Cenozoic tectonic evolution of Qaidam basin and its surrounding regions (Part 1): The southern Qilian Shan-Nan Shan thrust belt and northern Qaidam basin[END_REF][START_REF] Zhou | Cenozoic deformation history of the Qaidam Basin, NW China: Results from cross-section restoration and implications for Qinghai-Tibet Plateau tectonics[END_REF][START_REF] Zhuang | Cenozoic multiple-phase tectonic evolution of the northern Tibetan Plateau: Constraints from sedimentary records from Qaidam basin, Hexi Corridor, and Subei basin, northwest China[END_REF]. The Qilian Shan grew during late Oligocene -middle Miocene time (e.g., [START_REF] George | Miocene cooling in the northern Qilian Shan, northeastern margin of the Tibetan Plateau, revealed by apatite fission-track and vitrinite-reflectance analysis[END_REF][START_REF] Gilder | Oligo-Miocene magnetostratigraphy and rock magnetism of the Xishuigou section, Subei (Gansu Province, western China) and implications for shallow inclinations in central Asia[END_REF][START_REF] Métivier | Northeastward growth of the Tibet plateau deduced from balanced reconstruction of two depositional areas: The Qaidam and Hexi Corridor basins, China[END_REF][START_REF] Zheng | Erosion, fault initiation and topographic growth of the North Qilian Shan (northern Tibetan Plateau)[END_REF] as suggested by magnetostratigraphy, apatite fission-track (AFT) and (U-Th)/He cooling data, and sedimentary mass balance. This growth accelerated during the Pliocene [START_REF] Métivier | Northeastward growth of the Tibet plateau deduced from balanced reconstruction of two depositional areas: The Qaidam and Hexi Corridor basins, China[END_REF] and continued until Quaternary time as evidenced by active thrusting (e.g., [START_REF] Tapponnier | Active thrusting and folding in the Qilian Shan, and decoupling between upper crust and mantle in northeastern Tibet[END_REF][START_REF] Métivier | Northeastward growth of the Tibet plateau deduced from balanced reconstruction of two depositional areas: The Qaidam and Hexi Corridor basins, China[END_REF][START_REF] Zheng | 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[END_REF]Hetzel et al., 2004Hetzel et al., , 2019;;Hu et al., 2015[START_REF] Hu | Slip Rate and Paleoseismology of the Bolokenu-Aqikekuduk (Dzhungarian) Right-Lateral Strike-Slip Fault in the Northern Tian Shan, NW China[END_REF][START_REF] Du | Segmentation and termination of the surface rupture zone produced by the 1932 Ms 7.6 Changma earthquake: New insights into the slip partitioning of the eastern Altyn Tagh fault system[END_REF][START_REF] Xu | Slip Rate of the Danghe Nan Shan Thrust Fault from 10Be Exposure Dating of Folded River Terraces: Implications for the Strain Distribution in Northern Tibet[END_REF].

In the western DHNS piedmont, a conglomerate coarsening in the Tiejianggou Formation occurred at 13.7 Ma as determined from palaeomagnetic data [START_REF] Sun | Tectonic uplift in the northern Tibetan Plateau since 13.7 Ma ago inferred from molasse deposits along the Altyn Tagh Fault[END_REF] ; Figure 2). Both changes in sedimentary facies and variations in depositional rates suggest increased tectonic uplift between 13.7 and 9 Ma [START_REF] Sun | Tectonic uplift in the northern Tibetan Plateau since 13.7 Ma ago inferred from molasse deposits along the Altyn Tagh Fault[END_REF].

Similarly, the AFT and U-Th/He data obtained from sections of Cenozoic terrestrial clastic sediments and bedrock of Paleozoic granodiorites along the western DHNS (see location in Figures 1 and2) are consistent with a change in exhumation rate around 14 Ma, implying intensification of uplift at that time [START_REF] Lin | Detrital apatite fission track evidence for provenance change in the Subei Basin and implications for the tectonic uplift of the Danghe Nan Shan (NW China) since the mid-Miocene[END_REF][START_REF] Yu | Miocene Range Growth Along the Altyn Tagh Fault: Insights From Apatite Fission Track and (U-Th)/He Thermochronometry in the Western Danghenan Shan, China[END_REF].

Thrust-fault scarps in the Subei and Yanchi Wan basins, along the western and central DHNS piedmont, respectively, testify to active shortening and surface rupturing earthquakes [START_REF] Métivier | Northeastward growth of the Tibet plateau deduced from balanced reconstruction of two depositional areas: The Qaidam and Hexi Corridor basins, China[END_REF][START_REF] Van Der Woerd | Rapid active thrusting along the northwestern range front of the Tanghe Nan Shan (western Gansu, China)[END_REF][START_REF] Shao | Historical (Yuan Dynasty) Earthquake on the North Danghe Nanshan Thrust, Western Qilian Shan, ChinaHistorical (Yuan Dynasty) Earthquake on the North Danghe Nanshan Thrust, Western Qilian Shan, China[END_REF][START_REF] Xu | Slip Rate of the Danghe Nan Shan Thrust Fault from 10Be Exposure Dating of Folded River Terraces: Implications for the Strain Distribution in Northern Tibet[END_REF]; Figure 1). Thrusting along the western DHNS southwest of Subei is kinematically compatible with a compressive right step between the north strand of ATF and the Yema-Daxue Shan and Yema Nan Shan left-lateral strike-slip faults (Figures 1 and2). The kinematics of triple junctions predicts that the ATF sliprate decreases northeastward past the different junctions (e.g., [START_REF] Métivier | Northeastward growth of the Tibet plateau deduced from balanced reconstruction of two depositional areas: The Qaidam and Hexi Corridor basins, China[END_REF] as observed by geological and geodetic slip rates distribution along the ATF (Figure S2; [START_REF] Mériaux | The Aksay segment of the northern Altyn Tagh fault: Tectonic geomorphology, landscape evolution, and Holocene slip rate[END_REF][START_REF] Xu | Late Quaternary sinistral slip rate along the Altyn Tagh fault and its structural transformation model[END_REF][START_REF] Zhang | Late Quaternary and present-day rates of slip along the Altyn Tagh Fault, northern margin of the Tibetan Plateau[END_REF][START_REF] Seong | Constant slip rate during the late Quaternary along the Sulu He segment of the Altyn Tagh Fault near Changma, Gansu, China[END_REF][START_REF] Xiao | Structure and geometry of the Aksay restraining double bend along the Altyn Tagh Fault, northern Tibet, imaged using magnetotelluric method[END_REF][START_REF] Elliott | Persistent rupture terminations at a restraining bend from slip rates on the eastern Altyn Tagh fault[END_REF][START_REF] Liu | Late Quaternary slip rate of the Aksay segment and its rapidly decreasing gradient along the Altyn Tagh fault[END_REF]. Locally, other rates have been determined on thrust or strike-slip faults [START_REF] Van Der Woerd | Rapid active thrusting along the northwestern range front of the Tanghe Nan Shan (western Gansu, China)[END_REF][START_REF] Luo | Slip Rate of Yema River-Daxue Mountain Fault since the Late Pleistocene and Its Implications on the Deformation of the Northeastern Margin of the Tibetan Plateau[END_REF][START_REF] Zheng | 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[END_REF][START_REF] Xu | Slip Rate of the Danghe Nan Shan Thrust Fault from 10Be Exposure Dating of Folded River Terraces: Implications for the Strain Distribution in Northern Tibet[END_REF].

A few paleoseismic trenches have been dug along the DHNS thrust and adjacent strike-slip faults. West of the Subei junction, Li et al. ( 2016) described evidence for four events from a trench dug across the Aksay segment of the ATF. They constrained the ages of two events at 1180 a B.P. and 507 -230 a B.P. To the east, three events are identified in trenches near Shibao Cheng over the last 7000 years, with the most recent one between 2720 and 2800 a B.P. [START_REF] Luo | Spatial and temporal distribution of earthquake ruptures in the eastern segment of the Altyn Tagh fault, China[END_REF]. Along the western DHNS (site 3 in Figure 2), a thrust rupture with a vertical offset of ~1.7 m after 4300 a B.P. is documented [START_REF] Wu | Investigation of Late Quaternary Activity along Subei Fault[END_REF]. Along the central DHNS in the Yanchi Wan basin (Figure 1), [START_REF] Shao | Historical (Yuan Dynasty) Earthquake on the North Danghe Nanshan Thrust, Western Qilian Shan, ChinaHistorical (Yuan Dynasty) Earthquake on the North Danghe Nanshan Thrust, Western Qilian Shan, China[END_REF] M7.6 Changma earthquake (Figure 1). The surface rupture of this earthquake extends for 120 km with coseismic horizontal displacements up to 5.5 m [START_REF] Du | Segmentation and termination of the surface rupture zone produced by the 1932 Ms 7.6 Changma earthquake: New insights into the slip partitioning of the eastern Altyn Tagh fault system[END_REF][START_REF] Meyer | Mécanismes des grands tremblements de terre et du raccourcissement crustal oblique au bord nord-est du Tibet[END_REF]Peltzer et al., 1988). The potential for a large earthquake is suspected in the Subei basin along the DHNS as large several-meter-high seismic scarps are evidenced (see below).

