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Basin Research (2017) 1–20, doi: 10.1111/bre.12264 Topographic growth of the Jishi Shan and its impact on basin and hydrology evolution,NE Tibetan Plateau Joel E. Saylor,* Jessica C. Jordan,†,1 Kurt E. Sundell,* Xiaoming Wang,‡ Shiqi Wang§,¶ and Tao Deng§ *Department of Earth and Atmospheric Sciences, University of Houston, Houston, TX, USA †School of Earth Sciences and Environmental Sustainability, Northern Arizona University, Flagstaff, AZ, USA ‡Department of Vertebrate Paleontology, Natural History Museum of Los Angeles County, Los Angeles, CA, USA §Key Laboratory of Vertebrate Evolution and Human Origins, Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences, Beijing,China ¶Center for Excellence in Tibetan Plateau Earth Sciences, Chinese Academy of Sciences, Beijing, China

ABSTRACT Previous research demonstrates that large basins on the periphery of the northern edge of the Tibe- tan Plateau were partitioned during development of intrabasin mountain ranges. These topographic barriers segregated basins with respect to surface flow and atmospheric circulation, ponded sedi- ments, and formed rain shadows. However, complex mixing between airmasses and nonsystematic isotope-elevation lapse rates have hampered application of quantitative paleoaltimetry to determine the timing of development of critical topographic barriers. We address the timing and drivers for changes in surface connectivity and atmospheric circulation in the Linxia and Xunhua basins using a multidisciplinary approach incorporating detrital zircon geochronology, Monte Carlo inverse flexu- ral modelling, and published stable isotope data. Disruption of surface flow between the two basins during exhumation of the Jishi Shan preceded development of topography sufficient to intercept moisture-bearing airmasses. Detrital zircon data point to disruption of an eastward-flowing axial flu- vial network between 14.7 and 13.1 Ma, coincident with the onset of exhumation in the Jishi Shan. Flexural modelling suggests that by 13 Ma, the Jishi Shan had developed 0.3 0.1 km of relief; sufficient to disrupt eastward-flowing drainage networks but insufficient to intercept moisture-bear- ing airmasses. Stable isotope data indicate that, although surface connections between the Xunhua and Linxia basins were broken, the two basins continued to be dominated by a common climate regime until 9.3 Ma. Subsequent reintegration of surface flow between the basins occurred between 9.3 and 7.6 Ma. Divergence in the stable isotope and depositional environment records between the two basins suggests that at 9.3 Ma the paleo-Yellow River breached the growing Jishi Shan dam, and may have reintegrated surface flow between the two basins via erosion of the modern Yellow River gorge, which transects the Jishi Shan. The reintegration of the Xunhua and Linxia basins’ sur- face connection is confirmed by reintroduction of a Songpan-Ganzi flysch sediment source by 7.6 Ma. Continued exhumation and uplift of the Jishi Shan developed 0.8 0.2 km of relief by ca. 8 Ma capable of intercepting moisture-bearing airmasses; isolating and increasing aridity in the Xunhua Basin while decreasing it in the Linxia Basin. Our findings point to protracted development of the modern ca. 1 km of relief in the Jishi Shan between 14 and ca. 4.5 Ma followed by attainment of a topographic equilibrium which persists into modern times.

Correspondence: Joel E. Saylor, Department of Earth and Atmospheric Sciences, University of Houston, Houston, TX 77204, USA. E-mail: [email protected] 1Present address: Pioneer Natural Resources, Irving, TX 75039, USA

© 2017 The Authors Basin Research © 2017 John Wiley & Sons Ltd, European Association of Geoscientists & Engineers and International Association of Sedimentologists 1 J. E. Saylor et al.

d18 INTRODUCTION sourced from the Atlantic Ocean precipitation O val- ues are controlled primarily by seasonal temperature Regional surface uplift and growth of basin-bounding changes resulting in nonsystematic relationships between mountain ranges are inextricably linked to regional and elevation and stable isotopic compositions (Tian et al., local paleoenvironmental change in the northern Tibetan 2007). Due to these complexities, the magnitude of sur- Plateau (Dettman et al., 2003; Kent-Corson et al., 2009; face uplift needed to drive regional or local changes in Hough et al., 2014; Zhuang et al., 2014; Li et al., 2016). atmospheric circulation remains unconstrained. Upward and outward growth of the Tibetan Plateau has Here, we determine the middle Miocene–Pliocene been linked to simultaneous weakening of Indian Summer paleoelevation evolution of the basin-bounding Jishi Shan Monsoon circulation and strengthening of East Asian using a Monte Carlo approach to flexural modelling by Monsoon circulation (An et al., 2001; Tang et al., 2013). evaluating the response of the Linxia Basin to random The northeastern Tibetan Plateau is characterized by for- topographic loads (Fig. 1). Because flexural modelling mation and subsequent partitioning of large peripheral yields nonunique solutions, the growth of topography, basins (Metivier et al., 1998; Meyer et al., 1998; Tappon- and surface and atmospheric connectivity is interpreted nier et al., 2001; Horton et al., 2004; Carroll et al., 2010; by comparison to new and published detrital zircon Craddock et al., 2011; Horton, 2012; Bush et al., 2016). geochronology, thermochronology, and stable isotopic Exhumation and uplift of new basin-bounding ranges compositions of paleosol and lacustrine carbonates on formed topographic barriers to surface flow and atmo- both sides of the Jishi Shan. Flexural modelling indicates spheric circulation, promoting arid rainshadow conditions that the Jishi Shan developed ca. 75% of its modern relief within the new sub-basins (Sobel et al., 2003; Strecker between 13 and 7.6 Ma and resulted in disruption of et al., 2007; Hough et al., 2014; Pingel et al., 2014). Fol- atmospheric circulation patterns between the basins to the lowing surface uplift, formation of basin-margin topo- east and west of the range at ca. 8 Ma. Topographic graphic barriers and associated arid conditions inhibited growth was accompanied by cut-off of diagnostic south- erosional evacuation and resulted in sediment ponding. west-derived detrital geochronology age modes at 13 Ma. However, the current exposure of basin fill in many Subsequent reintroduction of these characteristic source northeastern Tibetan Plateau basins points to a subse- signatures by 7.6 Ma points to the establishment of the quent period of drainage integration and sediment evacu- modern course of the Yellow River, which transects the ation (Fang et al., 2003; Garzione et al., 2005; Lease Jishi Shan (Fig. 1). et al., 2007, 2012a; Craddock et al., 2011; Liu et al., 2013; Hough et al., 2014). Despite the importance of understanding the elevation GEOLOGICAL SETTING history of the northeastern Tibetan Plateau on monsoon Paleogene deformation and basin formation strength and aridity trends in central Asia, reconstructing the paleoelevation of basin-bounding mountain ranges Cenozoic deformation in the northeastern Tibetan Pla- remains difficult for several reasons. First, the in situ geo- teau occurs in an intraplate setting, more than 1,500 km chemical proxies that might be used to reconstruct from the nearest major plate boundary. Deformation in paleoelevation are often not preserved at high elevations the region is dominated by two stages: 1) north-south– due to erosion, and it is the mountain ranges that block oriented contraction in the Paleogene–early Neogene and moisture-bearing air masses and whose paleoelevation 2) east-west–oriented transpression in the middle must be determined. Second, application of stable iso- Miocene–Pliocene. In the first stage, Paleogene forma- topic lapse rates to quantitatively reconstruct paleoeleva- tion of the regionally integrated Longzhong Basin in tion is subject to multiple complicating factors (e.g. response to crustal loading by the West Qinling was Quade et al., 2007, 2011; Leier et al., 2009; Botsyun followed by progressive dissection and isolation of sub- et al., 2016). Complicating factors most relevant to the basins in the Neogene (Craddock et al., 2011; Lease northeast Tibetan Plateau include extensive recycling of et al., 2011, 2012a; Yuan et al., 2013; Lease, 2014). southern-derived moisture during advection over the The West Qinling fault is a north-verging, west-north- mass of the Tibetan Plateau, subcloud evaporation in arid west-trending, sinistral transpressional fault system on settings, mixing of multiple moisture sources including the southern edge of the Longzhong (now Linxia) the East Asian Monsoon and central Asian airmasses Basin (Fig. 1b, GBGMR, 1989; QBGMR, 1991). Apa- (Araguas-Araguas et al., 1998; Tian et al., 2001; Hren tite (U-Th)/He data from the hanging wall of the et al., 2009; Bershaw et al., 2012), and potentially the West Qinling fault reveal rapid and major cooling at impact of climate change on stable isotopic lapse rates ca. 45–50 Ma followed by relatively minor cooling at (Poulsen & Jeffery, 2011; Botsyun et al., 2016). North of 17–18 Ma, interpreted as two pulses of exhumation the Tibetan Plateau where moisture is dominantly (Clark et al., 2010). The West Qinling fault places

