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Earth and Planetary Science Letters 287 (2009) 298–310
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Earth and Planetary Science Letters
j ourna l homepage: www.e lsev ie r.com/ locate /eps l
The Neogene Xiyu Formation, a diachronous prograding gravel wedge at front of theTianshan: Climatic and tectonic implications
Julien Charreau a,b,⁎, Charles Gumiaux c, Jean-Philippe Avouac a,1, Romain Augier c,2, Yan Chen c,2,Laurie Barrier d,3, Stuart Gilder e, Stéphane Dominguez f,4, Nicolas Charles c,2, Qingchen Wang g
a Tectonics Observatory, California Institute of Technology, Mail Code 100-23, Pasadena CA 91125, USAb Centre de Recherche Pétrographique et Géochimique, 15 rue Notre Dame des Pauvres, 54501 Vandoeuvre lès Nancy, Francec Université d'Orléans, CNRS/INSU, Université François Rabelais — Tours, Institut des Sciences de la Terre d'Orléans — UMR 6113, Campus Géosciences, 1A, rue de la Férollerie,45071 Orléans Cedex 2, Franced Institut de Physique du Globe de Paris, 4 place Jussieu, 75252 Paris Cedex 05 Francee Ludwig Maximilians University, Department of Earth and Environmental Sciences, Geophysics Section, Theresienstrasse 41, 80333 Munich, Germanyf Laboratoire Dynamique de la Lithosphère; UMR CNRS/UMII 5573, Bâtiment 22 [cc. 60] Place Bataillon, 34095 Montpellier Cedex, Franceg State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China
⁎ Corresponding author. Centre de Recherche PétrograNotre Dame des Pauvres, 54501 Vandoeuvre lès Nancy,fax: +33 383511798.
E-mail addresses: [email protected], jcha(J. Charreau), [email protected] (C. Gum(J.-P. Avouac), [email protected] (R. Augie(Y. Chen), [email protected] (L. Barrier), gilder@ge(S. Gilder), [email protected] (S. [email protected] (N. Charles), qcwang@m
1 Tel.: +1 626 395 4239; fax: +1 626 395 1740.2 Tel.: +33 238417206; fax: +33 238636488.3 Tel.: +33 144274934; fax: +33 144277463.4 Tel.: +33 0467144589; fax: +33 0467523908.
0012-821X/$ – see front matter © 2009 Elsevier B.V. Adoi:10.1016/j.epsl.2009.07.035
a b s t r a c t
a r t i c l e i n f oArticle history:Received 25 February 2009Received in revised form 24 July 2009Accepted 28 July 2009Available online 28 August 2009
Editor: T.M. Harrison
Keywords:Xiyu FormationmagnetostratigraphyTianshangravel wedge progradationshortening rates
The Tarim and Junggar basins in central Asia are capped by a thick pile of conglomerates, called the XiyuFormation, that are commonly linked to a change in climate and/or accelerated uplift near the Plio-Pleistocene boundary. In order to better understand their origin and significance, we carried out a combinedstructural and magnetostratigraphic study in the Quilitage syncline (southern Tianshan), where the base ofthe Xiyu conglomerates is observed at both sides of the syncline. A balanced cross-section shows that, evenat a local-scale, the base of the Xiyu conglomerates cannot be regarded as a single continuous stratigraphiclayer. On the southern flank of the Quilitage syncline, we collected 172 samples collected formagnetostratigraphic dating identify 17 polarity chrons that date the new section from 5.2 to ~1.7 Maand constrain the base of the Xiyu conglomerate here at ~1.7 Ma. This is 4.2 Ma younger than the age of theXiyu previously found on the northern limb of the same syncline. Together with other magnetostratigraphicstudies carried out around the Tianshan, our study unambiguously demonstrates that the onset of depositionof the Xiyu conglomerates is diachronous, and that the conglomerates are systematically younger toward thebasin. Consequently, the Xiyu Formation should not be considered as a chronostratigraphic marker related toany particular tectonic or climatic event, but is instead a prograding gravel wedge that has prograded overthe underthrusting forelands. A synthesis of chronologic and structural results yields progradation rates overthe last 10 Ma on the order of ~2.0 mm/yr and ~3.9 mm/yr south and north of the Tianshan Mountainsrespectively. These rates are comparable to the shortening rate across the Tianshan range, suggesting thatunderthrusting is the main factor governing the progradation rate of the Xiyu Formation.
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1. Introduction
As a mountain range grows surface processes become more activeand result in a mass transfer from the uplifted zone to the flankinglowlands. This mass transfer depends on climate and can significantlyinfluence tectonics by contributing to uplift or subsidence throughisostatic compensation (Beaumont et al., 1988; Cobbold et al., 1993;Molnar and England, 1990; Schlunegger et al., 1997; Whipple andMeade, 2006), and bears on deformation and thermal structure by itseffect on heat advection (Avouac and Burov, 1996; Batt and Braun,1997; Beaumont et al., 1994; Jamieson and Beaumont, 1988).Mountain building is, therefore, a complex process resulting fromcoupling between tectonic deformation, climate and surface processes(Avouac and Burov, 1996; Burbank, 1992; Koons, 1987; Molnar andEngland, 1990). Foreland basin sediments thus provide a record of this
Fig. 1. (a) Map of central Asia with the repartition in grey of supposed Quaternary sediments and highlighted in yellow are the piedmont where the Xiyu Formation is identified.Black stars indicate locations of magnetostratigraphic studies that provide basal ages of the Xiyu Formation with (1) Charreau et al. (2009), (2) Charreau et al. (2005), (3) Charreauet al. (2009), (4) Huang et al. (2006), (5) Chen et al. (2002), (6) Heermance et al. (2007), (7) Zheng et al. (2000); (8) Bullen et al., 2001; (b) Geologic map of the northern Tianshanpiedmont with location of the Jingou He (Charreau et al., 2009) and Kuitun sections (Charreau et al., 2005). Black horse indicates the locationwhere fossil Equus Sameniensis has beenfound; (c) Geological map of the southern Kuche piedmont and location of the Yaha section (Charreau et al., 2006). (For interpretation of the references to color in this figure legend,the reader is referred to the web version of this article.)