Methods

Shortening constraints from uplifted area balancing

Abandoned, uplifted, and folded terraces may be used to constrain the kinematics of shortening over the time span of the terrace ages (e.g., [START_REF] Métivier | Northeastward growth of the Tibet plateau deduced from balanced reconstruction of two depositional areas: The Qaidam and Hexi Corridor basins, China[END_REF]Rockwell et al., 1998;[START_REF] Lal | Cosmic ray labeling of erosion surfaces: in situ nuclide production rates and erosion models[END_REF][START_REF] Van Der Woerd | Rapid active thrusting along the northwestern range front of the Tanghe Nan Shan (western Gansu, China)[END_REF][START_REF] Thompson | Late Quaternary slip rates across the central Tien Shan, Kyrgyzstan, central Asia[END_REF][START_REF] Daëron | Modeling the shortening history of a fault tip fold using structural and geomorphic records of deformation[END_REF][START_REF] Simoes | Slip rates on the Chelungpu and Chushiang thrust faults inferred from a deformed strath terrace along the Dungpuna river, west central Taiwan[END_REF]Hu et al., 2015[START_REF] Hu | Slip Rate and Paleoseismology of the Bolokenu-Aqikekuduk (Dzhungarian) Right-Lateral Strike-Slip Fault in the Northern Tian Shan, NW China[END_REF][START_REF] Guilbaud | Kinematics of active deformation across the western Kunlun mountain range (Xinjiang, China) and potential seismic hazards within the southern Tarim basin[END_REF][START_REF] Xu | Slip Rate of the Danghe Nan Shan Thrust Fault from 10Be Exposure Dating of Folded River Terraces: Implications for the Strain Distribution in Northern Tibet[END_REF]. Provided that the base levels can be reconstructed, the present height of the strath terraces above their base level is an estimate of the tectonic throw on the underlying thrust fault (e.g., [START_REF] Bull | Geomorphic responses to climatic change[END_REF][START_REF] Merritts | Long river profiles, tectonism, and eustasy: A guide to interpreting fluvial terraces[END_REF][START_REF] Lal | Cosmic ray labeling of erosion surfaces: in situ nuclide production rates and erosion models[END_REF][START_REF] Van Der Woerd | Rapid active thrusting along the northwestern range front of the Tanghe Nan Shan (western Gansu, China)[END_REF][START_REF] Thompson | Late Quaternary slip rates across the central Tien Shan, Kyrgyzstan, central Asia[END_REF]Wang et al., 2020). Using a mass conservation model (Figure 3) and assuming negligible erosion of the terrace, it is possible to estimate the amount of corresponding slip on the thrust fault at depth (S) to account for uplift and folding of the strath level (areas A1 and A2).

We infer the geometry of the thrust fault at depth from the surface geology and structural data, such as bedding of the sedimentary beds or fault dip, similarly to [START_REF] Métivier | Northeastward growth of the Tibet plateau deduced from balanced reconstruction of two depositional areas: The Qaidam and Hexi Corridor basins, China[END_REF], van der Woerd (1998), or Hu et al. (2015). We consider a simple geometry of the thrust fault at depth made of two connected planes, a sub-horizontal décollement, and a ramp (Figure 3). The dip of the frontal ramp without surface observation of the emerging thrust fault, may range between 45±15° as commonly observed or inferred from structural data, i.e., bedding of the thrusted sediment layers (e.g., [START_REF] Van Der Woerd | Rapid active thrusting along the northwestern range front of the Tanghe Nan Shan (western Gansu, China)[END_REF][START_REF] Gilder | Oligo-Miocene magnetostratigraphy and rock magnetism of the Xishuigou section, Subei (Gansu Province, western China) and implications for shallow inclinations in central Asia[END_REF]. The wavelength of the antiformal folding of the terraces may be used to constrain the depth where the ramp and the décollement meet (D 0 ; Figure 3, e.g., [START_REF] Métivier | Northeastward growth of the Tibet plateau deduced from balanced reconstruction of two depositional areas: The Qaidam and Hexi Corridor basins, China[END_REF]. The precise dip (α) or rooting depth (D α ) of the décollement is unknown, thus we use two end-member models, where the dip of the "flat" changes from near 0° (Model 1) to a certain dip (Model 2, 0 <α<45°; Figure 3). In Model 1, this depth remains constant until it eventually joins with other faults at depth. In Model 2, the décollement dips with an angle α and thus connects with other faults deeper (Figure 3).

In the case where several terrace levels are incrementally uplifted and folded, and without independent structural constraints on the geometry of the décollement, it is possible to adjust the dip angle α and thus depth D α by minimizing the shortening rate variations among the terraces. A sloping décollement has two consequences. First, the dipping décollement acts like a shallowly dipping ramp with uniform uplift (U in Model 2, Figure 3) above its entire length (e.g., [START_REF] Métivier | Northeastward growth of the Tibet plateau deduced from balanced reconstruction of two depositional areas: The Qaidam and Hexi Corridor basins, China[END_REF]Hu et al., 2015).

Second, increasing D α reduces the slip amount (S) necessary to account for the uplifted area (A2) above base level. To document the subsurface fault geometry and rupture history of the seismic scarp at site 2, we investigated the subsurface sedimentation and structure by opening a trench across the main fault scarp (Figures 6 and7). The exposed walls in the trench are cleaned and photographed with a digital camera (Figure 8). We used the structurefrom-motion method to mosaic the photos (e.g. [START_REF] Bemis | Ground-based and UAV-Based photogrammetry: A multi-scale, high-resolution mapping tool for structural geology and paleoseismology[END_REF][START_REF] Reitman | High-Resolution Trench Photomosaics from Image-Based Modeling: Workflow and Error AnalysisHigh-Resolution Trench Photomosaics from Image-Based Modeling: Workflow and Error Analysis[END_REF] before mapping the deformed and offset sedimentary units onto the orthophotos.

Sampling and dating methods

To constrain the age of the deformed terraces and fans, we combined three different dating techniques: terrestrial cosmogenic nuclide (TCN) dating, accelerator mass spectrometry for radiocarbon samples (AMS 14 C) and optically stimulated luminescence dating (OSL).

AMS 14 C and OSL were used to constrain the ages of sandy or silty sediments found either capping the coarser conglomeratic material of the alluvial deposits or sandy lenses in coarser sediments. The radiocarbon samples were processed and dated at Beta Analytic. Ages were calibrated with IntCal13 [START_REF] Reimer | IntCal13 and Marine13 radiocarbon age calibration curves 0-50,000 years cal BP[END_REF]. All OSL samples were processed and measured following the procedure described in [START_REF] Shao | Historical (Yuan Dynasty) Earthquake on the North Danghe Nanshan Thrust, Western Qilian Shan, ChinaHistorical (Yuan Dynasty) Earthquake on the North Danghe Nanshan Thrust, Western Qilian Shan, China[END_REF]. Fine-grained (4 -11 μm) quartz fractions were prepared and treated with 30% hydrogen peroxide (H 2 O 2 ) and 10% hydrochloric acid (HCl) to remove organic materials and carbonates, respectively. The purity of the prepared quartz was checked by the ratio of infrared-stimulated luminescence (IRSL) to blue light-stimulated luminescence (BLSL) (<5%). We used a Risø TL/OSL-DA-20 reader equipped with a 90Sr/90Y beta source (~0.12 Gy/s), blue LEDs (470 ± 20 nm, ~80 mW/cm2), and two Hoya U340 filters (290 -370 nm) to conduct irradiation, heating, and luminescence measurements. A simplified multiple aliquot regenerative dose (SMAR) was used to measure the quartz aliquots.

At site 1, depth profiles are dug into the loess covered surfaces of the terraces in relatively flat places away from any visible surface erosion or incised channels (Figure 4), except for one terrace, where the depth profile was sampled by refreshing its steep eastern riser (Figure 9). From each profile, we collected 6 -8 samples of quartz-rich gravels in the top 2 m of the several meter-thick gravels/cobbles deposits (Figures 9 and10). The age of abandonment of the underlying conglomerate is then determined by 10 Be depth profile modeling considering shielding of the overlying loess cover, which age is constrained with 14 C and OSL dating, following the approach developed in [START_REF] Shao | Late Pleistocene slip rate of the central Haiyuan fault constrained from optically stimulated luminescence, 14C, and cosmogenic isotope dating and highresolution topography[END_REF]. At site 2, only surface samples were collected from a boulder fan surface. Six 2.5 cm-thick rock samples were collected with a chisel and hammer from the top of six embedded granitic boulders (size > 0.5 m in diameter) (Figure 11).

The TCN samples were crushed and sieved, allowing the 250 -500 μm fraction to be treated by sequential ultrasonic leaches to obtain pure quartz. Beryllium oxide was extracted by chromatography from the sample solution and inserted into target holders at the Cosmogenic Isotope Laboratory of Institut Terre et Environnement de Strasbourg (ITES UMR7063). The isotopic ratios were determined at the ASTER-AMS facility at the Centre Européen de Recherche et d'Enseignement des Géosciences de l'Environnement (CEREGE, Aix-en-Provence, France). We used the online calculator of CRONUS-Earth calculator Version 3 [START_REF] Balco | A complete and easily accessible means of calculating surface exposure ages or erosion rates from 10Be and 26Al measurements[END_REF] to calculate the exposure ages of surface samples with the time-dependent scaling model of [START_REF] Lal | Cosmic ray labeling of erosion surfaces: in situ nuclide production rates and erosion models[END_REF][START_REF] Stone | Air pressure and cosmogenic isotope production[END_REF]. For subsurface samples, we modeled the ages based on the depth dependence of the concentration (e.g., [START_REF] Braucher | In situ produced 10Be measurements at great depths: implications for production rates by fast muons[END_REF][START_REF] Gosse | Terrestrial in situ cosmogenic nuclides: theory and application[END_REF][START_REF] Lal | Cosmic ray labeling of erosion surfaces: in situ nuclide production rates and erosion models[END_REF][START_REF] Yang | New constraints on the late Quaternary landscape evolution of the eastern Tibetan Plateau from 10Be and 26Al in-situ cosmogenic nuclides[END_REF][START_REF] Shao | Late Pleistocene slip rate of the central Haiyuan fault constrained from optically stimulated luminescence, 14C, and cosmogenic isotope dating and highresolution topography[END_REF]. The terrace conglomerate densities, which depend on composition, grain size and consolidation of terrace sediment were estimated from the exponential best fit of the depth profiles.