© 2017 The Authors 2 Basin Research © 2017 John Wiley & Sons Ltd, European Association of Geoscientists & Engineers and International Association of Sedimentologists Topography and basin evolution, NE Tibet

90˚E 95˚E 100˚E 105˚E 40˚N (a) AFTAFT Qilian Shan Legend Triassic- Permian Neoproterozoic Quaternary Granodiorite Alluvium/Loess Triassic Flysch/ Proterozoic Qaidam Undifferentiated Songpan-Ganzi NE Basin QSQS Depocenter HFHF Pliocene Jishi Fm. Proterozoic Permian carbonates Metamorphics 36˚N KSKS B late Miocene Upper Archean JSJS KFKF Hewangjia Fm. Carboniferous Metamorphics strata Legend KS Kunlun suture Elevation(m) Songpan early-late Miocene Thrust Fault BangongJS SutureJinsa suture Liushu-Jiaozigou Fm.. 5724 Ganzi Terrane Silurian plutons Suture zones QS QilianAmdo suture Strike slip Lhasa Terrane Undifferentiated earlyy KF Kunlun fault 0 250 500 Thrust fault Neogene/Paleogene Ordovician plutons AFTIndus AltynTsangpo Tagh Suture fault Normal fault HF Haiyuan fault 336 early Miocene Tala Fm. XiningXining Ordovician strata (b) 7 RiyueR Shan8 BasinBasin 6 Cretaceous strata Cambrian volcanic Stratigraphic iyu rocks sections e CatchmentCatchment ssamplesamples Sh LajiL Shan Jurassic strata andand ccatchmentatchment aarearea an aji S Lower Paleozoic 1616 han strata XunhuaXunhua 5,5, Jurassic plutons 0 25 50 100 km GuideGuide JSCJSC Plutons 36˚N BasinBasin JishiJ Shan i BasinBasin s XunhuaXunhua h MaxianMaxian Figure 3 GongheGonghe i MSCMSC ShanShan S

BasinBasin h 1414 1515 4x4x a n 4 LinxiaLinxia MaogouMaogou TongrenTongren WangjiashanWangjiashan BasinBasin WestW Qinling LinxiaLinxia BBasinasin es t Q WQCWQC in lin g

35˚N 101˚E 102˚E 103˚E 104˚E 105˚E

Fig. 1. (a) Digital elevation model of the northern Tibetan Plateau showing the location of the Linxia Basin relative to major tectonic elements of the northeastern Tibetan Plateau. Modified from DeCelles et al. (2007), Clark et al. (2010), Craddock et al. (2011) and Duvall et al. (2013). (b) Geologic map of the Linxia Basin region. Deposition of the Jishi through Tala formations is localized to Linxia basin, where present elsewhere indicates coeval deposition to the Linxia basin stratigraphy. Outlined numbered catchment samples are modified from Lease et al. (2007, 2012a), lettered samples are from this study. See Table S1 for details. Modified from Gansu Bureau of Geology and Mineral Resources (1989), Qinghai Bureau of Geology and Mineral Resources (1991), Weislogel et al. (2010), Hough et al.(2011), Lease et al. (2012a), and Liu et al.(2013).

Carboniferous–Permian carbonate rocks and banded disrupted the Longzhong Basin and isolated the Linxia/ siliceous limestone, Lower–Middle Triassic Songpan- Xunhua basins to the south from the Xining Basin to the Ganzi flysch, Lower Triassic volcanic rocks, Lower north (Lease et al., 2011; Duvall et al., 2013). The Laji Triassic–Jurassic granite, and Cretaceous conglomerate Shan is composed of Proterozoic phyllite, schist and dolo- in the hanging wall over Neogene strata of both the mite, Cambrian-Ordovician meta-andesite, Ordovician Linxia and Xunhua basins in the footwall to the north andesitic tuff and breccia interbedded with chert, slate (Fig. 1b, QBGMR, 1991; Sun et al., 1997). The West and phyllite, Ordovician plutons, Cretaceous conglomer- Qinling fault merges with faults bounding the Laji ate, and Paleogene Xining Group conglomerate Shan in the Riyue Shan thrust belt west of the Xunhua (QBGMR, 1991). The Maxian Shan is composed of Basin (QBGMR, 1991). Proterozoic metamorphic rocks, Neoproterozoic granodi- orite, Paleozoic plutons, and Cretaceous and Jurassic Neogene deformation and basin formation strata (CGIRCUG, 2006a,b,b). The second stage began in the middle Miocene when Deformation migrated ca. 60 km northward at ca. 22 Ma the Tibetan Plateau underwent an orogen-wide change in with the exhumation of the east–west trending Laji Shan kinematics. On the northern margin of the plateau this and Maxian Shan along a bivergent transpressional thrust change is marked by a transition from north-northeast– system (Fig. 1b, Dayem et al., 2009; Clark et al., 2010; oriented contraction to east-east sinistral shear and trans- Duvall et al., 2011; Lease et al., 2011, 2012a). This pressional faulting, while central Tibet accommodated exhumation and resultant rapid cooling persisted until ca. both northward contraction and east-west extension along 15 Ma, after which it transitioned to a slower cooling rate, conjugate strike-slip faults (Taylor et al., 2003; Lease which is inferred to persist through modern times (Lease et al., 2011, 2012a; Duvall et al., 2013; Yuan et al., 2013). et al., 2011). Exhumation of these mountain ranges Separation of the previously contiguous Xunhua/Linxia

© 2017 The Authors Basin Research © 2017 John Wiley & Sons Ltd, European Association of Geoscientists & Engineers and International Association of Sedimentologists 3 J. E. Saylor et al.

Basin into its two component basins during uplift of the calculations. Clastic lithofacies are assigned a porosity Jishi Shan completed the partitioning of the single Long- coefficient (1/km in depth), surface porosity (%) and a zhong Basin into multiple, isolated basins (Craddock grain density (kg/m3) modified from Sclater & Christie et al., 2011; Hough et al., 2011). (1980). Values for claystone are taken from those The Linxia Basin is a flexural basin (Fang et al., reported for shale by Sclater & Christie (1980) and val- 2003) which formed initially in response to exhumation ues for siltstone are calculated assuming a linear mix of of the West Qinling (Clark et al., 2010). However, 75% sandstone and 25% shale from Sclater & Christie Neogene subsidence in the Linxia Basin is driven pri- (1980). The surface porosity of siltstone is taken from marily by loading by the north-trending Jishi Shan Bond & Kominiz (1984) (Table S2). along range-bounding bivergent thrust systems (Fig. 1b, Fang et al., 2003; Lease et al., 2012b). Rapid Zircon U-Pb geochronology exhumation in the Jishi Shan began at 13–14 Ma and has continued until the present (Lease et al., 2011). Zircon U-Pb analyses were conducted primarily at the Prior to 14 Ma the rocks in the hanging wall of the University of Houston. A subset of the samples was anal- east-directed Jishi Shan thrust record slow cooling ysed at the LaserChron Center at the University of Ari- (Lease et al., 2011). The west-directed thrust on the zona (see Data repository text for details of samples western flank of the range is known to have been active analysis and Table S1 for details of which samples were after 13 Ma based on deformed Xunhua Basin strata analysed at each location). Zircon grains were separated dated at 9.3 Ma and partially reset apatite (U-Th)/He from fine- to coarse-grained sandstones following stan- ages as young as 8 Ma from a sample in the hanging dard mineral separation procedures of crushing, disc wall (Lease et al., 2012a). The northern third of the milling, water tabling and heavy liquids and magnetic Jishi Shan is comprised primarily of Paleozoic meta- separation. morphic rocks locally intruded by Ordovician granite 207Pb*/206Pb* provides greatest precision for ages and overlain by Cretaceous conglomerate. A small >1400 Ma, while 206Pb*/238Pb is more precise for ages swath of Cambrian volcanic rocks is present in the <1000 Ma. We place the cut-off between 207Pb*/206Pb* most northern portion of the Jishi Shan. Neoarchean– age and 206Pb*/238Pb age between 900 and 1000 Ma to Paleoproterozoic schist and Cretaceous conglomerate avoid bisecting grain populations (Gehrels et al., 2008). dominate the central portion while lower Paleozoic plu- Grains older than 600 Ma exhibiting greater than 20% tons characterize the southern segment (Lease et al., discordance or less than 5% reverse discordance are dis- 2012a; Liu et al., 2013). carded. A separate filter is applied to all grains younger than 600 Ma, such that grains whose 2r uncertainty envelope does not overlap the 15% discord are removed METHODS from further consideration. Grains that pass these filters Measured stratigraphic sections are plotted as probability density plots (PDPs) and his- tograms (Sircombe, 2004). Due to the possibility of Pb Linxia Basin-filling stratigraphy was measured at loss or core inheritance and the difficulty of interpreting decimeter scale at two locations: Wangjiashan (particularly young) slightly discordant ages when grains (35.628533˚ N, 103.058250˚ E) and Maogou (35.649383˚ are not cogenetic, geologic interpretations are guided by N, 103.273317˚ E, Fig. 1b). We conduct subsidence age modes comprising at least three overlapping grain analysis of the entire stratigraphic interval, but focus on ages (Dickinson & Gehrels, 2008). the 14–2.6 Ma interval for flexural analysis. The stratig- Comparison of detrital zircon age spectra is aided raphy was decompacted and backstripped using a by multidimensional scaling (e.g. Vermeesch, 2013). MATLAB-based code developed by Nestor Cardozo Multidimensional scaling (MDS) of detrital zircon data (Cardozo, 2016) which implements standard decom- attempts to transform the dissimilarity between sam- paction and backstripping methods (Angevine et al., ples to distance in N-dimensional space. Samples are 1990; Watts, 2001; Allen & Allen, 2005). Lakes in the represented as a point, typically in two-dimensional or Linxia Basin record are interpreted to be shallow and three-dimensional Cartesian space, with greater dis- have minimal impact on subsidence analysis and there- tances between two points indicating greater dissimi- fore no water depth correction was applied. Decom- larity between the two U-Pb age distributions. The paction accounts for sediment volume changes transformation is accomplished by iterative rearrange- associated with mechanical decreases in porosity by cal- ment of the data in N-dimensional space to minimize culating the exponential decrease in porosity with depth. the misfit (“stress”) between the calculated distances Deposition of a veneer of Pleistocene loess of variable and the disparities. Stress is calculated as P 2 0:5 thickness would have minimal impact on sediment com- ðÞfxðÞd ij P ij ij paction and was not incorporated into the decompaction 2 , where dij is the distance and f(xij) dij ij

