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Quaternary Research
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Magnetostratigraphy of deep drilling core SG-1 in the western Qaidam Basin(NE Tibetan Plateau) and its tectonic implications
Weilin Zhang a,b, Erwin Appel a, Xiaomin Fang b,⁎, Chunhui Song c, Olaf Cirpka a
a Department of Geosciences, Center for Applied Geoscience, University of Tübingen, Hölderlinstr. 12, 72074 Tübingen, Germanyb Institute of Tibetan Plateau Research, Chinese Academy of Science, P.O. Box 2871, Beijing 100085, Chinac Key Laboratory of Western China's Environmental Systems, Ministry of Education of China & College of Resources and Environment, Lanzhou University, Gansu 730000, China
The Qaidam Basin is the largest intermontane basin of the northeastern Tibetan Plateau and contains acontinuous Cenozoic sequence of lacustrine sediments. A ~1000-m-deep drilling (SG-1) with an average corerecovery of ~95% was carried out in the depocenter of the Chahansilatu playa (sub-depression) in thewestern Qaidam Basin, aimed to obtain a high-resolution record of the paleoenvironmental evolution and theerosion history. Stepwise alternating field and thermal demagnetization, together with rock magnetic results,revealed a stable remanent magnetization for most samples, carried by magnetite. The polarity sequenceconsisted of 16 normal and 15 reverse zones which can be correlated with chrons 1n to 2An of the globalgeomagnetic polarity time scale. Magnetostratigraphic results date the entire core SG-1 at ~2.77 Ma to~0.1 Ma and yielded sediment accumulation rate (SAR) ranging from 26.1 cm/ka to 51.5 cm/ka. MaximumSARs occurred within the intervals of ~2.6–2.2 Ma and after ~0.8 Ma, indicating two episodes of erosion,which we relate to pulse tectonic uplift of the NE Tibetan Plateau with subsequent global cooling.
The NE Tibetan Plateau has become an important region for testinghypotheses on the mechanisms of Tibetan Plateau uplift and climatechange (e.g., Harkins et al., 2007; Kent-Corson et al., 2009; Lease et al.,2007; Molnar et al., 2010). Prevailing tectonic models indicate that theNE Tibetan Plateau is the latest deformed and uplifted part of theplateau, predominantly affected since Pliocene (Tapponnier et al.,2001). Early hypotheses assumed that uplift of the Tibetan Plateau,associatedwith strong enhancement of erosion and silicateweathering,was responsible for global cooling,monsoon evolution and drying in theAsian interior (Broccoli and Manabe, 1992; Raymo and Ruddiman,1992; Ruddiman et al., 1989). Recent studies indicate that the growth ofthe NE Tibetan Plateau may have played an important role in theevolution of the Asian aridification and monsoon climate (e.g., An et al.,2001; Dupont-Nivet et al., 2007; Hövermann and Süssenberger, 1986;Yang et al., 2011). The erosion history has been considered keyinformation for revealing tectonic deformation processes and linkingtectonic uplift with climate change (e.g., Clark et al., 2010; Pan et al.,2009, 2010; Sun et al., 2010). Dating basin sediments and using theseresults to calculate sediment fluxes provide an important approach toreconstruct the erosion history at high (basin-scale) resolution.
ashington. Published by Elsevier In
The Qaidam Basin is the largest intermontane basin of the NETibetan Plateau and is therefore an ideal location to study the linkageof tectonic uplift and climate change. It is located at the transitionalregion between the arid inland zone and the East Asian monsoonregion and represents a closed inland basin which has received up to12 km of continuous Cenozoic sediments (Fang et al., 2007). In orderto avoid non-continuity of outcrops at basin margins, a Sino-Germanresearch team has carried out a joint scientific drilling program nearthe depocenter in the western Qaidam Basin since 2008, as part ofthe Sino-German TiP-TORP projects. In this paper, we present thepaleomagnetic results of the nearly 1000-m-deep sediment core SG-1and discuss its constraints on erosional events in this area. Themagnetostratigraphic dating represents the basic age framework forfurther publications of paleoclimate and paleoenvironment proxydata from this core.