We performed forward modeling to estimate the shielding effect of loess considering a density of 1.4 g/cm 3 and constant accumulation rates calculated by linearly fitting the age/depth relationship from up to three OSL and/or 14 C dates (similarly to other studies in the Qilian Shan area, e.g., Hetzel et al., 2004Hetzel et al., , 2019;;[START_REF] Shao | Late Pleistocene slip rate of the central Haiyuan fault constrained from optically stimulated luminescence, 14C, and cosmogenic isotope dating and highresolution topography[END_REF].

Results

The study sites 1 and 2 are situated in the DHNS bajada south of Subei along the main thrusts F1 and F2 (Figure 2). The first site (site 1) is located in the middle of the uplifted ledge at a place already described in van der Woerd et al. ( 2001) and composed of a set of four main uplifted and folded terrace levels by thrust F1 (Figure 4). The second site (site 2) is located 10 km to the east along the fault scarp of F2 crossed by the Dang He river. From the range front in the south to the bajada in the north, i.e., between F0 and F1 (Figure 4), all terraces show a similar pattern of uplift and deformation. Upstream they stand above the present stream bed with a slightly steeper slope as T0, while they form a clear antiform near their northern end (Figure 5). This shape is particularly clear for T4 and T2, which treads extend largely upstream, while for T1 and T3 only the folded bulge to the north remains. Minimum and maximum heights of terraces T1, T2, T3 and T4 above the present-day stream bed are 0, 18 ± 1, 27 ± 2, and 50 ± 3 m and 7. ± 0.5, 23 ± 1, 40 ± 2, and 60 ± 3 m, respectively (Figure 5; Table 1). From our mapping (Figure 2), the bajada in front of the fold are fan deposits (Tf2) corresponding mostly to terrace level T2. The corresponding maximum offset of the surface T2 relative to the Tf2 bajada is 15 ± 1 m (Table 1).

Site 2

Site 2 is located at the bottom of the Subei basin along the Dang He river-bed, approximately 7 km east of site 1 (Figures 2,5,6). There, thrust F2 can be traced for approximately 10 km outlined by a series of half-meter-to several-meter-high scarps.

To the northwest, the scarps, across the distal fans of the main bajada, are subparallel to F1 for 2 km (Figure 2). The scarps then veer eastward to strike east-west on average across a set of fans abandoned by the Dang He and a major tributary. Site 2 is located where the scarps reach up to several meters high. Farther east, the scarps continue along the southern rim of a major valley within the Yema Shan, and their traces become unclear in the high relief eastward. The Dang He and tributaries at site 2 incise in a set of imbricated fans, the oldest and highest fans (T3, Tf3) standing up to 60 m above the riverbed near their apex or where the Dang He leaves the gorge to the east (profiles pf8, pf9; Figure 6). Their slopes decrease progressively from the oldest to youngest. Presently, F2 thrust scarps cut through the lower parts of Tf3, Tf2, T1 and T2'. Across Tf2 and T2', the scarps heights range from 2.6 to 3.5 m, while across T1, the scarp is approximately 1.6 m high (Figure 6D). The frontal 3.5 ± 0.2 m-high scarp on surface Tf2 (profile pf5; Figure 6D) was targeted for trenching (Figure 8). Orthophotos of the two 13 m-long and 3.5 m-high trench walls exhibit similar units and faulting evidence. Three main units can be separated. Unit 1 (U1) is the top layer of approximately 1 m-thick silty loess thinning upstream, underlain by a 3 -5 cm thick bed of reddish coarse sand at the bottom. Unit 2 (U2) consists of approximately 1.5 m-thick light gray poorly sorted angular sandy gravels. Unit 3 (U3) is a layer of gray, poorly sorted angular sandy gravels.

Three main shallow-dipping (7° to 19°) thrust faults (f1-f3) can be seen on both walls cutting through units U1 to U3, with a minor back thrust splaying from f2 (Figure 7c). All three faults offset the top of unit U2; faults f2 and f3 also offset the loess unit U1. As a results, the bottom of U2 is displaced ~50 -55 cm and ~300 -310 cm by f1 and f2, respectively. The top of U2 is displaced ~50 cm, ~280 cm, and ~90 -120 cm by f1, f2, and f3, respectively.

Trench exposures show evidence of two events. The older event is indicated by a gravelly colluvial wedge in the footwall of fault f1, occurring before the deposition of unit U1. The younger event is manifested by faults f2 and f3, which offset the base of unit U1. The upward extending traces of faults f2 and f3 are unclear in the homogeneous loess unit. Therefore, it is difficult to determine whether there is more than one event after the deposition of the lower part of unit U1. 4). See text for unit details.

Taking a 20° dip, the summed dip slip of approximately 4.9 ± 0.2 m of the thrusts in the trench would account for 1.7 ± 0.2 m of uplift. This value is less than half of the 3.6 ± 0.2 m height of the frontal scarp of terrace Tf2 (profile pf5 in Figure 6). For these faults and slip amounts to be responsible for the 3.6 ± 0.2 m-high cumulative scarp of Tf2, they must steepen rapidly south of the trench to approximately 47°.

Terrace age determination

At site 1, we model the exponential depth distribution of 10 Be concentrations of amalgamated samples to determine the exposure ages. To average the variable inheritance among the individual rock pieces, our samples include a minimum of 60 distinct pebbles, a number usually considered large enough for this purpose (e.g., [START_REF] Anderson | Explicit treatment of inheritance in dating depositional surfaces using in situ 10Be and 26Al[END_REF][START_REF] Hancock | Dating fluvial terraces with 10Be and 26Al profiles: Application to the Wind River, Wyoming[END_REF][START_REF] Matrau | Late Pleistocene-Holocene Slip Rate Along the Hasi Shan Restraining Bend of the Haiyuan Fault: Implication for Faulting Dynamics of a Complex Fault System[END_REF][START_REF] Shao | Late Pleistocene slip rate of the central Haiyuan fault constrained from optically stimulated luminescence, 14C, and cosmogenic isotope dating and highresolution topography[END_REF].

Nevertheless, our depth profiles show scattered 10 Be concentrations with depth not simply modeled by an exponential fit. With scattered values largely above analytical errors and in the absence of vertical mixing, several reasons could be responsible for the dispersion observed, such as sequential deposits or not well-averaged inheritance (e.g., [START_REF] Perrineau | Incision rate of the Yellow River in Northeastern Tibet constrained by 10Be and 26Al cosmogenic isotope dating of fluvial terraces: implications for catchment evolution and plateau building[END_REF][START_REF] Dortz | Interpreting scattered in-situ produced cosmogenic nuclide depth-profile data[END_REF]. There are no field evidence for sequential deposits or unconformities within the conglomerates. Consequently, in the case of not well-mixed inheritance, the less concentrated samples in the profile are likely closer to the actual terrace exposure time (e.g., [START_REF] Dortz | Interpreting scattered in-situ produced cosmogenic nuclide depth-profile data[END_REF]. Figure 10 shows modeling fitting the less concentrated samples. Model solutions showing an average fit of all samples are also presented in Figures S3 -S6. Both models give similar results with differences up to 10%, which remain small with no systematic bias. We thus present our results below with our preferred modeling considering not well averaged inheritance. The depth profile in T1 includes 7 samples in the 2 m-thick deposit, which consist of a 20 cm-thick gravel deposit on top of a 10 cm-thick loess layer above a homogenous 1.7 m-thick conglomerate (Figure 10a). An OSL sample from the loess layer yields a date of 5.4 ± 0.2 ka, which we interpret as a minimum age of the terrace (Table 2). The best fit to the 10 Be concentration with depth explains four out of seven samples (R 2 = 0.99) and yields a model age of 8.7 ± 0.5 ka. The modeling suggests two depositional phases: a first phase of conglomerate accumulation since 8.7 ± 0.5 ka; after a hiatus, a second phase of 10 cm-thick loess accumulation since 5.4 ± 0.2 ka, and the gravel layer on top (Figure 10b). In the following, we use the 8.7 ± 0.5 ka as the age of T1 as it represents the major phase of terrace formation and because the top loess and gravel layer may post-date the onset of folding and could be derived from post-depositional surface reworking.

A ~3.6 m-deep pit in T2 reveals a loess layer of 145 cm above the conglomerate of the terrace (Figure 10c). Three OSL samples at depths of 50, 90, and 135 cm yield dates of 4.8 ± 0.2, 5.6 ± 0.2, and 5.9 ± 0.2 ka, respectively (Table 2). The samples are stratigraphically ordered, which implies that the 145 cm-thick loess deposits accumulated between 6.1 ka and 4.3 ka at a rate of ~72 cm/kyr. Because the 10 Be concentrations of 5 conglomerate samples distributed between 1.5 and 3.5 m depths are scattered, we fit only the 2 less concentrated samples at 1.5 and 2 m depths, thus yielding an exposure age of 9.5 ± 1 ka (Figure 10d). The 3 m-deep depth profile in T3 reveals a ~130 cm-thick loess layer on the top with a thin reddish silty soil layer at its base, indicating weathering and pedogenesis (Figure 10e). Two OSL samples at depths of ~40 and ~130 cm yield burial ages of 7.6 ± 0.4 ka and 5.2 ± 0.2 ka, respectively ( 10f). In any case, the considerable time lag implied by the TCN modeling (30 to 50 kyr) between the conglomerate deposit and the overlying loess cover indicates that the top terrace remained devoid of loess cover most of its time, consistent with the observation of soil development at its top (Figures 9 and10e). Given the significant uncertainty, the two models provide the upper and lower bounds on the age of T3 (41 -68 ka or 51.5 ± 13.5 ka).

The 2.8 m-deep pit in T4 reveals a ~45 cm thick loess layer with a thin reddish silty soil at its base above the terrace conglomerate (Figure 9). One OSL sample at the bottom of the loess yields a burial age of 8.2 ± 0.4 ka (Figure 10g, Table 2). The model yields an exposure age of 128 ± 20 ka by fitting four out of six conglomerate samples (Figure 10h). Samples SBTCN-T4-02 and SBTCN-T4-06 are considred outliers. Sample SBTCN-T4-02 is close to the top of the terrace deposit and may have been bio-or cryo-perturbated. Including sample SBTCN-T4-02 results in a similar age with larger uncertainties (Figure S6). Similar to terrace T3, the time lag between the deposition of the conglomeratic strath and the overlying loess cover (~100 kyr) is consistent with soil development at the top of the conglomerate and indicates no or minimal surface erosion before loess mantled its surface.

