Crustal‐Scale Duplex Development During Accretion of the Jiuxi Foreland Basin, North Qilian Shan

Understanding the propagation of shortening, especially the interaction of shallow and deep structural levels in space and time is important to understand the accretion process of a compressional orogen as well as to fully understand earthquake hazards to populated foreland basins. Here we combine evidence from geologic maps and stream‐terrace surveys to construct a set of retrodeformable cross‐sections of the western North Qilian Shan foreland. The uplifted, severely tilted Mesozoic and older rock units suggest the presence of both deep and shallow décollements in western and central part of our research area, and that these structures alternated activity since commencement of the latest phase of the North Qilian Shan uplift. Conversely, in the east, the absence of foreland fold‐and‐thrust belt and the moderately tilted Mesozoic rocks indicate the deformation is dominated by thick‐skinned uplift. Based on our cross‐sections, we estimate the long‐term shortening rate of the Jiuxi foreland basin of 1.2–1.8 m/Kyr. Deformed foreland terraces show that, from west to east in our research area, active deformation switches between different structural levels. This trade‐off between deformation styles in time and space shows that two décollement levels bound a crustal‐scale duplex as the foreland is incorporated into the orogen. We suggest the complex and out‐of‐sequence deformation pattern may relate to pre‐existing weakness within the basement rocks and is likely a common characteristic of the North Qilian foreland. This may impose an additional challenge for seismic hazard estimation of the region.

deformation.Some mountain belts show thick-skinned deformation that follows an initial, thin-skinned episode of shortening (Molinaro et al., 2005), whereas other examples exhibit thick-skinned deformation during the initial shortening stages (Lacombe et al., 2003).This sequencing of events may be difficult to establish, as later shortening will tend to further tilt and refold structures formed in earlier events.
We seek to better understand the interaction of thin-and thick-skinned deformation from the active foreland-basin setting of the Qilian Shan, located in the NE margin of Tibetan Plateau.The Qilian Shan orogen encompasses a region of more than 200,000 km 2 at an average elevation of ∼3 km.Though the Qilian terrane has experienced repeated deformation since the Proterozoic, the topography of Qilian Shan is mostly the result of Cenozoic tectonism, driven by the distant collision between the Indian and Eurasian Plates (Huang et al., 2015;Jolivet et al., 2001;Niu et al., 2003;Ritts & Biffi, 2001;S. Song et al., 2010S. Song et al., , 2013;;Vincent & Allen, 1999;Wu et al., 2017;Zhang et al., 2007;Zuza et al., 2018).Studies of the structural evolution of the Qilian Shan show that major reverse faults within the orogen have accommodated amounts of crustal shortening comparable to that at the range front fault (e.g., Hu, Cao, et al., 2021;Meyer et al., 1998;Shao, 2010;Shao et al., 2019;Xu et al., 2021;Yi et al., 2022).Previous research suggests that the western North Qilian Shan mountain front and its foreland exhibits a variety of shortening patterns and shortening rates along strike.At some locales, reverse fault slip is expressed as displacement both at a mountain front fault and within an adjacent foreland thrust belt (e.g., W. Chen, 2003;Hetzel et al., 2019;S. Yang, 2007;H. Yang et al., 2020).Conversely, in other areas the deformation is solely concentrated near the mountain front (e.g., Hetzel et al., 2006;Y. Wang et al., 2020;D. Zheng et al., 2010;Zuza et al., 2016).
In this study, we define a thin-skinned structure as thrust system deforming above shallow, low angle décollements which usually do not exceed 5-8 km depth, whereas a thick-skinned structure is characterized as steep thrust faults that cut across the upper crust and form large-scale basement uplifts (Pfiffner, 2006).By incorporating information of cumulative deformation from tilted bedrock units and ongoing deformation from stream terrace profiles, we differentiate deformation stages and build a set of permissible cross-sections of the western North Qilian Shan foreland between the Beida River and Baiyang River (Figures 1 and 2).From this analysis, we seek to answer the following questions: (a) What is the regional pattern of crustal shortening in North Qilian Shan and its foreland at present?(b) What is the sequence of deformation events and how do thin-and thick-skinned structures interact in foreland settings?(c) What is the mechanism behind such deformation patterns?

