Late Triassic paleomagnetic result from the Baoshan Terrane, West Yunnan of China: Implication for orientation of the East Paleotethys suture zone and timing of the Sibumasu-Indochina collision

Journal of Asian Earth Sciences 111 (2015) 350–364

Contents lists available at ScienceDirect

Journal of Asian Earth Sciences journal homepage: www.elsevier.com/locate/jseaes

Late Triassic paleomagnetic result from the Baoshan Terrane, West Yunnan of China: Implication for orientation of the East Paleotethys suture zone and timing of the Sibumasu-Indochina collision Jie Zhao a,b, Baochun Huang c,⇑, Yonggang Yan a,b, Donghai Zhang c a b c

State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China University of Chinese Academy of Sciences, Beijing 100049, China Key Laboratory of Orogenic and Crustal Evolution, Ministry of Education, School of Earth and Space Sciences, Peking University, Beijing 100871, China

a r t i c l e

i n f o

Article history: Received 22 February 2015 Received in revised form 27 June 2015 Accepted 30 June 2015 Available online 2 July 2015 Keywords: Baoshan Terrane Paleomagnetism Triassic Southeast Asia Paleotethys Paleogeography

a b s t r a c t In order to better understand the paleogeographic position of the Baoshan Terrane in the northernmost part of the Sibumasu Block during formation of the Pangea supercontinent, a paleomagnetic study has been conducted on Late Triassic basaltic lavas from the southern part of the Baoshan Terrane in the West Yunnan region of Southwest China. Following detailed rock magnetic investigations and progressive thermal demagnetization, stable characteristic remanent magnetizations (ChRMs) were successfully isolated from Late Triassic Niuhetang lava flows. The ChRMs are of dual polarity and pass fold and reversal tests with magnetic carriers dominated by magnetite and subordinate oxidation-induced hematite; we thus interpret them as a primary remanence. This new paleomagnetic result indicates that the Baoshan Terrane was located at low paleolatitudes of 15°N in the Northern Hemisphere during Late Triassic times. Together with available paleomagnetic data from the Baoshan Terrane and surrounding areas, a wider paleomagnetic comparison supports the view that the East Paleotethys Ocean separated the Sibumasu and Indochina blocks and closed no later than Late Triassic times. We argue that the currently approximately north-to-south directed Changning-Menglian suture zone is very likely to have been oriented nearly east-to-west at the time of the Sibumasu-Indochina collision. Ó 2015 Elsevier Ltd. All rights reserved.

1. Introduction The Sibumasu (comprising the Sino-Burma-Malaysia-Sumatra Block forming the eastern part of the Cimmerian continent) and Indochina blocks are the main tectonic units in Southeast Asia (Fig. 1a). Their collision has been related to closure of the eastern branch of the Paleotethys Ocean and resulted in the amalgamation of East Asia during formation of the Pangea supercontinent (Sengör, 1987; Brookfield, 1996; Metcalfe, 1996; Acharyya, 1998). Thus the timing of collision and the position of the ophiolitic belt are key issues for understanding the evolution of this sector of the Paleotethys and the paleogeographic reconstruction of East Asia, and hence their relationship with the main body of the Pangea supercontinent. In general, the Cimmerian continent has been considered to have drifted from Gondwana, the southern part of Pangea, in the Early Permian and subsequently collided with ⇑ Corresponding author at: School of Earth and Space Sciences, Peking University, No. 5, Yiheyuan Road, Haidian District, Beijing 100871, China. E-mail address: [email protected] (B. Huang). http://dx.doi.org/10.1016/j.jseaes.2015.06.033 1367-9120/Ó 2015 Elsevier Ltd. All rights reserved.