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process in which the respective influence of tectonics and climate isoften difficult to untangle.
The Tarim and Junggar basins in central Asia (Fig. 1) accumulateeroded materials from the nearby Tianshan mountains. As closedintracontinental basins, their thick Cenozoic sedimentary coverrepresents prime examples where one can estimate the mass erodedfrom the mountain and thus quantify transfer processes. Folding andthrusting on both the northern and southern piedmonts exposeexceptional outcrops of the sediments at numerous sections. In bothbasins, the top of the sedimentary fill consists of remarkably thickdark gravels, called the Xiyu Formation (Figs. 1 and 2), that one can beused to quantify mass transfer across the basins.
In 1953, Chinese geologist C.L. Fang and his colleagues found avertebrate fossil of Equus sanmeniensis in the Anjihaihe section at thetransition between the Dushanzi and the Xiyu Formations (Fig. 1).This fossil was dated from 2.48 Ma to the early Pleistocene (Chenet al., 1994; Feng and Dai, 2004; Gaboardi et al., 2005; Zhu et al.,2004). Xiyu-type conglomerates are widely distributed along theperiphery of the major mountain ranges in central Asia (Fig. 1), andare commonly lithostratigraphically assigned to the early PleistoceneXiyu Formation throughout central Asia (Huang et al., 2006; Sun et al.,2007; Sun et al., 2004). The age designation stemming from thefinding of E. sanmeniensis in the Anjihaihe section has thus hadconsiderable impact. Some workers argue that the conglomeratewould signify a late Plio-Pleistocene acceleration in uplift of theTianshan (Burchfiel et al., 1999) or of the Tibet Plateau (Zheng et al.,
Fig. 2. (a) Panorama of the Xiyu/Dushanzi transition in the Kuitun section with enlargemensection located ~20 km west of the Yaha section (see Fig. 1); (c) Panorama of the Xiyu/Kuch(d) Photograph of the Xiyu/Kuche transition of the Yaha section in south of the Quilitagconglomeratic layers are thinner toward south and evolve laterally toward sandstone; (e(f) Synthetic stratigraphic log from the Jingou He section showing the characteristic lithology
2000). Alternatively, since the Plio-Pleistocene transition is charac-terized by a transition to widespread glaciation after a relatively warmperiod during the Mio-Pliocene (Zachos et al., 2001; Zhang et al.,2001), other workers argue that the Xiyu Formation must be linked toclimate change (Liu et al., 1996; Zhang et al., 2001).
Recentmagnetostratigraphic studies show that the basal age of theXiyu conglomerate could be much older (by up to ~15 Ma) than theearly Pleistocene (Charreau et al., 2009; Charreau et al., 2005;Heermance et al., 2007). However, others studies still assume aPleistocene age (Huang et al., 2006; Sun et al., 2007; Sun et al., 2004).As the studied localities are often far apart and because the definitionof the Xiyu Formation is loose, the depositional age and the origin ofthe Xiyu conglomerates remain a matter of controversy. Resolvingthis debate (e.g. Charreau et al., 2008b) would bring importantimplications regarding regional tectonics and late Cenozoic climatechange.
The purpose of this paper is, therefore, to clarify the definition ofthe Xiyu Formation and accurately determine its age range. Ouranalysis is based on existing magnetostratigraphic constraints andnew results acquired from a section along the Yaha River, in thesouthern Tianshan piedmont, which, thanks to a singular tectonicsetting, unambiguously demonstrates the diachronism of the Xiyuconglomerates at a local-scale.
Hereafter, we first give an overview of the regional geologicalsetting. Next, we propose a strict sedimentological definition of theXiyu Formation — that should provide a basis for future work in the
t of the transition on the left; (b) Panorama of the Xiyu/Kuche transition in the Kuchee transition in the Yaha section north of the Quilitage syncline (see location in Fig. 4);e syncline (see location in Fig. 4) which shows lateral lithological variations as dark) topographic map of the Tianshan showing location of photograph and panorama;and facies of the Xiyu Formation; (g) Synthetic stratigraphic log from the Yaha section.
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region — and then move onto the presentation of the results withthe magnetostratigraphic dating of the Yaha section. Finally wediscuss the nature, tectonic and climatic significance of the Xiyuconglomerates.
2. Geological setting of the Tianshan range
The Tianshan is a 2500-km-long tectonically active mountainrange with an average altitude of 2500 m and summits reaching up to7000 m. Its geology consists of Paleozoic rocks related to a long-livedIsland Arc and subduction/collision history that started during theDevonian and ended during the Permian time (Burtman, 1975;Charvet et al., 2007; Gao et al., 1998; Lin et al., 2008;Wang et al., 2008;Windley et al., 1990). The present high topography mainly owes itsorigin to the Cenozoic reactivation of the range during the Oligoceneto late Miocene (Avouac et al., 1993; Bullen et al., 2003; Bullen et al.,2001; Dumitru et al., 2001; Hendrix et al., 1994; Métivier andGaudemer, 1997; Sobel et al., 2006; Sobel and Dumitru, 1997;Windley et al., 1990), under the influence of the ongoing India–Asiacollision. The Tianshan range is sandwiched between two largeintracontinental basins (Fig. 1) that contain the erodedmaterials fromthe uplifting range.
