Late Neogene magnetostratigraphy in the western Qaidam Basin (NE Tibetan Plateau) and its constraints on active tectonic uplift and progressive evolution of growth strata

Tectonophysics 599 (2013) 107–116

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Late Neogene magnetostratigraphy in the western Qaidam Basin (NE Tibetan Plateau) and its constraints on active tectonic uplift and progressive evolution of growth strata Weilin Zhang a, c,⁎, Xiaomin Fang a, Chunhui Song b, Erwin Appel c, Maodu Yan a, Yadong Wang d a

Key Laboratory of Continental Collision and Plateau Uplift, Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Beijing 100101, China Key Laboratory of Western China's Environmental Systems, Ministry of Education of China & College of Resources and Environment, Lanzhou University, Gansu 730000, China c Fachbereich für Geowissenschaften, Universität Tübingen, Sigwartstr. 10, 72076 Tübingen, Germany d Key Laboratory of Petroleum Resources Research, Institute of Geology and Geophysics, Chinese Academy of Sciences, Lanzhou 730000, China b

a r t i c l e

i n f o

Article history: Received 16 August 2012 Received in revised form 5 April 2013 Accepted 8 April 2013 Available online 29 April 2013 Keywords: Magnetostratigraphy Neogene Growth strata Qaidam Basin NE Tibetan Plateau

a b s t r a c t The Qaidam Basin as the largest intermontane basin of the NE Tibetan Plateau is the ideal place to provide constraints on depositional and tectonic patters. To determine its tectonic deformation history and progressive evolution of growth strata we conducted paleomagnetic study on the late-Neogene stratigraphic section in the western Qaidam Basin. A magnetostratigraphic study of the well exposed 805 m Qigequan section at the Qigequan anticline in the western Qaidam Basin reveals twelve pairs of normal and reversed polarity zones which can be readily correlated with chrons C1n-3Ar of the Geomagnetic Polarity Time Scale (GPTS). From this correlation we can conclude that the Shizigou and the Qigequan formations were formed at >6.9 Ma–2.5 Ma and 2.5–~0.4 Ma, respectively. Accumulation rates determined from our chronology, together with the occurrence of unconformities suggest four phases of tectonic uplift which began at about 3.6 Ma, 2.5 Ma, 1.1 Ma and 0.8 Ma. The results also suggest that offlap growth strata according to the limb rotation model on the anticlinorium started to occur at ~8.2 Ma. They progressively become younger from the frontal region of the Altyn Tagh Mts. (~8.2 Ma) to the southwestern basin (~2.5 Ma) and to further east of the Qaidam Basin (b2.5 Ma), caused by fault-propagation-folding in the Qaidam Basin, rapid uplift and fast exhumation of the NE Tibetan Plateau at those times. © 2013 Elsevier B.V. All rights reserved.

1. Introduction The Cenozoic sedimentary basins in the interior of the Tibetan Plateau are a direct result of the continental collision of India with Eurasia associated with deformation and the uplift of the Tibetan Plateau and movement along strike-slip faults (e.g. Altyn Tagh Fault, Kunlun fault and Qilian fault) (Dupont-Nivet et al., 2007; Fang et al., 2007; Horton et al., 2002; Métivier et al., 1998; Meyer et al., 1998; Molnar, 2005; Molnar et al., 2010; Sobel et al., 2001; Tapponnier et al., 2001; Yin et al., 2002). Under the dynamic regime of North–South directional compression, the Cenozoic strata of the basins have been folded and thrusted to some extent and developed synthetical folds, which provide the information on the depositional systems, erosional unroofing patterns, and kinematic histories of fold-thrust systems along the margins of the long strike-slip faults (Clark et al., 2010; Graham et al., 1993; Horton et al., 2002; Métivier et al., 1998; Sobel and Dumitru, 1997; Yin ⁎ Corresponding author at: Key Laboratory of Continental Collision and Plateau Uplift, Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Beijing 100101, China. Tel.: +86 10 84097090; fax: +86 10 84097079. E-mail address: [email protected] (W. Zhang). 0040-1951/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.tecto.2013.04.010

et al., 2002, 2007, 2008a, 2008b). Poor knowledge on the timing of the deformation of Cenozoic strata in these basins, however, greatly hampers our understanding of the evolution of the large strike-slip faults at the margins of the basins. The Qaidam Basin as the largest intermontane basin of the NE Tibetan Plateau is the ideal place to provide constraints on depositional and tectonic patterns. It is located at the northeastern margin on Tibetan Plateau and surrounded by the Qiman Tagh and Kunlun mountains to the south, the Altyn Tagh mountains to the northwest, the Qilian mountains to the northeast and the Ela mountains to the east (Fig. 1a). Recently, the magnetostratigraphic studies from the Qaidam Basin have been carried out only for the early Tertiary sediments on the Hongsanhan section in the northwestern (Sun et al., 2006), the Dahonggou section in the northern (Lu and Xiong, 2009) and the Huaitoutala section in the east (Fang et al., 2007), or for the Quaternary sediments from the drilling cores in the west (Shi et al., 2010; Zhang et al., 2012) and the east (Liu et al., 1998; Wang et al., 1986). However, the late Neogene magnetostratigraphy has not been studied in the Qaidam Basin. In order to better understand the structural and geomorphic processes that contributed to build the northeastern plateau, we present new late Neogene paleomagnetic