Geological setting
The rhomb-shaped Qaidam Basin with an area of 121,000 km2 isthe largest intermontane and sedimentary Cenozoic basin on the NETibetan Plateau. Gravel and sand deserts, salt lakes and marshes, andyardang (wind-blown) landforms are widely distributed in the basin.The basin is surrounded by the Qiman Tagh and East Kunlun Shan(Mts.) to the south, the Altyn Tagh (Mts.) to the northwest, the QilianShan (Mts.) to the northeast, and the Ela Shan (Mts.) to the east(Fig. 1). The interior of the basin has an average elevation of almost
Figure 1. Structural setting of the Qaidam Basin and adjacent regions (modified from Meyer et al., 1998). The SG-1 drilling site is located within the area marked by a rectangularbox (see Fig. 2 for enlarged map). Asterisks show locations of former related boreholes. Dotted and broken lines indicate roughly scopes of the north Qaidam marginal thrustbelt (NQMTB) and south Qaidam marginal thrust belt (SQMTB), respectively. Major thrust-folds: SX: Saishiteng Shan–Xitie Shan; LH: Leng Hu–Nanbaxian; EY: Eboliang–Yahu;KB: Kunbei; AR: Alaer; SY: Shizigou–Youshashan; GG: Ganchaigou; XY: Xianshuiquan–Youquanzi; YY: Yantan–Youdunzi; NH: Nanyishan–Huanggualiang; XD: Xiaoliangshan–Dafengshan; JJ: Jiandingshan–Jianshan.
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2800 m above sea level (m asl), with surrounding mountains reach-ing elevations of about 4000 to 5000 m asl (Figs. 1, 2).
Two NW-striking thrust-fold systems were developed along thenorthern and southwestern margins of the Qaidam Basin, respective-ly occurring at the foot of the South Qilian Shan and the EastKunlun Shan (Fang et al., 2007; Yin et al., 2008) (Fig. 1). The former iscalled the north Qaidam marginal thrust belt (NQMTB) and consistsof three large rows of thrust-folds basinwards (southwards), namelythe Saishiteng Shan–Xitie Shan, Leng Hu–Nanbaxian, and Eboliang–Yahu tectonic belts. The latter is called the southwest Qaidam mar-ginal thrust belt (SQMTB) and consists of ~nine rows of thrust-foldsbasinwards (northwards), i.e. the Kunbei, Alaer, Shizigou–Youshashan,Ganchaigou, Xianshuiquan–Youquanzi, Yantan–Youdunzi, Nanyishan–Huanggualiang, Xiaoliangshan–Dafengshan, and Jiandingshan–Jianshantectonic belts (Fang et al., 2006) (Fig. 1).
The Qaidam Basin received up to 12 km of Cenozoic sediments,mostly alluvial–fluviolacustrine–playa facies, from the surroundingmountains (Gu et al., 1990; Huang et al., 1996; Xia et al., 2001),recording the interaction of mountain uplift, erosion and climatechange. On the basis of pollen and ostracode analyses, the Cenozoicstratigraphy in the Qaidam Basin has historically been divided intoseven formations. In upward sequence these are the Lulehe Fm.(Paleocene to Eocene, E1–2), the Xia Ganchaigou Fm. (Oligocene, E3),the Shang Ganchaigou Fm. (early Miocene, N1), the Xia YoushashanFm. (mid Miocene, N2
1), the Shang Youshashan Fm. (late Miocene,N2
2), the Shizigou Fm. (Pliocene, N23) and the Qigequan Fm. (early
Pleistocene, Q1). Recent magnetostratigraphic and biostratigraphicresults of some outcrops along basin margins indicated that thebottom and top of the sediments are dated at ca. 54 Ma and 1.8 Ma,respectively (Fang et al., 2007; Lu and Xiong, 2009; Sun et al., 2005;Wang et al., 2007; Zhang, 2006). Paleomagnetic dating of some oilexploration boreholes at the present depocenter in the easternQaidam Basin shows that the Quaternary sediments can reach a
maximum thickness of 3200 m and were continuously depositeduntil almost present (Liu et al., 1998; Wang et al., 1986).
Basin analyses based on seismostratigraphy, drilling and sedi-mentological studies from Qinghai Oil Company demonstrated thatduring the early Cenozoic the depocenter of the Qaidam Basin waslocated in the western part of the basin along the East Kunlun Shanand Altyn Tagh (Fang et al., 2006). From the Oligocene to EarlyMiocene, the depocenter began to migrate eastwards, associatedwith the uplift of the Altyn Tagh. From the Late Miocene and Pliocene,this eastward migration accelerated, accompanied by basinwardspropagation of thrust-folds from the surrounding mountains, andshortening and uplifting the western Qaidam Basin (Fang et al., 2006;Wang et al., 2006; Xia et al., 2001; Yin et al., 2008). This processdisassembled the western Qaidam Basin into several sub-basins (sub-depressions) separated by the NW-propagating thrust-folds. Thesesub-basins developed salt lakes and playas since the Late Plioceneand Quaternary as with climate became more arid (Figs. 2a, b).Deposition in some of the sub-basins has been continuous from theirformation to the present, for example Gasikule Hu (salt lake) in thenorthern front of the East Kunlun Shan and Dalangtan Salt Lake in thesouthern front of the Altyn Tagh (Shen et al., 1993) (Fig. 1).