3. Geology of the piedmonts
Both piedmonts are tectonic wedges composed of fold and thrustbelts where early Mesozoic to Quaternary sediments that wereinitially deposited in the foreland basin, are now overthrustedand well exposed along numerous north–south-trending rivers(e.g. Kuitun He, Jingou He or Yaha, Fig. 1b and c). During the 1990s,several studies focused on the structural analysis of the Tianshanpiedmonts (e.g. Avouac et al., 1993; Burchfiel et al., 1999). Morerecently, numerous seismic line profiles made for exploration of gas–oil resources have greatly improved our knowledge of the deepframework of the basins (e.g. Charreau et al., 2008a; Heermance et al.,2007; Hubert-Ferrari et al., 2007). We focused our study in two areas,on both sides of the range: (1) in the northern Tianshan, west ofUrumqi, where the piedmont consists of three main fold and thrustsequences that are respectively offset in a left lateral sense from southto north (Fig. 1b), (2) in the southern Tianshan piedmont is a2000 km-long tectonic wedge extending from the Kuche basin to theKashi area. Our study was carried out in the Kuche area where twoanticlines can be recognized: the Quilitage anticline and the morefrontal Yakeng anticline (Fig. 1c). Seismic imaging defines theQuilitage anticline as a 225 km-long fold bend-fault while the Yakenganticline may be seen as a detachment fold (see seismic line in Fig. 4from Hubert-Ferrari et al. (2007)). Between these anticlines and thehigh range, the piedmont contains several other folds and thrustswhich mainly involve Paleozoic to Mesozoic sediments.
4. Definition of the Xiyu Formation
The Xiyu conglomerates were first described by Obruchev in the1930s. Later on, T. K. Huang later described their lithology in moredetail and classified the Xiyu (Western domain in Chinese) Formationas all piedmont-type, conglomeratic layers composed of dark-gray tobrown-colored, coarse (3 to N30 cm) and thick (50 to 3200 m)deposits that are intercalated with scarce yellow sandstone andmudstone layers (Zhou et al., 2000). This description suggeststhe Xiyu is a coherent and continuous unit lying at the top of thesedimentary pile in most of the central Asia's foreland basins.
The Xiyu conglomerates can be, however, more rigorouslycharacterized according to their lithology, color and sedimentarystructures that typify their depositional environment. They consist ofhomolithic, clast-supported, poorly to weakly sorted, massive tohorizontally- and cross-bedded conglomerates made up of pebble to
blocky clasts and a coarse-grained sandymatrix. Those conglomeratesare exposed in 1 to 10s of meters-thick and 1000s of meters-wideamalgamated strata with erosive basal boundaries. Occasionally, theycontain massive to cross-bedded, medium to coarse-grained sand-stones lyingwithin 0.1 to 1meter-thick and 1 to 10meter-wide lenses.Despite the poor sorting of the Xiyu deposits, their horizontal troughcrossbedded stratification indicate motion and deposition from grain-by-grain traction sedimentation under a unidirectional upper flowregime (Simons et al., 1965; Southard and Boguchwal, 1990). Thosesediments can thus be interpreted as bed load deposits at the base ofhigh-velocity and turbulent water flow that generate large-scalegravelly bed-forms as the bars formed into the gravely braidedstreams (Gloppen and Steel, 1981; Mack and Leeder, 1999; Miall,1977, 1978). The Xiyu Formation implies therefore the repeatedoccurrences of powerful stream flows typical of gravely braidedalluvial fans (Blair and McPherson, 1994; Nemec and Postma, 1993),which still exist in the Tianshan piedmont (Fig. 3).
At their base, the Xiyu conglomerates gradually covered theunderlying units (e.g. the Dushanzi, Kuche, Artux and Aksu forma-tions), which are composed of finer-grained deposits (heterolithic,clast-supported, massive and horizontal to cross-bedded conglom-erates alternating with claystone, siltstone and sandstone) due tostream flows and associated overbank flows into a braided fluvialsystem. The transition between the Xiyu Formation and theunderlying sediments can be easily recognized. It may be eithergradual, 100–200 m thick on average such as at the Jingou He section(e.g. Charreau et al., 2009) with a progressive coarsening up (Fig. 2fand g, see also Heermance et al. (2007)), or sharp but withoutevidence of any hiatus (e.g. the Kuitun He section, see Charreau et al.(2005)). This transition probably owes its origin to a shift related to anacceleration of the basinward progradation of the Xiyu alluvial fans. Astraightforward definition for the base of the Xiyu Formation is whenthe content of boulder-size fraction exceeds 95% in the sediments.
A similar faciological transition can be observed in the present-daydepositional landscape of the Tianshan piedmonts where the limitbetween alluvial fan and braided rivers can be easily outlined fromsatellite images and/or topographic maps (Fig. 3).
Below we present chronological constraints derived first from ournew geometrically-constrained magnetostratigraphic analysis of theupper Yaha section and then from previous studies based on therecognition of this stratigraphic limit following the definition pro-posed here.
