Magnetostratigraphy of the Kelasu section in the Baicheng depression, Southern Tian Shan, northwestern China

Journal of Asian Earth Sciences xxx (2015) xxx–xxx

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Magnetostratigraphy of the Kelasu section in the Baicheng depression, Southern Tian Shan, northwestern China Zhiliang Zhang a,b,⇑, Zhongyue Shen a, Jimin Sun b,c, Xin Wang a, Zhonghua Tian b, Xiaoqing Pan a, Linquan Shi a a b c

Department of Earth Sciences, Zhejiang University, Hangzhou 310027, China Key Laboratory of Cenozoic Geology and Environment, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China CAS Center for Excellence in the Tibetan Plateau Earth Sciences, China

a r t i c l e

i n f o

Article history: Received 21 January 2015 Received in revised form 31 May 2015 Accepted 16 June 2015 Available online xxxx Keywords: Magnetostratigraphy AMS Kelasu Baicheng Southern Tian Shan

a b s t r a c t In order to better constrain chronology of the Cenozoic sediments in the foreland basin of the Southern Tian Shan, we carried out a magnetostratigraphic study along the Kelasu River, in the Baicheng depression, northwestern China. This is the basis for future studies of the tectonic shortening history and paleoclimatic changes. Stepwise thermal demagnetization was used to isolate the high-temperature characteristic component (ChRM) from 1521 oriented samples collected along two overlapped sections. The ChRM directions are interpreted to be acquired at or close to the time of rock formation. A composite magnetostratigraphic column composed of 86 (45 normal and 41 reversed) polarity chrons can correlate with GPTS (CK95) from 54 Ma to 7.6 Ma. The basal ages of the Kumugeliemu, Suweiyi, Jidike and the Kangcun formations are 54 Ma, 46 Ma, 34 Ma and 9.7 Ma, respectively. The changes of anisotropy of magnetic susceptibility (AMS) parameters (Pj and T) cannot be used to reflect the Cenozoic uplift of the southern Chinese Tian Shan due to the parameters have a significant linear positive correlation with the bulk magnetic susceptibility (Km), suggesting a sedimentary provenance control. Based on the alignments of the AMS, we concluded that the paleo-river channel flowed from north to south, being similar to the present river flowing direction, suggesting that there was still a residue relief of the Tian Shan orogen after the long-term Mesozoic denudation or the reactivation of the Tian Shan may have been initiated as early as 54 Ma. Ó 2015 Elsevier Ltd. All rights reserved.

1. Introduction The Tian Shan orogen is one of the longest mountain ranges in central Asia, with peaks exceeding 7000 m and stretching about 2500 km from east to west (Fig. 1). This mountain has experienced multiple episodes of tectonic reactivation in the Cenozoic, as a response to the intracontinental deformation within the India– Eurasian collision zone (Tapponnier and Molnar, 1979; Coleman, 1989; Windley et al., 1990; Allen et al., 1991, 1994; Burchfiel and Royden, 1991; Avouac and Tapponnier, 1993; Liu et al., 1994; Lu et al., 1994; Carroll et al., 1995).

⇑ Corresponding author at: Key Laboratory of Cenozoic Geology and Environment, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China. E-mail address: [email protected] (Z. Zhang).

Thick Cenozoic terrigenous sediments denuded from the Tian Shan have been tectonically deformed and formed a series of fault-related anticlines, providing geological archives for studying the relationship between continental deformation, rock denudation and sediment deposition. The chronology of the Cenozoic successions is the basis for tectonic and paleoclimatic implications of the thick sediments. Magnetostratigraphy of the Cenozoic sediments in the foreland basins of Tian Shan has been widely studied during the last two decades (e.g., Teng et al., 1997; Sun et al., 2004, 2007, 2009; Charreau et al., 2005, 2006, 2009; Huang et al., 2006, 2010; Zheng and Meng, 2006; Ji et al., 2008; Sun and Zhang, 2009; Jing et al., 2011; Li et al., 2011; Zhang et al., 2014). However, due to the scarcity of paleontological fossils and volcanic rocks appropriate for accurately dating, there have been many debates about the magnetostratigraphic time scales in the foreland basins of the Tian Shan Range. For example, the magnetostratigraphy of the Cenozoic

http://dx.doi.org/10.1016/j.jseaes.2015.06.016 1367-9120/Ó 2015 Elsevier Ltd. All rights reserved.

Please cite this article in press as: Zhang, Z., et al. Magnetostratigraphy of the Kelasu section in the Baicheng depression, Southern Tian Shan, northwestern China. Journal of Asian Earth Sciences (2015), http://dx.doi.org/10.1016/j.jseaes.2015.06.016

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Z. Zhang et al. / Journal of Asian Earth Sciences xxx (2015) xxx–xxx

Fig. 1. Topographic map showing tectonic framework and the site location of the Kelasu section. EUR: Europe, KAZ: Kazakhstan, JUN: Junggar, TAR: Tarim, IND: India, SIB: Siberia, NCB: North China block, SCB: South China block, INC: Indochina, TFF: Takas-Fergana fault, KF: Karakoram fault, ATF: Altyn Tagh fault, KYTS: Kashgar-Yecheng Transfer System.