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Fig. 1. (a) Map of Tibetan Plateau and adjacent regions showing major structures; (b) Tectonic map of the Qaidam basin and adjacent regions within the northeastern Tibetan Plateau (modified from Yin et al. (2002)); (c) geological map of the study area in the western Qaidam Basin showing the location of Qigequan section (red asterisks). See Fig. 1b for its location within the basin. The regions (I & II) of gray broken lines indicate roughly scopes of the frontal regions of the Altyn Tagh (Mts.) thrust belt and the east Kunlun Shan (Mts.) thrust belt, respectively.

result of the Qigequan section and document the late Neogene history of sedimentation and related contraction in the western Qaidam Basin. 2. Geological setting The rhomb-shaped Qaidam Basin with an area of 121,000 km 2 is actively shortened in NE–SW direction in response to the ongoing

collision between India and Asia (Fig. 1a) (Kapp et al., 2011; Tapponnier et al., 2001; Wang and Burchfiel, 2004; Wang et al., 2006; Yin et al., 2007, 2008a, 2008b). The interior of the basin has an average elevation of almost 3000 m above sea level (a.s.l.), with surrounding mountains reaching elevations of about 4000 to 5000 m (Fig. 1a). This high relief between the Qaidam Basin and the surrounding mountains is controlled by large boundary faults between them (Fang et al., 2007; Wang et al., 2006; Yin et al., 2002) (Fig. 1a). The east Kunlun

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fault, derived from the Kunlun fault belt along the Qiman Tagh and the Kunlun Shan, bounds the southern margin of the Qaidam Basin and is a major strike-slip fault, thought to absorb and transfer deformation imposed by the collision of India with Asia (Molnar and Tapponnier, 1975; Tapponnier and Molnar, 1977). Due to the tectonic compression and uplift from the east Kunlun fault since early Cenozoic, NW-striking thrust-fold systems were developed in the southwestern Qaidam Basin (Fang et al., 2007; Wang et al., 2006; Yin et al., 2002) (Fig. 1b) consisting of a series of large rows of thrust-folds basinwards as F1k–F6k (Figs. 1b and 2). The impressive 1500 km long, NE–SW sinistral strike-slip Altyn Tagh fault cuts through the northwestern margin of the Qaidam Basin. Displacement between ~350 km and ~700 km along the Altyn Tagh fault (Chen et al., 2002; Meng et al., 2001; Peltzer et al., 1989; Ritts and Biffi, 2000; Tapponnier et al., 1982) has not only caused the formation of the Altyn Tagh mountains, but has also led to the depression and deformation of the northwestern margin and the interior region of the Qaidam Basin (Chen et al., 1999; Huang et al., 1996; Molnar and Tapponnier, 1975; Tapponnier et al., 2001; Wang and Burchfiel, 2004; Yin et al., 2002, 2007, 2008a, 2008b) (Figs. 1b and 2). Seismic data and field observations show that the south Altyn marginal thrust belts extend southwards into the interior of the Basin and mainly consist of four large rows of thrust-folds (F1a–F4a) at our study area (Figs. 1b and 2). At least since the Oligocene, the interior of the western Basin has experienced significantly stronger extensional to contractional deformation, in response to fast strike-slip of the Altyn fault and the Kunlun fault (Liu et al., 2009; Wang et al., 2007; Xia et al., 2001; Yin

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et al., 2002; Zhou et al., 2006), accompanied by uplift of the surrounding mountains due to an outward growth of the Tibetan Plateau (Fang et al., 2006, 2007; Métivier et al., 1998; Wang and Coward, 1990; Xia et al., 2001; Yin et al., 2008b). Contractional growth-strata are widely preserved within alluvial-fluviolacustrine -playa successions along the basin-margin structures and the interior basin. They are characterized by different dip angles of ~ 5°–50° (Fig. 2a–b–c) or by unconformities between individual segments in the growth anticlinal axes (Fig. 2d), and a notable decrease in the thickness of strata from the axes to the limbs of folds (Fig. 2a–b–c–d). These features of growth strata may be either caused by a single period of non-deposition or erosion associated with deformation or by erosion that gradually migrated basinwards as the synthetical folds developed (Ford et al., 1997; Sobel et al., 2003). The southern margin of the Qaidam Basin likely formed in early Miocene and has persisted without significant erosion since then (Sobel et al., 2003; Yin et al., 2008a, 2008b). Syncontractional sedimentations have been gradually or abruptly triggered by growth strata between the NW–SE striking anticlines in the southwestern Qaidam basin. Since Pliocene–Quaternary, molasse sediments were well-developed at the front of the Altyn fault and the parallel NW–SE striking anticlines were strongly uplifted associated with the uplift of the Altyn Tagh (Fang et al., 2006; Wang et al., 2007), leading to the preservation and exposure of the Cenozoic strata and triggering a fast eastward shift of the basin depocenter which ultimately withdrew from the western basin (Fang et al., 2007; Gu et al., 1990; Huang et al., 1996; Kapp et al., 2011; Métivier et al., 1998).