© 2017 The Authors 4 Basin Research © 2017 John Wiley & Sons Ltd, European Association of Geoscientists & Engineers and International Association of Sedimentologists Topography and basin evolution, NE Tibet is the disparity between the ith and jth element. Dispar- y w (a) ity is calculated as a linear (1:1) transformation of the Showing random inputs input dissimilarities. In a low-stress MDS plot, the dis- h tances between points linearly correlate with the dissimi- ρ c Te larities between samples. Metric nonclassical MDS was x implemented using an in-house MATLAB algorithm (Data S1 and S2). Dissimilarity was calculated as the Kuiper V value and the coefficient of nondetermination (calculated as the complement of the Cross-correlation 2 coefficient: 1-R , Saylor et al., 2012, 2013; Saylor & Sun- y dell, 2016). Because the V value and coefficient of nonde- (b) ρ Showing constant inputs and calculated outputs termination are different and are calculated differently, c h the resulting MDS plots are not expected to be identical. Rather, the MDS plots are expected to yield consistent h’ x intersample distances, consistent up-section trends and Δd complementary interpretations. Plots are interpreted in ρ } terms of relative up-section changes in similarity d i ρs between samples and sources with greatest weight given d1 d2 to those trends seen in MDS plots derived from both d3 metrics. Legend and inputs 3 E = 70 GPa ρi = 2400 kg/m 3 υ = 0.25; ; ρs = 3300 kg/m Flexural modelling = Filters (from backstripped d1 = 37.5 ± 2.5 km subsidence) d2 = 56.9 ± 2.5 km g = 9.81 m/s2 d = 81.5 ± 7.5 km Flexural modelling was implemented using a MATLAB- 3 based inverse Monte Carlo model to that calculates subsi- Fig. 2. Flexure model set up and key parameters. (a) dence profiles from prescribed topographic loads of Semi-infinite (broken) plate model and random variables. random sizes and densities (Data S3). The code incorpo- (b) Infinite (unbroken) plate model showing constant and rated two classes of models: lithosphere that is infinite in calculated variables. the x-direction and lithosphere that occupies a semi-infinite space in the x-direction (Fig. 2). In both cases the plate is assumed to extend infinitely in the z-direction (into the Vi ¼ q ghi ðÞxiþ1 xi ; ð3Þ plane of the page), rendering the problem two-dimen- ci sional. The infinite lithosphere model incorporates equa- where qci is load density, hi is the height and (xi+1xi)is tions modified from Wangen (2010) that calculates the width of ith block. In Eqn. 1, f(u) is calculated as cumulative lithospheric flexure induced by multiple topo- ðÞ¼ uðÞþ ; ð Þ graphic loads. The deflection at each individual point is fu e cos u sin u 4 then the sum of the deflections due to N applied loads cal- where culated as, ¼ jj a1 ð Þ X u x ci 5 ðÞ¼ N Vi ðÞ: ð Þ wx ¼ fu 1 th i 1 2 Dq g a and ci is the position of the centre of the i load block. The semi-infinite lithosphere model incorporates equa- In Eqn. 1, Dq is the difference in density between the tions from Hetenyi (1979). Like the infinite plate, deflec- q displaced substrate (Fig. 2 s: mantle) and the material tion is calculated as the cumulative effect of multiple that fills the depression created by the applied load (Fig. 2 topographic loads. The deflection at each individual point q a i: basin sediments). The variables g and are the acceler- is then the sum of the deflections due to N applied loads ation due to gravity and flexural parameter, respectively. calculated as The flexural parameter is calculated as XN 1 ðÞ¼ ðÞ; ð Þ 3 4 wx wi x 6 ETe a ¼ ; ð2Þ i¼1 3 Dq g ð1 m2Þ where wi(x) is calculated by Hetenyi (1979; equation 23a) where E is Young’s modulus, m is Poisson’s ratio and as Te is effective elastic thickness (Turcotte & Schubert, th Vi 1982). In Eqn. 1, Vi, the applied load of the i block wiðÞ¼x ðÞC þ 2D A 2ðÞC þ D B þ E: ð7Þ is calculated as 2 g Dqa

© 2017 The Authors Basin Research © 2017 John Wiley & Sons Ltd, European Association of Geoscientists & Engineers and International Association of Sedimentologists 5 J. E. Saylor et al.

Terms A–E in Eqn. 7 are calculated by Hetenyi (1979; was pinned at the west side of the Jishi Shan (Fig. 3). chapter 2.1, equation h) as follows: Each model run generated 100 000 random flexural pro- files which were then filtered to accept those that pro- x x x ¼ a þ ; ð Þ A e cos a sin a 8 duced the observed subsidence at the location of the measured sections at 13.1, 7.6, 6, 4.5 and 2.6 Ma. We x x accounted for the fact sediment loading adds to the mass B ¼ e a sin ; ð9Þ a of the tectonic load using Eqn. 13 (below, and Fig. 4), ci ci ci q C ¼ e a cos sin ; ð10Þ 0 ¼  i ; ð Þ a a h h d 1 q 13 c ci ci  ¼ a ; ð Þ where h is the mean height of the modelled load block, d D e cos a 11 is the subsidence at the centre of the load block and qi and q and c are the assigned infill density and mean load density, respectively. Conceptually, this approach decreases the jjc x jj jj i ci x ci x area of the basin infill beneath the tectonic load so that its E ¼ e a cos þ sin : ð12Þ a a density matches the density of the tectonic load. The resultant decrease in height of the infilling material was Young’s Modulus, Poisson’s ratio and gravitational subtracted from the height of the tectonic load block to acceleration were held constant at 70 Gpa, 0.25 and obtain the final topographic elevation. This is equivalent 9.81 m s 2, respectively, for all model simulations. to the more common approach of modelling basin infill as Load height randomly varied between 0.01 and 5 km. additional load blocks (Jordan, 1981; Yong et al., 2003; Load width ranged from 5 to 12.5 km for models which Fosdick et al., 2014), but is less computationally intensive loaded the Linxia Basin lithosphere with half of the (Fig. 4). topographic load of the Jishi Shan or 10–25 km for Flexural profiles were filtered using the calculated models that loaded the Linxia Basin with the full Jishi decompacted sediment thickness at two locations, and a Shan load. Effective elastic thickness (Te) was randomly third location which was treated as the distal edge of the assigned from 5 to 30 km. Load density (qc)was foredeep. The two stratigraphic sections used included assigned randomly between 2450 and 2850 kg m 3. Wangjiashan and Maogou (Fig. 1b). At each of these 3 Density difference (Dq = qs - qi) was set at 900 kg m locations we placed an uncertainty window (Dd, Fig. 2) assuming an average density for asthenosphere of of 5 km and accepted subsidence profiles which passed 3300 kg m 3 and a density of basin-filling sedimentary within the uncertainty window. The basin zero edge was rocks of 2400 kg m 3. The position of the load block taken at the edge of Cenozoic deposition within the

(a) (b)

Fig. 3. (a) Digital elevation model of the Linxia Basin showing the three control points used to filter subsidence profiles and the 10-km-wide swath profile. (b) Swath profile showing minimum, mean, and maximum elevations. Also shows the mean elevation for the Jishi Shan and Linxia Basin and the x-interval over which the means are calculated.