strata in the Kuqa foreland of the Southern Tian Shan completed by Huang et al. (2006) yielded an age range of 31–5.5 Ma. However, this chronology was challenged by Charreau et al. (2008). Recently, based on the Paleocene to Eocene foraminifera and calcareous nannofossil assemblages found at the bottom of the Kumugeliemu Formation within the same foreland basin (Hao et al., 2000; Guo et al., 2002; Su et al., 2003), Zhang et al. (2014) reinterpreted the magnetostratigraphy of Huang et al. (2006) and proposed a much older basal age. Additionally, the previous magnetostratigraphic investigations in the foreland basins of the Tian Shan mainly focused on the Neogene deposits (e.g., Charreau et al., 2005, 2006, 2009; Huang et al., 2006, 2010; Sun and Zhang, 2009; Sun et al., 2009; Jing et al., 2011; Zhang et al., 2014) (see the detailed locations in Fig. 2a). There is still a lack of magnetostratigraphic studies of Paleogene strata in the region. This paper aims to constrain the ages of the Paleogene to Neogene strata through establishing a detailed magnetostratigraphy and to discuss magnetic fabrics implications in the Baicheng foreland basin, northwestern China. 2. Geological setting, materials and methods There are several folding and thrusting fault belts, forming fault-related anticlines in the Baicheng depression in the foreland basin of the Southern Tian Shan (Fig. 2a). In this study, we focus on two neighboring anticlines (the Kumugeliemu to the north and the Kasangtuokai to the south) cut by the south flowing Kelasu River (Fig. 2b). The Cenozoic sediments of the two anticlines include five formations: the Kumugeliemu, Suweiyi, Jidike, Kangcun and the Kuqa formations from old to young (GMRMXUAR, 1993). Two overlapped sections (A and B) were sampled along the Kelasu River. Among them, section A, lying on the southern limb of the

Kumugeliemu anticline, consists of the Kumugeliemu and the Suweiyi formations, whereas section B lies on the northern limb of Kasangtuokai anticline and includes the Suweiyi, Jidike and the lower Kangcun formations (Zhang et al., 2013). The two sections are about 4 km apart (Fig. 2b). The Kumugeliemu Formation is characterized by a thick gray conglomerate bed in the bottom and alternations of reddish mudstone/siltstone and gravels in the lower and upper parts, while intercalated thin (usually less than 10 cm) gypsum occur in the middle part of the formation dominated by reddish mudstone. The overlying Suweiyi Formation is dominated by reddish mudstone and siltstone occasionally with gravel intercalations. The Jidike Formation conformably overlies the Suweiyi Formation, was previously attributed to a Neogene age (GMRMXUAR, 1993) and it is dominated by reddish brown siltstone to mudstone interbedded with thin gray-green sandstone and gray conglomerates. The Kangcun Formation is characterized by the alternations of brownish siltstone and gray gravels. There is a trend of towards coarsening and less-reddish accumulation from the bottom to the top of the section (Fig. 3). Generally, the paleomagnetic sampling intervals were between 0.2 m and 2 m, but they may be as large as 5 m where the beds were dominated by conglomerate or coarse sandstone. About 1600 oriented core-samples were drilled using a portable petrol-powered drill and oriented with a magnetic compass. All the cores were cut into standard samples (about 2 cm in length) in room, and then they were subjected to stepwise thermal demagnetization in 17–19 steps using an ASC TD-48 thermal demagnetizer. Remanent magnetization was measured using a 2G-755 cryogenic magnetometer in the Paleomagnetic Laboratory of Nanjing University. Demagnetization results were evaluated on Zijderveld diagrams (Zijderveld, 1967). Magnetic components were determined by principal component analysis (Kirschvink, 1980). The anisotropy of magnetic susceptibility

Please cite this article in press as: Zhang, Z., et al. Magnetostratigraphy of the Kelasu section in the Baicheng depression, Southern Tian Shan, northwestern China. Journal of Asian Earth Sciences (2015), http://dx.doi.org/10.1016/j.jseaes.2015.06.016

Z. Zhang et al. / Journal of Asian Earth Sciences xxx (2015) xxx–xxx

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Fig. 2. (a) Regional topography of the Kuqa and Baicheng depression, showing the anticlines and the positions of previous studies. (1) Jing et al. (2011); (2) Zheng and Meng (2006); (3) Sun et al. (2009); (4) Huang et al. (2006); (5) Huang et al. (2010); (6) Charreau et al. (2006); and (7) Zhang et al. (2014). (b) Detailed geological map of the studied area, the black–white columns indicates the sampling routes for the two overlapped sections along the Kelasu River.

(AMS) of 1600 oriented samples was measured by using an AGICO KLY-3S KappaBridge before demagnetization in the Paleomagnetic Laboratory of Zhejiang University. Representative samples were also chosen for rock magnetism analysis. 3. Results 3.1. Magnetic mineralogy Nine samples were used for thermomagnetic analysis (magnetic susceptibility versus temperature curves) by using an AGCIO KLY-3s KappaBridge coupled with a CS-3 apparatus. All the samples show irreversible behaviors during heating and cooling (Fig. 4a–c). Two decrease in the heating curves were apparent: a

minor one at 580 °C, which indicates the presence of Ti-poor magnetite (Charreau et al., 2009); the other one is around 700 °C, which corresponds to hematite. Regarding that the magnetic susceptibility of magnetite is much higher than that of hematite, therefore, the magnetic minerals are dominated by hematite, whereas magnetite presents as a subordinate ferromagnet. Acquisition of isothermal remanent magnetization curves (Fig. 4d–f) show rapid increase from 0 to 250 mT and 90% of the maximum magnetization were acquired at about 1000 mT. However, they are still not saturated when the applied field is as high as 2700 mT. The backfield to remove the IRMs acquired at the highest field is around 150 mT. The high saturation field and high coercivity indicates the presence of a magnetic mineral with relatively high coercivity, such as hematite.