Fig. 2. (a–b) Seismic profiles A–B and C–D (Fang et al., 2006); (c) cross-section E–F; (d) cross-section of the Qigequan section. See Fig. 1b for their locations within the basin and the definition of strata units. Note that the lower part of faults is mostly schematic and is not plotted at real vertical scale, the profile is vertically exaggerated; selected dip directions and dips are shown.

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The Qaidam Basin received up to 12,000 m thick Cenozoic sediments of mostly alluvial-fluviolacustrine-playa facies from the surrounding mountains (Gu et al., 1990; Huang et al., 1996; Xia et al., 2001). According to palynologic, paleontologic and lithostratigraphic studies (Gu et al., 1990; Huang et al., 1996; Qinghai BGMR, 1991; Yang et al., 1994), the Cenozoic stratigraphy before the early Pleistocene in the Qaidam Basin has been divided into seven formations with assigned ages (in upward sequence): the Lulehe Fm. (Paleocene to Eocene, E1–2), the Xia Ganchaigou Fm. (Oligocene, E3), the Shang Ganchaigou Fm. (early Miocene, N1), the Xia Youshashan Fm. (mid Miocene, N21), the Shang Youshashan Fm. (late Miocene, N22), the Shizigou Fm. (Pliocene, N23) and the Qigequan Fm. (early Pleistocene, Q1). The middlelate Pleistocene sediments, characterized by fine-grained conglomerate around the marginal areas of the basin and by fine-grained gypsum and saline lacustrine deposits elsewhere (Huang et al., 1996; Li et al., 2010; Qinghai BGMR, 1991), are widespread within the Qaidam Basin and mostly distributed with a maximum thickness of more than 2000 m in the eastern part of the basin (Fang et al., 2006). Until now, the middle-late Pleistocene sediments could not be further subdivided. Our paleomagnetic section lies along the southern limb of the Qigequan anticline against the southern edge of the Altyn Tagh, about 15 km north of Huatugou Town (Fig. 1b). In the sampled outcrop an 805 m section is exposed and located between N38°20.463′, E90°38.672′ and N38°21.18.3′3, E90°39.006′, comprising late Miocene to Quaternary sedimentary rocks of the Shizigou Fm. and Qigequan Fm. (Fig. 2c–d). The boundary between the 504 m thick Shizigou Fm. and the underlying Shang Youshashan Fm. is not met in the section, and there is a paraconformity (U2) with the overlying Qigequan Fm. (Fig. 2d). The lithology of the Shizigou Fm. is characterized by sandstone intercalated with siltstone and mudstone. The Qigequan Fm. is 301 m thick (from 504 m to 802 m) with an angular unconformity (U1) at 745 m, and is dominated by conglomerate intercalated with thin sandstone; in its uppermost part it is covered by Holocene conglomerate (Fig. 2d). 3. Sampling and measurements Oriented block samples were taken at intervals of 1–2 m by digging 0.5 to 2 m deep pits into the sandstone-mudstone layers. Siltstone or mudstone lenses were collected at intervals of 2–3 m in the conglomerate layers. From the block samples three oriented cubic specimens of 2 × 2 × 2 cm were cut in the laboratory, forming three parallel sets of samples for cross-calibration measurements. A total of 408 block samples and 408 × 3 specimens were obtained. Thirty-eight pilot samples were measured by the systematic stepwise thermal demagnetizations in fifteen to twenty discrete steps between room temperature and 690 °C (intervals of 50 °C below 550 °C and 10–20 °C above it). Twenty-two representative samples were selected for isothermal remanent magnetization (IRM) acquisition and back field measurements in the Paleomagnetic Laboratory of Lanzhou University. IRM was imparted using an ASC IM-10-30 pulse magnetizer with maximum fields of 2.3 T, and measured on an Agico JR5-A spinner magnetometer. Back field IRM measurements were carried out until the remanence reached to an opposite direction. All the specimens of the first and second sets of specimens were thermally demagnetized in 8 to 12 steps at 20– 50 °C intervals between 400 °C and 690 °C. Remanence measurements were done with a 2G Enterprises SQUID magnetometer in a magnetically shielded room. The first set of specimens was measured in the Paleomagnetism Laboratory of the Institute of Geology and Geophysics Beijing (Chinese Academy of Sciences) and the second set of specimens in the Paleomagnetism Laboratory of the Institute of Earth Environment (Chinese Academy of Sciences). Representative thermal demagnetization diagrams are shown in Fig. 3. Three components can be generally distinguished based on their contrasting directions and unblocking intervals. A low temperature