5. Diachronous deposition of the Xiyu conglomerates in theYaha section
The Yaha section is located on the southern flank of the Tianshanrange, about 30 km east of Kuche city, where the south-flowing YahaRiver cuts the Quilitage anticline (Fig. 1c). In the syncline locateddirectly north of the Quilitage anticline the contact between theXiyu conglomerates and the underlying fine grained sandstone(Kuche Fm.) can be clearly observed in the field on both limbs in acontinuous section, along the Yaha River (Fig. 2c and d). The section iscontinuously exposed and does not show any significant discordanceor faulting. This setting represents a remarkable case where thebasal age of the Xiyu Formation can be constrained at two locationsalong a single section. On the northern limb of the syncline, themagnetostratigraphic study of Huang et al. (2006) has yielded a basalage of the Xiyu conglomerate of 5.9 Ma (Fig. 2c). Note that in theirpaper Huang et al. (2006) identified the thick conglomerate asKuche formation, but we consider it as Xiyu following the abovedefinition. On its southern flank, Charreau et al. (2006) havemagnetostratigraphically dated a 2814-m thick section, in the lowerpart of the series, ranging from 12.6 to 5.2 Ma, but without reachingthe base of the Xiyu Formation (Fig. 4a and b).
Fig. 3. (a) Contour interval of 25 m superimposed on a Landsat satellite image of the Kuitun/Jingou He area; (b) geomorphic interpretation of the studied area showing major recentalluvial fans (c) topographic profiles along sections A and B (see location in panel b) which show a strong break in slope used to place the limit of Xiyu-like fan deposits.
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5.1. Structural constraints in the Quilitage syncline
The structure of the syncline was well constrained via 29 fieldmeasurements of bedding orientation regularly distributed along a4200-m-long section and accurately geo-referenced with GPS (Fig. 4band c). The strike direction remains relatively constant with a mean ofN83° (with a low circular variance of 0.071; Wan and Sandilands,1995), consistent with cylindrical folding. We built a sectionperpendicular to the mean strike with nearly all measurementsites lying at a distance less than 150m from the synthetic section(Fig. 4b and c). Apparent dips were projected on this synthetic sectionline yet almost equal to the true values measured on the field (Fig. 4c).These structural measurements were used to compute the cumulativethicknesses along the section: the Xiyu Formation amounts to~1052m on the northern limb and to only ~220m on the southernone.
A hand-balanced cross section is presented in Fig. 4c. Taking intoaccount the overall measurements within the conglomerates, and inparticular the ones surrounding the fold axis, the best-fit solutionresults in a simple, upright syncline fold with sub-parallel stratawithin the Xiyu formation. Such a structure has been extrapolated todepth in the section (cf. dash lines on Fig. 4c). The reference horizon(thick black line on Fig. 4c), which corresponds to the base of the Xiyuconglomerates at the southern limb, is structurally positioned farabove the base of the conglomerates on the northern flank. Moreover,the difference of cumulative thicknesses of the Xiyu formation, fromone limb to the other (~1052 and ~220 m), is rather consequent It istherefore clear that the stratigraphic depth of the transition to theXiyu conglomerates varies laterally.
Therefore, one cannot structurally extend the base of the Xiyuconglomerates through a single horizon (Fig. 4c) which strictlyrequires lateral sedimentological facies variations from the homolithicXiyu conglomerates toward a finer grained heterolithic Formation(i.e. Kuche Fm.) from north to south (i.e. range toward basin). Weobserved these stratigraphic features at the southern contact betweenthe Xiyu and Kuche Formations (Fig. 3d). This figure shows a laterallithological variation within the series highlighted by thinning of theconglomerate beds and a relative thickening of the sandstones fromnorth to south. Thus, from a structural viewpoint the base of the Xiyuconglomerate cannot be regarded as a synchronous and laterally-continuous, single stratigraphic layer.
5.2. Sampling, rock magnetism and magnetostratigraphy of the upperYaha section
To determine the age of the base of the Xiyu Formation in thesouthern flank of the Quilitage syncline, we carried out a newmagnetostratigraphic section to extend upward the previouslypublished section of Charreau et al. (2006). To ensure stratigraphiccontinuity with the previous section, the new magnetostratigraphicsection was carried out in a lateral tributary of the main Yaha River(Fig. 4b). In addition, the structural study, unraveling the cylindricalarchitecture of the fold allows a perfect correlation between themagnetostratigraphic and the structural sections (Fig. 4c). The newsampled section spans 1630 m in stratigraphic thickness spanning ahorizontal distance of 5900 m (Fig. 4b). It begins at the top end ofCharreau et al.'s (2006) section and progress upwards until reachingthe contact between the Xiyu and Kuche formations. 386 cores were
Fig. 4. (a) Simplified cross section of the Quilitage anticline and syncline with the locations of the differentmagnetostratigraphic studies which provide bound on the basal ages of theXiyu Formation; (b) Topographic map of the Yaha river with location of the magnetostratigraphic sampling sites of Charreau et al. (2006) and this study; (c) Detailed balanced crosssection of the Quilitage syncline with location of the 29 structural measurements and the exact position of the two lithological transitions between the Xiyu conglomerates andunderlying Neogene Kuche Formation.
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drilled then oriented with magnetic and sun compasses. Two or threecores per horizon were collected with an average distance betweenhorizons of about 8 m. Each horizon was positioned using a GPSinstrument. Samples were collected in the upper part of the Kuchegroup were composed mainly of red-muddy sandstones intercalatedwith siltstone and green-gray sandstones. The average grain sizebecomes progressively coarse-grained upward in the sequence, untilreaching the Xiyu conglomerates. As mentioned above, no strati-graphic duplication, unconformities or faults were visible either infield or in seismic images (see seismic line in Fig. 4 fromHubert-Ferrari
et al. (2007)), suggesting that sedimentation was relatively con-tinuous. Spore and pollen, such as Deltoudospora sp., Ephedripites sp.Pinuspollonites sp., have been identified and assign a Pleistocene toPliocene age to the upper part of the Kuche group (Bureau of theGeological and the Mineral Resources of the Xinjiang Uygur Autono-mous Region, 1985).