At site 2, six embedded granitic boulders from surface Tf2 were targeted for TCN dating (Figure 11). Four (LP02, 03, 04, 06) samples are clustered at approximately 16 ka, with an average age of 16.2 ± 2.5 ka (youngest age of 15.3 ± 1.4 ka) (Table 2;

Figure 11). Two samples yielded older dates than the rest of the samples (~25 -35 ka), probably owing to longer exposure in the catchment upstream resulting in larger inheritance and are thus considered outliers. Since the ages of the four youngest samples are well-clustered, erosion and inheritance may be considered negligible (e.g., van der Woerd et al., 1998;[START_REF] Ryerson | Applications of morphochronology to the active tectonics of Tibet[END_REF][START_REF] Matrau | Late Pleistocene-Holocene Slip Rate Along the Hasi Shan Restraining Bend of the Haiyuan Fault: Implication for Faulting Dynamics of a Complex Fault System[END_REF]. The position of Tf2 relative to the other terrace implies that the date of 16.2 ± 2.5 ka is a maximum emplacement age of the fan surfaces at site 2, indicating that these terraces and fans formed at the end of the last glacial maximum with the return of a more humid climate (e.g., [START_REF] Van Der Woerd | Uniform postglacial slip-rate along the central 600 km of the Kunlun Fault (Tibet), from 26Al, 10Be, and 14C dating of riser offsets, and climatic origin of the regional morphology[END_REF]Hetzel, 2013).

In the trench, one radiocarbon sample collected at the bottom of the upper loess unit U1 in the west wall was dated at 8040 -8190 years B.P.. This age is overall concordant with the base of the loess units at site 1 and even farther west (this study;

Tables 2 and4; see also [START_REF] Van Der Woerd | Rapid active thrusting along the northwestern range front of the Tanghe Nan Shan (western Gansu, China)[END_REF]. When we consider the bottom burial age of the loess covering the terrace conglomerate at ~8 ka, we can bracket the age of Tf2 between 8 and 16.2 ka (12 ± 4 ka).

Figure 11. 10 Be dating of surface samples at site 2. (a) Samples were collected from the surface of imbedded granitic boulders. Hammer for scale. See Figure 8 and Table 3 for location details. (b) 10 Be cosmogenic nuclide concentration and age distributions of TCN samples at site 2 (Table 3). Thin curves are single sample density distribution, and gray shaded area is cumulative density distribution. Four youngest ages cluster with overlapping error bars and constrain alluvial fan (Tf2) abandonment age.

Uplift and shortening rates estimates

Site 1 along catchment r5 has been targeted because it is one of the above average sized catchments along the western DHNS range with moderate incision, which has preserved a set of four major terrace levels (Figure 4; e.g., [START_REF] Van Der Woerd | Rapid active thrusting along the northwestern range front of the Tanghe Nan Shan (western Gansu, China)[END_REF].

The tops of terraces T1 to T4 stand 7 to 60 m at their highest point above the present stream bed (Figure 6). The highest terrace T4, which can be mapped up to the range front, has a slightly steeper slope upstream (7 -8°) than the present stream bed (6 -7°), indicating progressing slope lowering with time. In addition, sedimentation at the front of the fold and burial of the older fan deposits also need to be considered when reconstructing older base levels [START_REF] Van Der Woerd | Rapid active thrusting along the northwestern range front of the Tanghe Nan Shan (western Gansu, China)[END_REF]. Indeed, the present bajada is dominated by deposits related to T2 (see Figure 4), i.e., Holocene fan deposits, and by the present-day active stream outlets and fans that develop farther downslope (Figures 2 and4; see also [START_REF] Van Der Woerd | Rapid active thrusting along the northwestern range front of the Tanghe Nan Shan (western Gansu, China)[END_REF]. Because the stream gradient changes across the length of the terraces (2° between F0 and F1, i.e., approximately 3 km) and because the past base level geometry is not available, we take the present-day stream bed river profile and its gradient variations as a template for reconstructing past base levels. Given the steeper slope of upstream terrace T4, we thus rotate the present-day stream bed profile to fit the steeper gradient of terrace T4.

For terrace T3, we choose an intermediate gradient between that of T4 and the present-day gradient. For T2 and T1, the base level is that of the present-day stream r5 (Figure 12).

The frontal ramp is considered to reach the surface with a dip close to the bedding of the frontal red beds, i.e., approximately 40 -50° (van der [START_REF] Van Der Woerd | Rapid active thrusting along the northwestern range front of the Tanghe Nan Shan (western Gansu, China)[END_REF]. The wavelength of the antiformal folding of the terraces averages approximately 1000 m, thus constraining the depth where the ramp and the décollement meet, i.e., at 1000 m depth (D 0 ; Figures 3 and12, Table 6). For Model 1, this depth then remains constant until it joins with thrust F0. For Model 2, the décollement dips with an angle α and thus joins F0 at a deeper depth (approximately 1900 m; Figure 12,Table 6).

Shortening results for models 1 and 2 are shown in Table 6. The areas considered are measured between the top present-day folded and uplifted surface of the terraces and the reconstructed base level (Figure 12), neglecting the impact of loess cover and thickness of the conglomerates as their thickness remains small compared to the amounts of incision. Overall, the thickness of the conglomeratic deposit above the strath is approximately 2 -4 m for T1 and T2 and reaches 5 -10 m for T3 and T4. Figure 12. Reconstructed uplifted areas A1 and A2 for the 4 terraces at Site 1 considering end-member models with (Model 2) and without (Model 1) dip of the décollement. Blue lines are reference base levels above which uplifted area is outlined by terrace surface profiles (Figures 4 and5). See text and Table 6 for details.

In Model 1 shortening is absorbed solely by the warped frontal part of the terraces, and the upstream parts of the terraces are considered to have little or no uplift.

The incision of the river is thus driven mainly by progressive base level adjustment (see [START_REF] Van Der Woerd | Rapid active thrusting along the northwestern range front of the Tanghe Nan Shan (western Gansu, China)[END_REF]. For this model, the shortening rate varies from 0.8 -1.0 mm/yr for T1 and T2 to 0.5 mm/yr for T3 and T4 (Table 6).

Model 2 considers a dipping décollement before it ramps up as fault F1. In this case, shortening is absorbed by both the warped frontal part of the terraces and their uplifted upstream part. The incision of the river is now a combination of both progressive base level adjustment and overall uplift and folding of the terraces between F0 and F1. With this model, adjusting the dip of the décollement allows to minimize shortening rate differences throughout the set of terraces. An almost constant shortening rate of 0.5 ± 0.1 mm/yr for all terraces can be calculated with a dip of 23.5°, the corresponding uplift rate is 1.1 ± 0.3 mm/yr (Table 6).

At site 2, the 4.9 ± 0.5 m total slip on all branches of fault F2, as revealed from trenching may correspond to slip on a 47° dipping ramp to account for the 3.6 ± 0.2 m cumulative uplift and approximately 3.4 ± 0.2 m of shortening of Tf2. Dividing the slip values by the age of Tf2 (12 ± 4 ka) yields uplift and shortening rates of 0.5 ± 0.1 mm/yr on fault F2.

Considering the less well constrained shortening rate along thrust F2, and accounting for some shortening accommodated along thrust F0, total uplift and shortening rates along the western Danghe Nan Shan range front, south of Subei, for the last 130 ka could reach 1.4 ± 0.4 mm/yr and 0.8 ± 0.2 mm/yr, respectively.

Discussion

Shortening rate bias due to unsteady base level

Using terrace heights in the hanging wall of active thrusts to estimate shortening rate requires knowledge of a reference base level (e.g., Merrits et al., 1994;[START_REF] Guilbaud | Kinematics of active deformation across the western Kunlun mountain range (Xinjiang, China) and potential seismic hazards within the southern Tarim basin[END_REF]Hetzel et al., 2004;[START_REF] Lal | Cosmic ray labeling of erosion surfaces: in situ nuclide production rates and erosion models[END_REF][START_REF] Van Der Woerd | Rapid active thrusting along the northwestern range front of the Tanghe Nan Shan (western Gansu, China)[END_REF]Daeron et al, 2007;[START_REF] Hu | Slip Rate and Paleoseismology of the Bolokenu-Aqikekuduk (Dzhungarian) Right-Lateral Strike-Slip Fault in the Northern Tian Shan, NW China[END_REF]. Although changes with time of the base level are almost always a dynamic adjustment to the sediment supply or the transport capacities of a stream, they can be triggered by various causes, such as climate change, stream capture, subsidence, uplift, and earthquake-triggered changes in sediment supply [START_REF] Tapponnier | Active thrusting and folding in the Qilian Shan, and decoupling between upper crust and mantle in northeastern Tibet[END_REF]Merrits et al., 1994;[START_REF] Van Der Woerd | Uniform postglacial slip-rate along the central 600 km of the Kunlun Fault (Tibet), from 26Al, 10Be, and 14C dating of riser offsets, and climatic origin of the regional morphology[END_REF][START_REF] Bull | Tectonic geomorphology of mountains: a new approach to paleoseismology[END_REF][START_REF] Poisson | Holocene hydrological changes inferred from alluvial stream entrenchment in north Tian Shan (northwestern China)[END_REF][START_REF] Rizza | Post Earthquake Aggradation Processes to Hide Surface Ruptures in Thrust Systems: The M8.3, 1934, Bihar-Nepal Earthquake Ruptures at Charnath Khola (Eastern Nepal)[END_REF]. Long-term tectonic uplift is a cause of changes in base level when erosion and incision are not able to compensate for uplift. This is the situation along the western DHNS range front, where small and steep catchments are uplifted [START_REF] Van Der Woerd | Rapid active thrusting along the northwestern range front of the Tanghe Nan Shan (western Gansu, China)[END_REF]. As a consequence, aggradation occurs in the bajada south of Subei, as shown by the youth of the fan deposits (fan Tf2 of Holocene age) burying older deposits.