Tectonics
The Qilian Shan is an actively shortening mountain range located between the Qaidam basin to the southwest and the Hexi Corridor to the northeast and bounded by the Altyn-Tagh Fault (ATF) to the northwest (Figure 1).It forms the northeastern margin of the broader Tibetan Plateau.The Qilian terrane records a complete history from continental breakup to ocean basin evolution, followed by ocean-closure and continental collision (Huang et al., 2015;S. Song et al., 2010S. Song et al., , 2013;;Wu et al., 2017;Yin & Harrison, 2000).The Qilian Ocean opened during the Neoproterozoic and closed during the Ordovician (B.Li et al., 2021;Niu et al., 2003;S. Song et al., 2013;Wu et al., 2021;Yin et al., 2008;Yin & Harrison, 2000), while the intensive orogenic activity continued into the late Silurian and early Devonian, resulting in exhumation of ultra-high-pressure rocks in the North Qilian Shan (Fu et al., 2023; X. Y. Song et al., 2006;S. Song et al., 2013;Xiao et al., 2009;Yin & Nie, 1993;Zhang et al., 2007).Pervading south-dipping foliations in Paleozoic and older rocks and seismic fabrics that extend to mid-lower crust, discovered through geological survey and seismic reflection profiles across Qilian Shan, are likely the evidence of the Paleozoic collision and subduction (Figure 2; Gansu Geological Bureau, 1989;Gao et al., 2013Gao et al., , 2022;;Ye et al., 2021;Zuza et al., 2018).
Three ESE-striking major active faults deform the westernmost North Qilian Shan and its foreland.The southernmost is the Changma Fault (F3 in Figure 2), an oblique slip (left-lateral and reverse) fault that bounds an intramontaine basin within range.The left lateral slip-rate increases to the east and reaches its peak around Baiyang River area (Du et al., 2020;Institute of Geology, China Earthquake Administration, 1993;Luo et al., 2013;W. Zheng, 2009;W. Zheng et al., 2013).Further east, in the Beida River area, the fault is inactive at least since the late Pleistocene (Du et al., 2020;Y. Wang et al., 2020).The Hanxia-Dahuanggou Fault (F2 in Figure 2) is the active frontal fault of the North Qilian Shan west of Dahuanggou, which then turns into the range and becomes inactive at the longitude of the Beida River.Field observation suggests the fault dips 40-50° with dominantly reverse slip (Institute of Geology, China Earthquake Administration, 1993; Y. Wang et al., 2020).The Yumen Fault (F1 in Figure 2) is the northernmost main fault strand of the North Qilian Shan.Dipping at 50°, it serves as the frontal fault of the North Qilian Shan east of Yaoquan, with a vertical uplift rate of 1.1 ± 0.3 mm/year (Y.Wang et al., 2020;Y. Wang & Oskin, 2022).To the west, this fault becomes part of the foreland-basin fold and thrust belt, with a vertical uplift rate of 1.0 ± 0.3 mm/year (W.Chen, 2003).
The Hexi Corridor is a ∼200 km wide foreland-basin system formed immediately north of the North Qilian Shan.As the westernmost component of the Hexi Corridor, the Jiuxi Basin is located west of Jiuquan, bounded by the ATF to the west, the Hei Shan to the north, and the North Qilian fault system to the south (Figures 1 and 2).A series of WNW-ESE to E-W striking active thrust faults and fold belts are developed within the basin, which are related to the northward propagation of the North Qilian Shan orogen (Fang, 2005;Hetzel et al., 2006;W. Zheng et al., 2013).Magnetostratigraphy and sedimentology evidence from exposures within the Yumen (Laojunmiao) anticline suggest a ∼3.6 Ma onset time of deformation of the Jiuxi foreland (Fang, 2005;J. Li et al., 2014;C. Song et al., 2001;D. Zheng et al., 2017).
Based on seismic reflection profiles of the Jiuxi Basin, S. Yang (2007) suggests that the western part of the North Qilian hinterland includes multiple detachment levels, and that active deformation is transferring from F2 to F1 into to Hexi Corridor through a shallow décollement, with blind thrust ramps (F1) and fault-related folding of the foreland-basin sediments.Some studies of the foreland thrust belt in both the Jiuxi basin as well as the Jiudong basin (east of Jiuquan) to the east (W.Chen, 2003;Hetzel et al., 2019;H. Yang et al., 2020) also suggest that faulting and folding of the western North Qilian Shan foreland is controlled by shallow structures.Conversely, there is also evidence shows that the shallow décollement beneath the Yumen anticline is no longer active, and that instead uplifted stream terraces indicate deformation above a deeper décollement level beneath the foreland (Hetzel et al., 2006;R. Liu et al., 2017).Similarly, our recent study on fill terraces preserved inside the mountain range along the Beida River revealed a long wavelength (>30 km) fold within the western North Qilian Shan, suggesting that the active range-front structure of the North Qilian Shan should be a steep basement fault that soles into a ∼10° décollement at ∼15-17 km depth (Y.Wang et al., 2020).Such a fault geometry is also consistent with thermochronology and structural modeling of the Jingangsi region, located adjacent to the Jiudong basin (D.Zheng et al., 2010;Zuza et al., 2016).

Stratigraphy
The scope of our study encompasses three areas spanning the transition from the North Qilian Shan to the Jiuxi Basin: Qingtoushan (cross-section I-I′), Yaoquan (II-II′), and Dahuanggou (III-III′) (Figures 2 and 3).The stratigraphic section exposed in the Qingtoushan area is the most complete of the three.The oldest rocks exposed  (Gansu Geological Bureau, 1989;C. Li et al., 2019) and our field survey.Major unconformities are denoted in the stratigraphic column as wavy lines.
here consist of the Pre-Cambrian to Early Paleozoic crystalline rocks, metasedimentary rocks, and island-arc volcanic rocks, overlain by Silurian flysch, Devonian conglomerates, Carboniferous shale with coal interbeds, and Permian continental-marine transitional facies strata.Mesozoic rocks present here consist of Triassic to Cretaceous conglomeratic sandstone units.The Cenozoic sequence in the Jiuxi Basin consists of up to 4 km thickness of non-marine sedimentary rocks deposited since the Late Eocene (Huoshaogou Fm.) through to the present.In the Qingtoushan area, only the upper part of the Oligocene Baiyanghe Fm., the Miocene Shulehe Fm., and the Quaternary Yumen and Jiuquan conglomerates are exposed (Dai et al., 2005;Fang, 2005;Gansu Geological Bureau, 1989;Hsü et al., 1995;J. Li et al., 2014).Further to the east, Jurassic and Cretaceous rocks are not exposed in the Yaoquan area nor in the Dahuanggou area but rocks this age may be present at depth.
Four major unconformities exist in our research area and are important for unraveling its structural history (Figure 2).The oldest lies between Silurian and Carboniferous/Devonian units.This angular unconformity is exposed in Dahuanggou area (III-III′) at the mountain front.The second unconformity is interpreted to lie at the base of Cretaceous rock units.Within the mountainous area south of the Qingtoushan (I-I′), Cretaceous reddish sandstone that dips 25° toward NE unconformably overlies Ordovician greenish slate.Further south, the Cretaceous rocks unconformably overly both Ordovician and Proterozoic rocks.The third major unconformity is located at the base of the Oligocene Baiyanghe Fm.In the Qingtoushan and Dahuanggou area at the mountain front, Oligocene Baiyanghe Fm. unconformably overlies Silurian through Triassic rocks.The youngest unconformity is between the Quaternary sediments (Yumen Conglomerate) and the Baiyanghe Fm.In between Yaoquan (II-II′) and Dahuanggou area, near the mountain front, Quaternary Yumen Conglomerate unconformably overlies Baiyanghe Fm., and the Miocene Shulehe Fm. is missing from this section.Apart from these four major unconformities, there are minor unconformities within the Shulehe Fm., Baiyanghe Fm.Yumen Fm. and other Quaternary conglomerates that are related to the ongoing uplift of the North Qilian Shan (Fang, 2005;J. Li et al., 2014).