Curie point analyses show an important drop in magnetic suscep-tibility near 580 °C, followed by a progressive decrease in susceptibilityuntil 680 °C, signaling the presence of both magnetite and hematite(Fig. 5a). Both thermal and alternating field demagnetization yield
Fig. 5. (a) Curie point analyses and (b) Hysteris curves of representative samples; (c to f) Representative Zijderveld diagrams (Zijderveld, 1967) obtained from both thermal andalternating field (AF) demagnetization (in situ coordinates).
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similar results, although thermal demagnetization seemed moreefficient (Fig. 5). Most samples possess two components — one at lowtemperatures or small peak fields (b200 °C or b10 mT) that is northand downward directed in in-situ coordinates and does not decaytoward the origin on orthogonal diagrams, and another at hightemperatures or high peak fields (300° to 680 °C or 10 to 120 mT)that is of dual polarity and does decay toward the origin (Fig. 5c, d, eand f). We considered the high temperature/field component as thecharacteristic remanent direction (ChRM).
For the about 200 samples we demagnetized, 185 possess a stablehigh temperature/field component, identified as the ChRM. 52 sampleshave normal polarities with an average ofD=2.8°, I=54.5°, α95=3.8°,while 103 samples have reversed polarities with an average ofD=177.4° I=58.8°, α95=3.3°. The remaining 30 samples lie withinreversal boundaries and their directions were likely acquired in
transitional fields. The reversal test is positive (Class B, McFadden andMcEhlinny (1990)). Moreover, the lower sequence in Charreau et al.(2006) passed the fold test, so we conclude, that the high temperature/field component represents a primary remanent magnetization.
A magnetostratigraphic sequence was established from 172samples (horizons) possessing the high temperature/field componentwhich defined nine normal (a to i) and eight reverse (1 to 8) polaritychrons (Fig. 6a, b and c), with each chron being defined by at least twosamples from two different sedimentary horizons, except for thereverse chron 1 with only one sample. We correlatedthe magnetostratigraphic sequence to the Lourens et al. (2004)reference-scale (Fig. 6d) continuing from the top of the section ofCharreau et al. (2006) at 5.2 Ma. Fig. 6e represents our preferredcorrelation, which provides an almost unique fit in terms of intervalnumber and relative duration. The only doubt in the correlation
Fig. 6. Paleomagnetic declination (a) and inclination (b) obtained using principal component analysis; (c) Magnetostratigraphic column from this study; (d) Reference polarity timescale after Lourens et al. (2004); (e) Age versus depth plot of the upper Yaha section, using the data and correlation from c and d.
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concerns the reverse chron 1 defined by just one sample. Toward thetop of the section, sedimentation becomes coarser and it becomesharder to sample suitably fine-grained material within the coarseconglomerates. The magnetic remanence becomes less stable as well.That is why the number of samples with ChRM directions decreasestoward the top. Despite this, the correlation appears robustand delimits the sampled sediments in time from 5.2 to 1.7 Ma witha remarkably stable sedimentary accumulation rate of 0.48±0.21 mm/yr (Fig. 6e). This new section is perfectly continuous with theunderlying section (Fig. 7) of Charreau et al. (2006) which thusstrongly supports their original correlation and not the reinterpretationof Huang et al. (2006) as explained in Charreau et al. (2008b). This~4500-m-thick total section ranges from 12.6 to 1.7 Ma. It identifies arather young, 1.7 Ma, age for the base of the Xiyu conglomerates that lieon top of the section and shows a distinct accumulation rate change at~11 Ma. A similar acceleration rate was also observed on the northflank of the Tianshan range at ~11 Ma (Charreau et al., 2008a). Hubert-Ferrari et al. (2007) have shown that the Quilitage anticline has beenactive over the last ~5 Ma. The growth of the fold must have impactedthe sedimentary record and accumulation rates by either reducing theaccommodation space above the growing structure or increasing theaccommodation space behind the structure (through trapping ofsediments in the piggy back basin). This effect can be correctedaccording to a fold growth model (Charreau et al., 2009; Daëron et al.,2007). In the present studywewere not able tomodel this effect due tothe complicated structure of the Quilitak antlicline.
6. The Xiyu conglomerates: a prograding diachronousgravel wedge
The Yaha section clearly shows clearly that the lithological contactbetween Xiyu conglomerates and their underlying sandstones
observed on the southern and northern flanks of the Quilitagesyncline, just 4.7 km apart (Fig. 4), lies at different stratigraphicdepths. In addition, their estimated depositional ages are diachronousby 4.2 Ma from one limb to the other (Fig. 4c and d). These featuresshow that the Xiyu conglomerates grade laterally, away from themountain front (Figs. 3 and 4). A similar pattern is observed along theBoguzihe section, in the western part of the southern Tianshan (Chenet al., 2002), where the base of the Xiyu conglomerates can beobserved at two locations across the same Atushi–Talanghe anticline,with depositional ages estimated at 2.8 and 1.9 Ma in less than ~5 kmapart. An analysis of a seismic line running across the Yakeng andDushanzi anticlines shows important changes in seismic character(Fig. 8a and b) that may be interpreted as due to a sedimentologicalfacies change from the homolithic Xiyu conglomerates to the under-lying heterolithic and finer grained Neogene deposits mainly com-posed of sandstones (He et al., 2005). Geometry of contacts betweenthese two lithologies further implies a depositional diachronism andyounger age toward the basin.