© 2017 The Authors 6 Basin Research © 2017 John Wiley & Sons Ltd, European Association of Geoscientists & Engineers and International Association of Sedimentologists Topography and basin evolution, NE Tibet basin (35.6796875˚ N, 103.5083˚ E, Fig. 1b) (CGIR- observed tectonic episodes when using this chronology. CUG, 2006a,b). The uncertainty window on the zero- Figure S1 presents basin subsidence profiles using the edge location was set at 15 km and has minimal impact chronology of Deng et al. (2013). on the accepted flexural profiles. Formation ages are We focus on five dated horizons which post-date the based on both biostratigraphic and magnetostratigraphic onset of exhumation in the Jishi Shan as well as post-dat- dates (Qiu & Qiu, 1990; Qiu et al., 1990, 2002; Gu ing the cessation of exhumation in the West Qinling and et al., 1995a,b; Fang et al., 1997, 2003; Deng et al., Laji Shan: 2.6, 4.5, 6, 7.6 and 13.1 Ma (Clark et al., 2010; 2013). The chronology for and Wangjishan section Lease et al., 2011, 2012a). Magnetostratigraphy provides between 12 and 29 Ma is based on lithostratigraphic independent constraints on chronology for each of the correlation with the Maogou section rather than on sections in question for this time interval. In this work, magnetostratigraphy from the section itself (Fang et al., the Hujialiang Formation is included in the Dongxiang 2003). Recognizing the need for independent constraints Formation because the magnetostratigraphic chronology on basin chronology, we utilize the chronology of Fang was developed prior to the formation’s recognition et al. (2003) due to the coincidence between enhanced (Fang et al., 2003). Each of these time slices was filtered subsidence episodes and changes in sediment prove- for the cumulative tectonic subsidence since 14 Ma at the nance observed in the basin fill and independently three locations.

3 (a) Density of sediment loads: 2400 kg/m Width of sediment load blocks: 1000 m –3000 3 Height of crustal load Density of crustal load: 2650 kg/m –2500 Width of crustal load block: 16000 m Height of crustal load: 2876 m –2000 Final elevation of crustal load Effective elastic thickness: 10557 m (after summing –1500 to deflection) Summed absolute difference (deflected profile - control points): 65 m –1000 Final load elevation: 1029 m –500

0 Sediment loads 500 Control points No infill

Deflecon/Elevaon (m) Deflecon/Elevaon 1000

1500

2000

2500 0 10 000 20 000 30 000 40 000 50 000 60 000 70 000 80 000 90 000 100 000 Distance from end of plate (m) (b) Density of infill: 2400 kg/m3 3 –3000 Density of crustal load: 2650 kg/m Width of crustal load block: 16000 m –2500 Height of crustal load: 1224 m Effective elastic thickness: 10557 m –2000 Height of crustal load Summed absolute difference Final elevation of –1500 crustal load (deflected profile - control points: 32 m (after summing to “compacted” infill) Final load elevation: 1049 m –1000

–500

0 Fig. 4. Comparison of flexural models Infill where the basin-filling sediment load is 500 Control points modelled either as (a) a tapered series of (m) Defelecon/Elevaon 1000 load blocks or (b) as filling the space cre- ated by the deflected plate. Both profiles 1500 Infill volume reduced by “compaction” until ρinfill = ρload were created by minimizing the summed 2000 absolute difference between the deflected 2500 plate and the three control points and 0 10 000 20 000 30 000 40 000 50 000 60 000 70 000 80 000 90 000 100 000 result in final load elevations that are Distance from end of plate (m) within 2% of each other.

© 2017 The Authors Basin Research © 2017 John Wiley & Sons Ltd, European Association of Geoscientists & Engineers and International Association of Sedimentologists 7 J. E. Saylor et al.

Model calibration was conducted by comparing the the initial calibration we considered both a semi-infinite modelled 2.6 Ma subsidence profile and associated topog- and infinite plate, as well as considering subsidence due to raphy under a number of different model parameters to the full width of the Jishi Shan topography versus loading the observed modern topography in the Jishi Shan. For by the Jishi Shan half-width. Model results were

Wangjiashan 2.6 Ma 35.61485°N Jishi Fm. 103.07910°E section n = 9 3.6 M Braided 4.5 Ma 1200 Hewangjia Fm. Floodplain

n = 10 6 Ma Braided

Liushu Fm. Floodplain

n Legend = 9 7.6 Ma Paleocurrent orientation & n = 9 number of measurements Detrital zircon U-Pb sample Carboante nodule Lacustrine Gypsum crystals Covered interval Massive (pedogenetically altered) Dongxiang Fm. Laminated Carbonate Imbricated Massive (depositional disorganized)

13.1 Ma

600 Anastomosing Maogou section 35.67265°N 103.348733°E Stratigraphic height (m) Shangzhuang Hewangjia Fm. Floodplain Fm. 500 6 Ma Braided Braided n = 13 Liushu Fm. 14.7 Ma Floodplain

7.7 Ma Hujialiang Fm. Braided

n = 19 Dongxiang Lacustrine Anastomosing 13.1 Ma Fm. Jiaozigou Fm. Marginal Shangzhuang Lacustrine n Fm. = 13 14.7 Ma 200 500 700 800 900 1000 1100 Braided 200 Marginal Jiaozigou Fm. Lacustrine Stratigraphic height (m) n = 11 21.4 Ma 21.4 Ma Braided Braided Marginal Lacustrine Tala Fm. 35.62853°N Tala Fm. Anastomosing n 103.058250°E = 10 35.6494383°N

10300400 0100 29 Ma 29 Ma 103.273317°E 10300400 0100 cy sss cgm cy sss cgm Grain size Grain size

Fig. 5. Stratigraphic sections for the Wangjiashan and Maogou locations. See Fig. 1 for locations.

© 2017 The Authors 8 Basin Research © 2017 John Wiley & Sons Ltd, European Association of Geoscientists & Engineers and International Association of Sedimentologists Topography and basin evolution, NE Tibet compared to observed modern mean topographic relief (a) Wangjiashan and Maogou subsidence Age (Ma) across the Jishi Shan to determine which combination of 30 28 26 24 22 20 18 16 14 12 10 8 6 4 2 0 factors best reproduces the modern observations. A mean 0 16 m/Myr 17 m/Myr 14 m/Myr difference in elevation of 1.0 0.4 km (1r) was calcu- 200 46 m/Myr lated between mean Jishi Shan elevations (3.1 0.4 km, 49 m/Myr 16 m/Myr 400 58 m/Myr 1r) and the mean elevation of the Linxia Basin 29 m/Myr 140 m/Myr (2.1 0.2 km, 1r) along a 10-km-wide swath (Fig. 3). 600 The model which best reproduced the observed modern Maogou Compacted Sediment 800 Thickness

elevation difference was then used for each preceding Thickness (m) Maogou Decompacted Sediment 50 m/Myr time step. 1000 Thickness Wangjiashan Compacted Sediment 95 m/Myr Thickness 1200 62 m/Myr Wangjiashan Decompacted Sediment Thickness RESULTS 1400

(b) Post-14 Ma decompacted sediment accummulation Measured stratigraphic sections Age (Ma) 20 18 16 14 12 10 8 6 4 2 0 The greatest thickness of Cenozoic stratigraphy of the 0 Linxia Basin is at least 1.2 km, located in the western por- 100 tion of the basin adjacent to the Jishi Shan. Strata thin 200 eastward to at least 500 m at the Maogou section, the 300 middle of the basin (Fig. 5; Fang et al., 2003). There is an overall upward-fining trend from the Oligocene Tala 400