We have no subsurface data of the deposits in the footwall of fault F1. However, according to our mapping (Figure 3), fan Tf2 deposits in the footwall correspond to terrace T2 in the hanging wall, which indicates that the pre-Holocene corresponding footwall terrace deposits are buried. This is considered in Model 2 (Figure 12). Therefore, the heights of terraces T3 and T4 above their respective base levels are possibly greater than 13 and 22 m, as suggested in Model 2 (Figure 12; Table 6).

Consequently, the estimated shortening is minimized.

Shortening model implementation

Contrary to large mountains belt incised by large rivers (>50 km long), like in the North Qilian Shan, the Western Kunlun Shan, the Tian Shan, the catchment lengths west of Subei barely exceed about 5 km. As a consequence, the intermittent streams that incise the western DHNS are far from being stabilized graded rivers over long time periods. Shortening models as proposed, for instance, by Hu et al. (2015), [START_REF] Thompson | Late Quaternary slip rates across the central Tien Shan, Kyrgyzstan, central Asia[END_REF], are well adapted for the development of folds along stable rivers with almost no large base level changes and negligible slope changes. This is not the case west of Subei, where, as explained above, we still fail to reconstruct a reliable base level throughout the deposition time span (130 ka) of all the terraces. We thus choose to adapt our simplified structural model to this situation by considering two end-members, one without dipping décollement (Model 1) and another with a dipping décollement (Model 2) (Figure 3). Both situations are equally possible as illustrated by the variety of well documented fold and thrust belts across central Asia (e.g., [START_REF] Tapponnier | Active thrusting and folding in the Qilian Shan, and decoupling between upper crust and mantle in northeastern Tibet[END_REF][START_REF] Avouac | Active thrusting and folding along the northern Tien Shan and late Cenozoic rotation of the Tarim relative to Dzungaria and Kazakhstan[END_REF][START_REF] Métivier | Northeastward growth of the Tibet plateau deduced from balanced reconstruction of two depositional areas: The Qaidam and Hexi Corridor basins, China[END_REF][START_REF] Van Der Woerd | Kinematic Coupling between Active Strike-Slip and Thrust Faults in Northeast Tibet[END_REF][START_REF] Van Der Woerd | Rapid active thrusting along the northwestern range front of the Tanghe Nan Shan (western Gansu, China)[END_REF][START_REF] Poisson | Holocene hydrological changes inferred from alluvial stream entrenchment in north Tian Shan (northwestern China)[END_REF][START_REF] Hubert-Ferrari | Mechanisms of active folding of the landscape (southern Tian Shan, China)[END_REF][START_REF] Guilbaud | Kinematics of active deformation across the western Kunlun mountain range (Xinjiang, China) and potential seismic hazards within the southern Tarim basin[END_REF].

Making the right assumptions for assigning parameters to estimate shortening rate is challenging (Figure 3; see also, for instance, [START_REF] Suppe | Geometry and kinematics of fault-propagation folding[END_REF]Lave and Avouac, 2000;[START_REF] Thompson | Late Quaternary slip rates across the central Tien Shan, Kyrgyzstan, central Asia[END_REF]Hu et al., 2015). Here, we use the topographic profiles (Figure 5) and ages (Figure 10) of 4 incrementally folded terraces together with both a changing base level with time (slope decrease) and a changing décollement dip (from flat to dipping) (Figures 3 and12). The 2 proposed endmember models capture a large range of the variability in shortening rate estimates (0.4 to 1 mm/yr, Table 6) although, as explained above, these rates might still be minimum rates (Figures 2,13,14).

Link with long-term cumulative shortening and exhumation

Accelerated range growth between 13.7 Ma and 9 Ma inferred from magnetostratigraphy [START_REF] Sun | Tectonic uplift in the northern Tibetan Plateau since 13.7 Ma ago inferred from molasse deposits along the Altyn Tagh Fault[END_REF] is correlated with the deposition of coarse conglomerate deposits along Tiejangou (see location in Figure 2). Provenance change and lag-time variation of detrital apatite fission-track data in the same section corroborate a tectonic pulse at 14 Ma [START_REF] Lin | Detrital apatite fission track evidence for provenance change in the Subei Basin and implications for the tectonic uplift of the Danghe Nan Shan (NW China) since the mid-Miocene[END_REF]. Consistently, apatite fissiontrack and apatite (U-Th)/He data from three bedrock transects in the western DHNS (see location in Figure 2) suggest a period of rapid exhumation of approximately 0.1 to 0.3 mm/yr from 16 to 9 Ma [START_REF] Yu | Miocene Range Growth Along the Altyn Tagh Fault: Insights From Apatite Fission Track and (U-Th)/He Thermochronometry in the Western Danghenan Shan, China[END_REF].

Balanced geological sections indicate cumulative shortening of 10 -20 km by thrusts F0 and F1, based on structural constraints along several river cuts with an onset of shortening likely at 9 -14 Ma based on the age of the uppermost coarse conglomerate [START_REF] Gilder | Oligo-Miocene magnetostratigraphy and rock magnetism of the Xishuigou section, Subei (Gansu Province, western China) and implications for shallow inclinations in central Asia[END_REF][START_REF] Van Der Woerd | Rapid active thrusting along the northwestern range front of the Tanghe Nan Shan (western Gansu, China)[END_REF]. The average shortening rate is thus estimated at 1.5 ± 0.7 mm/yr since the late Miocene. This estimate may indicate a near-constant shortening rate since 14 Ma when compared to our late Pleistocene rate. In addition, it indicates that F1 and F2 accommodate most of the shortening responsible for the present-day uplift of the western DHNS.

Conversely, it may suggest that F0 or the southern DHNS thrust do not accommodate much shortening, although more work is probably needed on these mostly bedrock faults to assess their slip history. 6.4. Slip accommodation at the Subei junction: geometric and kinematic 3D complexities

The fault geometry in 3D

The Subei junction is particularly complex and involves numerous active faults, including strike-slip faults, thrusts, and normal faults (e.g., [START_REF] Métivier | Northeastward growth of the Tibet plateau deduced from balanced reconstruction of two depositional areas: The Qaidam and Hexi Corridor basins, China[END_REF]. The N70°E striking north strand of the ATF (NATF) connects the Aksay section with the Shibao Cheng section to the northeast (Figure 1). West of Subei, the N135°E-striking and south-dipping thrusts of the western DHNS veer to the west and connect with the NATF. Although the NATF can be traced continuously, thrusting also occurs north of it but west of the junction (e.g., [START_REF] Van Der Woerd | Rapid active thrusting along the northwestern range front of the Tanghe Nan Shan (western Gansu, China)[END_REF]. East of the junction and north of NATF, normal faults run parallel to the ATF for 20 km across the Subei basin and Dang He (Figures 2 andS2). Slip transfer from NATF to the DHNS, and rigid block kinematics explain the extensional component northeast of the junction (e.g., [START_REF] Métivier | Northeastward growth of the Tibet plateau deduced from balanced reconstruction of two depositional areas: The Qaidam and Hexi Corridor basins, China[END_REF]. In the long term, these kinematic interactions also explain the long-lived Subei trough along the NATF.

The geological and topographical continuity of the arcuate shape of the western DHNS range, underlined by the continuous trace of the south strand of the ATF (SATF), testifies to the time-progressive accumulation of deformation both by strike-slip faulting along the ATF and shortening across the DHNS range. This long-term coupling of strike-slip faulting and thrusting accommodated the strain at the margin of the Tibet plateau contributing to continental plateau growth and widening (e.g., [START_REF] Métivier | Northeastward growth of the Tibet plateau deduced from balanced reconstruction of two depositional areas: The Qaidam and Hexi Corridor basins, China[END_REF][START_REF] Tapponnier | Oblique Stepwise Rise and Growth of the Tibet Plateau[END_REF][START_REF] Van Der Woerd | Kinematic Coupling between Active Strike-Slip and Thrust Faults in Northeast Tibet[END_REF][START_REF] Van Der Woerd | Rapid active thrusting along the northwestern range front of the Tanghe Nan Shan (western Gansu, China)[END_REF]. The two sub-parallel, arcuate ranges of Guai Zhang Shan (GZS) and DHNS, with their contrasted morphology, further depict this long-term coupling. GZS summits are only slightly below the peaks of DHNS, although it is not bounded by active thrust faults. On the contrary, valleys and mountain slopes merge gently with the surrounding sedimentary basin fills, indicating no tectonic activity but passive erosion. We thus infer that GZS grew in the early Miocene and stopped growing as strain stepped northeastward along the ATF (Figure 12). Describing the full 3D geometry of the Subei junction is beyond the scope of this study, and we only stress part of this complexity to address the difficulty of balancing the kinematics across the junction. Figure 13. Simplified tectonic section across western Danghe Nan Shan with possible fault geometry at depth (see location of faults and section in Figures 1 and2). Thrust fault north of Guai Zhang Shan (black dashed line) is inferred. Blue labels present fault strikes.

Although the structural link in map view is clear between the thrusts F0', F0 and F1 with the N70°E striking ATF, how thrusts faults that root onto shallow dipping ramps and décollement at the crustal level (e.g., [START_REF] Métivier | Northeastward growth of the Tibet plateau deduced from balanced reconstruction of two depositional areas: The Qaidam and Hexi Corridor basins, China[END_REF] are linked with subvertical lithospheric scale strike-slip fault (e.g., [START_REF] Wittlinger | Teleseismic imaging of subducting lithosphere and Moho offsets beneath western Tibet[END_REF] is not straightforward. Our mapping (Figures 1 and2) and section (Figure 13) suggest a transition from strike-slip to thrusting as faults progressively deviate from the N70°E direction of the main ATF and become perpendicular to the main stress direction (N45°E) as depicted in thrust focal mechanisms (Figure 1) or the average geodetic displacement field direction [START_REF] Wang | Present-day crustal deformation of continental China derived from GPS and its tectonic implications[END_REF]. Southeast of Subei, other strikeslip faults emerge (YDS or YNS) at the eastern fold tip implying kinematic links and slip transfer from thrusting to strike-slip faulting (Figures 2, 13 and 14).