Please cite this article in press as: Zhang, Z., et al. Magnetostratigraphy of the Kelasu section in the Baicheng depression, Southern Tian Shan, northwestern China. Journal of Asian Earth Sciences (2015), http://dx.doi.org/10.1016/j.jseaes.2015.06.016

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Z. Zhang et al. / Journal of Asian Earth Sciences xxx (2015) xxx–xxx

Fig. 3. Composite cross section of the sedimentary succession exposed by the Kelasu River in the studied area. The black stars represent the sampling sites for fold test. The detailed data are shown in Table 1. E1 2km: the Kumugeliemu Formation; E2 3s: the Suweiyi Formation; N1j: the Jidike Formation; N1k: the Kangcun Formation; N2k: the Kuqa Formation.

ks137 Height: 223 m section A

20

Susceptibility(

15

10

5

Relative magnetic moment

(d)

25

-6

10 SI)

(a)

ks842 Height: 215 m section B

1.2 1 0.8 0.6

0.4 0.2

0 0

100

200

300

400

500

600

700

-150 -100 -50

0

500

1000

1500

2000

2500

3000

Applied field (mT)

(e)

40 ks582 Height: 752 m section A 30

20

10

Relative magnetic moment

Susceptibility(

10 - 6 SI)

(b)

ks906 Height: 339 m section B

1.2 1 0.8 0.6

0.4 0.2

0 0

100

200

300

400

500

600

700

-150 -100 -50

0

500

1000

1500

2000

2500

3000

20

16

ks1444 Height: 1225 m section B

12

8

4

(f) Relative magnetic moment

(c) Susceptibility( 10 - 6 SI)

Applied field (mT) ks1430 Height: 1199 m section B

1.2 1 0.8 0.6

0.4 0.2

0 0

100

200

300

400

500

600

700

-150 -100 -50

0

500

1000

1500

2000

2500

3000

Applied field (mT) Fig. 4. The rock magnetism analyzing results of representative samples. (a)–(c) Temperature dependence of magnetic susceptibility (j–T curves). The samples were heated from room temperature to 700 °C in argon gas atmosphere and then cooled to room temperature. (d)–(f) Acquisition curves of saturated isothermal remanent magnetization (on the right) and coercive force curves of remanence (on the left).

Please cite this article in press as: Zhang, Z., et al. Magnetostratigraphy of the Kelasu section in the Baicheng depression, Southern Tian Shan, northwestern China. Journal of Asian Earth Sciences (2015), http://dx.doi.org/10.1016/j.jseaes.2015.06.016

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Z. Zhang et al. / Journal of Asian Earth Sciences xxx (2015) xxx–xxx

(a)

(c)

(b)

(f)

(e)

(d)

(g)

(h)

(i)

Fig. 5. Orthogonal vector diagrams of representative samples. Directions are plotted in-situ; solid and open circles represent inclination and declination, respectively.

3.2. Paleomagnetic results and analysis One thousand five hundred and twenty-one samples were subjected to stepwise thermal demagnetization. About 75% of the total samples could isolate stable high temperature directions. Fig. 5 shows the Zijderveld diagrams of representative samples. Most samples have two magnetic components: one at low temperature (<300 °C/350 °C) with random directions, another high-temperature characteristic component (ChRM) is of dual polarities and decays toward the origin. Among the samples, 1079 samples have stable high-temperature components. The mean directions of all the normal and reversal polarities after tilt-correction are DN = 357.6°, IN = 42.7°, a95N = 2.9°, jN = 5.5 and DR = 175°, IR = 42.8°, a95R = 2.5°, jR = 6.6, respectively (Fig. 6). The mean directions of the dual polarities pass the reversal test (McFadden and McElhinny, 1990) with a classification (the angle between mean directions of the normal and reversal polarities is 1.91°, which is smaller than 5°). The sampling sites (KL01–KL22) were used for fold test and the majority of samples yielded well-defined

high-temperature components. Details of the 22 sites are listed in Table 1. Eigen fold test (Tauxe and Watson, 1994) was applied to evaluate the data, with the results shown in Fig. 7. The maximum in s1 and optimal concentration occur when the bed is unfolded 103–112%, suggesting a pre-folding remanence with a high degree of confidence. The positive results of reversal test and fold test together indicate that the ChRMs were acquired before the time of deforming. 3.3. Magnetostratigraphy The ChRM directions were used to yield virtual geomagnetic poles (VGP) latitudes, in order to define the magnetic polarity zones. Forty-five normal and 41 reversed polarity zones were identified, except the zones defined by just one sample. The two sections overlap 400 m in thickness. We used the following criteria to constrain the time range of the two composite sections: (1) the relative long magnetic chrons (N3, N4, R6–R11, and N30–N35), (2) sediments in the studied area are mapped as Paleogene to Miocene (Fig. 2b), (3) fossils of bivalves