Our joint Sino-German drilling (SG-1) was performed at N38°24′35.3″ and E92°30′32.6″ in the depocenter of the Chahansilatu sub-basin (or playa plain) between the Eboliang anticline and theJianshan anticline in the western Qaidam Basin. These anticlinesare the final tectonic belts of two thrust-fold systems (NQMTB andSQMTB) (Figs. 1, 2). The surface of the Chahansilatu sub-basin is veryflat and covered by thick (2–3 m) salt crust. Prospecting wells andseismic sections demonstrated that the Cenozoic sedimentarysequence at the drilling site is horizontal and has a thickness of upto 6 km.
The drilling campaign was carried out in 2008, reaching a depth of938.5 m with an average recovery rate of ~95%. A three-in-one tube
Figure 2. (a) Geologic map of the study area in the western Qaidam Basin showing the location of the SG-1 borehole. See Fig. 1 for its location within the basin. (b) Cross-sectionalong A–B shown in the map.
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technology was used. In this approach a transparent hard plastic liner(2.5 m long, 80 mm diameter) is inserted into an inner steel tube,which is itself inserted into an outer rotating steel tube. This rotatingtube does the actual drilling. The sampled core is directly pushed intothe plastic liner during drilling, avoiding reduction or loss of corematerial (Fig. 3).
The penetrated sedimentary sequence comprised clay, clay–silt,and siltstone, intercalated with salt layers (mainly halite), marl bedsand thin or scattered gypsum crystals, showing depositional cycles ofclay/silt and halite/marl. These cycles may have resulted from long-term paleoclimate trends and fluctuations. In total, we observed morethan 100 salt layers of various thicknesses; most were thin, but about20 were as thick as 2–3 m. The sediments from surface to a depth of326.7 m were characterized by cycles of thick gray-white salts andgrayish black clay–siltstone with a salt/clay ratio of about 1:3.Gypsum crystals or thin gypsum layers were observed frequentlywithin the clay–siltstone layers. The sediments in the middle partfrom depths 326.7 to 723.1 m were mainly gray-white thin salt layers(obviously reduced in thickness compared to the upper part) anddark-gray or bluish-gray mudstone, muddy siltstone and silty clayintercalated with scattered gypsum crystals and some thin marllayers. The lower part of the core from depths of 723.1 to 938.5 m
mainly contained gray siltstone/mudstone and marl layers interca-lated with same lithologic layers, but with salt nodules and scatteredgypsum crystals.
Sampling and measurements
The plastic liners with the core sediments were cut into twohalves for photographic documentation, lithologic description andsub-sampling (Fig. 3). For paleomagnetic analyses, cubic samples of2-cm length were taken at intervals of 25 cm along the whole SG-1core (3 samples per each level). In total, 11,260 oriented sub-sampleswere collected. A total number of 18,770 bulk sub-samples formagnetic parameters and other paleoclimate proxy analyses weretaken at 5-cm intervals. The samples for paleomagnetic analyseswere sealed and stored under cool conditions (~5°C) in a cabinetrefrigerator. Pollen and grain-size samples were conserved in air-conditioned rooms, and other samples for geochemical indicators andmineral magnetic parameters were frozen in cabinets at stable cooltemperature (−18°C).
Stepwise thermal demagnetization (15 steps up to 610°C atintervals of about 20–50°C) was done by an MMTD shielded furnaceon 24 pilot specimens selected according to different sediment types.
Figure 3. (a) Photograph of the drilling site. (b) Sliced drill cores. (c) Drilling devices for three-in-one tube technology, which guarantees that the core is directly drilled into theinnermost plastic tube. (d) Core with plastic tube being cut into two halves.
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Magnetic susceptibility was measured after each heating step tomonitor possible changes of magnetic phases. Thermomagnetic runsof magnetic susceptibility (k–T curves) for 21 samples were per-formed using a Kappabridge KLY-3 (AGICO) equipped with a furnaceand a CS3 low temperature unit. A total of 52 pilot samples wastreated by 16 steps of alternating field demagnetization (AfD) from5 mT to 100 mT at intervals of 5 mT below 50 mT, and 10 mT above60 mT. For remanence measurements, a 2G Enterprises SQUIDmagnetometer (2G 760R with a noise level b0.01 mA/m for 10-cm3
samples) was used. AfD was performed by a 2G degausser attached tothe magnetometer. All paleomagnetic measurements were done atthe Center for Applied Geoscience, Tübingen University.