Laurasia, the northern sector of Pangea by Jurassic times (Metcalfe, 1996; Wang et al., 2001a,b, 2010; Sone and Metcalfe, 2008). However, the timing of collision between the Sibumasu and Indochina blocks is still debated due to the scarcity of reliable quantitative paleogeographic constraints such as high-quality paleomagnetic results for the terranes and blocks comprising the Southeast Asia collage (Collins, 2003; Scotese, 2004; Metcalfe, 2013; Stampfli, 2013). Some studies argue for an Early to early Middle Triassic collision based on thermotectonic activity in Vietnam dated 258 to 242 Ma (Carter et al., 2001; Nam et al., 2001; Jian et al., 2009) although it is suspected to be related to the collision between the Indochina and South China blocks (Lepvier et al., 2004; Maluski et al., 2005) or closure of back-arc basin between the Sukhotai arc and the Indochina Block (Sone and Metcalfe, 2008) and paleontological affinities between the two blocks (Shi and Archbold, 1998; Ueno, 2003). In contrast a Middle to Late Triassic collision has been suggested from the evidence of the youngest pelagic sediments from the Changning-Menglian Suture Zone (Liu et al., 1993; Sone and Metcalfe, 2008), ages of the collision-correlated Lincang granite

J. Zhao et al. / Journal of Asian Earth Sciences 111 (2015) 350–364

351

Fig. 1. (a) Schematic tectonic map of Eurasia showing the Cimmerian continent after Sengör (1987); (b) the structural map around the Baoshan area modified from Leloup et al. (1995) and Shen et al. (2005). XLGSZ: Xuelongshan shear zone, DCSSZ: Diancangshan shear zone, CSSZ: Chongshan shear zone, SGF: Sagaing fault, MMB: Mogok metamorphic zone, CMS: Changning-Menglian Suture zone, 1: Biwu granite, 2: Linong granite, 3: Lunong granite, 4: Xin’anzhai monzogranite, 5: Tongtiange leucogranites, 6: Xiaodingxi and Manghuihe basaltic rocks, 7: Lincang granite, 8: Metamorphic rocks from Kannack complex.

(Dong et al., 2013), and post-collisional basaltic rocks which crop out in the Lancangjiang Tectonic Zone (Wang et al., 2010). Paleomagnetic study is the only approach able to provide fully-quantitative paleogeographic data for these continental blocks. Unfortunately this area has experienced strong post-collisional deformation and intense magmatism especially following the India-Asia collision (Tapponnier et al., 1990; Morley, 2002; Otofuji et al., 2012), which produced a pervasive remagnetization in pre-Cenozoic rocks (Yang and Besse, 1993). For this reason only a few Late Paleozoic to Early Mesozoic paleomagnetic data from Southeast Asia can meet minimal requirements satisfying paleomagnetic reliability criteria (e.g. Van der Voo, 1990); this is in spite of many reconnaissance paleomagnetic investigations performed over the past three decades (Chan et al., 1984; Fang et al., 1989; Huang and Opdyke, 1991; Yang and

Besse, 1993; Li et al., 2004; Ali et al., 2013; Kornfeld et al., 2014). Correspondingly, the scarcity of available Late Paleozoic to Early Mesozoic paleomagnetic poles from Southeast Asia has also resulted in serious disagreement between paleogeographic reconstructions of East Asia during formation of the Pangea supercontinent (Collins, 2003; Scotese, 2004; Golonka, 2007; Stampfli, 2013). In this paper, we report a paleomagnetic study of Late Triassic basaltic lavas from the southern area of the Baoshan Terrane in the northernmost part of the Sibumasu Block. Together with available paleomagnetic constraints from the Baoshan Terrane and the surrounding region, the results enable us to reconstruct the orientation of the East Paleotethys suture zone prior to intra-continental deformation related mainly to the Cenozoic India-Asia collision. This further enables us to estimate the timing and position for the Sibumasu and Simao-Indochina collision and evaluate the

352

J. Zhao et al. / Journal of Asian Earth Sciences 111 (2015) 350–364

paleogeographic setting of the Southeast Asian terranes within the main body of the Pangean supercontinent.