– 500

Formation through the top of the middle late Miocene Thickness (m) Dongxiang Formation. This is followed by a minor Wangjiashan Section 600 upward-coarsening trend through the end of the Pliocene. Maogou Section Basin strata are dominated by mudstone with subordinate 700 sandstone and conglomerate beds. 800 Both the Wangjiashan and Maogou sections display two maxima in sediment accumulation rates: the first in Fig. 6. (a) Cumulative compacted and decompacted sediment the middle Miocene and the second in the late Mio- accumulation curves for the Wangjiashan and Maogou locations cene (Fig. 6 and Table S6). Both sections show mini- since 29 Ma. (b) Cumulative compacted and decompacted sedi- mal Paleogene sediment accumulation. Decompacted ment accumulation curves for the Wangjiashan and Maogou sediment accumulation rates in the Wangjiashan section locations since 14 Ma. Post-14 Ma subsidence is attributed pri- increase from ≤17 m Myr1 prior to 21.4 Ma to an marily to flexural loading due to exhumation and uplift of the Jishi Shan and is used in flexural modelling. average of ca. 50 m Myr 1 between 21.4 and 14.7 Ma. However, this early Miocene increase in the Wangji- ashan section is not mirrored by a similar increase in Potential sediment source samples’ zircon U-Pb age the Maogou section. There is an approximately three- spectra are dominated by peaks between 200–300, 400– fold increase in sediment accumulation rates at both 500, 1700–2100, and 2300–2600 Ma ranges. Potential the Wangjiashan and Maogou locations between 14.7 sediment sources including the Cretaceous sandstone and and 13.1 Ma. Sediment accumulation rates return to Jishi Shan are dominated by early Paleozoic zircons. The early Miocene values in both locations from 13.1 to northern sources, including samples from the Laji Shan, 7.6 Ma, before increasing again between 7.6 and 6 Ma. Maxian Shan and Riyue Shan, are also dominated by early Sediment accumulation rates begin to tail off after Paleozoic zircon ages but include minor age modes in the 6 Ma, and, by Pliocene time, the sediment accumula- Permo-Triassic and Paleoproterozoic. The West Qinling tion rates are essentially zero. to the south of the Linxia Basin includes very narrow peaks with a dominant Permo-Triassic mode, a secondary Zircon U-Pb geochronology early Paleozoic age mode, and very minor additions of Paleoproterozoic ages. Finally, Triassic strata of the A total of 2266 new zircon ages from 12 samples are northeast depocenter of the Songpan-Ganzi complex reported here (Table S3 and S4). We combine our sam- (hereafter referred to as the Songpan-Ganzi source) ples with samples from published sources (Lease et al., exposed in the West Qinling and farther south features 2007, 2012a; Weislogel et al., 2010) for plotting as proba- three sub-equal broad peaks slightly overlapping Permo- bility density plots (PDPs). The PDPs show each age and Triassic and early Paleozoic age modes and a 1700– its uncertainty as a normal distribution, and sum all ages 2100 Ma Paleoproterozoic peak. This source also has a from a sample into a single curve (Figs 7, 8 and 9). fourth major Neoarchean–Paleoproterozoic peak between

© 2017 The Authors Basin Research © 2017 John Wiley & Sons Ltd, European Association of Geoscientists & Engineers and International Association of Sedimentologists 9 J. E. Saylor et al.

Source samples Maogou section samples 100 30 (a) Songpan-Ganzi flysch 80 (a) Hewangjia Fm. (6 Ma) NE depocenter 60 n = 112 20 N = 13 40 10 n = 681 20 0 0 West Qinling catchments 80 (b) (b) Liushu Fm. (7.6 Ma) N = 4 60 20 n = 130 n = 337 40 10 Southern and southwestern 20 Number of grains 0 0 (c) Northern sources 120 N = 5 Dongxiang Fm (13.1 Ma) 80 (c) 20 n = 306 n = 99 40 10 Number of grains 0 0 (d) Jishi Shan catchments 160 Relative probability N = 3 120 (d) Shangzhuang Fm. (14.7 Ma) 20 n = 283 80 n = 119 40 10 0 Cretaceous sandstone 80 0 Northern and northwestern (e) n = 106 60 (e) Jiaozigou Fm. (21.4 Ma)

Relative probability 20 40 n = 90 20 10 0 0 500 1000 1500 2000 2500 3000 0 Age (Ma) (f) Tala Fm. (25 Ma) 20 n = 72 Fig. 7. Probability density plots and histograms for source sam- 10 ples. Includes data from this study, as well as from Weislogel et al. (2010) and Lease et al. (2007). See Table S1 for details of 0 Basal Tala Fm. (29 Ma) samples included in each source. (g) 20 n = 95 10

2300 and 2600 Ma comprised of slightly lower proportion 0 0 500 1000 1500 2000 2500 3000 of grain ages. Age (Ma) Multidimensional scaling shows a systematic varia- tion between southern sources versus northern/eastern Fig. 8. Probability density plots and histograms for Maogou sources (Figs 10 and 11 and Table S5). In the section samples. See Table S1 for details of samples included Wangjiashan section there is a systematic increase in here. similarity between basin samples and southern sources between 29 Ma and 14.5 Ma seen in both the Cross- correlation coefficient and the Kuiper V value. Simi- larly, in the Maogou section, there is an increase in Flexural modelling similarity between detrital zircon age spectra from The calibrated flexural model successfully reproduced basin sediments and southern sources, particularly observed topography using a broken plate loaded with between 21 Ma and 14.5 Ma (Fig. 10). Between the full load width of the Jishi Shan (Fig. 12 and 14.5 Ma and 13.1 Ma, there is an abrupt increase in Table S7). Modelled relief using these parameters was the similarity to northern/northwestern sources and 1.3 0.3 km (1r). We model the 13, 7.6, 6 and the basin samples at both locations coincident with 4.5 Ma time slices using a broken plate loaded with the the onset of exhumation in the Jishi Shan. At the full load of the Jishi Shan. Average effective elastic Wangjiashan location, samples continue to decrease in thickness for these models was 10.5 0.5 km. This is similarity with Songpan-Ganzi as well as West Qin- comparable to estimates of 10–20 km calculated from ling sources through the late Miocene while continu- remotely sensed geophysical data (Braitenberg et al., ing to become more similar to northern and eastern 2003; Jordan & Watts, 2005; Deng et al., 2014; Chen sources; particularly, the Cretaceous sandstone and et al., 2015). In comparison, models involving half the Northern Sources. However, this trend is not seen at load width of the Jishi Shan or an infinite plate both the Maogou location. Here, the similarity between produce excess topographic relief. The broken plate basin and northern/northwestern source samples does model loaded with half of the load width of the Jishi Shan not increase after 7.5 Ma. Instead the 6 Ma sample yielded an overestimate of relief (2.6 0.7 km), but reflects a mixture of all potential sediment sources.

© 2017 The Authors 10 Basin Research © 2017 John Wiley & Sons Ltd, European Association of Geoscientists & Engineers and International Association of Sedimentologists Topography and basin evolution, NE Tibet