component (LTC) was removed between 50 °C and 250 °C. This LTC has no stable direction (Fig. 3b) and is interpreted as a secondary remanent magnetization. The intermediate temperature component (MTC) shows a gradual decay between 350 °C and 500 °C and an obvious intensity drop at 585 °C (Fig. 3a). The high temperature component (HTC) decays from 585 °C to 670 °C and attains a minimum remanence value at 690°C (Fig. 3a). The MTC and HTC are likely residing in magnetite and hematite, respectively. For most samples throughout the stratigraphy, a characteristic magnetization (ChRM) is clearly isolated and points to the origin, either representing an MTC or an HTC (Fig. 3b). The IRM acquisition curves show a steep rise below 300 mT; they attain ~70% of the saturation IRM (SIRM) at 300 mT and do not fully saturate at 2.5 T (Fig. 3c). This further confirms the presence of low and high coercivity minerals as magnetite and hematite. The ChRM directions of the MTC and HTC samples were determined, and about 8 steps of thermal demagnetization (between ~400 °C and 690 °C) were performed using principal component analysis. Samples with maximum angular deviation (MAD) >15° were excluded for further interpretations. Virtual geomagnetic polarity (VGP) latitudes were calculated from the ChRM directions. Specimens with VGP latitude values less than 30° were also rejected for polarity determination. According to these criteria a total of 77 stratigraphic levels were omitted. The results of the remaining specimens were averaged for each level and then used to calculate VGPs (Figs. 4 and 5). 4. Magnetostratigraphy The accepted tilt-corrected ChRM directions were used for a reversal test. The mean direction of the normal-polarity sites (D = 359.9°) is antiparallel to the mean direction of the reversed-polarity sites (D = 181.5°) (Fig. 4a). A statistical bootstrap technique (Tauxe, 1998) has been used to test whether the distributions of the ChRM vectors are possibly non-Fisherian, and to characterize the associated uncertainties for both normal and reversed ChRM directions. The histograms of the Cartesian coordinates of bootstrapped means (Tauxe, 1998) allow us to determine a 95% level of confidence (ovals around the means in Fig. 4a) and to demonstrate that the bootstrap reversal test is positive (Fig. 4b). For performing a fold test after Tauxe and Watson (1994), 188 representative high quality (MAD b 5) site-mean ChRMs from different parts of the section were used. Within 95% confidence limit the tightest grouping of data occurs between 71% and 104% untilting (Fig. 5c), which indicates a positive fold test. The tiltcorrected normal and reverse polarity ChRM directions cluster around antipodal means (Fig. 5a–b). Fig. 6 shows a VGP plot along the stratigraphic section resulting from the accepted ChRM directions. A total of 12 normal and 12 reverse polarity intervals are recorded (Fig. 6). The predominantly reverse polarity interval from R1 to R3 corresponds to the Matuyama reverse polarity epoch while the normal polarity zones N2 and N3 can be readily correlated with the Jaramillo and Olduvai normal subchrons (events) (Fig. 6). The normal polarity zones N4, N5 and N6 make up the Gauss normal polarity epoch and the reverse zones R4 and R5 therefore should represent the Kaena Event and Mammoth Event (Fig. 6). The striking reversed intervals R6–R10 are correlated straightforward with the five reverse zones within the Gilbert reverse polarity epoch and the four short normal polarities for N7-N10 represent the Cochiti, Nunivak, Sidufjall and Thvera subchrons (Fig. 6). N11-R11-N12-R12 are regarded as the chrons C3An and C3Ar in the Epoch-5 (Fig. 6). Accepting this correlation with the GPTS, the top and bottom age of the sampled Qigequan section is estimated at 6.9 Ma to 0.4 Ma from the sediment accumulation rate between the lowest (highest) two polarity boundaries (Fig. 6). Based on the accumulation rates, the age of the stratum hiatuses of the two unconformities U1 (745 m) and U2 (501 m) is about 0.78–0.71 Ma and 2.95–2.4 Ma, respectively, and the age of the stratigraphic boundary between the Shizigou Fm. and the Qigequan Fm. is estimated between

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Fig. 3. Results of thermal demagnetization and for representative samples of the Qigequan section. (a) Remanence intensity curves versus temperature; (b) Zijderveld diagrams; open (closed) symbols represent vertical (horizontal) projections; (c) IRM acquisition curves with back field measurements.