2. Geological setting and sampling The northern part of Sibumasu, the Baoshan Terrane (Wopfner, 1996; Ueno, 2003) is located in southwest China. It is bounded by the Gaoligong Suture Zone (GLGSZ) to the west, the Chongshan Suture Zone (CSSZ) and the Changning-Menglian Belt to the east (Fig. 1b). In common with other parts of the Cimmerian continent, it is believed to have formed a part of the Gondwana Supercontinent before the middle Early Permian (Metcalfe, 1996; Jin, 2002; Wang and Sugiyama, 2002; Huang et al., 2008b; Sone and Metcalfe, 2008) and accreted to the Simao-Indochina Block during the early Mesozoic (Metcalfe, 2006; Wang et al., 2010). Subsequent major deformation is considered to be related to intra-continental strike-slip faulting and thrusting caused by Cenozoic penetration of India into the Asian collage (Tapponnier et al., 1990; Tanaka et al., 2008; Otofuji et al., 2010; Cao et al., 2011a,b). Late Cambrian to Jurassic platform carbonates and clastics are well developed in the Baoshan Terrane and intervening volcanic episodes occurred in the Early Permian, Late Triassic, and sparsely during the Middle Jurassic and Cenozoic (Shi and Archbold, 1998; Wang et al., 2001a, 2002; Jin, 2002; Ueno, 2003; Jin et al., 2011). In general, Middle and Late Triassic rocks overlie the Permian platform carbonates and clastics with angular unconformity, and are disconformably overlain by Middle Jurassic clastics. The Triassic System is composed, in ascending order, of the Middle Triassic Hewanjie Formation (limestones) and the Upper Triassic Niuhetang Formation (volcanics and intercalated sediments) succeeded by the (Upper Triassic) Dashuitang and Nanshuba formations (clastic sediments); the Lower Triassic is regionally absent. The Upper Triassic Niuhetang Formation has a wide distribution in the southern part of the Baoshan Terrane. As shown in Fig. 2a, d–e, this formation unconformably overlies the Middle Triassic Hewanjie limestones but is overlain by the Dashuitang and Nanshuba clastics by a disconformity (YBGMR, 1984). Furthermore, this formation can be subdivided into three members. The lower member consists chiefly of basalts and intercalated andesites; the middle member consists chiefly of rhyolites; whilst the upper member is composed mainly of basalts intercalated with terrestrial sediments. Plant fossils such as Equisetites cf. sarrani (Zeiller) Harris, E. sp., Dictyophyllum aff. nathorsti Zeiller, Dorathophyllum sp., Neocalamites carrerei (Zeiller) Halle, ?N. carrerei (Zeiller) Halle, Podozamites aff. lanceolatus (L. et. H.), P. sp., Sinoctenis sp., Taeniopteris sp., T.? sp., Petrophyllum ptilum Harris have been found in the intercalations, indicating a Late Triassic age and an intraplate setting for the basaltic eruption (YBGMR, 1984). Since the basaltic lavas have lower and upper portions showing almond-shaped vesicular textures, the basalt succession can be readily subdivided into different lava flow units in the field (Fig. 2c). In addition, the underlying Middle Triassic Hewanjie Formation is composed primarily of limestones and contains an abundant conodont, brachiopod and lamellibranch fauna including Costatoria cf. radiata hsuei Chen, Entolium discites Schlotheim, Posidonia wengensis Wissmann, Placunopsis., Daonella? Sp., Maxillirhynchia? Sp., Ninglangothyris sp. indicative of a Middle Triassic age. The paleontology, lithofacies and contact relationships indicate a Late Triassic age for the Dashuitang and Nanshuba clastic facies which are in contact the underlying Niuhetang Formation following a short time break (YBGMR, 1984). Using this precise stratigraphic control the Late Triassic Niuhetang volcanic rocks were chosen for paleomagnetic sampling. In general, eight to twelve individual cores were drilled from each