Field Mapping
We conducted extensive regional mapping of the North Qilian Shan and Jiuxi Basin in the area between the Beida River and Baiyang River, the two major rivers draining across the North Qilian Shan into Jiuxi Basin along the eastern and western edge of our research area (Figure 2).This mapping refines the China National Digital Geological Map at the 1:200,000 scale (Gansu Geological Bureau, 1989;C. Li et al., 2019).Our surveys were mostly focused on verifying contacts and measuring bedding orientations of the Mesozoic and younger strata that outcrop in the foreland and adjacent hinterland close to the mountain front.
To obtain information of the active deformation of both the foreland and hinterland, we mapped and surveyed uplifted stream terraces and alluvial fan sequences using a laser rangefinder (∼0.3 m distance accuracy, 0.25° inclination accuracy) and differential GPS (Globle Positioning System; 0.1-0.5 m accuracy).We also mapped and extracted terrace elevations and the profile of the Baiyang River from an 8 m resolution digital elevation model (DEM) produced by the Polar Geospatial Center (Shean, 2017).(Dai et al., 2005;Fang, 2005;Gansu Geological Bureau, 1989;J. Li et al., 2014;C. Song et al., 2001).I-I′, II-II′, and III-III′ refer to cross-section locations on Figure 2. Red wavy lines represent unconformities.

Fault Kinematic and Cross-Sections Restoration
The detailed field mapping and surveying were used to produce three NNE-oriented cross sections spaced 7-10 km apart (Figure 1).The subsurface continuation of contact relationships is in accordance with the altitudes and styles of the surface structures and interpreted using geometrical methodologies (e.g., Suppe, 1983;Suppe et al., 1992;Suppe & Medwedeff, 1990), while the small-scale deformation within the fold-thrust belt has been simplified.Orientation and contact relationships of most of the Paleozoic and older bedrock shown in our sections are based on existing geologic maps (Gansu Geological Bureau, 1989).Due to severe erosion, very few Mesozoic or younger strata are preserved in the North Qilian hinterland.We therefore constrain our cross section to the foreland and the mountain front.The cross sections were reconstructed to different stages through un-tilting and unfolding thrust sheets and stratigraphic layers, mainly the Devonian-Jurassic units and Cenozoic basin deposits, and through comparing the deformed strata with terrace profiles.Orientations, cross-cutting relationships, stratigraphic onlap, and repetition of pre-Cenozoic units allows for the restoration of cross sections to the states prior to the Cenozoic orogenic event, while the tilting, thinning, or absence of the Cenozoic basin deposits reveal the kinematic history of the North Qilian and Jiuxi basin since the recent North Qilian uplift.The deformed terrace profiles represent active structural movements at the mountain front, therefore discrepancies between underlying strata and terrace profiles indicate changes of deformation pattern.We did not restore deformation prior to Cenozoic North Qilian uplift, due to erosional removal of Mesozoic strata in the mountain-range interior.Our detailed cross-sections focus on relationships within the upper 8 km, where structures resulting from thin-skinned and thick-skinned deformation overlap and their activity may be inferred from deformed river terraces.We estimate the depth of the deeper-level décollement, where active, using a listric fault-bend-fold model (Hu et al., 2015;Lavé & Avouac, 2000;Y. Wang et al., 2020).Limited by severe erosion and lack of age constraints on key structures, the structural models we presented in this paper are a permissible solution based on our best understanding of the structural relationship and the deformation history of the area.
The shortening of the mountain front since the latest North Qilian uplift is estimated based on the restored cross-section with constant line-length assumption.Due to erosion, the shortening of the hinterland portion of our cross-section is estimated by assuming minimizing fault slip, therefore the total shortening is most likely an underestimation.

Qingtoushan Area (I-I′)
At Qingtoushan we identified an uplift in the foreland where the Devonian-Jurassic rock units are exposed and dip steeply (74-85°) toward the SSW (Figures 4a and 5a).Carboniferous rocks are exposed at the northernmost part of the uplift, which are faulted against Pleistocene-Holocene basin deposits.The exposed bedrock becomes younger toward the south, and at the southernmost part of this area Devonian rocks are juxtaposed against Jurassic rocks along a bedding-parallel fault.The Oligocene and Plio-Pleistocene Baiyanghe and Yumen Fm. partially cover the Carboniferous rocks at the northern end of this area; the Yumen Fm. exposed at the hangingwall dips 20° toward NNE.Further north, at the footwall, Quaternary fluvial sediments dip ∼5° toward NNE.At the southern end of the area, the Baiyanghe Fm. is unconformably deposited upon near-vertical dipping Devonian strata.Here the Baiyanghe Fm. dips ∼30° toward SWS over a distance of ∼500 m, then becomes horizontal to the south.This suggests that even though the Paleozoic and Mesozoic rocks all dip to the south, deformation of younger strata indicates that this part of the section has been deformed into an anticline.Further south, inside the North Qilian Shan, Cretaceous sedimentary rocks unconformably overlap Silurian and Ordovician rocks, whilst juxtaposed against Silurian rocks to the west by an NNE-SSW striking Cretaceous normal fault (Figure 2).
A small river incising next to and parallel to the line of section formed strath terraces preserved on both sides of its valley.Profiles extracted from a differential GPS survey (Transect A of Figures 2 and 4a) indicate that these terraces have been tilted by folding above a fault (F1) underlying the anticline.The relative height between terraces reaches its maximum to the north, near where this blind reverse fault presumably projects to the surface.These terrace levels gradually merge with each other toward the south (Figure 4a).Combining terrace profiles and strata orientations, we suggest the length of this fold backlimb is ∼2.5 km.
Three kilometers to the west, along the Baiyanghe River, the extracted terrace profiles (Figure 6) show that the two most continuous terrace levels (T2 and T3) have been folded.This large-scale fold is 20 km-wide, extending from the foreland to around 12 km into the hinterland, with the fold crest located at the bedrock mountain front (Figure 6d).