Paleomagnetic and rock magnetic data analysis
Results of thermal demagnetization for representative samplesare shown in Figure 4. In most cases, a remanent magnetization com-ponent was readily removed below 120°C, accompanied by a change ofthe remanence direction (Fig. 4a, b) and amarked decrease of magneticsusceptibility after heating (Fig. 4c). Destruction of a ferrimagneticphase was probably responsible for this low-temperature componentrather than a breakdown of goethite, as the latter would not create alarge susceptibility decay. A characteristic remanent magnetization(ChRM) was clearly separated above 400°C, linearly pointing towardthe origin in thefigure and reaching complete demagnetization at about580°C. Most samples showed a steady decrease of magnetic suscepti-bility, but several samples revealed a peak at higher temperature closeto 580°C (Fig. 4c). The remanence intensity was near zero at 580°C(Fig. 4a) indicating magnetite. Low-temperature runs of magneticsusceptibility confirmed this by the occurrence of the Verwey transitionat about −150°C (Fig. 5). The marked decrease of k values within therange −196 to −160°C may be explained by ultrafine particles thatbecame ferrimagnetic at lower temperatures.
For most of the samples, heating led to a dramatic increase ofk values due to transformation of paramagnetic Fe-sulfides or Fe-bearing clay minerals into magnetite (Roberts, 1995). The trans-formation starts at ~400°C. Only one Curie temperature at ~580°C isrevealed (magnetite) and the shape of the k–T heating curves (beforethe transformation starts) demonstrates that original magnetite exists
in the samples. All these results clearly point out that magnetite is thedominant contributor to the ChRM.
Based on the results of the pilot samples, AfD with twelvesteps between 0 and 60 mT was selected for treatment of theremaining samples. Samples at an interval of ≥1 m were selectedand measured (in critical parts of the core the sample densitywas increased). About 55% of the measured samples showed a verystable demagnetization behavior with only one component (termedB component) (Fig. 6a). The rest of the pilot samples showedtwo components (A, B) and a residual small remanence whichcould not be removed by AfD (Fig. 6b, c). The first component (A) wasdemagnetized at lower fields of about 15 mT, related to softermagnetite particles or to a viscous overprint in the samples. The secondcomponent (B) showed stable demagnetization behavior; it couldbe clearly separated between about 15 and 60 mT (Fig. 6b) andmost likely corresponded to the remanence of the single-componentsamples (therefore we term both component B). Component B doesnot point exactly toward the origin in the figure. The small residualremanence at 60 mT is either residing in highest coercive magnetiteor in hematite.
Principal-component analysis was used to calculate the ChRMdirections for component B. Directions with maximum angulardeviation (MAD) of more than 15° were rejected. Results from 1128samples out of 1257 measured ones were accepted for furtherinterpretation (by rejecting results with MAD >10° we would onlylose another 92 samples). From the analyzed ChRM directions, onlythe inclination is significant, as there was no control on the coreazimuth during drilling; relative adjustment of consecutive samples isnot helpful because of the short (2.5 m) core segments retrievedduring drilling. The present axial dipole field inclination at the SG-1drilling site is relatively large (I=58°) and according to the apparentpolar wander path of Eurasia (Besse and Courtillot, 2002) it was alsoquite similar during the last 3 Ma. The large difference betweeninclination values of normal and reverse polarity remanences allowsdetermining the polarity from inclinations only with high certainty.Inclination values are shown in Figure 7 by a histogram with 3°intervals. The antipodal distribution supports a primary nature of theB component, at least for the predominant majority of the samples.
The distribution of the measured inclinations is slightly skewedtoward a higher density at low inclination values. Such a skewness is
Figure 4. Results of thermal demagnetization for representative samples of core SG-1 (SG1-iii shows the depth of the samples). (a) remanence intensity curves versus temperature;(b) Zijderveld diagrams; open (closed) symbols represent vertical (horizontal) projections; (c) magnetic susceptibility measured after each heating step.