sampling site with each one distributed within a different lava flow. In total, sixteen lava flows were collected from two sections (Fig. 2a, d–e) with twelve, one, and three flows selected from the lower (site YY045-050 and YZ196-201), middle (site YY054), and upper (site YY051–YY053) members of the formation respectively. Bedding attitudes define a plunging fold axis dipping 29.7° towards N148.9°E (a95 = 5.1°, N = 10, Fig. 2b). All the core samples were collected using a portable gasoline-powered drill and orientated using a sun compass. Where possible cores were orientated by both sun and magnetic compasses in order to identify any local magnetic effects on the magnetic compass. The average difference between readings of sun and magnetic compasses was of 0.99° ± 5.1° (n = 60, 2r), which is generally consistent with local declination (359.0°) calculated from the International Geomagnetic Reference Field (IGRF) model for the sampling locality at 24.0°N, 99.0°E. Thus magnetic compass readings can be used with a 1.0° correction for local declination in this study. 3. Rock magnetic investigations Field core samples were cut into cylindrical specimens 2.0–2.2 cm in length and some selected fresh end materials were subjected to rock magnetic analysis. Following sample preparation, eleven samples were subject to magnetic experiments comprising acquisition of isothermal remanent magnetization (IRM), back-field demagnetization of saturated IRM (SIRM), hysteresis loops, and thermomagnetic analysis in order to better understand magnetic mineralogy. The acquisition of IRM, back-field demagnetization of SIRM, and hysteresis loops were performed using a Micromag 3900 alternating gradient magnetometer. Magnetization versus temperature curves (J–T curves) were measured by a VFTB in an equivalent DC field of 1.0 T. Rock magnetic measurements and subsequent paleomagnetic experiments were performed in the Paleomagnetism and Geochronology Laboratory (PGL) of the Institute of Geology and Geophysics, Chinese Academy of Sciences. Rock-magnetic measurements on 11 pilot samples can be classified into two categories. The first category (including 9 samples) is characterized by approximately reversible J–T curve with unblocking temperatures of 580 °C (Fig. 3a). Together with typical low-coercivity (32.0 mT) subtracted from the IRM acquisition and back-field demagnetization of SIRM curves (Fig. 3b–c), magnetite is identified as the main magnetic mineral. The hysteresis parameters for 9 pilot samples (Fig. 4) show that the magnetic particles are resident in the pseudo-single-domain (PSD) range (Day et al., 1977; Dunlop, 2002). The second group also exhibits approximately reversible J–T curves during the heating–cooling run but with the unblocking temperature of remanence above 650° (Fig. 3e). The IRM acquisition and back-field demagnetization curves (Fig. 3f–g) show significant presence of two magnetic components: a low coercivity component with B1/2 of 56.0 mT and a high coercivity component with B1/2 of 708.0 mT and distribution width (DP) of 0.35 (Kruiver et al., 2001). Together with the significant wasp-waisted hysteresis loop (Fig. 3h) we speculate that this sample is ferromagnetically-dominated by both magnetite and hematite. 4. Paleomagnetic results and analysis Following evaluation of rock magnetic behaviors resolved from pilot samples, all the 149 specimens were subjected to progressive thermal demagnetization using a TD-48 thermal demagnetizer with residual magnetic field minimized to less than 10 nT inside the cooling chamber. Demagnetization intervals were 50–100 °C at lower temperatures, and subsequently reduced to increments

353

J. Zhao et al. / Journal of Asian Earth Sciences 111 (2015) 350–364

(a)

(b)

(c)

(d)

(e)

scale

Fig. 2. (a) Geological map of southern Baoshan Terrane in West Yunnan (Metcalfe, 1996, 2002, 2013; Sone and Metcalfe, 2008; Wang et al., 2010); (b) lower hemisphere equal-area projection for bedding attitudes of the sampled sites defining a fold axis plunging 29.7° towards N148.9°E; (c) photograph showing field outcrops in the studied section in which individual lava flow could be readily identified by almond-shaped vesicular textures; (d, e) composite cross section of the Niuhetang Formation in Zhenkang and Yongde showing stratified units and distribution of sampling sites. T2h, T3n and T3d represent the Hewanjie, Niuhetang and Dashuitang formations, respectively.

as small as 5 °C at higher temperatures as the maximum unblocking temperatures of the remanence carriers were approached. All remanence measurements were performed on a 2G-755 cryogenic magnetometer. Both demagnetizer and magnetometer are installed in a magnetically shielded space with the field inside

minimized to less than 300 nT. Demagnetization results are plotted onto orthogonal diagrams (Zijderveld, 1967) and stereographic projections with the former used to resolve components by principle component analysis (Krischvink, 1980); mean directions were calculated using standard Fisher statistics (Fisher, 1953) or the