Please cite this article in press as: Zhang, Z., et al. Magnetostratigraphy of the Kelasu section in the Baicheng depression, Southern Tian Shan, northwestern China. Journal of Asian Earth Sciences (2015), http://dx.doi.org/10.1016/j.jseaes.2015.06.016

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Z. Zhang et al. / Journal of Asian Earth Sciences xxx (2015) xxx–xxx

N

(a)

(b)

N

N 1079

W

E Tilt-corrected

In-situ

S

S

Fig. 6. Equal area projections of 1059 ChRM directions before (a) and after (b) tilt-correction. These data defines an A-class reversal test (for details in the text) at the 95% confidence level. The solid squares and open triangles represent lower hemisphere and upper hemisphere projections respectively. The red stars represent the mean directions after tilt-correction, and the gray ellipses are the a95 values. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Table 1 Summary of the ChRM directions of the 22 sampling site used for fold test on the two sections in the studied area. Site ID

Strike/dip

n/n0

Dg

Ig

Ds

Is

jg/js

a95g/a95s

KL01 KL02 KL03 KL04 KL05 KL06 KL07 KL08 KL09 KL10 KL11 KL12 KL13 KL14 KL15 KL16 KL17 KL18 KL19 KL20 KL21 KL22

166/70 151/62 172/66 170/65 160/65 167/68 158/69 170/74 160/71 160/65 160/65 145/70 0/45 0/45 350/54 350/54 350/54 355/62 355/62 340/65 350/60 350/60

5/7 9/10 4/6 6/6 5/5 5/7 5/6 7/8 7/8 8/8 7/10 13/15 14/15 7/10 6/8 5/7 8/9 13/15 9/10 8/10 10/12 6/11

190.8 182.6 159.2 2.8 350.8 12.8 360 337.6 352.7 358.5 174.7 162.4 247.9 331.2 98.3 69.1 354.1 189.1 189.3 122.6 330.7 347.1

26.3 25.8 29 22.8 11.1 39.9 38.9 30.3 38.9 19.1 37.2 40.6 85.8 54.9 62.8 68.5 77.2 83.5 77 73.6 69.1 60

194.4 188.3 158.2 6 18.2 6.5 357.1 335.5 351.5 6.6 176.7 159.8 185.4 231.2 24.4 14.4 168.6 352.8 350.8 356.7 180.4 178.1

37.8 29.8 35.6 41.2 72.8 21.7 26.7 42.1 30.9 46 36.1 27.3 43.3 69.2 39.7 29.4 48.8 34.8 40.6 38.9 49.3 59.5

12.8/12.8 34/25.9 32.6/35.8 21.7/20.8 35.7/35.7 21/23 25.2/25.2 27.9/27.8 35.5/35.5 30.3/32.8 7.9/8 13.2/13.2 30.3/30.4 21.5/21.4 12.7/12.7 25.1/22 20.3/20.3 15.1/9.6 24/24.9 34.1/40.8 26.1/26.1 21.9/21.8

26.7/26.7 9/10.3 4.9/4.9 14.7/15 13/11 14.2/12 15.5/15.5 5.9/5.8 10.3/10.3 4.3/3.8 22.9/22.8 11.9/11.9 7.3/7.3 13.3/13.3 19.5/19.5 15.2/10.2 8.4/8.4 9.8/15.7 10.5/10.5 13.3/12.1 9.6/9.6 14.6/14.7

Note: Site ID, sampling site identification; strike/dip, the strike azimuth and dip of sampling beds; Dg, Ig, jg and a95g (Ds, Is, js and a95s) are declination, inclination, precision parameter and 95% confidence limit of fisher statistics before (after) tilt-correction.

and ostracoda. According to previous studies, the Kumugeliemu Formation is further divided into three formations (Talake, Xiaokuzibai and Avate) (Yin et al., 1998 and references therein). The Talake formation was considered to be late Paleocene to early Eocene in age based on the ostracoda (Sinocypris, Neocyprideis galba, Loxoconcha sp. Laculata, Mandelstam) and the marine lamellibranchia fossils (Leda crispate, Ukrainica sokolov, Corbula subpisum) (GMRMXUAR, 1993; Zhang et al., 1986). The bivalves (Polamides sp.) and ostracoda (Neocyprideis galba and foraminifera)

found in the Xiaokuzibai Formation were assigned to an age of middle Eocene (Yin et al., 1998 and references therein). The Suweiyi Formation contains Cyclocypris cavernosa, Cyprideis littoralis, Darwinula silentiosa and Euoypris longa, its age should range from Eocene to Oligocene (Ye and Huang, 1990; GMRMXUAR, 1993; Editing Committee of the Stratigraphy of China, 1999). The Jidike Formation has fossils of Tungtingichthys sp. and Candona leei, Paracandona euplectella, Potamocypris reflexa, Cypria acutus, Eucypris longa, E. concinna, E. kaktalensis, E. inftata, E.