Yaoquan Area (II-II′)
Bedrock exposure in Yaoquan area is not as extensive for the other two areas.At the mountain front, the frontal reverse fault (F2′) places a slice of Carboniferous-Triassic units that dip 55° toward WSW against Quaternary fan deposits (Figures 2, 4c, and 5c).These Carboniferous-Triassic units, partially covered by Oligocene Baiyanghe and younger strata, are dissected and duplicated by inactive high angle faults.Further south, Silurian and Ordovician units are uplifted and juxtaposed against the Carboniferous-Triassic rocks along another reverse fault (F2).
In the foreland, the surface trace of the northernmost reverse fault steps northward by ∼2 km compared to the Qingtoushan area, forming a salient (Figures 2 and 4a-4c).This fault, designated F0, offsets Neogene sediments and uplifts Pleistocene alluvial fans and terraces by more than 50 m.Close to the fault (F0), the Oligocene Baiyanghe Fm. is tilted ∼80° toward the SW, then the dip angle quickly reduces to ∼20° southward further away from the fault.The Miocene Shulehe Fm. in this area is tilted to 15-20° toward SSW around 1-2 km south of the fault (Figure 5b).∼2 km south of F0, slip on F1 deforms the Shulehe Fm., forming an anticline which likely is the continuation of the Qingtoushan anticline.Along the trends of the forelimb and backlimb of the Qingtoushan anticline, the Shulehe Fm. is tilted ∼20° toward north and ∼23° toward SE, respectively (Figures 2 and 4b).
We extracted two transects of terrace-elevation profiles using differential GPS on either side of our line of section II-II′ (transect B and C in Figures 2,  4b, and 4c).A major range-front scarp occurs in both profiles at the location of F0.At ∼2-3 km south of the range-front scarp, another scarp is identified where the dip direction of the Shulehe Fm. changes from SE to NW.This fault cuts the T2 terrace, but no offset could be found through the adjacent youngest T1 terrace.This second scarp lies close to the projection of the F1 fault of Qingtoushan and its related fold, evident in transect A. Further east of transect C, we find F0 merges with a branch of F1 (Figures 4c and 4d).
Compared to transect A, folding of fluvial terraces in transects B and C exhibits a wider axial top (∼5 km length), and shorter backlimb (∼1.5 km).The backlimb hinge position lies on the same trend evident adjacent transect A, however the dip angle of the Shulehe Fm. at the backlimb in this section (15-20° and 23°) is smaller than the backlimb dip in Baiyanghe Fm. in section I-I′ (30°).

Dahuanggou Area (III-III′)
In the Dahuanggou area, foliation within the Silurian and Ordovician metasedimentary rocks generally dips 50° toward the SSW.We also identified multiple sets of bedrock faults and small-scale folds within the mountain range as we surveyed transect D and along the Beida River, located ∼4.5 km east of our line of section.However, none of these structures have deformed the fluvial terraces preserved within the mountain range (Figure 4d).At the mountain front, Carboniferous through Triassic rock units generally dip 30°-50° to the SW and are structurally duplicated along a high angle reverse fault crossing bedding at a 20°-30° angle (Figure 5d).This bedrock fault is presently covered by fluvial gravel, the continuous terrace surface indicates the fault is no longer active.These rock units are collectively deformed into a syncline-anticline pair, where the NE limb of the syncline dips ∼50° toward SW and the SW limb of the syncline dips ∼20° toward the NE.The SW limb of the syncline is truncated by a reverse fault that places Silurian rocks directly against the Carboniferous through Triassic rocks (Figures 2 and 4d).The Carboniferous through Triassic units are juxtaposed against Oligocene Baiyanghe Fm. by another presently inactive reverse fault.The Baiyanghe Fm., unconformably overlying Silurian units, is uplifted by the active F1 frontal thrust.
Terrace profiles of transect D indicate that at present, slip occurs only along the frontal fault, F1 (Figure 4d).The ∼1 km long terrace profile shows no signs of tilting or folding within the fault hangingwall.North of the frontal fault F1, several fault scarps of half meter to a meter height cutting across a Late Quaternary alluvial fan surface are identified as branches of the F1 fault (Figure 4d).