J. Zhao et al. / Journal of Asian Earth Sciences 111 (2015) 350–364 1

8

(a) IRM/SIRM

4

2

0.08

0.6 0.4 0.2

0 100 200 300 400 500 600 700

0.2

Temperature (oC) 0.3

YY050-4

0

0.1

2

-800 -0.4

3

1

(f)

(g)

0.4

0.4

(h)

0.6 0.4

0 0 0.5

0

Temperature ( C)

1

0

1.5

0

-1

0.2

0 -0.3 -0.2 -0.1

0.2

1

0.8

0.6

o

0

Field (T)

Log Applied field (mT)

0.2 100 200 300 400 500 600 700

-0.2

10

gradient

0.2

1

2

1

(e)

0

-400

Field (T)

IRM/SIRM

J(Am2/kg)

0.04

1

0.4 0.6 0.8

0.8

0

0.06

0

-0.1 -0.05 0

(d)

400

0.02

0

0

800

(c)

J (Am 2)

J(Am2/kg)

6

0.1

(b)

0.8

gradient

YY200-8

J (Am2)

354

1 10

Field (T)

2

3

-2

4

-0.8

Log Applied field (mT)

-0.4

0

0.4

0.8

Field (T)

Fig. 3. Rock magnetic results for representative samples from the Niuhetang Formation: (a, e) J–T curves for pilot samples, all the measurement were conducted in an air atmosphere; (b, f) acquisition curves of isothermal remanent magnetization (IRM) and back-field demagnetization curves of saturated IRM; (c, g) examples of IRM component analyses (open circle: raw data, red line: Comp.1, green line: Comp.2); (d, h) hysteresis loops for pilot samples collected from the Late Triassic Niuhetang Formation, Baoshan area of West Yunnan. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

1

SD

0.5

0

20%

0

0.3

40% 20%

0.2

60%

30nm

40%

SP+SD

Mrs/Ms

60%

80%

0.1

25nm 20nm

PSD 10nm

80%

15nm

7nm 90%

90%

SD+MD

0.05

95%

95%

0.03 0.02 100%

100%

MD

0.01

0.005

1

2

3

5

10

20

30

50

100

Bcr/Bc Fig. 4. The Day diagram for pilot samples from the Niuhetang basaltic lavas, in which Mr and Ms are saturation remanence and saturation magnetization; Bc and Bcr are coercivity and coercivity of remanence; SD: single domain, PSD: pseudo single domain, MD: multi domain; nm: nanometer. Numbers along curves are volume fractions of the soft component (SD or MD) in mixtures with SD grains.

method of combined remagnetization circles and direction observations (McFadden and McElhinny, 1988). From demagnetization results exhibited on the orthogonal diagrams (Zijderveld, 1967), all the demagnetized specimens could be

classified into three groups. The first group contains 127 specimens and all these specimens identify a high-temperature characteristic remanent magnetization (ChRM) after removal of a viscous component in the initial stage of demagnetization and/or a

J. Zhao et al. / Journal of Asian Earth Sciences 111 (2015) 350–364

low-temperature component by demagnetization temperatures up to 250–300 °C (Fig. 5a–c, f). The high-temperature characteristic component was generally unblocked by temperatures 550–600 °C (Fig. 5b–f), and in a minority of specimens at higher temperatures up to 650 °C (Fig. 5a). This behavior suggests that the ChRM is carried by magnetite and subordinate hematite, which is concordant with rock magnetic results for the pilot samples (Fig. 3). Noticeably, thermal demagnetization for specimens from site YZ201 showed some differences, in which the low-temperature component was completely removed by temperatures as high as 450–520 °C. Correspondingly, the high-temperature ChRM was subtracted either between 400/450 °C and 575/580 °C in 4 out of 8 demagnetized specimens or between 450/540 °C and 675/680 °C in the remaining 4 specimens (Fig. 5g). The second group contains 12 out of 149 specimens demagnetized. These specimens exhibited a high-temperature demagnetization trajectory