Wangjiashan section samples the effective elastic thickness and width of the Jishi Shan 40 load do not change systematically with time and have (a) Hewangjia Fm. (6 Ma) 30 means of 10.5 0.5 km (1r) and 16.8 0.2 km (1r), n = 130 20 respectively. As with all inverse problems, this set of solu- 10 tions is nonunique. Therefore, below we compare the 0 results of our flexural model to the detrital zircon and (b) Liushu Fm. (7.6 Ma) 30 N stable isotope records from the Linxia and Xunhua = 2 20 n = 234 basins. 10 0 (c) 11.5 Ma 30 INTERPRETATION n = 100 20 Number of grains 10 Zircon U-Pb geochronology 0 We attribute the increase in similarity between basin (d) Dongxiang Fm (13.1 Ma) 30 n samples and southern (West Qinling and Songpan- = 109 20 10 Ganzi) sources between 29 and 21.4 Ma to erosion of 0 this highland following the Eocene onset of exhuma- Shangzhuang Fm. (14.7 Ma) tion (Clark et al., 2010). Following exhumation of the Relative probability (e) 30 N = 2 20 Laji Shan at ca. 22 Ma (Lease et al., 2011; Duvall n = 229 10 et al., 2013) and cut-off of potential northward sedi- 0 ment dispersal systems, sediment would have been (f) Jiaozigou Fm. (21.4-21.5 Ma) 30 routed axially from the Xunhua Basin in the west east- N = 2 20 ward towards the Linxia Basin, following the approxi- n = 203 10 mate course of the modern Yellow River. This 0 sediment load would have included sediment shed (g) Tala Fm. (29-25 Ma) 80 from the Laji Shan to the north but also contained an N = 2 60 admixture of southern sources, characterized by zircons n = 189 40 20 with U-Pb ages >1700 Ma. The decrease in similarity 0 between basin samples and southern sources between 0 500 1000 1500 2000 2500 3000 14.7 Ma and 13.1 Ma reflects the cut-off of this source Age (Ma) due to the onset of exhumation in the Jishi Shan Fig. 9. Probability density plots and histograms for Wangji- (Lease et al., 2011, 2012a). Currently, only short, steep ashan section samples. Includes data from this study, as well as drainages flow from the West Qinling to Linxia Basin from Lease et al. (2012a). See Table S1 for details of samples and hence do not integrate the broad Paleoproterozoic included here. range of zircon ages present in the Songpan-Ganzi (Figs 1b and 7). Reintroduction of the >1700 Ma age peak at 7.6 Ma yielded an acceptable elastic thickness of 12.0 0.9 km. at the Maogou location and its strengthening by 6 Ma Models involving an infinite plate loaded with the full may represent either dissection of the Jishi Shan to the width of the Jishi Shan produce 1.9 0.6 km of relief west and reestablishment of the paleo-Yellow River with an unacceptably low elastic thickness of course, or direct sourcing of sediment from the West 7.0 0.6 km. Qinling to the south. We favour the former interpreta- Flexural models are constrained by sediment accumu- tion because eastward oriented paleocurrent indicators lation histories and hence pulses of relief generated by in this interval (Fig. 5) implicate a source to the west. flexural modelling of time slices at 13, 7.6, 6 and 4.5 Ma In addition, the last major tectonic activity in the West mirror the pulses of sediment accumulation observed in Qinling occurred at 45–50 Ma (Clark et al., 2010). the subsidence profiles. The surface uplift rate was Major changes in drainage organization in the recently 0.31 0.06 mm yr1 from 14 to 13 Ma. From 13 to exhumed Jishi Shan present a more plausible explana- 7.6 Ma surface uplift rates decrease by more than half to tion for the changes in Linxia Basin than a similar rates of 0.09 0.04 mm yr1, before accelerating again reorganization in the West Qingling >35 Myr after tec- to 0.16 0.25 mm yr1 between 7.6 and 6 Ma. After tonic activity in that range. Permo-Triassic plutons are 6 Ma, surface uplift rates tail off progressively and reach primarily exposed west of the Wangjiashan section uplift rates of zero between 4.5 and 2.6 Ma. By 4.5 Ma, (south of Xunhua Basin, Fig. 1b) while both Permo- relief is greater than, although within uncertainty of, the Triassic and Paleoproterozoic zircons are present in the modern. In contrast to the continuous increase in relief, Songpan-Ganzi sedimentary rocks (Fig. 9a). The

© 2017 The Authors Basin Research © 2017 John Wiley & Sons Ltd, European Association of Geoscientists & Engineers and International Association of Sedimentologists 11 J. E. Saylor et al. detrital zircon ages from the Maogou section suggest 10 Ma, the proportions of Permo-Triassic and an increasing southern/southwestern contribution after >1700 Ma grains do not significantly vary in the Hua- 13.1 Ma, first with the increase in ca. 1700 Ma zircons long section (Lease et al., 2012a), suggesting that the at 7.6 Ma and then with the increase in both Permo- Xunhua Basin was not cut-off from the southern Triassic and Paleoproterzoic zircons at 6 Ma (Fig. 10a– source during the Neogene as was the Linxia Basin. c). This is mirrored by an increase in the southwestern We interpret the diminished proportion of >1700 Ma source in the Wangjiashan section through 7.6 Ma grains at the Wangjiashan location after 7.6 Ma as a (Fig. 11b). However, that trend is reversed in the result of being off-axis relative to the now entrenched Wangjiashan section between 7.6 and 6 Ma, when the integrated sediment dispersal system cutting through relative abundance of both Permo-Triassic and ca. the uplifted Jishi Shan. Sediments at the Wangjiashan 1700 Ma zircons decrease (Fig. 11a–b). Where data are location are dominantly locally sourced from the wind- available from the Xunhua Basin between 19 and ward (eastern) side of the Jishi Shan.

(a) Cross-correlation coefficient West Qinling Shepard Plot, 0.4 stress = 0.18506 Jiaozigou (21.4 Ma) Distances Dongxiang (13.1 Ma) 1.2 Disparities Basal Tala (29 Ma) 1:1 line 0.2 1 Jishi Shan Tala (25 Ma) 0.8 0 0.6 Hewangjia (6 Ma) Songpan 0.4 Northern Sources Ganzi –0.2 Distances/Disparities 0.2

0 Liushu (7.6 Ma) 0 0.5 1 Dissimilarities –0.4 Cretaceous Ss Chronological order Shangzhuang (14.7 Ma) of samples Onset of exhumation –0.4 0 0.4 in Jishi Shan

(b) Kuiper V value 0.4 Jishi Shan Shepard Plot, stress = 0.12321 Distances Disparities 1 1:1 line 0.2 West Qinling Dongxiang (13.1 Ma) 0.8

Songpan Basal Tala (29 Ma) 0.6 Ganzi Northern Sources 0 Hewangjia (6 Ma) 0.4

Shangzhuang Distances/Disparities 0.2 (14.7 Ma) Liushu (7.6 Ma) Tala (25 Ma) 0 Jiaozigou (21.4 Ma) 0 0.5 1 –0.2 Dissimilarities Cretaceous Ss

–0.4 0 0.4

Fig. 10. Two-dimensional multidimensional scaling plot of the source and Maogou section samples. Dissimilarity is based on (a) the complement of the Cross-correlation coefficient or (b) the Kuiper V value. Both metrics show increased similarity between basin sam- ples and southern (Songpan-Ganzi) sources between 29 and 14.7 Ma, followed by an abrupt increase in similarity to northern sources between 14.7 and 13.1 Ma. The late Miocene samples become marginally more similar to southern sources, likely reflecting a well- mixed source area.

© 2017 The Authors 12 Basin Research © 2017 John Wiley & Sons Ltd, European Association of Geoscientists & Engineers and International Association of Sedimentologists Topography and basin evolution, NE Tibet

(a) Cross-correlation coefficient Shepard Plot, Songpan-Ganzi stress = 0.14384 0.4 Distances Cretaceous Ss Disparities 1:1 line Hewangjia (6 Ma) s 1 0.2 Shangzhuang (14.7 Ma) Northern Sources 0.5 0 Jiaozigou Liushu (21.4 Ma) (7.6 Ma) Dongxiang (11.5 Ma) Distances/Disparitie

–0.2 Dongxiang (13.1 Ma) Jishi Shan 0 0 0.5 1 Tala (25 Ma) Dissimilarities

–0.4 Chronological order West Qinling of samples Onset of exhumation –0.4 0 0.4 in Jishi Shan

(b) Kuiper V value 0.6 Shepard Plot, stress = 0.12182 Distances Jishi Shan 1 Disparities 0.4 1:1 line s 0.8

0.6 0.2 West Qinling 0.4 Liushu (7.6 Ma) Songpan Ganzi Dongxiang (11.5 Ma)

Distances/Disparitie 0.2 0 Northern Sources Shang- Dongxiang (13.1 Ma) zhuang 0 (14.7 Ma) 0 0.5 1 Hewangjia (6 Ma) Dissimilarities –0.2 Jiaozigou Tala (25 Ma) (21.4 Ma) Cretaceous Ss

–0.4 0 0.4

Fig. 11. Two-dimensional multidimensional scaling plot of the source and Wangjiashan section samples. Dissimilarity is based on (a) the complement of the Cross-correlation coefficient or (b) the Kuiper V value. Both metrics show increased similarity between basin samples and southern (Songpan-Ganzi) sources between 29 and 14.7 Ma, followed by an abrupt increase in similarity to northern sources between 14.7 and 13.1 Ma. The late Miocene samples become more similar to northern/northwestern sources, likely reflect- ing continued dominance of the Jishi Shan as a source for this location. The absence of a basin response to early Miocene Subsidence analysis exhumation suggests that the Laji Shan was the primary We attribute the increase in sediment accumulation and subsidence driver, but that the flexural effect of exhuma- subsidence rate in the Wangjiashan section at ca. 21.4 Ma tion of loading by the Laji Shan was attenuated in the to the onset of exhumation in the Laji Shan possibly in Maogou location and only affected proximal locations. combination with resumed exhumation in the West Qin- Exhumation of the Maxian Shan appears to have had little ling (Clark et al., 2010; Lease et al., 2011). Although effect on sediment accumulation rates in Linxia Basin. thermochronology results point to early Miocene The Laji Shan experienced rapid cooling initially, fol- exhumation of the Maxian Shan (Duvall et al., 2013), the lowed by slow cooling after 18 Ma (Lease et al., 2011), increase in sediment accumulation rates at 21.4 Ma is not suggesting that while exhumation of the Laji Shan influ- observed in the nearby Maogou section. Furthermore, the enced basin subsidence, it was not the primary driver in basin fill thins from the Maogou location towards the middle Miocene. We attribute the middle Miocene the Maxian Shan (i.e. towards the modelled zero edge). increase in sediment accumulation, which is observed in

© 2017 The Authors Basin Research © 2017 John Wiley & Sons Ltd, European Association of Geoscientists & Engineers and International Association of Sedimentologists 13 J. E. Saylor et al.