In the southern Tianshan, across the foreland of the Kashi basin,in the western Tarim basin, several basal ages of the Xiyu conglom-erates were magnetostratigraphically dated from 15.5 up to 0.7 Ma(Heermance et al., 2007). In the northern Tianshan piedmont, at theJingou He section, the limit between the Dushanzi and the Xiyuconglomerates was dated at ~7.6 Ma (Charreau et al., 2009), while atthe Kuitun section ~20 km to the north (basinward), the base of theXiyu conglomerates were dated at 4.8 Ma and 4.2 Ma, respectively(Charreau et al., 2005) (Fig. 8c). In the Chu basin in the northernTianshan, the Sharpyldak formation, which is analogous to the XiyuFormation, was magnetostratigraphically dated at 3 Ma (Bullen et al.,2001).
This growing database shows that the deposition of the XiyuFormation, and probably all similar alluvial fan deposits around the
Fig. 7. Age versus depth plot of the total Yaha section, using data from this study andCharreau et al. (2006).
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Tianshan, is highly diachronous at both local and regional scales.Fig. 9a shows the initial depositional age of the Xiyu conglomerates atvarious locations around the Tianshan as a function of the presentdistance from the range front. Because the range front topography ishard to define everywhere, the distance was estimated using thelocation of the tectonic contact between the Paleozoic and Mesozoicunits as a reference. This plot shows a clear trendwith decreasing agesfrom the mountain range toward the basin (Fig. 9a).
All these observations suggest that the Xiyu conglomerates are atime-transgressive alluvial gravel wedge unit that has prograded overthe foreland as it was incorporated into the orogenic wedge (Fig. 10).The diachronous character of the Xiyu Formation in the Kashi areacould reflect the effect of individual structures uplifting and causingreworking of sediments, and hence facilitating its basinwardprogradation (Heermance et al., 2007). However, in central Tianshan,no clear relationship exists between the depositional ages of theconglomerates and the activation of individual structures. Moreover,the predominance of clasts in the conglomerate that were derivedfrom the high range suggests minor reworking of sediments due toemerging structures in the piedmont.
While the conglomerates prograded over the foreland, deforma-tion propagated toward the basin as well (Charreau et al., 2008a;Daëron et al., 2007; Hubert-Ferrari et al., 2007). Therefore, the XiyuFormation may unconformably overlie previously deformed units insome places while in other places it lies conformably over underlyingdeposits, or exists as syn-kinematic growth strata (Charreau et al.,2008a). Thus, the stratigraphic relationship between the conglom-erates and the underlying finer grained fluviatile units cannot be usedas a criterion to define the base of the Xiyu Formation.
7. Tectonic implications: a record of underthrusting rate?
The nature of the Xiyu Formation as a prograding gravel wedge canbe used to make tectonic inferences. First, the diachronism of the Xiyuconglomerates implies that they are not related to any particularepisode of thrusting and uplift in and around the Tianshan or theTibetan Plateau in the Plio-Pleistocene, as previously thought(Burchfiel et al., 1999; Zheng et al., 2000). For example, Zheng et al.(2000) dated the base of the Xiyu conglomerates at ~3.5 Ma in theYecheng section in the western Kunlun piedmont, and inferred thatthe northern Tibetan plateau experienced rapid uplift at that time. Thisis likely incorrect, since the Xiyu Formation is most probably alsotransgressive at front of the western Kunlun. This particular deposi-tional age of the conglomerates in Yecheng is probably only valid thereand has no particular significance at regional-scale. However, becauseof the time-transgressive faciesmigration, the oldest settlement of thisfaciesmight bemeaningful with regard to the age of reactivation of therange (Brozovic and Burbank, 2000). The oldest Xiyu conglomerateswere dated at ~15.5 Ma in the Kashi piedmont (Heermance et al.,2007) suggesting that at this time the topography of the range hadbeen sufficiently rejuvenated for erosion processes to lay downgravelsalong the piedmonts. This timing is consistent with the observationthat the sediment accumulation rate accelerated at about that time, asseen in several sections around the range (Charreau et al., 2009;Heermance et al., 2007), and with thermochronological studies whichsuggest that exhumation in the range started earlier, possibly around~25 Ma (Dumitru et al., 2001; Hendrix et al., 1994; Sobel et al., 2006).
The plot in Fig. 9a shows a trend with the older Xiyu-type con-glomerates lying closer to the mountain front. As this plot disregardspotential deformation of the foreland, the ~2 mm/yr rate provides aminimum estimate of progradation of the conglomerates shed by theTianshan range. To better quantify the progradation history of theconglomerate wedge, the present distances from the mountain frontwould need to be corrected for the amount of tectonic transport thatmay have taken place at each location posterior to deposition (asillustrated in the inset of Fig. 10). The distance of the transition fromdeposition of Xiyu-like conglomerates to finer grained fluviatilesediments depends primarily on two factors: (1) the transportcapacity of the drainage system, and hence on topography andclimate, and (2) the rate at which the foreland is moving toward themountain front (Simoes and Avouac, 2006). So, the progradation ratewithin a particular section, Vpr, measures the rate at which the studyarea has moved toward the range due to underthrusting (V1 inFig. 10), added to a general term that accounts for changes in to-pography and climate:
Vpr = V1 + f ðtopography; climateÞ ð1Þ
The total shortening rate across the range is the sum of allshortening across individual faults (Vi) and would be the under-thrusting rate V1 plus the overthrusting rate (V2 in Fig. 10; i.e., the rateat which rocks within the range are advected with respect to thesteady-state topography):
∑Vi = V1 + V2 ð2Þ
Both V1 and V2 have the same reference, which is the limit be-tween the foreland and the basin (Fig. 10), but V1 is positive from thebasin toward the range while V2 is negative. When erosion rates arelow as in central Asia, V2 represents only a small part of the thrustingrate across the range and can be neglected (Avouac, 2003). Withmeasured shortening rates across individual faults, one can estimateV1 and consequently constrain the influence of climate and topogra-phy. Thanks to several recent studies, long-term shortening rates areknown in both Tianshan piedmonts.