Slip accommodation by the western Danghe Nan Shan

If the amount of shortening along the DHNS results entirely from slip transfer from the north strand of ATF, then a similar amount of left-slip rate is lost for the north strand of ATF eastward from the Subei junction. Our estimated shortening rate on fault F1 is approximately 0.5 mm/yr (Table 6), and the total, including fault F2 and possible additional slip on F0, could amount to 1 mm/yr. There is no doubt that the rate of slip of the north strand of the ATF decreases toward the east (e.g., [START_REF] Xu | Late Quaternary sinistral slip rate along the Altyn Tagh fault and its structural transformation model[END_REF][START_REF] Wang | Present-day crustal deformation of continental China derived from GPS and its tectonic implications[END_REF][START_REF] Xu | Slip Rate of the Danghe Nan Shan Thrust Fault from 10Be Exposure Dating of Folded River Terraces: Implications for the Strain Distribution in Northern Tibet[END_REF]Figure S2), however, where and how the slip is distributed remains difficult to constrain.

If all the left-slip displacement along the ATF is absorbed in the Qilian Shan region in shortening or left-lateral faulting [START_REF] Tapponnier | Active thrusting and folding in the Qilian Shan, and decoupling between upper crust and mantle in northeastern Tibet[END_REF][START_REF] Peltzer | Magnitude of late Quaternary leftlateral displacements along the north edge of Tibet[END_REF][START_REF] Burchfiel | Intracrustal detachment within zones of continental deformation[END_REF][START_REF] Métivier | Northeastward growth of the Tibet plateau deduced from balanced reconstruction of two depositional areas: The Qaidam and Hexi Corridor basins, China[END_REF][START_REF] Tapponnier | Oblique Stepwise Rise and Growth of the Tibet Plateau[END_REF] and the knowledge of slip rate on each fault is known, one should be able to build an equilibrated kinematic budget between slip on the ATF and the other faults to the southeast (Figure 14). However, slip rates on faults are variably documented in the Qilian Shan region (Figure S2, Table S1).

Figure 14 suggests an overall slip-rate decrease along ATF from about 10 mm/yr west of Aksay to a few mm/yr east of the Qilian Shan junction at 97°E. At the Subei junction, our shortening rates and published rates on adjacent faults may point to kinematic incompatibilities. On one hand (Figure 14a), if indeed YDS and YNS faults connect southeast of thrusts F1 and F2, our maximum total shortening of 1 mm/yr is low compared to the published rates on YDS (2 -3 mm/yr;[START_REF] Luo | Slip Rate of Yema River-Daxue Mountain Fault since the Late Pleistocene and Its Implications on the Deformation of the Northeastern Margin of the Tibetan Plateau[END_REF] and YNS (3 mm/yr; [START_REF] Zheng | 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[END_REF], as well as shortening along DXS (1-3 mm/yr;[START_REF] Métivier | Northeastward growth of the Tibet plateau deduced from balanced reconstruction of two depositional areas: The Qaidam and Hexi Corridor basins, China[END_REF][START_REF] Luo | Slip Rate of Yema River-Daxue Mountain Fault since the Late Pleistocene and Its Implications on the Deformation of the Northeastern Margin of the Tibetan Plateau[END_REF] (Figure 14a). The rates on YDS and YNS may suggest additional connection to the ATF in addition to thrust F1 and F2, which might be found in thrust faults within the western DHNS, mapped as faults F0' and F0 (Figures 2 and13). Thrusts faults F0' and F0 cut through western DHNS bedrock and may have been overlooked. On the other hand (Figure 14b), rates along YDS and YNS may be overestimated and no additional slip transfer from faults F1 and F2 is needed.

In this case, the ATF slip-rate east of Subei remains larger and more slip is transferred in the junctions eastward, at Shibao Cheng/Changma and Qilian (Figures 1 and14).

Complex fault systems are subject to slip variations as faults interact in time and space during their seismic cycle (e.g., [START_REF] Dolan | Extreme multi-millennial slip rate variations on the Garlock fault, California: Strain supercycles, potentially time-variable fault strength, and implications for system-level earthquake occurrence[END_REF][START_REF] Matrau | Late Pleistocene-Holocene Slip Rate Along the Hasi Shan Restraining Bend of the Haiyuan Fault: Implication for Faulting Dynamics of a Complex Fault System[END_REF]Bennet et al., 2004). Building a kinematic budget as proposed in Figure 14 implies that rates are known over similar temporal periods or that their rates remain constant over longtime scale. Very few rates are determined for time scales larger than the last 10 ka (Table S1). Knowing that the return time of large magnitude 7 -8 earthquake in the region is on the order of a few 1000 years (Figure 1; e.g., [START_REF] Luo | Spatial and temporal distribution of earthquake ruptures in the eastern segment of the Altyn Tagh fault, China[END_REF], makes these rates highly dependent on their seismic cycle. Discussing variations in slip-rate in space and time, even along the ATF that is the most documented fault in the area, remains challenging.

Overall, the rate we found along the western DHNS near Subei is comparable to the rate estimated eastward along the same range in Yanchi Wan (Figure 1; [START_REF] Xu | Slip Rate of the Danghe Nan Shan Thrust Fault from 10Be Exposure Dating of Folded River Terraces: Implications for the Strain Distribution in Northern Tibet[END_REF]. It is also comparable to other rates along the northern Qilian Shan using similar approaches (e.g., [START_REF] Tapponnier | Active thrusting and folding in the Qilian Shan, and decoupling between upper crust and mantle in northeastern Tibet[END_REF]Hetzel et al., 2004;Hu et al., 2015;[START_REF] Yang | Active fold deformation and crustal shortening rates of the Qilian Shan Foreland Thrust Belt, NE Tibet, since the Late Pleistocene[END_REF]Wang et al., 2020), which indicates that the NE-SW directed shortening in the DHNS, together with other thrust faults, contributes to overall crustal shortening in the Qilian Shan region. Figure 14. Schematic kinematic budget along ATF (see Figure S2, and Table S1). Width of color band is rate. Red bars are shortening rates connected to strike-slip faults. (a) Published rates on YDS and YNS are larger than our determined rates on F1 and F2. Possibly, rates of F0 and F0' are underestimated. (b) Rates on YDS and YNS are overestimated as well as the shortening rates along DXS and YNS. Strikeslip rate is larger on ATF east of Subei, and shortening rates larger on TLS and QS. NATF: north strand of Altyn Tagh fault, SATF: south strand of Altyn Tagh fault, SDNS: south Danghe Nan Shan thrust, WDNS: western Danghe Nan Shan thrust, CF: Changma fault, TLS: Tuolai Shan thrust, NQS: Northern Qilian Shan thrust, DXS: Daxue Shan thrust, YNS: Yema Nan Shan thrust.

Seismic hazards

Evaluating seismic hazards in the Subei area becomes critical due to population growth and the new traffic line across the Danghe Nan Shan thrust and the Altyn Tagh fault (Figure 2). There is no historical record of large earthquakes in the Subei basin related to faulting along the northern DHNS thrust. However, multiple 0.6 to 2.7 m-high fault scarps across the fans in front of the frontal folded hills (e.g., site 3 of van der [START_REF] Van Der Woerd | Rapid active thrusting along the northwestern range front of the Tanghe Nan Shan (western Gansu, China)[END_REF]; site 2 of this study; Figure 2) suggest several surface rupturing events in the Holocene. A paleoseismic trench across one of these scarps on a lower terrace by [START_REF] Wu | Investigation of Late Quaternary Activity along Subei Fault[END_REF] shows evidence of slip on a 20 -25° dipping fault plane and ~1.7 m vertical offset after ~4.2 ka B.P. The ~1.5 m high scarp across the same terrace has been suggested to result from one seismic event, while the 2.6 -3.3 m cumulative vertical offset of this terrace, approximately twice the height of the scarp, implies a record of two earthquakes [START_REF] Van Der Woerd | Rapid active thrusting along the northwestern range front of the Tanghe Nan Shan (western Gansu, China)[END_REF]. Although no evidence of a penultimate event was described in the trench [START_REF] Wu | Investigation of Late Quaternary Activity along Subei Fault[END_REF], we cannot rule out the possibility of the occurrence of one older event in the frontal ramp and folding in the hanging wall.

At site 2, similar 1.5 -3.6 m scarp heights are observed across various terraces (Figures 5,6). The young terraces (pf2 and pf3) show smaller offsets (1.5 m) than the older terraces (pf4-pf5 with offsets up to 3.5 -3.6 m), suggesting the occurrence of two events, with the latest event occurring after emplacement of the youngest terrace.

The ages of these two events are loosely constrained by limiting ages at site 2. In the trench, a radiocarbon sample under the younger event horizon provides an age of 8.1 ka for the loess unit (U1). The exposure age of the terrace defines a maximum age of 16.2 for the gravels unit (U2). We then can bracket the occurrence time of the older event between 16.2 and 8.1 ka and the younger event after 8.1 ka. According to the ages of these events, it is possible that the younger event at site 2 correlates with the recent event at site 3. Earthquake magnitude can be estimated by the empirical relationship of rupture parameters (e.g., [START_REF] Wells | New empirical relationships among magnitude, rupture length, rupture width, rupture area, and surface displacement[END_REF]. There is no strong evidence indicating that sites 2 and 3 (15 km apart) along two different structures (F1 and F2) have ruptured together during the same seismic events. The sizes of the minimum slip events at both sites, however, are compatible with concomitant 20 km-long ruptures.