Please cite this article in press as: Zhang, Z., et al. Magnetostratigraphy of the Kelasu section in the Baicheng depression, Southern Tian Shan, northwestern China. Journal of Asian Earth Sciences (2015), http://dx.doi.org/10.1016/j.jseaes.2015.06.016

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Z. Zhang et al. / Journal of Asian Earth Sciences xxx (2015) xxx–xxx

(a)

(c)

103 - 112 Percent Unfolding

1.0

In situ

0.8

0.6

(b) 0.4

0.2 Tilt-corrected

0.0 -20

0

20

40

60

80

100

120

140

160

% Untilting Fig. 7. The result of fold test. Equal area projections of 22 sites ChRM directions before (a) and after (b) tilt-correction. (c) Eigenvalues of the orientation matrix for 20 paradata sets of the data shown in Table 1, as they undergo incremental unfold. The red lines represent 20 eigenvalues (s1) versus unfold (%); the green line indicates confidence degree fraction (CDF). The maximum s1 is reached near 100% unfolding, which indicates a positive fold test. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

simplex, Hemicyprinotus, Pseudoeucy-pris kerkiensis in the lower part suggest that its basal age cannot be later than Oligocene (Editing Committee of the Stratigraphy of China, 1999; Guan and Guan, 2002). Sun et al. (2009) found vertebrate fossil (Hipparion chiai) which have an age range of 11–9 Ma in the upper part of the section in the nearby Kuqa foreland basin. Therefore, the Jidike Formation should last from Oligocene to Miocene. The ostracoda found in the Kangcun Formation (Candona (Lineocypris), C. (Typhlocypris) lonus, C. (Pseudocandona) subequalis, P. euplectella, Cyclocypris cavernosa, Ilyocypris Cornae) suggest that it should be assigned to a late Miocene age (Editing Committee of the Stratigraphy of China, 1999). The Kuqa Formation contains ostracoda fossils of Candona (Pseudocandona) rostrata, C. (Lineocypris) asseptis, C. (Candona) neglecta, Sublacypris subtilis, Paracandona euplectella, Cyclocypris regularis, Cypeidopsis vidua, Zonocypris membranae, Limnocythere lenuie, Eucypris inflata suggesting that its age should be Pliocene (Editing Committee of the Stratigraphy of China, 1999). Based on these biostratigraphic age assignments, the magnetic polarity column constructed by all the polarity zones was then correlated with CK95 (Cande and Kent, 1995) geomagnetic polarity time scale (GPTS) (Fig. 8). The magnetostratigraphic columns are characterized by three relatively long normal polarities (N30–N35 on section A, N3 and N4 on section B) and several reversed polarities (such as R32– R34 on section A and R6–R11 on section B) as shown in Fig. 8. In the upper part of section A (550–775 m), it is dominated by normal polarity zones punctuated by several relatively short reversed polarities (R27–R31). We correlate them with C15n–C18.2n of the GPTS CK95 (Cande and Kent, 1995), despite some reversed polarities are missing. The absence of these chrons may be due

to large sampling spans caused by coarse-grained sediments. At the middle part of the section B, the most obvious characteristic is the thick zone dominated by reversed polarity punctuated by several thin normal polarities (R6–R11). They can be correlated to the zones between C6n and C8n.2n on GPTS. Hence, the age range of the composite section along Kelasu River is between 54 Ma and 7.6 Ma.

4. Discussion 4.1. The age constraints on the Cenozoic strata in the Baicheng depression, northwestern China Due to the absence of mammalian fossils and radio chronological constraints, magnetostratigraphy becomes the most useful tool for constraining the ages of the Cenozoic terrigenous sediments. Although several magnetostratigraphic results have been published in recent years in the nearby region (e.g., Teng et al., 1997; Charreau et al., 2006, 2009; Huang et al., 2006, 2010; Zheng and Meng, 2006; Jing et al., 2011; Zhang et al., 2014), no agreement has been acquired. Teng et al. (1997) carried out a magnetostratigraphic study in the Kuqa section, which is about 80 km to the east of our studied section. The basal ages of the Suweiyi, Jidike and the Kangcun Formation were assigned to 38 Ma, 24.4 Ma and 5.3 Ma, respectively. However, due to large sampling spaces, they cannot yield high-resolution magnetostratigraphy. A similar work was carried out by Huang et al. (2006) near the Kuqa River, which is in the same structural belt as our studied area. The formations were assigned to younger ages than our results. Their results were once challenged by Charreau et al. (2008).

Please cite this article in press as: Zhang, Z., et al. Magnetostratigraphy of the Kelasu section in the Baicheng depression, Southern Tian Shan, northwestern China. Journal of Asian Earth Sciences (2015), http://dx.doi.org/10.1016/j.jseaes.2015.06.016

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Z. Zhang et al. / Journal of Asian Earth Sciences xxx (2015) xxx–xxx Height

Polarity

VGP Latitude -90

-45

0

45

N1

Kangcun Fm

1350 1300 1250

GPTS Ma

90

R1 N2 R2

C3An.1n C3An.2n C3Bn

6

C4n.2n

8

C4An

10

N3

C5n.2n

1200 R3

1150 1100 N4

C5An.1n

12

C5ACn C5ADn

14

C5Cn.1n

16

1050 1000 R4 950 R5

900

Jidike Fm.

N6 850 800 750

N7

C5Dn

18 C5En

N5

R6

C6n

20

C6An.1n C6An.2n

22

R7 N8 R8

24

N9 C7n.2n

600

C8n.2n

R11

C9n

N13

C10n.1n C11n.1n C11n.2n

550

R12

Polarity

30 32

N15 C13n

450 R13 N16

400

R14

350

C15n C16n.1n C16n.2n

34 36

C17n.1n

N18 N19 R16 N20

C18n.1n

250

R17 N22 R18 N23 N24

C19n

SWY Fm.