Structure of the Foreland Fold-And-Thrust Belt
Dip angles of the Devonian-Jurassic units of the foreland indicate that the Qingtoushan area (I-I′) may have experienced at least two tilting events, whilst Yaoquan (II-II′) and Dahuanggou area (III-III′) may have only experienced one tilting event.In Qingtoushan area, the Devonian-Jurassic units are exposed in the foreland fold-and-thrust belt, ∼80° dipping, structurally duplicated, and unconformably covered by ∼30° dipping Oligocene Baiyanghe Fm. in the backlimb.Restoring the 30° tilting of the Baiyanghe Fm. would also restore the ∼80° dipping Devonian-Jurassic units to 30-50°.This suggests the units were already duplicated and tilted to 30-50° before the Oligocene, and were then eroded and covered later by basin deposits (Figures 7a, 8a,  and 9a).Similar duplicated sections of Carboniferous through Triassic strata which dip 30-50° to the south are found approximately along strike in the Yaoquan and Dahuanggou areas, but here they are at the mountain front and uplifted by the frontal thrust (F2 in Yaoquan and F1 in Dahuanggou, respectively) (Figures 2, 4c,  4d, 5c, and 5d).This along-strike relationship indicates that sometime prior to the latest uplift of the North Qilian Shan, Devonian-Jurassic units in our research area were duplicated and tilted to 30-50°.We further surmise that this earlier event might have involved both thin-and thick-skinned deformation: the thin-skinned deformation duplicated and tilted the Devonian-Jurassic units, whereas the thick-skinned deformation uplifted the Silurian and older rocks to the surface (Figures 7a, 8a, and 9a).This interpretation introduces thick-skinned deformation to explain several key observations in the North Qilian Shan and in the fold-and-thrust belt: the absence of Devonian-Jurassic units inside the mountain range south of the Qingtoushan area between the Cretaceous and Silurian-Ordovician units, and the absence of Carboniferous-Triassic units at the mountain front of Dahuanggou between the Neogene deposits and the Silurian units (Figures 2, 4d, and 9).Though due to the complex deformation history and erosion of the region, it is impossible for us to accurately reconstruct this early deformation, we present this thin-and thick-skinned model as a permissible solution for the deformation prior to the latest Cenozoic uplift.The second tilting event likely started no later than the deposition of Yumen Fm. (Late Pliocene-Early Pleistocene), judging from the growth strata in both Qingtoushan and Yaoquan, we therefore suggest this event is In Qingtoushan area (I-I′), the location where the terraces converge and where the Oligocene deposits change the dip angle from 30° to 0° represents the location of the lower fold hinge of a fold backlimb.The 3 km width of the fold crest, formed in the hangingwall of F1, requires the presence of an additional thin-skinned structure at shallow depth.By assuming the dip angle of F1 to be 50°, based on observations in the neighboring Beida River and Yaoquan (II-II′) areas (W.Chen, 2003;Y. Wang et al., 2020), we can deduce the presence of a shallow-level décollement connecting F2 to F1.This décollement should dip 9° and meet F1 at less than 2 km depth to produce the observed fold backlimb (Figures 7c, 8b, and 8c).The thick package of Cenozoic sediments deposited within the Jiuxi Basin indicate a large amount of subsidence has occurred in the foreland since the uplift of the North Qilian Shan (Fang, 2005;J. Li et al., 2014;S. Yang, 2007).The Oligocene through Pleistocene basin deposits exposed in Yaoquan also suggest that large amount of sediment has been deposited on top of the Devonian-Jurassic rocks.To accommodate this subsidence and bring the Devonian-Jurassic rocks to surface, we built two alternative structural models.In the first model (Figure 7b), deep slip along F2 is transferred through a shallow décollement to a foreland fault-bend fold.The lower, thick-skinned portion of fault F1 activates later.This model can produce the ∼80° dipping bedrock units in the foreland, however, to uplift the bedrock to the surface requires slip above a deeper décollement and therefore a longer backlimb than observed from the orientation of Neogene strata and deformed terrace profiles (Figure 4a).The second, preferred model (Figure 7c) reverses the order of faulting, starting with a thick-skinned deformation by uplifting of the foreland along F1 in the first stage, followed by transferring slip from F2 to F1 along the shallow décollement to produce a foreland fault-bend fold.This model produces both the ∼80° dipping bedrock in the foreland, as well activity above a shallow décollement to produce the folded terraces we observe in the field.Apart from these two models, a third model is that these exposed Devonian through Jurassic rock units in the foreland Qingtoushan area may be transported to the foreland from the hinterland along a long and shallow, south dipping décollement (S.Yang, 2007).We rule out this possibility because in the Yaoquan and Dahuanggou area, similar exposures of Carboniferous to Triassic units at the mountain front unconformably overly Silurian rocks, which suggests these were uplifted by steeply dipping faults, instead of being transported from far to the south along a shallow décollement.We conclude that the second scenario (Figure 7c) best explains the deformation history in the area.Our preferred cross-section interpretation suggests that there has been ∼6.3 km or 40% of shortening in the foreland and mountain front of the Qingtoushan area since the development of the fold-and-thrust belt.
The folded Baiyang River terrace profiles (Figure 6) agree with earlier findings from the area (R. Liu et al., 2017), and are comparable to the fold that we documented in the hinterland along the Beida River (Figure 7 in Y. Wang et al., 2020).The folded profiles show similar asymmetric geometry but slightly smaller (20 vs. >30 km) wavelength and a gentler forelimb than the Beida River fold, which suggests that an active thick-skinned structure is also present in the foreland here, at least in the Qingtoushan-Baiyang River area (I-I′).To deduce the fault geometry at depth, we built a kinematic model under the assumption that F1 near the surface and its décollement at depth share the same dip angles (50° and 10°, respectively) as the fault geometry revealed by the Beida River terraces (Y.Wang et al., 2020).The model (Figure 6e) shows that to produce the long-wavelength fold recorded by the Baiyang River terraces, the 50° planar fault likely turns into a listric fault at ∼5 km depth; this fault then soles into the 10° dipping décollement at ∼13 km depth.That no Mesozoic or older rock units are exposed in the foreland here except at the thin-skinned Qingtoushan anticline suggests relatively recent activation or reactivation of this thick-skinned structure.We propose that the Qingtoushan area is the present transition zone between active thin-and thick-skinned foreland deformation levels.
Similar to the Qingtoushan area (I-I′), Yaoquan area (II-II′) also has been affected by the same set of faults, leading to tilting of the Oligocene to Pleistocene basin deposits to 15-30° in the backlimb and 20-80° in the forelimb, and uplifting and folding of the fluvial terraces.The continuity of the lower hinge of the fold backlimb, indicated by the terrace profiles (Figures 5a-5c), suggests that the short wavelength folds of the Qingtoushan and Yaoquan area are developed above the same shallow décollement, likely merging into F2 at depth (Figures 7 and 8).Different from Qingtoushan area, this décollement impinges upon F1, transferring slip onto F1 during an earlier stage, which resulted in folding of the Neogene and Quaternary deposits and uplift of the T3 terrace south of F1; afterward, the shallow décollement crosscut F1, and presently breaks through surface strata as fault F0.Another difference between the Yaoquan and Qingtoushan is that no bedrock units are exposed in the foreland fold-and-thrust belt here, suggesting that the amount of uplift in Yaoquan is likely smaller than at Qingtoushan.Similarly, we built two cross-section models for the Yaoquan area (Figure 8).The first model (Figure 8b) only exhibits thin-skinned structures active in the foreland, while in the second model, slip along F1 and F2 uplifts Mesozoic and older rock units and Cenozoic strata during the first stage (Figure 8c), then in the second stage, the deeper part of F1 becomes inactive and slip along F2 steps southward along a shallow décollement, cross-cutting F1 and connecting to the ramp F0 (Figure 8d).Both models can explain the deformation in this area, however, considering the spatial relationship between Yaoquan and Qingtoushan, we suggest the second model is more likely.Our preferred cross-section interpretation suggests that there has been ∼5.4 km or 36% of shortening in the foreland and mountain front of the Yaoquan area since the development of the fold-and-thrust belt.
In Dahuanggou area (III-III′), the fold-and-thrust belt formed in the hangingwall of fault F1 has been incorporated into the hinterland.The foreland of the Dahuangou area only exhibits a few fault scarps cutting across alluvium, each less than 1 m high and located ∼2 km away from the frontal fault F1.Terrace profiles of the Beida River in the foreland to the east of Dahuanggou also show no obvious sign of folding or tilting (Y.Wang et al., 2020).This evidence suggests the absence of foreland fold-and-thrust belt in the area.Due to erosion, most evidence of earlier foreland deformation has been erased from the hangingwall of fault F1.We note that uplifted Carboniferous-Permian units at the mountain front are exposed continuously with 30-50° dip from Yaoquan (II-II′) to Dahuanggou, and therefore likely were deformed similarly.Terrace profiles along transect D and the Beida River further indicate that this area is dominated by thick-skinned uplift.Therefore, we suggest that this area has most likely only experienced slip above a deep décollement since the North Qilian orogen expanded to this region (Figure 9).Our preferred cross-section interpretation suggests that there has been ∼4.2 km or 41% of shortening at the mountain front of the Dahuanggou area, though lack of hangingwall cutoff information means this is likely an underestimation.