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expected for a normal distribution of poles (due to secular variation)and the dipole field geometry and therefore supports a primary(detrital) origin of component B. Mean values for positive andnegative inclinations are +51.9° and−41.4°, respectively, represent-ing normal and reverse polarity directions (Table 1). The meaninclination values are lower than the dipole field direction during thelast 3 Ma and therefore indicate the presence of inclination shallow-ing. This is in agreement with earlier studies reporting anomalouslylow inclinations during the Pliocene and Pleistocene in the QaidamBasin (Dupont-Nivet et al., 2002; Lu and Xiong, 2009; Wang et al.,1986), in the Guide Basin (just east of the Qaidam Basin) and otherparts of central Asia (Cogné et al., 1999; Thomas et al., 1994; Yan etal., 2005). Occurrence of inclination shallowing is another indicationfor a detrital origin of the B component. The 10.5° difference of themean positive inclination (normal polarity) and negative inclination
Figure 5. Magnetic susceptibility versus temperature (high temperature heating and coolinSG-1 (SG1-iii shows the depth of the samples). As strong new formation of magnetite occurrat low temperature within the gray circles indicates Verwey transition of magnetite at ~−starts) indicates superposition of a plateau-like shape due to original magnetite and a para
(reversal polarity) values (Table 1) points out that part of the samplesare significantly overprinted in the present Earth magnetic, resultingin higher inclination values (reaching +58° for complete overprint).The wider distribution of positive inclinations can be explainedby superposition of samples with a detrital remanence (i.e., with alower inclination) and samples with a recent field-parallel overprint(i.e. with an inclination of +58°). In contrast, samples with negativeinclinations should predominantly represent detrital directions.Therefore, as a first-order model, the distribution of inclination valueswas fitted by stacking three Gaussian curves representing (a) normaland (b) reverse polarity (primary) remanences and (c) an additional(secondary) present-field component (mean inclination+58° accordingto the dipole field). Fitting was performed by minimization of squaredresiduals using theMelder–Nead simplex algorithm implemented in theMatlab function “fminsearch” (Lagarias et al., 1998). Inclination value
g curves; low temperature curves connected to heating curves) for samples from coreed at elevated temperature, expanded heating curves are additionally shown. The peak150°C; the heating curve below about 300°C (i.e., before formation of new magnetitemagnetic 1/T trend.
Figure 6. Zijderveld diagrams of alternating field demagnetization for representative samples of core SG-1 (SG1-iii shows the depth of the samples); open (closed) symbolsrepresent vertical (horizontal) projections.
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reflection at 90° was considered for fitting the data (as north andsouth pointing remanence directions in one hemisphere both countto the same inclination value). As further boundary conditions oneof the means was fixed to +58° (i.e. a complete overprint in thepresent field), and the other two means were constrained to havesame absolute values but with opposite sign (representing primarydetrital remanences with antiparallel directions). The resulting fityields mean inclinations of ±45° for the antiparallel primary compo-nents (Fig. 7), indicating a mean inclination shallowing of ~13°. Theresulting percentage of overprinted samples (with +58° inclination)is about 3%.
The mean NRM intensity for samples with I>0 (7.60 mA/m) ishigher than for samples with Ib0 (6.47 mA/m), which points out thatoverprinted samples have a tendency toward higher NRM intensities.We therefore separated the data set into two groups with lower andhigher NRM intensities. A threshold of 5 mA/m still keeps sufficientsamples in the lower intensity group (Table 1) and the magnetostrati-graphic result based on samples with NRM intensities b5 mA/mdelivers a less noisy polarity sequence.
Figure 7. Histogram of inclinations (3° windows); positive and negative inclinations denofitted to the accumulative distribution with the following boundary conditions: two componthe same mean absolute value, and one additional component with mean inclination of +variations (σ) and contributions (weight i.e., integral of the Gaussian curves) are listed.
Magnetostratigraphic dating
Figure 8 shows the interpreted polarity sequence recorded for thecore SG-1. Intervals with mixed positive and negative inclination arefrequently occurring, and samples with low inclinations are lesssignificant for polarity determination. For the final interpretationsingle outliers were ignored. In case of at least two subsequentsamples with same polarity we further considered these data as theymight represent an excursion or event. Sixteen normal and 15reversed polarity zones, marked as N1–N16 and R1–R15, respectively,were determined (Fig. 8). Taking into account that the drilling site islocated in the center of a just desiccated playa (with uneroded flatsurface and deep salt crust), which geomorphologically belongs to thepresent depression and depocenter zone of the western QaidamBasin, the sediments at the surface must be of sub-recent age. The topof the core SG-1 should pre-date the age of the high lake levels ofpaleo-lakes in the eastern basin due to the eastwards shift of thedepocenter since 2.6 Ma (Fang et al., 2007; Shen et al., 1993; Wang etal., 2006). On the other hand, it must be younger than the surface ages
te downward and upward pointing remanence vectors, respectively. Gaussian curvesents with negative (reverse polarity) and positive (normal polarity) inclinations having58° (present dipole field component). Values of mean inclinations (μ), their standard
Table 1Statistics for magnetic inclination (incl.) data from core SG-1 (analyzed by inclination only statistics using the PmagPy software package of LisaTauxe at: http://magician.ucsd.edu/~ltauxe/); N, k, α95 are sample data, precision parameter, and 95% confidence limit.