300

200 150 100

38 C18n.2n

40

42 44

0

0

45

90

R25 N29 R26 R27 N30 N31 R28

800 750 700

N32

650

R29

N33 R30N34 R31 N35

600 550 500

R32 N36

450 400

N37 R34 300

C21n

48

Lithology

-45

350 46

R22 N26

-90

R33

C20n

R20 N25

50

Height

VGP Latitude

28

C12n

N14

500

26

SWY Fm.

650

R9 N11 R10

C22n

50

N38 250 R35 N39 R36

52

R37N42 R38 N43 R39 N44 R40

54

N45

C23n.2n C24n.1n C24n.3n

KMGLM Fm.

700

200 150 100 50

R41

0

Lithology

Fig. 8. Lithology and magnetostratigraphic results of the two sections in the Baicheng depression defined by VGP latitudes plotted versus thickness. KMGLM Fm., SWY Fm. and JDK Fm. are the abbreviations of the Kumugeliemu, Suweiyi and the Jidike Formation, respectively.

Charreau et al. (2008) argued that (1) the interpretations of the sedimentation rate changes they proposed were partially invalid. In addition, the reinterpretation by Charreau et al. (2008) yielded an acceleration in sedimentation at 10–11 Ma, rather than 7 Ma; and (2) the way Huang et al. (2006) interpret the AMS data was not correct. The changes in AMS cannot reflect tectonic strain at the time of deposition because the AMS has been affected by Pliocene–Pleistocene deformation. Zhang et al. (2014) reinterpreted the magnetostratigraphy (Huang et al., 2006) and determined the age ranges of the different formations. However, Zhang et al. (2014) also made a mistake, because the mammalian fossil that they used to constrain the age of the lower part of Kangcun Formation was actually found in the upper part of the Jidike Formation (Sun et al., 2009) from the Kuqatawu section, Though the distinction between the lithology of the Jidike and Kangcun Formation is obvious. Another detailed

magnetostratigraphic and rock magnetic study of the Yaha section, was reported by Charreau et al. (2006). The results suggest that the age of the whole section was from 12.6 to 5.2 Ma. However, Huang et al. (2006) proposed that the age should ranges from 10 to 2 Ma, which is about 3 Ma younger than the previous results. Although the thickness varies in different areas, the unique lithology of the Suweiyi and Jidike Formation makes it possible to correlate the Cenozoic successions in the Baicheng and Kuqa foreland basins. The reasonable sampling intervals (<2 m on average) ensure the accurate of our magnetostratigraphy because geomagnetic reversal occurred frequently in some periods of the Cenozoic. According to our results, the age range of the Kelasu section is from 54 Ma to 7.6 Ma and we well defined the boundaries of each formation. The age ranges of the Kumugeliemu, Suweiyi, Jidike, Kangcun formations are 54–42 Ma, 42– 34 Ma, 34–9.7 Ma, respectively.

Please cite this article in press as: Zhang, Z., et al. Magnetostratigraphy of the Kelasu section in the Baicheng depression, Southern Tian Shan, northwestern China. Journal of Asian Earth Sciences (2015), http://dx.doi.org/10.1016/j.jseaes.2015.06.016

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Z. Zhang et al. / Journal of Asian Earth Sciences xxx (2015) xxx–xxx -6

Pj

Km ( 10 SI)

Height 0

100

300

500

1

1.04

T 1.08

1.12

-0.6

-0.2

0

0.2

0.6

1

800 750 700 650 600 550 500 450 400 350 300 250 200 150 100 50 0

Lithology

(a)

(b)

(c)

Fig. 9. The curves of AMS parameters (Km, Pj and T) versus thickness of section A. The red lines show the average of a sliding window shifted every 20 samples. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

4.2. The changes of anisotropy of magnetic susceptibility (AMS) parameters Magnetic parameters can be very useful in analyzing sediment sources, depositional environment, hydrodynamic conditions, tectonic strain, etc., (e.g., Charreau et al., 2005, 2006, 2009; Huang et al., 2006). The variations and geological implications of the mean magnetic susceptibility (Km) and the two parameters (Pj and T) of AMS were discussed. Km is sensitive to the concentration of magnetic minerals, especially the magnetite, because the susceptibility of magnetite is 1000 times greater than other common minerals (Collinson, 1983). On the two sections, Km ranges from 10 to 500 lSI, with the lowest values in the lowest part (bellow the thickness of 150 m) and at the thickness of 500–525 m on section A (Figs. 9a and 10a). The lowest values of Km correlated to the coarse gray conglomerates and then the less magnetic minerals of the Kumugeliemu Formation. The corrected degree of anisotropy (Pj) (Jelinek, 1981) was thought to be sensitive to lithology (e.g., clay content) or strain (Hrouda, 1982). Figs. 9b and 10b show that the variations of Pj roughly follow the variations of Km (magnetite content). This can be further supported by the linear correlation analysis, which shows that correlation coefficients (Pearson’s r) of the two sections are 0.62 and 0.61, respectively (Fig. 11). The results indicate that the changes of Pj are positively related to Km (affected by the