Spatial and Temporal Relationship of the Fold-And-Thrust Belt
Cumulative deformation recorded by Devonian-Quaternary strata suggests that since the latest North Qilian uplift, our research area has experienced an early stage of thick-skinned deformation, resulting in uplifting of Devonian-Jurassic units and thinning of the Oligocene and younger basin deposits.These strata were further deformed during a later thin-skinned deformation stage, forming a duplex in the foreland of Qingtoushan (I-I′) and Yaoquan (II-II′).During this stage, the original ∼30-50° dipping Devonian-Jurassic units were tilted to near vertical in Qingtoushan, while the Oligocene and Pliocene strata were tilted to 30° and ∼20° in Qingtoushan and Yaoquan area, respectively.During this later stage, the foreland and hinterland of the Dahuanggou area (III-III′) show little sign of thin-skinned deformation.The present stage is recorded by deformed terrace profiles, showing both thin-and thick-skinned deformation in Qingtoushan, pure thin-skinned deformation in Yaoquan, and pure thick-skinned deformation in Dahuanggou area.The extent of the foreland fold-and-thrust belt can be determined by comparing the cumulative deformation of the bedrock with deformed fluvial terraces.The tilted Oligocene through Pleistocene strata (Baiyanghe through Yumen Fm.) in the foreland shows that an anticline with a wavelength between 3.5 and 7 km is present from Yaoquan (II-II′) in the east to the Baiyang River in the west.Further west, previous research suggests this structure continues to the Yumen area as the Yumen (Laojunmiao) anticline, based on exposed and folded growth strata of the Baiyanghe through Yumen Fm. (Fang, 2005; Institute of Geology, China Earthquake Administration, 1993; R. Liu et al., 2017).Displaced and tilted fluvial terraces in the foreland at Qingtoushan (I-I′) and Yaoquan show that this anticline is still active, though the foreland activity in Yaoquan has stepped basin-ward from F1 to F0.To the east, this fold-and-thrust belt terminates at the Dahuanggou area (III-III′).To the west, folding of the Yumen anticline is no longer active based on terrace profiles of Shiyou River (Hetzel et al., 2006), suggesting that the extent of the active, shallow foreland thrust-related folding to the west is presently limited to the Baiyang River area (Figure 10b).
Apart from the thin-skinned structure and short wavelength fold-and-thrust belt, thick-skinned faulting and folding has also affected the topography of the foreland.Folding of the Baiyang River terraces suggests that in Qingtoushan-Baiyanghe area (I-I′) both deep and shallow décollements are active contemporaneously, forming an active duplex in the foreland, as also suggested by S. Yang (2007) from seismic reflection profiles.Combined with evidence of an active, deep décollement in the Yumen area (Hetzel et al., 2006), we suggest the foreland thick-skinned flat-ramp system is active at least from the Yumen area in the west to Qingtoushan area in the east.Further to the east, in the Yaoquan area (II-II′), foreland terrace profiles show little evidence of long wavelength folding and terraces tend to converge at the mountain front (Figures 5b and 5c), suggesting that the deeper décollement is inactive in this area.Further east, in the Dahuanggou area (III-III′), uplift along the F1 fault, which soles into the 15-17 km deep décollement (Y.Wang et al., 2020), has incorporated the former foreland into the North Qilian Shan hinterland (Figure 10b).
Combining all the spatial and temporal evidence, we suggest the Late Cenozoic shortening of the North Qilian Shan and Juixi basin is characterized as the formation of a duplex involving at least two major frontal faults, F1 and F2, and two décollement levels.During this shortening event, we define three stages, featuring alternative activation of different parts of the duplex (Figure 10).During the first stage, fault F1 activated and was connected to a deep décollement.This faulting uplifted Devonian through Jurassic units to surface in Qingtoushan (I-I′) (Figures 7c and 9b) and led to erosional thinning of the Oligocene Baiyanghe Fm. and absence of Miocene Shulehe Fm. in some areas.During the second stage, activity along the deeper part of fault F1 in the Qingtoushan and Yaoquan (II-II′) areas waned and deformation backstepped to fault F2, while activity of F1 likely persisted in Dahuanggou area (III-III′).A shallow décollement connected to F2 took over most of the shortening in the foreland, linking to F1 and F0 at a shallow depth.This led to development of the foreland fold-and-thrust belt in the Qingtoushan and Yaoquan area, and correlates to the development of the Yumen anticline further west (Figures 7d and 8d).The third, present deformation stage is marked by waning of activity along the shallow part of F2, and reactivation of the lower décollement and the deeper part of F1 in the Qingtoushan-Yumen area.This three-stage history is not evident in the Dahuanguo area, where it appears that the F1 basement fault has been continuously active.Sedimentological analyses of the Yumen anticline (Fang, 2005;J. Li et al., 2014;C. Song et al., 2001;D. Zheng et al., 2017) suggest that the North Qilian Shan has experienced rapid uplift and the Yumen anticline started fast upheaval since 3.6 Ma ago.We suggest this may be the beginning of the first stage in our research area.However, we also cannot rule out the possibility of an earlier onset of stage 1 with relatively slow thick-skinned uplift, followed by a rapid, thin-skinned deformation stage (stage 2) starting ∼3.6 Ma ago.
Our structural analysis also indicates that there was a compressional event prior to the Late Cenozoic North Qilian uplift, which led to the initial tilting and duplication of Devonian to Jurassic units in our research area.Previous studies have identified three compressional events that could explain this deformation: a Late Triassic to Early Jurassic event and an Early Cretaceous event (Cheng et al., 2019;Han et al., 2023;Jolivet et al., 2001;Tong et al., 2020;Y. Wang et al., 2022), or a possible Eocene-Oligocene early event that defined the early extent of the Tibetan Plateau (Clark, 2012;Wu et al., 2021;Zuza et al., 2019Zuza et al., , 2020)).The unconformity between Oligocene and Devonian to Jurassic units supports that this event took place prior to the Oligocene; omission of Jurassic units in Yaoquan and Dahuanggou areas may favor a Late Triassic to Early Jurassic age for this event.However, without Cretaceous units in the fold-and-thrust belt in our research area it is difficult to discern whether a Late Triassic-Early Jurassic event or an Early Cretaceous event is responsible for the initial deformation.