It is thus tempting to suggest a minimum magnitude of 6.5 -7 (1 -2 m of slip over 20 km).

We suggest that both thrusts F1 and F2 merge into one ramp and décollement at depth (Figure 13). Mapping of the surface trace of the thrusts and the deformed terraces and fans in their hanging wall constrain in part the geometry of the ramp and décollement at depth (e.g., [START_REF] Métivier | Northeastward growth of the Tibet plateau deduced from balanced reconstruction of two depositional areas: The Qaidam and Hexi Corridor basins, China[END_REF][START_REF] Stockmeyer | Seismic hazards of multisegment thrust fault ruptures: Insights from the 1906 Mw 7.4-8.2 Manas, China, earthquake[END_REF]. The combination of co-seismic thrust scarp measurements and the cumulative deformation of the thrusts makes it possible to suggest the occurrence of events with multi-fault ruptures. Taking the length of the thrust system (20 km), the total width of the ramps and décollement above a depth of 15 km (20 km), and a total slip at depth of 4 m distributed on F1 and F2 would result in a seismic moment of 7.9x10 19 N.m with the shear modulus of 3.3x10 10 N/m -2. Such a moment is compatible with a moment magnitude of Mw=7.2 (e.g., [START_REF] Kanamori | The energy release in great earthquakes[END_REF].

Complex ruptures on multiple faults are not uncommon, as described above. In Subei, thrust faults F1 and F2 are connected with strike-slip faults to the west and to the east (see Figures 2 andS2). These connected faults could rupture together similarly to the 2016 Mw7.8 Kaikōura complex earthquake rupture (e.g., [START_REF] Klinger | Earthquake damage patterns resolve complex rupture processes[END_REF]. Previous paleoseimic investigations show that temporally consistent events ruptured on these faults [START_REF] Zhang | Late Quaternary and present-day rates of slip along the Altyn Tagh Fault, northern margin of the Tibetan Plateau[END_REF]He et al., 2018;[START_REF] Luo | Spatial and temporal distribution of earthquake ruptures in the eastern segment of the Altyn Tagh fault, China[END_REF]. For instance, both the Subei-Old Aksay section of the north strand of Altyn Tagh fault, the Yema-Daxue Shan fault ruptured in 6.3 -6.8 ka [START_REF] Zhang | Late Quaternary and present-day rates of slip along the Altyn Tagh Fault, northern margin of the Tibetan Plateau[END_REF]He et al., 2018), and the east section (east of Subei) of the north strand of Altyn Tagh fault also ruptured in 6.4 -7.2 ka [START_REF] Luo | Spatial and temporal distribution of earthquake ruptures in the eastern segment of the Altyn Tagh fault, China[END_REF]. The section from Old Aksay to Shibao Cheng of the north strand of Altyn Tagh fault ruptured in 2.6 -2.8 ka, which is consistent with the most recent event of the Danghe Nan Shan thrust. While the date uncertainties may only indicate close in time ruptures, it is not possible to rule out simultaneous ruptures on several faults. Thus, if combined ruptures involving both the thrust system and strike-slip faults are considered, then our magnitude estimation above is a minimum.

One such earthquake could potentially damage local infrastructure. More structural and paleoseismic investigations involving all the active faults, are needed to assess the seismic hazard of the complex fault system in and around the Subei basin.

Conclusion

Based on field investigations and high-accuracy topographic surveys of multiple folded terraces along the western DHNS, we constrain the cumulative and recent activity of two thrusts. Considering 2D mass balance across the fold and thrust belt, with either a flat décollement or a dipping décollement allows us to constrain the amount of accumulated shortening. Integrating the terrace ages constrained by cosmogenic nuclides, OSL, and 14 C dating methods, we estimate a minimum shortening rate of 0.8 ± 0.2 mm/yr for the northern thrusts of the western DHNS for the last 130 ka. This rate is also compatible with the long-term shortening estimate, which implies that the shortening rate across the DHNS has remained constant since the upper Miocene. Our results indicate that the NE-SW-directed shortening along the western DHNS thrusts contributes to slip-rate decrease along the main ATF and sliptransfer to other thrust and strike-slip faults to the east. Evidence of surface seismic ruptures and large earthquakes is clear from scarps and trenches along the western DHNS. We suggest the possible occurrence of Mw7+ earthquake southwest of Subei.

Considering the structural links between the thrust system of the western DHNS and the strike-slip faults of ATF to the northwest or Yema-Daxue Shan to the southeast, earthquake ruptures may involve both strike-slip and thrust faults allowing for larger magnitude events.

  used high-resolution topographic data and trenches to describe a late event that ruptured between A.D. 1289 and 1395, with horizontal shortening of ∼1.3 m, inferred to be the A.D. 1289 event that occurred during the Yuan Dynasty according to historical records. Along the Yema-Daxue Shan fault, He et al. (2018) excavated three paleoseismic trenches and found evidence of three events since 7000 a B.P., with the latest event occurring after 2010 ± 30 a B.P. Recent seismicity in the western Qilian Shan region is mostly characterized by moderate earthquakes (Figure 1). For instance, nine earthquakes with magnitudes of M4 -6 have occurred along the DHNS since 1960. The focal mechanisms of these earthquakes are both thrust and strike-slip. A M5.5 earthquake occurred recently on 26 Aug. 2021 with an epicenter along the southern piedmont of DHNS. Its focal mechanism is similar to the Mw5.4 1980 event and, given the uncertainty in hypocenter depth and location, compatible with faulting along north-dipping thrusts subparallel to the range. The largest known event in the area is the 25 December 1932

  Figu corr dips

4. 1 .

 1 The geometry of uplifted and folded alluvial surfaces4.1.1. Site 1Southwest of Subei, thrust F1 sharply disrupts the bajada to form a NW-SE trending topographic scarp. Upstream of the scarp, uplifted reaches of late Pleistocene strath terraces are deposited unconformably above Tertiary reddish sandstones and conglomerates. Variably thick loess covers the sediments of the terraces. Two to three kilometers further southwest, the terraces are overthrust by the steeply faceted Paleozoic bedrock of the DHNS along thrust F0 (Figure2; van der[START_REF] Van Der Woerd | Rapid active thrusting along the northwestern range front of the Tanghe Nan Shan (western Gansu, China)[END_REF]. As a response to recent and active uplift, numerous streams have grown into subparallel catchments of relatively small size, deeply incising across the uplifted ledge and depositing alluvial fans in front of thrust F1. While older deposits are preserved upstream as narrow terrace remnants, downstream, younger deposits have buried the older sediments[START_REF] Van Der Woerd | Rapid active thrusting along the northwestern range front of the Tanghe Nan Shan (western Gansu, China)[END_REF].Site 1 is located in the central part of thrust F1 along a stream of one of the largest catchments along the range front (stream r5 in van der[START_REF] Van Der Woerd | Rapid active thrusting along the northwestern range front of the Tanghe Nan Shan (western Gansu, China)[END_REF]. A set of four terraces (T1 to T4) are preserved upstream of F1, whereas on the downstream side, only one main fan level (Tf2) is present (Figure4). To constrain the geometry of uplifted and folded terraces upstream of F1, a set of topographic profiles has been leveled along different terraces on the western bank of stream r5 (Figure4).All profiles are projected in a N45°E direction perpendicular to the average N135°E strike of thrust F1 in Figure5.

  Figure 4. Uplifted and folded terraces along fault F1 at site 1. (a) ZY3 satellite image of site 1. (b) Geomorphic interpretation from image in A and field observations. South of main thrust fault F1, four main terrace levels T1 to T4 are abandoned by stream r5. Dashed and dotted lines are topographic profile locations acquired with total station (van der Woerd et al., 2001) and DGPS (this study), respectively (see Figure 5). Triangles are pit locations for cosmogenic isotope depth profile dating. Star is charcoal sample location in van der Woerd et al. (2001). (c) Field view to north of uplifted and folded terraces at site 1 south of Subei. Terraces are loess covered several-meter-thick greyish conglomerate unconformable on red-orange south dipping

Figure 5 .

 5 Figure 5. Uplift measurements at site 1. (a) N45°E projected topographic profiles (see location in Figure 4, same labels). Terrace heights measured above active stream bed T0. Main thrust position inferred. (b) Detail of folded terraces at ledge front. Fan Tf2 in footwall corresponds to T2 in the hanging wall. Terrace maximum heights measured above active stream bed T0 in hanging wall and above Tf2 in footwall.

Figure 6 .

 6 Figure 6. Geomorphic interpretation of site 2. (a) ZY3 satellite image of site 2. (b) Geomorphic map. Black outline is drone based Lidar survey in (c). Dang He has abandoned a set of terraces T1, T1', T2, T2' and T3 as it leaves its gorge across southwest Yema Shan. Tf2 and Tf3 are alluvial fans from smaller tributaries. Terraces and fans are cut and offset by active thrust F2 (in red). (c) High resolution (0.5 m) DEM from drone based Lidar survey. Dashed lines are topographic profiles in (d) and (e). Solid lines represent terrace riser tops, ticks are on lower side. (d) Topographic profile across F2 scarps. Piercing lines (dashed) fit far field slopes of surfaces. (e) Left, topographic profiles extracted from ~6 m resolution DEM, and right, from drone DEM, located in (b) and (c), respectively.

Figure 7 .

 7 Figure 7. Field views of faulted alluvial fans and terraces by thrust F2 at site 2 (See Figure 6 for locations). Small arrows indicate top of fault scarp. Person is ~1.7 m tall.. (a)-(c) ~1 -2 m-high scarp west of Dang He. (c) ~2.6 m-high scarp across Tf2. (d)

Figure 8 .