355

following a great circle permitting ChRM isolation by a remagnetization great circle between temperatures of 80/100 °C and 500/525 °C (Fig. 5d–e). Site-mean directions of these four sites are therefore calculated by the McFadden and McElhinny (1988) method (Table 1). The remaining 10 specimens formed the third group in which thermal demagnetization exhibited an erratic high-temperature demagnetization behavior following removal of the viscous and/or low-temperature overprints so that no meaningful characteristic component could be isolated. In summary, the low-temperature component was isolated from 67 out of 149 demagnetized specimens. This component has an in-situ mean direction of D = 7.5°, I = 39.3° (k = 45.4, a95 = 2.6°) with significant deterioration of data grouping after tilt correction (ks/kg = 0.27) producing a negative fold test. We interpret this low-temperature component as a recent overprinting in the light of conformity to the Present Geomagnetic Field (PGF).

N,N

E,Up

Fig. 5. Typical orthogonal and stereographic vector plots illustrating progressive thermal demagnetization structures for basaltic rocks from the Niuhetang Formation in the Yongde and Zhenkang areas. Orthogonal directions (Zijderveld, 1967) are plotted in-situ; whilst stereographic directions are plotted in stratigraphic coordinates. Solid/open symbols represent endpoints projected onto horizontal or vertical planes, respectively.

356

J. Zhao et al. / Journal of Asian Earth Sciences 111 (2015) 350–364

Table 1 Summary of characteristic remanent magnetizations from Late Triassic Niuhetang Formation, Baoshan area of West Yunnan. Site ID

ks (°N)

us (°E)

Strike/dip

n/n0

Dg (°)

Upper member of the formation YY051 23.94 99.31 YY052 23.94 99.31 YY053 23.94 99.31

5.5/41 5.5/41 5.5/41

10/10 8(2)/8 6(4)/9

326.3 319.3 323.2

Middle member of the formation YY054 23.95 99.24

2/38

7/7

Lower member of the formation YY045 23.94 99.30 YY046 23.94 99.30 YY047 23.94 99.30 YY048 23.94 99.30 YY049 23.94 99.30 YY050 23.94 99.30 YZ196 23.72 99.15 YZ197 23.73 99.15 YZ198 23.73 99.15 YZ199 23.73 99.15 YZ200 23.73 99.15 a YZ201 23.73 99.15

5/48 5/48 5/48 5/48 5/48 5/48 79/36 79/36 84/35.5 84/35.5 67/22 67/22 Normal

Sub-mean

Ig (°)

Reversal

up (°E)

53.1 51.6 55.6

305.3 302.3 302.3

313.9 310.9 310.9

20.8 17.3 21.9

260.1 86.2 55.2

3.0 6.0 9.1

337.0 340.9 338.6

33.2 31.9 30.4

159.6

60.9

125.3

133.7

35.1

300.4

3.2

149.5

27.5

12/12 10/10 11/11 8/10 3/8 10/10 10(4)/10 8/8 10(2)/10 10/10 8/8 8/8

142.5 134.9 147.3 145.2 143.4 143.4 66.5 91.0 65.1 84.9 95.0 86.7

57.3 65.2 62.6 60.2 59.7 61.6 55.5 46.1 41.4 40.5 42.3 1.1

119.6 111.8 118.2 119.0 118.5 117.4 113.9 117.5 98.7 111.0 108.3 85.8

129.8 122 128.4 129.2 128.7 127.5 110.7 114.4 94.9 107.1 107.5 85.0

17.0 21.6 22.6 20.1 19.2 20.7 47.6 29.9 42.8 31.5 29.6 6.2

259.2 121.5 926.3 767.3 719.0 1036.8 78.1 251.3 191.4 320.9 213.3 26.3

2.7 4.4 1.5 2.0 4.6 1.5 5.5 3.5 3.5 2.7 3.8 11.0

161.8 164.3 159.9 160.6 161.4 161.4 154.6 163.9 164.5 166.5 167.4 194.0

31.1 23.3 28.2 29.7 29.6 28.1 5.2 14.6 5.6 8.0 8.9 3.3

(12/13)