Fig. 8. (a) Seismic line across the Yakeng anticline (after (Hubert-Ferrari et al., 2005), see Fig. 1 for location) showing the basinward progradation of the Xiyu conglomerate above themore layered Kuche formation; (b) Seismic line North of the Dushanzi anticline in the Kuitun He river (for location see Figs 1 and 8c) showing also the basinward progradation of theXiyu conglomerate; (c) Cross section of the Dushanzi and Huerguosi anticline with location of the Jingou He and Kuitunmagnetostratigraphic sections that provide constraint on theXiyu basal age.
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Based on detailed constraints of the structure and age control, werestored the Yaha section. This yields a horizontal minimum distanceof ~4700 m between two points at the base of the Xiyu from thenorth and south limbs of the fold (Fig. 4c), which leads to aprogradation rate of ~1.2 mm/yr. Over that same period, shorteningacross the Yakeng and Quilitage anticlines (both located south of theYaha section, Fig. 1) absorbed ~0.6 mm/yr and ~0.2 mm/yr from~5.5 Ma to 0.2–0.3 Ma, respectively (Hubert-Ferrari et al., 2007). Thisadds ~0.8 mm/yr to 1.2 mm/yr, making it comparable to theprogradation rate of 2.0 mm/yr. Hubert-Ferrari et al (2007) alsofound evidence for an increase of shortening rate across the two
Fig. 9. (a) Plot of the initial depositional age of the Xiyu conglomerates against their presePaleozoic rocks and Meso-Cenozoic foreland deposits, which probably represents old strucdepositional age of the Xiyu conglomerates found in the Northern Tianshan against their re
anticlines by a factor of 5 to 10 over the last 0.2–0.3Myr. Yet, from theQuilitage anticline, the present position of the Xiyu-like conglomer-ates is about ~20 km to the south, which implies a recent and strongincrease in progradation rate. This might indicate short timescalefluctuations of shortening rates and/or climate that could not beidentified by our analysis.
In the northern Tianshan, the basal age of Xiyu conglomerates hasbeen dated at 2.5 Ma, ~4.8 Ma, and ~7.6 Ma along the Anjihai (Zhouet al., 2000), Kuitun (Charreau et al., 2005) and Jingou He (Charreauet al., 2009) sections, respectively. It is possible to estimate thepaleodistances of these points with respect to a common reference
nt day distance from the front range. The front range is defined as the limit betweentures inherited from the Paleozoic orogeny within the Tianshan; (b) Plot of the initialstored distance from the N44°30′ latitude supposed fixed in the Junggar basin.
Fig. 10. (a) Schematic section of a half-range showing the relationship between sediment progradation rates, underthrusting rates and shortening across the structures that composethe piedmont and the range; (b) Schematic restored section of a piedmont from which one can estimate the sediment progradation rates of different facies. In both figures the Xiyuformation corresponds to the dark gray pattern within the foreland prism.
308 J. Charreau et al. / Earth and Planetary Science Letters 287 (2009) 298–310
line in the foreland (here line that parallels the mean strike ofpiedmont's structures and fixed at latitude around 44°30′N, seeFig. 3b) to restore the sections. The current distances were correctedassuming ~1.2 km (Daëron et al., 2007) and ~4 km (Avouac et al.,1993; Burchfiel et al., 1999; Molnar et al., 1994) of shortening acrossthe Dushanzi anticline along the Anjihai and Kuitun sections,respectively, and ~10 km across the Huerguosi anticline (Charreauet al., 2008a). When plot on an age against distance diagram (Fig. 9b)those three restored paleodistances align well with the distance ofHolocene fans in the area (Fig. 9b). We thus infer an averageprogradation rate of the Xiyu Formation over the Junggar basin of~3.9 mm/yr over the last 10 Myr (Fig. 9b). As the Kuitun section doesnot lie directly downstream of the JingouHe section, and because it isbased on only four data points, this estimate should be taken withcaution. For comparison the shortening rate across the Huerguosi andAnjihai anticlines (Fig. 1) is estimated at a minimum of ~1.5 mm/yrover the last 4Myr, and a minimum of ~1.1 mm/yr from 4 to ~10 Ma(Charreau et al., 2008a). However, some fraction of the shorteningacross the piedmont is missing since it ignores shortening across theQigu anticline and across a fault-tip fold in the footwall of theHuerguosi thrust fault (Charreau et al., 2008a). It seems thereforereasonable to assume that, here too, the progradation rates of the XiyuFormation is comparable to the shortening rates.
For comparison in the Kashi area, restoring distance fromshortening, Heermance et al. (2007) came to somewhat largerprogradation rates of 10–11 mm/yr for the same time period. Aprogradation rate larger in the Kashi area than in the Yaha section areais consistent with the westward increase of the shortening rate acrossthe range (Avouac et al., 1993; Reigber et al., 2001).