Full load width, Broken plate and aridity on the windward and lee sides of the develop- ing mountain range. Divergence in the isotopic record of Full load width, Infinite plate previously contiguous basins can therefore be used to Half load width, Broken plate track the timing of surface uplift of basin-dissecting ranges (Kleinert and Strecker, 2001; Hough et al., 2011). Modern Jishi Shan relief Cooling and adiabatic expansion of water vapour leads to

2.5 3 3.5 isotope fractionation between the liquid and vapour phases while vapour ascends the windward side of a mountain range, causing a decrease in the d18O values of 2 18 precipitation (d Op values; Dansgaard, 1964; Rozanski et al., 1993; Rowley, 2007). Relief can also produce a 18 decrease in d Op values on the windward side of Relief (km)

(a) Xunhua Basin Linxia Basin δ18O (‰, VPDB) δ18O (‰, VPDB) –12 –8 –4 0 –10 –8 –6 –4 –2 0 6 6

00.511.5 2 01234567891011121314 2 Age (Ma) 8 8

Fig. 12. Comparison of the final load elevation from flexural Age (Ma)

1 Age (Ma) modelling and modern relief in the Jishi Shan. Models with either 1 a half load width or infinite (unbroken) plate overestimate the relief generated for the 2.6 Ma step. The full load with a semi- 10 Lacustrine 10 infinite (broken) plate model best reproduces modern relief and is used as the basis for modelling relief generation through time. the Wangjiashan and Maogou sections and also in Xun- (b) 0 0 hua Basin to a pulse of exhumation in the Jishi Shan that 2 2 began at ca. 14 Ma (Lease et al., 2011, 2012a). The subse- 4 4

quent decrease in sediment accumulation rates, also 6 Floodplain6 Floodplain observed at both the Wangjiashan and Maogou locations, 8 8 suggests that the rate of topographic growth in the Jishi Shan may have decreased between 13.1 and 7.7 Ma. If 10 10 accurate, this complements thermochronological evidence Age (Ma) 12 12 Lacustrine Fluvial for continued rapid exhumation of the Jishi Shan between 14 Fluvial 14 ca. 14–13 Ma and ca. 6 Ma (Lease et al., 2011) and sug- 16 16 gests that increased erosion may have offset rock uplift in Age (Ma)

18 Lacustrine18 Lacustrine Lacustrine the late Miocene. The increase in sediment accumulation 20 20 Fluvial Fluvial rates at ca. 7.6 Ma is coincident with additional tectonic Fluvial 22 activity in the region including an interpreted 10° degree –16 –12 –8 –4 0 18 Marginal clockwise rotation of the Linxia Basin (Fang et al., 2003) δ O (‰, VPDB) Lacustrine 24 and also with eastward propagation of the Kunlun and Xunhua Basin 26 Haiyuan faults and incorporation of the Linxia region into Fluvial 28 the Kunlun-Haiyuan transpressional corridor (Duvall 30 et al., 2013). –12 –10 –8 –6 –4 –2 0 δ18O (‰, VPDB) Linxia Basin

DISCUSSION Fig. 13. Oxygen isotopic data for the Linxia and Xunhua Development of relief and rainshadows basins. (a) Shows the two post-9.3 Ma divergent trends in d18O values. (b) Shows the pre-9.3 Ma lock-step evolution of the two Development of relief can drive divergence in precipita- stable isotope records. Data are from Hough et al. (2014) and 18 18 tion amount, d O values of precipitation (d Op values) Dettman et al. (2003).

© 2017 The Authors 14 Basin Research © 2017 John Wiley & Sons Ltd, European Association of Geoscientists & Engineers and International Association of Sedimentologists Topography and basin evolution, NE Tibet mountain ranges by an increase in the absolute precipita- Basin subsequently remain low and show no trend tion amount (the “amount effect”: e.g. Vuille et al., 2003, through 6 Ma (range = 7.2 to 9.4 &). 2005). Rainout of water vapour in the windward side In the modern system, there is a 1.2 & difference in results in development of an arid rainshadow on the lee the d18O values of summer (May–September) precipita- side of the mountain range. A decrease in relative humid- tion in the Linxia and Xunhua basins (Hough et al., ity following topography-induced changes in atmospheric 2011). Barring a significant change in the seasonality of 18 circulation may cause an increase in d Op values due to precipitation or carbonate formation, the 0.9 & increase 18 subcloud evaporation and in surface waters due to evapo- in paleosol d Oc values between 8.83 and 6 Ma in the rative enrichment of 18O (Bershaw et al., 2012, 2016). Xunhua Basin is consistent with development of 75% of Divergence in stable isotope data (compiled from Dett- the modern rainshadow effect across the Jishi Shan. Fur- 18 man et al. (2003) and Hough et al. (2014)) and deposi- thermore, the increase in carbonate d Oc values in the tional environments between the Xunhua and Linxia Xunhua Basin corresponds to rapid topographic growth basins provide evidence for hydrological separation of in the Jishi Shan between ca. 8.8 and 6 Ma (Fig. 12). 18 these two basins followed by development of a rainshadow However, the increase in d Oc values does not linearly in the Xunhua Basin. In both basins, the minimum, mean, correlate with increasing calculated paleorelief, although and maximum d18O values of paleosol and lacustrine car- quantification of the relationship between the rate of rain- 18 bonates (d Oc values) increase from 18–19 Ma to ca. shadow development and topographic development is 9.3 Ma (Fig. 13). There is a gap in the stable isotopic beyond the scope of this work. In making this interpreta- record in the Xunhua Basin from 16.22 to 10.5 Ma. How- tion, we note that the observed 0.9& increase in the car- 18 ever, following the hiatus, between 10.5 and 9.3 Ma, the bonate d Oc values, although statistically robust, may be Xunhua and Linxia sections share similar most-negative the result of incomplete sampling of an extremely noisy 18 d Oc values (8.7 and 9.3 &, respectively, Fig. 13). record and that the modern stable isotope record to which Furthermore, ANOVA analysis indicates that the mean it is compared was collected over ca. 1.5 years (Hough 18 d Oc values from the two data sets between 10.5 and et al., 2011). 9.3 Ma cannot be discriminated at the >95% confidence level using a standard t test (mean and variance of Relief generation and basin evolution 4.8 3.8 & and 7.0 7.7 &, for Xunhua and Lin- xia basins, respectively, t(8) = 1.97, p = 0.08). Following Changes in the provenance, stable isotopic, sediment previous studies, we interpret these changes as respond- accumulation history, and flexural model results link the ing to regional climate changes (e.g. Dettman et al., exhumation and topographic evolution of the Jishi Shan. 2003). At ca. 22 Ma, prior to the onset of Jishi Shan exhuma- Following 9.3 Ma, the stable isotope records in the tion, exhumation of the Laji Shan cut-off northward Xunhua and Linxia basins show two divergent trends sediment dispersal systems (Fig. 14a). This resulted in possibly correlated the growth of Jishi Shan topography. rerouting of sediment dispersal systems axially from the 18 The initial divergence occurs at ca. 9 Ma when d Oc Xunhua Basin in the west, eastward towards the Linxia values in the Xunhua Basin decrease to minimum values Basin, following the approximate course of the modern of 9.4 & (mean for 9–8.75 Ma = 8.4 &). Over the Yellow River (Hough et al., 2014). Geochronological 18 same interval mean d Oc values in the Linxia Basin analysis of detrital zircons from the modern Yellow remain unchanged. The second divergent trend between River above Linxia shows a prominent age mode 18 the two basins marked by an increase in mean d Oc val- between 1500 and 2100 Ma (Nie et al., 2015). Axial ues in the Xunhua Basin and decreasing mean and invari- routing of these southern-sourced sediment dispersal 18 ant minimum d Oc values in the Linxia Basin. After systems accounts for the increase in proportion of zir- 18 8.83 Ma paleosol carbonate d Oc values in the Xunhua cons with U-Pb ages >1700 Ma between 29 and 18 Basin increase at a rate of 0.28 &/Myr. Mean d Oc val- 14.5 Ma (Figs 8 and 9). It also accounts for the increase ues for the final 0.5 Myr where the basins share records in similarity between basin samples and the Songpan- (6.5–6.0 Ma) are 7.5 &: 0.9 & higher than when the Gonzi flysch compilation (Figs 7, 10 and 11). increasing trend began (9–8.75 Ma). This trend is The increase in similarity with northern and north- robust, with samples from the final 0.5 Myr being differ- western sources between 14.7 Ma and 13.1 Ma reflects entiable from the first 0.5 Myr at the 95% confidence the cut-off of the southern source and enhancement of level based on a t test (mean and variance of 8.3 0.8 contributions from northern and northeastern sources & and 7.4 0.4 &, for the beginning and ends, due to the onset of exhumation in the Jishi Shan (Lease respectively, t(10) = 2.22, p = 0.04). In contrast to this et al., 2011). However, the stable isotopic record in both 18 18 record, maximum and mean d Oc values from the Linxia basins trends towards more positive d Oc values suggest- Basin decrease abruptly at ca. 8 Ma while minimum val- ing that both basins were dominated by a similar climate 18 ues remain unchanged. The d Oc values in the Linxia regime. Flexural modelling indicates development of