Fig. 4. (a) Equal-area projections of the characteristic remanent magnetization (ChRM) directions and mean directions (with oval of 95% confidence limits) for the Qigequan section determined with a bootstrap method (Tauxe, 1998). Downward (upward) directions are shown as closed (open) circles. α95, N, k are 95% confidence limit, number of samples and precision parameter. (b) Histograms of Cartesian coordinates of bootstrapped means of para-data sets of the Qigequan section; in the plots, the reversed polarity mode has been flipped to the antipode. The intervals containing 95% of each set of components are shown (dashed and solid lines). The confidence bounds from the two data sets overlap in all three components, the means of the reversed and normal modes cannot be distinguished at the 95% level of confidence, indicating a positive bootstrap reversal test (Tauxe, 1998).

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the jackknife technique of Tauxe and Gallet (1991) to quantify the reliability of the magnetostratigraphic result. The parameter J calculated from the accepted specimen-mean directions has a value of − 0.079 (Fig. 7), which falls within the range of 0 to − 0.5 indicating that more than 95% of polarity intervals have been recovered. The test therefore supports a robust magnetostratigraphic data set. Fig. 8a presents a thickness-vs.-age plot of the main chrons. The accumulation rate well corresponds to the lithological changes, for example, it is more than 20 cm/kyr for the conglomerate and sandy conglomerate in the upper and middle part of the section (~ 1.5 Ma and ~ 3.6 Ma) while it is less than 10 cm/kyr for the fine-grained mudstone in the lower part (Fig. 8a–b). These sedimentary features in the Qigequan section further support our magnetostratigraphic interpretation.

5. Discussion 5.1. Constraints of magnetostratigraphy on active tectonics

Fig. 5. Fold test for the 188 representative high quality site-mean ChRMs from different parts of the Qigequan section using the method of Tauxe and Watson (1994). (a) Equal-area projection of site-mean directions in situ coordinates; (b) equal-area projection of site mean directions after 100% untilting. (c) Fold test results; variation of the principal eigenvector τ1 with various degrees of unfolding is depicted with dashed lines for different para-data sets; the distribution of maxima for these data sets is shown by the histogram; the 95% confidence interval ranges from 71 to 104% untilting.

2.4 Ma and 2.95 Ma. The latter is supported by the 2.5 Ma age interpreted from the previous magnetostratigraphic result of the Huaitoutala section in the eastern Qaidam Basin (Fang et al., 2007). Fossil mammals, named Huaitoutala Fauna, were found in the Shizigou Fm. at the eastern Qaidam Basin, containing typical Pliocene north China components such as Orientalomys/Chardinomys and Mimomys, which belong to the Yushean Land Mammal Age (Fang et al., 2007; Wang et al., 2007) and which are estimated to occur in the Shizigou Fm. at ~4.8 Ma (Fang et al., 2007). The magnetostratigraphy of the Neogene Huaitoutala section showed that the Shizigou Fm./Shang Youshashan Fm. and Qigequan Fm./Shizigou Fm. boundaries occur at ages of ~8.2 Ma and ~2.5 Ma, respectively. All these data from the eastern Qaidam Basin support our interpretation of the Qigequan section. Finally, we used

Fig. 6. Magnetostratigraphic results of the Qigequan section. VGP: virtual geomagnetic polarity; GPTS: geomagnetic polarity time scale of Cande and Kent (1995).

The two unconformities (U1 and U2) at about 0.78 Ma and 2.5 Ma (Figs. 2d, 6 and 8a) are interpreted as significant tectonic and plateau uplift events. The average accumulation rate of the Qigequan section derived from the magnetostratigraphic results is about 12.3 cm/kry. Two intervals of highest sediment accumulation start at about 3.6 Ma and at 1.1 Ma (Fig. 8a). Climatic change might also increase or decrease wind speed to affect the accumulation rate (Kapp et al., 2011), but high accumulation rate has been considered to be caused by the tectonic events in our studying area (Fang et al., 2007; Wang et al., 2006; Yin et al., 2002). In previous studies of other basins on the Tibetan Plateau (An et al., 2001; Bloemendal and DeMenocal, 1989; Fang et al., 2005a, 2005b, 2007) the occurrence of unconformities and the increase in accumulation rates were also related to sudden tectonic events. According to this interpretation we can identify at least four phases of tectonic uplift events for the Qaidam basin, i.e. at about 3.6 Ma, 2.5 Ma, 1.1 Ma and 0.8 Ma. The first increase of the accumulation rate at 3.6 Ma is accompanied by the appearance of conglomerate in the studied section (Fig. 8), which is indicative of a synchronous folding of the Qigequan anticline by the backward propagation F2k fault from the forethrust F4k due to a push by the east Kunlun fault (Figs. 1b and 2c). A fast eastward shift of the depocenter in the Qaidam Basin was estimated to occur at about 3.6 Ma (Fang et al., 2007; Gu et al., 1990; Huang et al., 1996). Due to this tectonic uplift, huge and thick boulder conglomerate first appeared around the rim of the whole Tibetan Plateau (Burbank and Johnson, 1982; Fang et al., 2005a, 2005b, 2007; Li et al., 1997; Liu et al., 2010; Song et al., 2001, 2005; Zheng et al., 2000). Initiation of E–W extension, formation of graben systems in northern Tibet and the formation of the Kunlun Pass basin are reported to have occurred at about 3.6 Ma (Li et al., 2001; Song et al., 2001, 2005; Yin et al., 1999). The second tectonic activity at 2.6 Ma is accompanied by the unconformity U2 and is synchronous with the occurrence of a change of stratum dipping from 45° to 23° (Figs. 2d and 6). Close to the margin of the Altyn fault in the western Qaidam Basin, a clear angular unconformity between the Shizigou Fm. and the overlying Qigequan Fm., corresponding to phase B of the Qingzang movement (Li and Fang, 1999, Li et al., 1996), is widely observed at about 2.6 Ma in outcrops and seismostratigraphic sections (Fang et al., 2006, 2007; Yin et al., 2002). Meanwhile, unconformities were widely distributed and depositional hiatuses became numerous after 2.6 Ma, not only in the Hexi corridor, Kunlun pass basin, Linxia basin and Guide-Gonghe basin at the northeastern edge of the Tibetan Plateau, but also in the Zhada basin, Jilong basin and Woma basin at the southern Tibetan Plateau (Fang et al., 2003, 2005a; Li et al., 1997; Liu et al., 2010; Wang et al., 2008; Yue et al., 2004).