 8 Figure 8. Trench walls photo mosaic (a, c) and interpretation (b, d) at Site 2 (see location in Figure 5). East wall (c and d) is mirrored. Three main units U1, U2 and U3 are faulted. Solid and dashed thin black lines are minor layers. Red lines f1, f2 and f3 are shallow dipping faults, dashed when inferred. Offsets of units are indicated in green. Charcoal sample and calibrated age is indicated (Table4). See text for unit details.

Figure 9 .

 9 Figure 9. Field views of terraces and sampling sites at site 1 (locations of views and pits in Figure 4). (a) Westward view across T1 and T3. Position of depth profile in top 2 m of a loose conglomerate in T1, 7 m above the river bed. Orange colored outcrops below unconformable grey surface conglomerate are south-dipping Tertiary red beds. (b) Close-up view of depth profile in T1. Sandy gravels and pebbles are collected every 20 cm along the profile. (c) Pit dug in terrace T2. Conglomerate is capped by 145 cm-thick loess deposit. (d) Landscape view taken from upper terrace T4 eastward across site 1, Subei county in background left, and site 2 to the right. Location of pit dug in T3 indicated. (e) View of sampling pit in T3 with top 130 cm-thick loess, separated from bottom conglomerate by a reddish silty soil layer. (f) 3 m-deep pit in terrace T4. The top 45 cm-thick layer is made of loess covering a few cm-thick reddish silty soil.

Figure 10 .

 10 Figure 10. Depth profile description, sample type and position, and 10 Be age models for T1, T2, T3, and T4 at site 1. (a), (c), (e), and (g) are simplified stratigraphic logs with sample positions and average layer densities. (b), (d), (f), and (h) are 10 Be depth profile models "d" stands for density in g/cm 3 . See also alternative models in Figures S3 -S6.

  Figure 10. Depth profile description, sample type and position, and 10 Be age models for T1, T2, T3, and T4 at site 1. (a), (c), (e), and (g) are simplified stratigraphic logs with sample positions and average layer densities. (b), (d), (f), and (h) are 10 Be depth profile models "d" stands for density in g/cm 3 . See also alternative models in Figures S3 -S6.

Figure 2 .

 2 Figure 2.

  Figure 3.

  Figure 4.

  Figure 5.

  Figure 6.

  Figure 7.

  Figure 8.

  Figure 9.

Table 2

 2

	), indicating loess deposition since

Table 1 .

 1 Terrace heights and vertical offset estimates

	Terrace	Profile	Height above	Maximum	Vertical offset relative to
			river bed	height	downstream fan
			upstream (m)	above	(m)
				river bed
				(m)
	Site 1				Relative to Tf2
	T1	b-b'		7.5±0.5	-
	T2	p3/c-c'	18±1	23±1	15±1
	T3	p4/d-d'	27±2	40±2	31±2
	T4	P2/e-e'	50±3	60±3	50±3
	Site 2
	T1	pf2			1.5±0.2
	T1'	pf3			1.6±0.2
	T2'	pf4			3.5±0.5
	Tf2	pf1 pf5			2.6±0.3 3.6±0.5

Table 3 .

 3 10 Be analytical data for depth profiles at site 1 and surface boulders at site 2 1326

	Samples	Mass quartz	Be Carrier	Latitude	Longitude	Altitude Depth Thickness Sample	10Be/9Be	10Be/9Be error	10Be ± error	Exposure age ± uncertainty
		(g)	(mg)	(N)	(E)	(m)	(cm)	(cm)	type		(%)	(atom/g)	(yr)
	Site 1
	T1 depth
	profile
	T101	19.082	0.259	39.4644	94.8154	2550	4	5	gravel	1.99743E-13	2.84493542	139215±3966	See model
	T102	18.3446	0.2622	39.4644	94.8154	2550	23.5	5	gravel	2.34437E-13	3.16617704	179736±5698	/
	T103	24.6633	0.2619	39.4644	94.8154	2550	38.5	5	gravel	2.79777E-13	2.81553863	165707±4672	/
	T104	23.5114	0.2604	39.4644	94.8154	2550	66.5	5	gravel	2.11103E-13	3.21317004	122006±3925	/
	T105	16.2753	0.2628	39.4644	94.8154	2550	112	5	gravel	1.58258E-13	4.50561624	120855±5453	/
	T106	19.9296	0.2597	39.4644	94.8154	2550	162	5	gravel	1.06914E-13	4.78682755	52823±2532	/
	T107	24.7446	0.2595	39.4644	94.8154	2550	202	5	gravel	1.63803E-13	3.32730129	82378±2745	/
	T2 depth
	profile
	T201	26.985	0.2572	39.46584	94.81676	2543	146.5	5	gravel	2.88702E-13	2.80433717	156347±4390	See model
	T202	18.6568	0.2644	39.46584	94.81676	2543	160	5	gravel	2.16697E-13	2.79024612	164281±4590	/
	T203	29.7611	0.2625	39.46584	94.81676	2543	192.5	5	gravel	1.96218E-13	10.6339378	90175±9602	/
	T204	24.0209	0.2505	39.46584	94.81676	2543	221.5	5	gravel	2.47250E-13	3.86466734	150008±5805	/
	T207	29.7902	0.2592	39.46584	94.81676	2543	342.5	5	gravel	1.57479E-13	4.58115034	72964±3347	/
	T3 depth
	profile
	T301	30.002	0.2603	39.46587	94.81474	2571	132.5	5	gravel	1.79830E-12	5.06926063 1019912±51771	See model
	T302	30.1499	0.2621	39.46587	94.81474	2571	162.5	5	gravel	1.77618E-12	1.48973645 1009077±15053	/
	T305	30.1379	0.261	39.46587	94.81474	2571	252.5	5	gravel	5.58156E-13	7.55396228	300382±22721	/
	T4 depth
	profile
	T402	30.0074	0.265	39.46608	94.81159	2616	68.5	5	gravel	3.13757E-12	1.77849721 1831441±32615	See model
	T403	31.7724	0.2616	39.46608	94.81159	2616	98.5	5	gravel	3.08259E-12	1.3182097 1677233±22139	/
	T404	29.6724	0.2619	39.46608	94.81159	2616	131.5	5	gravel	1.98844E-12	1.77949615 1152668±20539	/
	T405	31.3068	0.2554	39.46608	94.81159	2616	161	5	gravel	1.25525E-12	4.89009314	665693±32596	/
	T406	30.4697	0.2693	39.46608	94.81159	2616	191	5	gravel	1.29302E-12	3.78535521	743514±28182	/
	T407	32.4745	0.2627	39.46608	94.81159	2616	221	5	gravel	5.47390E-13	2.53345878	277465±7039	/
	Site 2
	surface
	LP01	30.7098	0.308	39.47123	94.90786	2265	0	2.5	boulder 1.02453E-12	6.32872596	670639±42500	28133±3040
	LP02	30.2138	0.3154	39.47112	94.90761	2265	0	2.5	boulder 5.48760E-13	5.84434137	366152±21428	15826±1661

Acknowledgments

This work is jointly funded by the National Key Research and Development Program of China (2017YFC1500100), the Second Tibetan Plateau Scientific Expedition and Research Program (STEP) (2019QZKK0901), and the National Natural Science Foundation of China (41802228, U1839203), the Basic Research Project of Lanzhou Institute of Seismology (2018IESLZ01). YS has been partly supported by a grant from K.C. Wong Foundation and CNRS. We thank ASTER Team (D. Bourlès, G. Aumaître, Vincent Godard, Régis Braucher and K. Keddadouche) at ASTER-CEREGE for 10 Be AMS measurements. ASTER AMS national facility (CEREGE, Aix-en-Provence, France) is supported by INSU/CNRS, ANR through "Projets thématiques d'excellence" program for "Equipements d'excellence" ASTER-CEREGE action, and IRD.

Data Availability Statement

The historical earthquakes are from the China Earthquake Data Center (CEDC, http://data.earthquake.cn/index.html). The raw datasets of DEM derived from satellite images (https://doi.org/10.5281/zenodo.7045176) and LiDAR point cloud (https://doi.org/10.5281/zenodo.7045169) are shared in zenodo.org, and as well as the terrace survey data of site 1 (https://doi.org/10.5281/zenodo.7070457). The focal mechanisms come from the Global Centroid Moment Tensor project (CMT; http://www. globalcmt.org).

He Wen-gui, Zhang Bo, Wu Ming, Wang Peng-tao, Zou Xiao-bo, Gao Xiao-dong. (2018). Paleoseismology on the Yemahe segment of the Yemahe-Daxueshan fault revealed by trench study. Seismology and Geology, 40 (1): 261-275. Hetzel, R. (2013). Active faulting, mountain growth, and erosion at the margins of the Tibetan Plateau constrained by in situ-produced cosmogenic nuclides.

Tectonophysics, 582, 1-24. https://doi.org/10.1016/j.tecto.2012.10.027 Hetzel, R., Tao, M., Stokes, S., Niedermann, S., Ivy-Ochs, S., Gao, B., et al. (2004, December 1). Late Pleistocene/Holocene slip rate of the Zhangye thrust (Qilian Shan, China) and implications for the active growth of the northeastern Tibetan Plateau. https://doi.org/10.1029/2004TC001653 Hetzel, R., Hampel, A., Gebbeken, P., Xu, Q., & Gold, R. D. (2019). A constant slip rate for the western Qilian Shan frontal thrust during the last 200 ka consistent with GPS-derived and geological shortening rates. Earth and Planetary Science Letters, 509, 100-113. https://doi.org/10.1016/j.epsl.2018.12.032 Horton, B. K., Dupont-Nivet, G., Zhou, J., Waanders, G. L., Butler, R. F., & Wang, J. https://doi.org/10.1029/2003JB002913 Hu, X., Pan, B., Kirby, E., Gao, H., Hu, Z., Cao, B., et al. (2015). Rates and kinematics of active shortening along the eastern Qilian Shan, China, inferred from deformed fluvial terraces. Tectonics, 34(12), 2478-2493. Hu, X., Cao, X., Li, T., Mao, J., Zhang, J., He, X., et al. (2021)