Implications of the North Qilian Deformation
The analysis of the fold-and-thrust belt evolution history suggests that the deformation style within the North Qilian Shan foreland varies both temporally and spatially along strike.During the Cenozoic shortening event, both shallow and deep ramp-flat faults and folds formed during earlier deformation stages were reactivated and crosscut by later-formed thin-and thick-skinned faults, resulting in complex relationships between observed fold and fault geometry (Figures 7-9).In our research area, we thus identify two related deformation styles: (a) Purely thick-skinned deformation of the foreland and hinterland, with little or no thin-skinned deformation in the foreland, as observed in Dahuanggou (III-III′).(b) Thick-skinned uplift of the hinterland with alternating or coeval activating of thick-and thin-skinned deformation of the foreland, as observed in Qingtoushan (I-I′) and Yaoquan (II-II′), and the Yumen area to the west (Hetzel et al., 2006).We suggest the various deformation styles are the result of alternative activation of different parts of a crustal scale duplex.Importantly, the structural evolution of our research area exhibits out-of-sequence deformation, where the deeply rooted reverse fault forms first, cutting the foreland, and then the deformation steps back to the hinterland, with shunting of slip to the foreland via a shallow décollement (Figure 10).
Field studies and numerical modeling show that thick-skinned structures and out-of-sequence deformation can form under the conditions of critical taper theory in order to keep the frontal wedge at a critical state (e.g., Kellett et al., 2009;Pajang et al., 2022;Ruh et al., 2013;Schuller et al., 2015).However, in this scenario, thick-skinned structures would form inside the hinterland or at the rear of the wedge, while in the Qilian Shan, we find that thick-skinned structures initiated within the foreland.Paleozoic and Mesozoic (and possibly early Cenozoic) shortening and extension events have left this region with abundant pre-existing structures with favorable orientations to reactivate.This pre-conditioning favors initial formation of thick-skinned structures in the foreland.In fact, the steep frontal faults of North Qilian Shan (Institute of Geology, China Earthquake Administration, 1993) and south-dipping seismic fabric that extends to mid-lower crust (Gao et al., 2013(Gao et al., , 2022) ) suggest that these Cenozoic high angle reverse faults are likely reactivated Paleozoic structures.Studies have also shown that even if not active, discontinuities across pre-existing faults can lead to stress concentration and generation of new faults (e.g., Boutoux et al., 2014;Scisciani, 2009;Tricart & Lemoine, 1986).Similar examples may be found in the foreland of the Alps, Taiwan, Andes, and elsewhere (e.g., Camanni et al., 2014;Cristallini & Ramos, 2000;Jourdon et al., 2014;Kley et al., 1999;Mescua & Giambiagi, 2012;Mouthereau et al., 2002;Perez et al., 2016).
We suggest that the out-of-sequence deformation and the duplex structure may characterize many areas of the North Qilian mountain front and its adjacent foreland.There are regions, for example, Qingtoushan-Beida River area (this study), Jinggangsi area (D.Zheng et al., 2010;Zuza et al., 2016), where direct or circumstantial evidence suggests long-term thick-skinned out-of-sequence deformation.In addition, major foreland anticlines, that is, the Yumen anticline (this study; Fang, 2005;J. Li et al., 2014), Fodongmiao-Hongyazi anticline (Hetzel et al., 2019;Institute of Geology, China Earthquake Administration, 1993;H. Yang et al., 2020), Yumu Shan (Hu et al., 2017(Hu et al., , 2022;;Hu, Ji, et al., 2021), Yonggu anticline (Zhong et al., 2020), and Yongchangnanshan (Nanying) anticline (Hu et al., 2015;Lei et al., 2020), are distinct from one another and show different structural characteristics.For example, the Yumen Anticline is presently a mixture of thin-skinned and thick-skinned deformation, forming a duplex (this study), the Fodongmiao-Hongyazi anticline has transformed from a thin-skinned fold-and-thrust belt to a thick-skinned structure (H.Yang et al., 2020), and the Yonggu and Yongchannanshan anticlines, though adjacent, have significantly different décollement depths and shortening rates (Hu et al., 2015;Lei et al., 2020;Zhong et al., 2020).It is possible that these areas are dominated by similar duplex structure, and the alternative activation exhibits various deformation styles that we observed today.The pre-existing weak structures, likely arising from the complex earlier deformation history, favor the adoption of out-of-sequence deformation and a variety of deformation styles along strike.In addition, strikeslip faults that cut through the Qilian Shan interior, that is, Changma Fault (F3), Haiyuan Fault and Riyueshan Fault, have been playing an important role in North Qilian deformation history, whereas many segments of the North Qilian frontal fault, especially in the East Qilian, have shown evidence of oblique slip.This adds to the complexity of the thrust belt and may also contributed to the diverse deformation styles of the North Qilian mountain front.