SG-1 Incl. (°) N k α95 (°) Mean value for NRM (mA/m)
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further west of our study region because of the rapid wind erosion inthe western basin since late Quaternary (Kapp et al., 2011). Somemagnetostratigraphic results are available from areas adjacent to the
Figure 8. Magnetostratigraphic results of core SG-1. GPTS: inclination vs. depth,interpreted polarity sequence and correlation to the geomagnetic polarity time scale(GPTS 2004 of Gradstein et al., 2004). Black dotted lines represent the optimal correlationsbetween the observed polarity zones andGPTS, gray dotted lines represent the alternativemodels of the B/M and M/G boundaries. Small symbols (not connected by lines withneighboring data) represent single samples within otherwise opposite polarity zones.Italicswithin the Brunhes Epoch represent short excursions documented in Langereis et al.(1997), Champion et al. (1988), Lund et al. (2006), Laj and Channell (2007) and Thouvenyet al. (2008).
SG-1 site including optically stimulated luminescence (OSL) dating atthe top. OSL ages are 82–73 ka at Gaihai lake to the east (Fan et al.,2010a, 2010b) and ~0.111 ka at Dalangtan about 80 km to thenorthwest (Hou et al., 2010, 2011; Shi et al., 2010). The age at the topof core SG-1 can be therefore estimated at about 0.1 Ma. Preliminaryresults of OSL dating on samples from the SG-1 core and a pit sectionnearby confirm this age.
The observed polarities can be correlated with chrons 1n–2An ofthe global geomagnetic polarity time scale GPTS 2004 (Gradstein etal., 2004) (Table 2 and Fig. 8). The predominantly normal polarityzones N1 to N4 likely correspond to the Brunhes Normal PolarityEpoch (B) and the sequence comprising the reverse R4 to R13intervals is interpreted as the Matuyama Reverse Polarity Epoch (M),with the B/M boundary at a depth of 297.9 m. The very short reversedzones R1, R2 and R3 within the Brunhes Epoch (at depths of 70 m,236 m and 258 m) may represent the Portuguese margin event atca. 290 ka (or CR0 at ca. 260 ka), the Delta event at ca. 680 ka and theUn-named (q) event at ca. 700 ka, respectively (Champion et al.,1988; Laj and Channell, 2007; Langereis et al., 1997; Lund et al., 2006;Thouveny et al., 2008) (Fig. 8). The short normal polarity zones N7,N8, N9–N10 and N12 within the Matuyama Epoch can be readilycorrelated with Jaramillo, Cobb Mountain, Olduvai, Reunion normalsubchrons (events) and a short excursion at 2.19 Ma, respectively; N5and N13 may be records of the Kamikatsura or Post-Jaramillo event at0.85 Ma and the X-event at 2.44 Ma (or a short excursion at 2.39 Ma),respectively (Champion et al., 1988; Gradstein et al., 2004; Heirtzleret al., 1968; Valencio et al., 1970) (Figure 8). The rest of the core ischaracterized by mostly normal polarity zones N14–N16, which likelyrecord the uppermost part of the Gauss Normal Polarity Epoch (G)determining the M/G boundary at a depth of 877.5 m (Table 2 andFig. 8). The observed polarities between R4–R6 and N5–N6 showrather frequent alternations of positive and negative inclinations. Thecause of these alternations is unclear, however, it does not reject ourotherwise reasonable correlation with the GPTS. Consequently, thebottom of core SG-1 is estimated at ~2.77 Ma based on extrapolationof the sediment accumulation rate between the lowest two polarityboundaries (Table 2 and Fig. 9).
The correlation to the GPTS is not unambiguous. The B/Mboundary could be also placed at a higher level (bottom of N2 orN3) (Fig. 8). Correlating the B/M boundary to the bottom of N3,however, implies that the relatively long N4 interval represents anormal remagnetization (if the B/M boundary is at the bottom of N2,then also N3 must be overprinted). The M/G boundary might be alsofurther down i.e., at the top of N16 (Fig. 8), which requires a normalremagnetization of N14 and N15. Rock magnetic properties of N2, N3,N14 and N15 are not different from other parts of the sequence,making these alternative correlations less probable. As a conse-quence, the sediment accumulation rates would change (see below).Future detailed studies on paleoenvironment proxy data mightprovide further constraints for fixing the boundaries.