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Fig. 7. Magnetostratigraphic jackknife analysis (Tauxe and Gallet, 1991) for the Qigequan section; the value of J = −0.079 predicts that the section has recovered over 95% of the true number of polarity intervals.

The third tectonic activity at 1.1 Ma is regarded as an expression of strong tectonic deformation and uplift due to the left-lateral movement along the Altyn Tagh fault and north–south compression. This tectonic activity led to the formation of the parallel northwest– southeast striking anticlines, the en-echelon s-shaped structure in the interior basin, and the rapid shifts of the fold axes trend from NWW to NW in the western basin. It is synchronous with the termination of the Guide paleo-lake and Dongshan paleo-lake at the northeastern margin of the Tibetan Plateau (Li et al., 1996, 1999; Fang et al., 2003; Song et al., 2005) and the appearance of the large unconformity in the west Hexi Corridor (Zhao et al., 2001b; Fang et al., 2005b; Liu et al., 2010). The fourth tectonic activity at about 0.8 Ma is indicated by the unconformity U1. It again triggered the larger fluctuations on the dip-magnitude changes since 0.8 Ma (Figs. 2d and 6), and a fast eastward shift of the basin depocenter which ultimately withdrew from our study region toward the Qaerhan region (Gu et al., 1990; Shen et al., 1993). The fast uplift of the Altyn Tagh (Mts.) caused very rapid denudation and deposition of conglomerate in the western basin at about 0.8 Ma (Yin et al., 2002; Wang and Burchfiel, 2004; Wang et al., 2006). As a result of these tectonic movements at 0.8 Ma, the accumulation started to terminate in the Kunlun pass basin and in the Zhada basin at the interior and southern Tibetan Plateau, respectively (Song et al., 2005; Wang et al., 2008). Sharp angular unconformities and great incisions frequently appeared on the whole

Tibetan Plateau, which all were dated at about 0.8 Ma by magnetostratigraphy and loess-paleosol sequences (e.g., Fang et al., 2005a, 2005b, 2006; Li et al., 1996, 1997; Liu et al., 2010; Pan et al., 2010). 5.2. Constraints of magnetostratigraphy on growth strata The inherent synchroneity of growth strata and coupled fold or fault activity makes growth strata crucial to interpret fold-andthrust geometry and kinematics (Anastasio et al., 1997; Suppe et al., 1992). Paleomagnetic studies across the Qaidam Basin indicate no rotation along its northern margin and ~ 16–20° clockwise rotation in the southwestern basin in the Cenozoic (Chen et al., 2002; DupontNivet et al., 2002; Halim et al., 2003; Sun et al., 2006). The en-echelon s-shaped structure of the basin occurred in the late Neocene due to left lateral slip of the Altyn fault (Fang et al., 2006; Wang et al., 2006). The accumulation rates changed strongly during different periods (Fang et al., 2007; Shen et al., 1993; Yin et al., 2002), indicating that growth strata in the western Qaidam Basin developed by limb rotation associated with marked changes of dip and thickness of growth strata on the forelimb, rather than kink-band migration with no variation in dip or thickness (Ford et al., 1997; Poblet and Hardy, 1995; Suppe et al., 1992). Seismic and structural interpretation, balanced cross-sections, unfolding techniques and forward modeling techniques propose that the Cenozoic growth strata initiated at 65–50 Ma in the western basin against the Altyn fault and at 24 Ma in the eastern