concentrations of magnetic minerals). Therefore, the changes of Pj cannot be simply related to tectonic stress but to lithology. Another anisotropy parameter, T (shape parameter) (Hrouda, 1982), can reflect the information related to hydrologic regime and transport conditions experienced by the sediments (Kissel et al., 1997; Gilder et al., 2001; Charreau et al., 2005, 2006, 2009), because T is sensitive to the preferred orientation of particles during deposition (Charreau et al., 2009). Similar relationships are also observed between T and Km (Figs. 9c and 10c), which is roughly in accordance with the changes of lithology. Although the AMS parameters (Pj, T, and q) were widely used to reflect changes of tectonic stress in central Asia (e.g., Gilder et al., 2001; Charreau et al., 2005, 2006, 2009; Huang et al., 2006; Tang et al., 2012), it is worth to stress that care must be taken when using AMS as tectonic parameters because they are related to the provenance changes, rather than tectonic deformation on our section. Three types of magnetic fabrics have been divided in previous researches (Hrouda, 1991; Tarling and Hrouda, 1993; Kanamatsu et al., 1996; Parés and Van der Pluijm, 2002; Parés, 2004; Jia et al., 2007; Luo et al., 2014): (1) sedimentary magnetic fabric, which is characterized by the discrete distributions of Kmax and Kint on the bedding plane, whereas Kmin is perpendicular to the bedding plane; (2) preliminary deformation magnetic fabric, which indicates that sediments are affected by stress parallel to the bedding. Kmax will be clustered perpendicular to the stress and Kint will

Please cite this article in press as: Zhang, Z., et al. Magnetostratigraphy of the Kelasu section in the Baicheng depression, Southern Tian Shan, northwestern China. Journal of Asian Earth Sciences (2015), http://dx.doi.org/10.1016/j.jseaes.2015.06.016

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Z. Zhang et al. / Journal of Asian Earth Sciences xxx (2015) xxx–xxx -6

Km ( 10 SI)

Height 0

100

300

Pj 500

1

1.02

T 1.04

1.06

-0.8

-0.4

0

0.4

0.8

1350 1300 1250 1200 1150 1100 1050 1000 950 900 850 800 750 700 650 600 550 500 450 400 350 300 250 200 150 100 50 0 Lithology

(a)

(b)

(c)

Fig. 10. The curves of AMS parameters (Km, Pj and T) versus thickness of section B. The red lines show the average of a sliding window shifted every 20 samples. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

concentrate in the dip direction; and (3) pencil-shaped magnetic fabric. If the sediments undergo stronger stress, Kmax will concentrate in the strike direction and Kmin will discrete in the stress direction. The equal-area stereographic projections of section A and B are shown in Fig. 12. Only one type of magnetic fabrics can be recognized. The characteristics of AMS fabric types in the Baicheng depression are sedimentary fabrics. On the southern limb of the Kumugeliemu anticline (section A), Kmax scatter on the bedding plane, whereas Kmin is perpendicular to the bedding plane (the mean Kmax and Kmin after tilt-correction are Dmax/Imax = 209.6°/10.4° and Dmin/Imin = 356.1°/77.6°, respectively), which indicates a sedimentary magnetic fabric. The plots of T versus Pj (Fig. 12f) show that most of the samples are in the oblate area indicating sedimentary compaction. The projections of magnetic fabrics on section B are a little different from section A both in the geographic and stratigraphic coordinates. Kmax clustered in NEE-SWW trend, roughly parallel to the fold axes and Kmin inclined to the southeast (Fig. 12b). The mean directions of Kmax and Kmin after tilt-correction are Dmax/Imax = 67.0°/1.3° and Dmin/Imin = 161.3°/72.8°, respectively. The majority of samples are

in the area of 0 < T < 1 which suggest oblate ellipsoid caused by compaction normal to the bedding. The shape of susceptibility ellipsoid, combined with the low value of Pj (with an average of 1.021) suggests a primary sedimentary fabric that has not been affected by tectonic strain. The cluster of Kmax in NEE-SWW direction on section B was caused by the strengthening of hydrodynamic conditions reflected by the coarse-grained trend towards the top (Figs. 8 and 10). 4.3. Reconstruction of the paleocurrent direction Since the early recognition of preferred orientations of magnetic minerals (Baas et al., 2007 and references therein), AMS has long been used as a tool for reflecting paleocurrent direction in both natural and laboratory redeposited sediments (e.g., Ellwood and Ledbetter, 1977, 1979; Ledbetter and Ellwood, 1980; Abdeldayem et al., 1999; Liu et al., 2001, 2005). The magnetic lineation (the preferred direction of Kmax) is parallel or perpendicular to the paleocurrent direction, depending on the hydrodynamic regime and depositional surface (Parés et al., 2007). Weak hydraulic forces tend to align elongated particles parallel to the direction of the

Please cite this article in press as: Zhang, Z., et al. Magnetostratigraphy of the Kelasu section in the Baicheng depression, Southern Tian Shan, northwestern China. Journal of Asian Earth Sciences (2015), http://dx.doi.org/10.1016/j.jseaes.2015.06.016

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Z. Zhang et al. / Journal of Asian Earth Sciences xxx (2015) xxx–xxx

(a)

1.09

section A

1.040

1.08

(b)

section B

1.035

1.07 1.030

Pj

Pj

1.06 1.05

1.025 1.020

1.04 1.03

1.015 Pearson's r=0.61

Pearson's r=0.62

1.02

1.010 1.01 50

100

150

200

250

300

350

200

150

Km ( 10 6 SI)