Implications for Shortening Rate
Our preferred cross-section reconstructions suggest the total shortening of the fold-and-thrust belt since the onset of contraction are 6.3, 5.4, and 4.2 km for Qingtoushan (I-I′), Yaoquan (II-II′), and Dahuanggou (III-III′) areas, respectively.It is worth noting that the cross-sections we reconstructed for the hinterland are incomplete and the shortenings are underestimates, due to lack of hangingwall cutoff information in these areas.Assuming the onset time of Yumen anticline (Fang, 2005;J. Li et al., 2014;C. Song et al., 2001;D. Zheng et al., 2017), 3.6 Ma, as the onset time of deformation of the foreland in our research area, the minimum long-term shortening rate would be 1.2-1.8m/Kyr.This falls within the 1-2 m/Kyr shortening rate range inferred from deformed Late Pleistocene-Holocene geomorphic markers of the Western North Qilian Shan (e.g., Hetzel et al., 2019;Q. Liu et al., 2021;Y. Wang et al., 2020;H. Yang et al., 2018).Various deformation styles exhibited along the strike of the North Qilian Shan may potentially lead to discrepant shortening rate estimates at different sites.In addition, the possible existence of multiple active structures and duplexes in the North Qilian foreland indicates the impor tance of assessing the structural style of both foreland and hinterland when quantifying shortening rate.

Conclusions
Retrodeformable cross-sections of the western North Qilian Shan foreland, constructed from geologic mapping and constrained from deformed stream-terrace survey data, support that present tectonic activity here is mainly accommodated by the deeply seated frontal fault system.Our results further suggest that the activity of deep and shallow décollement levels varied both through time and spatially as the deformation front migrated into the Jiuxi foreland basin.The coexistence of deep and shallow décollement levels suggests that broadly, the foreland-hinterland fault system of the North Qilian Shan forms a crustal-scale duplex as accretion of the foreland basin proceeds.The out-of-sequence deformation styles exhibited by this duplex may be due to pre-existing weaknesses beneath the Hexi Corridor foreland-basin system.The coexistence of different, simultaneously active deformation levels results in variation of shortening style along strike of the North Qilian Shan, with each style contributing differently to the total shortening rate.This structural complexity and the presence of deep, active faults beneath foreland basins pose an acute hazard to urban regions that may be difficult to recognize and quantify from surface geology.

Figure 2 .
Figure 2. Geologic map of a portion of the western North Qilian Shan, compiled based on 1:200,000 scale geological mapping (Gansu Geological Bureau, 1989; C. Li et al., 2019) and our field survey.Major unconformities are denoted in the stratigraphic column as wavy lines.

Figure 4 .
Figure 4. Left: Terrace maps and locations of survey points of transect (a-d).See Figure 1b for transect locations.Terraces are numbered from youngest to oldest and may not correlate from transect to transect.Right: terrace and riverbed profiles (y axis on the left), and underlying strata (y axis on the right) of transect (a-d); (a) and (b) are field survey points projected to a vertical plane striking 016°; (c) and (d) are field survey points projected to a vertical plane striking 050°.Projection planes denoted as black lines on each terrace map at left.Annotations on the strata: Q2, Mid-Pleistocene (Jiuquan Fm.); Q1, Lower Pliocene-Upper Pleistocene (Yumen Fm.); N, Miocene-Upper Pliocene (Shulehe Fm.); E, Oligocene (Baiyanghe Fm.); J, Jurassic; T, Triassic; P, Permian; C, Carboniferous; D, Devonian; S, Silurian.

Figure 6 .
Figure 6.Baiyang River terrace map and profile.(a) Survey points along the river.(b) Baiyang River terrace map based on remote sensing and field observation.(c) Terrace and riverbed profiles extracted from 8 m digital elevation model produced by the Polar Geospatial Center (Shean, 2017).Note the perturbed river profile due to sediment infill upstream of reservoir.(d) Detrended terrace and riverbed elevations relative to a linear fit of the riverbed between 0 and 20 km distance.(e) Underlying fault geometry deduced with a listric model; dashed lines are the location of the backlimb hinges projected from the deformed terrace profiles.

Figure 7 .
Figure 7. Kinematic model of the foreland anticline in section I-I′: the initial stage (a), deformation scenario 1 (b) and scenario 2 (c).

Figure 8 .
Figure 8. Kinematic model for the foreland anticline of section II-II′: the initial stage (a), deformation scenario 1 (b) and scenario 2 (c).

Figure 9 .
Figure 9. Kinematic model for section III-III′.(a) Initial stage before the Cenozoic uplift of the North Qilian Shan.(b) Cenozoic uplift of the hinterland through activity along F2 and F1.

Figure 10 .
Figure 10.(a) Schematic Cenozoic evolutionary model for section I-I′ to III-III′, and a possible model for Yumen anticline.(b) Map view with corresponding structures of stage 3.