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Fig. 8. (a) Thickness-vs.-age plots of the magnetic polarity chrons for the Qigequan section, together with the resulting accumulation rates. (b) Variation of occurrence of conglomerate and sandstone beds, calculated for 50 m stratigraphic intervals using 20 m moving window increments.

basin against the eastern Kunlun fault (Zhou et al., 2006; Yin et al., 2008a, 2008b; Liu et al., 2009), with amplitudes ranging from ~170 km in the west to ~50 km in the east (Yin et al., 2008a, 2008b). The thickest Cenozoic strata are located in the center of the basin with a depth >15 km while the regions toward the basin margins along the Altyn Tagh Mts. have only thin strata with b3 km (Huang et al., 1996; Yin et al., 2007). The accumulation rate gradually increased from west to east with a maximum value > 30cm/ka in the Qarhan Lake (Fang et al., 2006, 2007; Shen et al., 1993). Furthermore, unconformities associated with growth folds widely developed in the western Qaidam Basin either due to absence of deposition, or to erosion associated with deformation since ~8.2 Ma, or to erosion that gradually migrated from the western to the eastern basin after ~2.5 Ma (Fang et al., 2007; Liu et al., 2009). Under the compression and thrusting basinwards the Cenozoic strata along the Altyn Tagh Mts. have formed faulted nose and steep-tip structures since ~8 Ma (Duan et al., 2009; Fang et al., 2006; Wang et al., 2010). These features indicate that the uplift rate of the anticlinorium in the western Qaidam basin by folding growth (Ru) varied since late Neogene and was higher or lower than the relative rate of sedimentation (Rs) (Liu et al., 2009; Wang et al., 2010). In the frontal I region of the Altyn Tagh (Mts.), known as the Altyn slope area (Fig. 1b) (Duan et al., 2009; Fang et al., 2006), faulted nose structures widely existed, accompanied by strong tectonic activities of the Altyn fault with an axis of high relief extending from the east to west (Wang and Burchfiel, 2004; Yin et al., 2002) parallel to the frontal thrust-faults F1a–F3a and other branches of the Altyn fault (Figs. 1b and 2c). Conglomerate intercalated with gravel sandstone first appeared at ~ 8.2 Ma in the Shizigou Fm. (Fang et al., 2006, 2007), in contrast to the lithologies of the lower Shangyou Shan Fm.

with a higher fraction of carbonate-mudstone (Liu et al., 2009; Zhang et al., 2010). Growth strata, steeply tilted basinwards, are typically related to syntectonic sedimentation at the front of the faulted nose structures (Fig. 9a). Slip and uplift on the Altyn faults result in a topographically active monoclinal ramp, prone to erosion along the Altyn Tagh Mts. and its frontal deposition at the frontward basin (Fang et al., 2007; Yin et al., 2007). The Rs was high with values ranging from 20 to 30 cm/kyr since ~ 8.2 Ma in the east and center of the Qaidam Basin (Fang et al., 2007; Shen et al., 1993), but very low with only 5 cm/ka at our study site (Fig. 8). In the I region, the low Rs, the decreased deposition or even its absence at the frontal area of the thrust fault belts and the erosion or denudation at the top of the thrust fault belts (Fig. 9a) can be explained by less deposition due to higher elevation or the rapid uplift of Altyn mountains. According to the limb rotation model, ongoing slip and/or propagation of the fault tip towards the surface resulted in block rotation of older sediments and angular offlap growth of subsequent deposits since ~ 8.2 Ma in the I region (Fig. 9a). In the II region, the thrust-folds basinwards (as F1k–F6k) and huge boulder conglomerate started to develop in the Qigequan Fm., and an unconformity formed at ~ 2.5 Ma at the boundary between the Qigequan Fm. and Shizigou Fm. (Fig. 2d). Cross-section across our Qigequan section and across the Yousha Shan, Youquanzi and Nanyishan anticlinoriums at the southeastern segments show that the backlimbs of the anticlinorium are all sub-horizontal since ~ 2.5 Ma (Fang et al., 2006; Yin et al., 2007, 2008a, 2008b) (Fig. 1b). Seismostratigraphy and outcrops also show that thick strata of the Qigequan Fm. deposited in the limbs of anticlinorium, while lack of deposition or even erosion appeared in the center regions of anticlinorium (Fang et al., 2006; Liu et al., 2009; Yin et al., 2007) (Fig. 2c). This indicates that the ratio of Ru/Rs increased in the anticlinorium and Ru exceeded Rs since ~ 2.5 Ma. Based on the classic fault-bend fold model of Suppe (1983, 1992) and Ford et al. (1997), offlap growth of this flat decollement occurred at ~ 2.5 Ma (Fig. 9) due to the high-angle thrusts (as F1k–F6k) from Kunlun thrust fault belt (Figs. 1b and 2c). This matches with the age of the above described tectonic activity in our Qigequan Fm. Off growth strata in the anticlinorium become progressively younger from the frontal region of the Altyn Tagh Mts. (~ 8.2 Ma) towards our study site (~ 2.5 Ma) and further east (b2.5 Ma) in the Jianshan anticline and Yahu anticline. This is verified by gradual younging of the Cenozoic strata on the top of the center regions of the anticlinorium from west to east (Qinghai BGMR, 1991). It is also confirmed by paleomagnetic data and the accumulation rate from the deep drill-cores SG-1 and the Yahu section (Fang et al., 2008; Shen et al., 1993; Zhang et al., 2012), and by the large differences of the strata thicknesses detected by drilling results of PetroChina (Liu et al., 1998). This overall feature is synchronous with the large-scale eastward shift of the depocenter since ~2.5 Ma (Fang et al., 2006, 2007; Shen et al., 1993; Zhang et al., 2012) and the appearance of unroofing erosion after ~0.4 Ma in our study region (Fig. 9a–b). 6. Conclusion Our magnetostratigraphic results from late-Neogene sediments of the Qigequan section provide new insight on the timing and character of tectonic deformation in the western Qaidam Basin region, related to the development of the Indo-Asia continental collision. The 805 m thick section comprises an age span from about 6.9 Ma to ~ 0.4 Ma; the boundary of the Shizigou and the Qigequan formations is detected at 2.5 Ma. Accumulation rates determined from our chronology, together with the existence of unconformities, allow distinguishing four phases of enhanced tectonic uplift (starting at about 3.6 Ma, 2.5 Ma, 1.1 Ma and 0.8 Ma). According to the limb rotation model, off growth strata in the anticlinoriums developed from ~ 8.2 Ma onward with