250

300

350

Km ( 10 6 SI)

Fig. 11. The results of correlation analysis between Km and Pj (a and b), Km and T (c and d) after sliding average. The red lines are the best fitting curves. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

N

N

(b)

(a)

(c) T

90

270

1

1.000

1.055 Pj

-1

section B

(e)

(d)

(f) T

90

270

1

1.000

1.129 Pj

-1

section A 180

180

Fig. 12. Equal area projections of all the samples collected from the composite sections. The left, middle and right columns are the projections in geographic coordinate, after tilt-correction and Pj–T plot, respectively.

current, resulting in a superimposed lineation on the bedding plane and the platy minerals (such as hematite) will be slightly imbricated (Tarling and Hrouda, 1993; Liu et al., 2001). The preferred orientation of Kmax can represent the paleocurrent direction. On the contrary, particles transported by high velocity currents are hydrodynamically stable when the Kmax are normal to the paleocurrent direction, whereas the other two axes will be randomly oriented along a plane parallel to the paleocurrent direction (Veloso et al., 2007 and references therein). Roll-down of particles on a slope due to gravity can orient Kmax axes close to the bedding

strike (Veloso et al., 2007). In this case, the azimuth of Kmin axes can indicate the paleocurrent direction as Kmax and Kmin axes are perpendicular to each other. The type of magnetic fabrics (Fig. 12) indicates that the sediments on section A deposited under low velocity of flows. The Kmax axes distributed within the depositional plane and Kmin axes tilted slightly to the north, resulting in oblate magnetic ellipsoids (Fig. 12e). The lineation (Kmax axes) was roughly in N–S direction, representing the paleocurrent direction (Fig. 13a). The magnetic fabrics of section B suggest strong current. The lineation resulted

Please cite this article in press as: Zhang, Z., et al. Magnetostratigraphy of the Kelasu section in the Baicheng depression, Southern Tian Shan, northwestern China. Journal of Asian Earth Sciences (2015), http://dx.doi.org/10.1016/j.jseaes.2015.06.016

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Z. Zhang et al. / Journal of Asian Earth Sciences xxx (2015) xxx–xxx

N

N

(b)

(a)

Fisher Concentrations % of total per 1.0% area

E

W

S

S

section A

section B

0.00~1.00% 1.00~2.13% 2.13~3.25% 3.25~4.38% 4.38~5.50% 5.50~6.63% 6.63~7.75% 7.75~8.88% 8.88~10.00% >10.00%

Fig. 13. Contoured stereograms showing the paleocurrent direction. (a) The distribution of Kmax of the section A; (b) the distribution of Kmin of the section B.

from the combination of hydraulic and gravitational forces are perpendicular to the flow (Tarling and Hrouda, 1993). In this case, the azimuth of Kmin represents the paleocurrent direction. The contour plots of section A and B (Fig. 13) suggest that the paleocurrent was roughly in N–S direction, being similar to the present river flows. Although the Tian Shan range is a late Paleozoic orogen, Central Asia was fully amalgamated by the Early Permian (Burtman, 1975; Windley et al., 1990; Avouac et al., 1993), after which time a less active tectonic regime and erosion to low-relief prevailed until the collision of India and Eurasia in the Early Tertiary (55– 50 Ma, Molnar and Tapponnier, 1975). The similar river flowing directions between the paleo-river channel and present, suggest that there was still a residue relief of the Tian Shan orogen after the long-term Mesozoic denudation or the reactivation of the Tian Shan may have been initiated as early as 54 Ma. 5. Conclusions Our magnetostratigraphic research on the Eocene-late Miocene terrestrial sequences provides a detailed and long-term chronological framework for the Baicheng depression. The age range of the studied Kelasu section (not the whole section due to impossible sampling during the high flow period for the uppermost part) is between 54 Ma and 7.6 Ma, and the basal ages of the Kumugeliemu, Suweiyi, Jidike, and the Kangcun formations are 54 Ma, 46 Ma, 34 Ma and 9.7 Ma, respectively. AMS analysis suggests that the types of magnetic fabrics in the two sections are primary sedimentary fabrics that have not been affected by tectonic stress. The AMS parameters (Pj and T) cannot be simply used to imply tectonic deformations because they are related to the concentration of magnetic minerals of the source rocks. To be sedimentary fabric, the alignments of the AMS can be used to infer paleo-river flowing direction. The reconstructed flowing direction was southward in the studied area, similar to the present river regime. Such results suggest that or there was still a residue relief of the Tian Shan orogen after the long-term Mesozoic denudation or the reactivation of the Tian Shan orogen may have been initiated as early as 54 Ma in response to the Cenozoic India–Eurasia collision. Acknowledgments This study was supported by National Science and Technology Major Projects (Grant 2011ZX05009-001) and National Natural

Science Foundation of China (Grants 41290251 and 41272203). We thank Qin, Y.P., Li J.P. and Zhang, H.F. for the experimental helps and Li, S.H. and Gong, Z.J. for the helpful discussions. We also thank Tang, P.C., Yu, Y.L. and Zhao, B. for the helps during the field expedition.

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Please cite this article in press as: Zhang, Z., et al. Magnetostratigraphy of the Kelasu section in the Baicheng depression, Southern Tian Shan, northwestern China. Journal of Asian Earth Sciences (2015), http://dx.doi.org/10.1016/j.jseaes.2015.06.016