These observations suggest that the progradation rate of the XiyuFormation is primarily controlled by tectonics reflecting the rate ofunderthrusting of the foreland beneath the mountain front (Fig. 10).Note that, if so, the progradation rates observed north and south ofthe range should not add to the total shortening rate across the rangesince ovethrusting rates are not estimated and since there is alsoconcomitant intermountain shortening, in particular within theBayanbulak basin (Fig. 1). In any case, this study shows thatthe underthrusting rate at mountain fronts (the rate at which thebasement of the foreland basin is thrust under the range) might beinferred from sedimentary facies migration provided that the effectsof climate and eventual changes of the topography and sedimentarysystems can be neglected or known independently.
8. Climatic implications
Zhang et al. (2001) and Molnar (2004) analysed the sedimentaryarchive of several large basins around the world, including Junggarand Tarim. They observed a systematic and rapid increase insediment shed to the basins at ~4–2 Ma, around the Plio-Pleistocenetransition. They linked this increase to the onset of glaciation thatmay have enhanced erosion rates and consequently impactedmountain building. In the case of central Asia, much of the datasupporting this hypothesis are derived from 1D local accumulationrates or from the work of Métivier and Gaudemer (1997), whoreconstructed regional sediment accumulation budgets based on 1Ddrilling logs. However, Métivier and Gaudemer (1997) concludedthat a gradual, not sharp or punctual, increase in accumulation ratesoccurred since the early Miocene. Moreover, their sediment fluxestimates must be handled with caution as their reconstruction isbased on lithostratigraphic correlations that are probably biased bypoorly constrained depositional ages. The inference of an increase inthe sediment flux (Métivier and Gaudemer, 1997) and localaccumulation rates (Zhang et al., 2001) at 4–2 Ma is, for example,linked to the strong assumption that the widespread Xiyu conglom-erates were everywhere Plio-Pleistocene in age, which, as shownabove, is no longer valid. Moreover, some magnetostratigraphicstudies carried out around the Tianshan have revealed constantaccumulation rates for the last 10–11 Ma (Charreau et al., 2009;Charreau et al., 2005; Charreau et al., 2006). The same is true for theNeogene basins, of marginal seas along the Himalayan range front,where no major timing-dependant increase of the sediment supplywas found in the depositional history (Clift, 2006) during this timeperiod.
On the other hand, the estimates of (Clift, 2006) may not representthe total material eroded from the Himalayas since a significant part ofthe sediments could have been trapped in the Himalayan foreland(i.e. Siwaliks), and, more importantly, it ignores the Bay of Bengal.Moreover, accumulation rates derived from magnetostratigraphicstudies are often carried out in syn-tectonic sediments across activefolds, and must therefore be corrected for the effect of fold growth(Charreau et al., 2008a). Finally, some studies show evidence for localincreases in accumulation rate during the Plio-Pleistocene. Forexample, magnetostratigraphy in the JingouHe section suggest thatthe sedimentation rate in the piggyback basin increased abruptlyfrom ~0.4 to ~0.7 mm/yr at ca. 4 Ma (Charreau et al., 2008a).
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Magnetostratigraphic dating of the Kashi town sections in the Kashiforeland, in the western Tarim basin, also shows a significantaccumulation rate acceleration ca. 4 Ma (Heermance et al., 2007).However it might be questionable that the 1D accumulation profilesderived from the Jingou He and Kashi sections can be extrapolatedto sediment supply over the whole of central Asia given the pos-sible influence of local tectonic factors. Therefore, one can naturallyconclude that no strong evidence exists, yet, for a Quaternary sedi-ment flux acceleration in the Tarim and Junggar basins, or for arelationship between the Xiyu conglomerate and the onset ofQuaternary glaciation in central Asia.
9. Conclusions
The Xiyu Formation is a thick conglomeratic unit that has inspiredvarious inferences with regards to the timing of mountain buildingand the effect of climate change in central Asia. The available datafrom magnetostratigraphic, sedimentological and structural studiesof this formation reveal that it constitutes a prograding gravelwedge with diachronous ages ranging between ~15.5 and ~0.7 Ma.The Xiyu Formation cannot be considered as a chronostratigraphicunit marker throughout the continental basins of central Asia. Wesuggest that the progradation rate of the Xiyu Formation primarilyreflects the rate at which the foreland is thrust under the mountainrange but that it must also reflect the evolution of topography andclimate.
Given the depositional diachronism of the Xiyu conglomerates,no evidence exists for a regional accumulation rate acceleration atthe Plio-Pleistocene transition to support the idea that a change inregional climate occurred as reflected in the onset of Quaternaryglaciation, or of a sudden increase in topographic relief in central Asia.The impact of the Plio-Pleistocene climatic transition on mountainbuilding and mass transfer remains, therefore, an open debate incentral Asia. We suggest that the Tarim and Junggar intracontinentalbasins may represent remarkable test cases to address this key issuein the future because sediment flux could potentially be estimated inthis closed system. In this regard, more chronological data and goodconstraints on basin geometry are strongly needed to reconstructaccurate sediment volumes.
Acknowledgments
This study was financed by the French ECLIPSE program, theChinese project kzcx3-sw-147 and 973 No 2005CB422101, PRA (T05-02/T06-04), the French ANR (ANR-05-BLAN-0143-01) and theGordon Betty Moore Foundation. We present our thanks to Dr. S.Peng and K. Chen for their supports to magnetostratigraphic samplingand to M. Laumonier and E. de Oliveira for their help in the laboratory.This is Caltech Tectonics Observatory contribution 123.
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