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Fig. 9. Two models of offlap growth in the western Qaidam Basin. (a) Angular offlap growth occurred before 6.9 Ma in the I region from the frontal of the Altyn Tagh (Mts.); (b) offlap growth of the decollement presented since ~2.5 Ma in the II region. See Fig. 1c for their location.

progressive younging from the frontal region of the Altyn Tagh Mts. (~ 8.2 Ma) to the southwestern basin (~ 2.5 Ma) and further east of the Qaidam Basin (b2.5 Ma), due to fault-propagation-folding in the Qaidam Basin, rapid uplift and fast exhumation of the NE Tibetan Plateau at those times. Acknowledgments This study was co-supported by the (973) National Basic Research Program of China (2013CB956400, 2011CB403000), the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDB03020400), the NSFC Grants (41021001, 40920114001, 41172032, 40702006) and the German Research Foundation within the Priority Programme 1372 “TiP”. We thank Long Xiaoyong, Gao Hongshan, Sun Ranhao, and Zhao Yande for field assistance, and Dai Shuang, Gao Donglin, Xu Xianhai and Li Lili for laboratory assistance. Special thanks are due to Prof. Zhu Rixiang and Prof. Qiang Xiaoke for their laboratory supports, to the Qinghai Petroleum Company of China National Petroleum Corporation for access to seismostratigraphic data and to Prof. Emilio L. Pueyo and Cor Langereis for their constructive comments which have helped to improve the manuscript. References An, Z.S., Kutzbach, J.E., Prell, W.L., Porter, S.C., 2001. Evolution of Asian monsoons and phased uplift of the Himalaya-Tibetan Plateau since Late Miocene time. Nature 411, 62–66. Anastasio, D.J., Erslev, E.A., Fisher, D.M., 1997. Fault-related folding. Journal of Structural Geology, Special Issue 19, 243–602. Bloemendal, J., DeMenocal, P.B., 1989. Evidence of a change in the periodicity of tropical climate cycles at 2.4 Myr from whole-core magnetic susceptibility measurements. Nature 342, 897–900. Burbank, D.W., Johnson, G.D., 1982. Intermontane-basin development in the past 4 Myr in the north-west Himalayas. Nature 298, 432–436. Cande, S.C., Kent, D.V., 1995. Revised calibration of the geomagnetic polarity timescale for the Late Cretaceous and Cenozoic. Journal of Geophysical Research 100, 6093–6095. Chen, W.P., Chen, C.Y., Nábelek, J.L., 1999. Present-day deformation of the Qaidam basin with implications for intra-continental tectonics. Tectonophysics 305, 165–181. Chen, Y., Gilder, S., Halim, N., Cogné, J.P., Courtillot, V., 2002. New paleomagnetic constraints on central Asian kinematics: displacement along the Altyn Tagh fault and rotation of the Qaidam Basin. Tectonics 21, 1042. http://dx.doi.org/10.1029/2001TC901030. Clark, M.K., Farley, K.A., Zheng, D., Wang, Z., Duvall, A.R., 2010. Early Cenozoic faulting of the northern Tibetan Plateau margin from apatite (U–Th)/He ages. Earth and Planetary Science Letters 296, 78–88.

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