Tectonic implications
From the depth versus age distribution of polarity changes, theaverage sediment accumulation rates (SAR) were calculated from ourfavored correlation model (I) and the alternative model (II) (Fig. 9). For
Table 2Depths and ages of major stratigraphic horizons and magnetic polarity (sub-)chronboundaries for core SG-1 and determined sediment accumulation rates (SAR). Italicsrepresent the alternative models of the B/M and M/G boundaries.
Depth (m) Observed polarity GPTS 2004 Age (Ma) SAR (cm/ka)
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determining the SARs we only consider major polarity boundaries ofchrons and subchrons, as the possible identification of events andexcursions must be considered with great caution. The average SARalong the entire core is 35.1 cm/ka with relatively higher valuesbetween 2.6 and 2.2 Ma (51.5 cm/ka for model I or 43.1 cm/ka formodel II) and after 0.8 Ma (43.8 cm/ka for model I and 34.4 cm/ka formodel II) (Fig. 9). Our favoredmodel I yields amuch stronger differencein SARs than the alternative model II. Based on magnetostratigraphy,higher SARs in the Brunhes Epoch are also observed in the coresZK3208, ZK402 and ZK05 in the western Qaidam Basin (Shen et al.,1993; Sun et al., 2000; Shi et al., 2010), ZK701 in the center of the basin(Shen et al., 1993; Shi et al., 2010), and Shui-6 in the eastern basin (Shenet al., 1993; Liu et al., 1990, 1998) (see Fig. 1 for their localities). Weinterpret these two phases of increased SARs as two erosional episodesdue to rapid uplift pulses of the NE Tibetan Plateau and global climaticcooling.
Restoration of balanced sections in the northeastern Qaidam Basindemonstrated that the shortening rate in the Cenozoic speeded upduring the Quaternary accounting for about 40% of total shortening inthe basin (Fang et al., 2006; Wang et al., 2009; Zhou et al., 2006).Close to the margin of the Altyn Tagh fault in the western QaidamBasin, a marked angular unconformity occurs between the underlyingShizigou Fm. and the Qigequan Fm. In the hanging wall, it is detectedin outcrops and by seismostratigraphy. Magnetostratigraphic datingof this unconformity revealed a stratigraphic gap between ca. 5.3 to2.6 Ma (Fang et al., 2006). This fastest and largest shortening revealed
Figure 9. Depth-age plot for core SG-1 based on correlation with the geomagnetic polarityfrom the sediment thickness between boundaries of correlated major chrons. Black and grarespectively (see Fig. 8).
in the Qaidam Basin was also widely recorded in other parts of the NETibetan Plateau. For example, the sinistral Kunlun fault bordering theQaidam Basin in the south began to slip and created the Kunlun ShanPass pull-apart basin at ca. 2.6 Ma, causing the basin to be down-faulted rapidly at this time (Song et al., 2005). The North Qilian Shanfault to the north of the Qaidam Basin began to propagate into theJiuquan Basin rapidly at ca. 2.6 Ma, causing a further fast rising of theLaojunmiao thrust-anticline and the formation of a minor angle-unconformity in the stratigraphy there (Fang et al., 2005a). The GuideBasin to the east of the Qaidam Basin experienced a similar tectoniccompression as the Jiuquan Basin, where magnetostratigraphic andpaleontological results revealed that the southern Laji Shan fault(equivalent to the eastern extensional part of the southern QilianShan fault bordering the northern Qaidam Basin) began to rapidlypropagate into the Guide Basin, causing a rise of the southernmostpart of the Laji Shan and the formation of a parallel unconformity inthe basin margin (about 0.2 Ma of strata was eroded) (Fang et al.,2005b). Further east to the Linxia Basin, the Pliocene boulder con-glomerate bed (Jishi Fm.) was significantly deformed at ca. 2.6 Ma,which was named as phase B of the Qingzang movement (Li and Fang,1999; Li et al., 1996).
Global cooling may have also contributed to the enhancederosional activity starting at ca. 2.6 Ma. The northern hemisphereentered into a large-scale glaciation at ca. 2.6 Ma, the first since theMesozoic (Shackleton et al., 1990; Zachos et al., 2001). This fastcooling could have triggered a significant deterioration of vegetationand could have enhanced mountain glaciation, thereby intensifyingsurface erosion. However, climate records show that global coolingcontinued from 2.6 Ma onward throughout the whole Quaternary(Shackleton et al., 1990; Zachos et al., 2001). SARs observed in thecore SG-1 indicate that the erosion did not enhance after 2.6 Ma, butdecreased significantly at ca. 2.2 Ma and further dropped rapidly atca. 1.8 Ma (Fig. 9). This suggests that pulse tectonics rather thanglobal cooling was the major factor to drive the erosional event at ca.2.6 Ma.