© 2017 The Authors Basin Research © 2017 John Wiley & Sons Ltd, European Association of Geoscientists & Engineers and International Association of Sedimentologists 15 J. E. Saylor et al.

Decreasing d18O values and a change from lacustrine to (a) Laji Shan 22-14 Ma c -Exhumation of Laji fluvial depositional environments in the Xunhua Basin West Qinling Shan Shan & West suggest a breaching of the Jishi Shan dam at 9.3 Ma. Qinling Shan Prior to 9.3 Ma the Xunhua Basin was dammed, but -Routing of sediment hydrologically open as indicated by the scarcity of evap- to Linxia Basin 18 orite deposits and lack of correlation between d Oc and 13 d Cc values during this time interval (Lease et al., Sediment routing Slight relief/ minor sediment source 2012a; Hough et al., 2014). The transition to fluvial Thrust faults Extreme relief/ deposition in the basin is complete by at 8.8 Ma (Hough major sediment source Lacustrine Extent of moisture et al., 2014). Following our preferred interpretation out- Fluvial advection lined above, reintroduction of the >1700 Ma age peak

(b) 14-9.3 Ma by 7.5 Ma at the Maogou and Wangjiashan locations -Exhumation of Jishi points to upstream–downstream connectivity between Laji Shan Shan the Xunhua and Linxia basins (Lease et al., 2012a). The -Cut-off of southern Jishi Shan increase in this population by 6 Ma at Maogou, and its Xunhua Basin sediment source to Linxia Basin decrease at Wangjiashan likely represents reestablish- Linxia Basin ment and entrenchment of the paleo-Yellow River course (Fig. 14c). This entrenchment resulted in bypass West Qinling Shan of sediment from the southern sources at the Wangji- ashan location which becomes less similar to southern/ southwestern sources after 7.6 Ma. The end of lacustrine conditions in the Xunhua Basin (c) 9.3-~8.9 Ma and transition to fluvial conditions at 8.8 Ma is accom- -Re-establishment of panied by an apparent increase in aridity in the basin. Laji Shan southern sediment source to Linxia This aridity is not expressed in the Linxia Basin record Jishi Shan d18 Xunhua Basin Basin as the lowest O values remain unchanged through -Establishment of this interval (Fig. 13), while that in Xunhua Basin Yellow River gorge 18 Linxia Basin shows a slight increase in d O values. The syn- chronously increasing aridity on the leeward side of the West Qinling Shan Jishi Shan, and decrease in mean d18O values and stabil- ity in minimum d18O values in the Linxia Basin d18O record (e.g. Fan et al., 2007) suggests that by 8 Ma the Jishi Shan achieved sufficient elevation to begin to inter- (d) 8.9-2.6 Ma -Continued uplift of cept moisture-bearing airmasses, and that hydrologic Laji Shan Jishi Shan separation occurred later than previously thought -Hydrological isolation Xunhua Jishi Shan (Fig. 14d; Hough et al., 2011, 2014). Basin of Xunhua Basin Flexural modelling indicates that there was Linxia Basin 0.8 0.2 km of relief in the Jishi Shan by 8 Ma. This is consistent with the 1 km of relief needed to induce oro- West Qinling Shan graphic precipitation in the Himalaya and Andes (Bookhagen & Burbank, 2006; Bookhagen & Strecker, 2012). After 8 Ma, flexural modelling shows that relief continued to increase until ca. 4.5 Ma, after which it sta- bilized. Continued development of relief in the Jishi Shan Fig. 14. Schematic paleogeographic maps showing the Neo- between 8 and 4.5 Ma is also consistent with increased gene evolution of the Linxia and Xunhua basin pair. influx of Jishi Shan-sourced sediment into the proximal Linxia Basin (the Wangjiashan location) reflected by minor relief (0.3 0.1 km) by 13 Ma suggesting that, increased similarity between basin sediments and north- although sufficient relief had been generated to disrupt ern/northwestern sources (Fig. 10). and pond the fluvial systems, it was insufficient to inter- cept moisture-bearing airmasses. Between the onset of exhumation of the Jishi Shan at CONCLUSIONS 14 Ma and 9.3 Ma, the lacustrine systems of the Xun- hua and Linxia basins experienced a similar evolution: This multidisciplinary study combines sediment prove- effected by similar climate regimes (Fig. 14b). nance, depositional environment reconstruction, stable

© 2017 The Authors 16 Basin Research © 2017 John Wiley & Sons Ltd, European Association of Geoscientists & Engineers and International Association of Sedimentologists Topography and basin evolution, NE Tibet isotope geochemistry, and flexural modelling to recon- SUPPORTING INFORMATION struct the paleorelief of the Jishi Shan. Detrital zircon U-Pb data document increasing contributions Additional Supporting Information may be found in the from south–southwest sources including the Qinling online version of this article: Shan and northeast depocenter of the Songpan-Ganzi flysch complex prior to 14 Ma. Exhumation of the Data Repository Text. Detailed zircon U-Pb geochro- Jishi Shan beginning at ca. 14 Ma cut off the south- nology methods. ern sediment fairways, and resulted in domination of Figure S1. Cumulative compacted and decompacted the Linxia Basin record by northern–northwestern sediment accumulation curves for the Wangjiashan and sediment sources from 13.1 to ca. 8 Ma. Flexural Maogou locations since 29 Ma using the alternative modelling shows that at 13 Ma, the Jishi Shan had chronology of Deng et al. (2014). developed ca. 0.3 km of relief: sufficient to disrupt Table S1. List of samples used in this study. eastward-flowing drainage networks but insufficient to Table S2. Compaction coefficients. intercept moisture-bearing airmasses. Hence, the Table S3. U-Pb data table from University of Houston. stable isotopic records in the Xunhua and Linxia Table S4. U-Pb data table from University of Arizona. basins remain coupled until ca. 9.3 Ma. Specifically, Table S5. MDS results presented in Fig. 10 and 11. d18 mean and minimum Oc values for the interval Table S6. Subsidence results presented in Fig. 6. 10.5–9.3 Ma are indistinguishable in the two basins. Table S7. Flexural modelling results presented in We propose that by 9.3 Ma the paleo-Yellow River Fig. 12. reestablished its current course, permanently recon- Data S1. MATLAB (.m) files for MDS algorithm necting the fluvial network between the Xunhua and (DZmds). Linxia basins. Continued exhumation and uplift of Data S2. DZmds.exe file. the Jishi Shan resulted in sufficient relief to intercept Data S3. MATLAB (.m) files for flexural modelling. moisture-bearing airmasses at ca. 8 Ma, resulting in isolation and aridification of the Xunhua Basin and a decrease in aridity in the Linxia Basin. Flexural mod- REFERENCES elling indicates that the Jishi Shan had developed ca. 0.8 km of relief before triggering this transition. ALLEN,P.&ALLEN, J. (2005) Basin Analysis: Principles and Appli- Relief continued to be generated, with >95% of total cations. 2nd edn, Blackwell Scientific Publications, Oxford. relief generated by 4.5 Ma. During this time the Jishi AN, Z.-S., KUTZBACH, J.E., PRELL, W.L. & PORTER, S.C. 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