There are a number of evidences indicating a very strong uplift ofthe NE Tibetan Plateau during the middle Pleistocene. At the marginsof the Qaidam Basin, a very clear unconformity is widely distributedbetween the early Quaternary Qigequan Fm. and the younger strata,for which isotope dating yielded an age of about 0.8 Ma (Ge et al.,2006). The Kunlun fault slipped fast at ca. 0.8 Ma, and the Kunlun ShanPass pull-apart basin (formed at 2.6 Ma) was strongly uplifted at thattime (Cui et al., 1996; Song et al., 2005). Sharp angular unconformities
time scale of Gradstein et al. (2004). Sediment accumulation rates (SAR) are calculatedy lines represent the SARs deduced by the optimal (I) and alternative (II) correlations,
147W. Zhang et al. / Quaternary Research 78 (2012) 139–148
and strong incisions were frequently observed in many other parts ofthe NE Tibetan Plateau, which all were dated at ca. 0.9–0.8 Ma bymagnetostratigraphy and loess–paleosol sequences (e.g., Fang et al.,2005a; Li et al., 1996, 1997; Liu et al., 2010; Pan et al., 2010). All theseresults indicate that tectonic uplift of the NE Tibetan Plateau wasstrong at ca. 0.9–0.8 Ma, which should have contributed to the en-hancement of erosion observed at ca. 0.8 Ma in core SG-1. Neverthe-less, global cooling during the so-called Mid-Pleistocene Revolution atca. 0.9 Ma (Berger et al., 1993; Zhao et al., 2001) may also have addedpower to this Mid-Pleistocene erosional period.
Variation of SARs determined from core SG-1 and other availableresults presented above suggest that both rapid uplift of the NETibetan Plateau and global climatic cooling caused the higher SARsduring ~2.6–2.2 Ma and after 0.8 Ma. Relatively higher elevation ofthe surrounding mountains due to uplift provided the potentialsediment source for our study site since 2.6 Ma. Global cooling since2.6 Ma and Asian aridity, increasing wind speed since late Pliocene(Kapp et al., 2011; Pan et al., 2004; Zan et al., 2010), might haveintensified surface erosion. Thrust-fold systems and a progressiveeastward transposition of the depocenter have started to speed up inthe western Qaidam Basin since about 2.6 Ma. Rapid tectonic uplift at0.8 Ma again triggered a rapid SAR and a fast eastward propagation ofthe western basin depocenter which ultimately shifted from ourstudy region toward the eastern Qaidam Basin, causing exhumationand denudation in the western basin after about 0.1 Ma.
Conclusions
1) Magnetostratigraphy dates the 938.5 m deep core SG-1, located inthe central part of the desiccated playa at Chahansilatu in thewestern Qaidam Basin, at between ca. 2.77 Ma and 0.1 Ma;Brunhes/Matuyama and Matuyama/Gauss boundaries are locatedat depths of 297.9 m and 877.5 m, respectively.
2) The results provide a fundamental age framework for studyingpaleoenvironment proxies on core SG-1.
3) The age model results in an average sediment accumulation rate(SAR) of about 35.1 cm/ka for the entire core and reveals twointervals with enhanced SARs between ~2.6–2.2 Ma and after0.8 Ma, suggesting two episodic erosional periods probably causedby pulse tectonic uplift of the NE Tibetan Plateau and possibly alsoinfluenced by global cooling at those times.
Acknowledgments
This study was co-supported by the NSFC grants (40920114001,41021001, 40702006), the Knowledge Innovation Program of theChinese Academy of Sciences, Grant No. KZCX2-YW-Q09-04, the (973)National Basic Research Program of China (2011CB403000), theGermanResearch Foundation (DFG) (No. AP34/34-1,2) and theGermanMinistry for Education and Research (BMBF) (No. 03G0705A). It is partof the DFG Priority Programme 1372 “Tibetan Plateau: Formation,Climate, Ecosystems (TiP).” We thank Liu Dongliang, Hu Sihu, ZhangQibo, Yang Yibo, Yang Yongbiao and Li Shiyuan for field drillingassistance, and Wolfgang Rösler, Hu Shouyun, Ulrich Blaha, BorjaAntolin and Ursina Liebke for constructive discussions and laboratoryassistance. We especially thank the Qinghai Jingcheng Geology andResource Ltd. Company for its perfect drilling technology support, andthe journal's Associate Editor and Prof. François Demory and Ute Frankfor their constructive comments which have helped to improve themanuscript.
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