112.8

59.2 120.3

28.3 28.6

311.9

20.0 20.0

122.8

26.6 26.9

14.3 51.2 32.5 767.0 780.6 780.6 15.9 53.5 34.3

11.9 6.1 7.7 4.5 4.4 4.4 9.9 5.3 6.6

160.9 K = 39.9

(3/3)

322.9

53.5 303.3

Formation-mean

a95 (°)

Ds (°)

115.2

(15/16)

119.8

Is (°)

j

Db (°)

58.6 117.0

kp (°N)

21.7 A95 = 6.1

Abbreviations are: site ID, site identification; Strike/dip, strike azimuth and dip of bed; n/n0, number of samples used for the calculation/demagnetized, numbers showing in the parentheses indicate number of remagnetization circles used; Dg, Ig (Db, Ds, Is), declination and inclination of direction in-situ (declination after general tilt-correction, declination and inclination after tilt-correction with plunging fold axis (N148.9°E, 29.7° with a95 = 5.1°); ks, us, latitude and longitude of the sampling site; kp, up, latitude and longitude of corresponding virtual geomagnetic pole (VGP) after tilt-correction with plunging fold axis. a Site-mean direction discarded from further calculation due to large deviation to the formation mean.

The high-temperature ChRM is of dual polarity and can be well determined from all the 16 sampling sites. Excluding consideration of site-mean observation from site YZ201 which deviates markedly from the other results both before and after tilt correction (Fig. 6a– c), the remaining 15 sites yield a formation mean of D = 119.8°, I = 58.6° (a95 = 9.9°, k = 15.9) before and D = 117.0°, I = 26.6° (a95 = 5.3°, k = 53.5) after tilt correction (Table 1 and Fig. 6a–b). The application of a two-step unfolding procedure (Stewart and Jackson, 1995) to correct for the plunging fold axis (N148.9°, 29.7° with a95 = 5.1°, Fig. 2b) before unfolding about a horizontal axis yields a corrected mean direction of D = 122.8°, I = 26.9° with k = 34.3 and a95 = 6.6° (Fig. 6c). Although this grouping by two-step unfolding yields a grouping of site-mean directions with slightly diminished grouping compared to the result from single tilt adjustment (Table 1), the Watson and Enkin (1993) fold test presents a noticeably increased optimal concentration of the ChRMs from 84.8 ± 3.8 following tilt adjustment to 92.5 ± 4.1 percent unfolding after tilt-correction with plunging fold axis. This suggests that the two-step unfolding correction is the proper procedure for the data set from this study. The traditional McElhinny (1964) fold test is positive at the 95% confidence level with the ratio ks/kg = 2.16, larger than the statistical threshold of 1.84. The reversal test (McFadden and McElhinny, 1990) is positive with an angular difference of 13.6° between the two-step unfolding corrected directions of each polarity, which is smaller than the threshold of 15.6° and yields a class C reversal test result. 5. Discussions 5.1. Origin of the high-temperature characteristic remanence Both rock magnetic experiments and thermal demagnetization indicate that the high-temperature ChRM is carried by magnetite

in a majority of samples and by both magnetite and hematite in a minority. The Day plot (Day et al., 1977; Dunlop, 2002) for pilot samples dominated by magnetite reveals that this mineral resides mostly in a PSD state. On the other hand, the high-temperature ChRM yields both a positive fold test (McElhinny, 1964; Watson and Enkin, 1993) and reversal test (McFadden and McElhinny, 1990). This indicates a pre-folding origin for the ChRM, namely prior to the two main regional folding phases comprising the Yanshanian and Himalayan recognized in the study area (Huang and Opdyke, 1991; Liao et al., 2003). The corresponding paleomagnetic pole derived from the two-step unfolding-corrected high-temperature ChRMs is located at 160.9°E, 21.7°S with A95 = 6.1°, and is significantly different from Jurassic and Oligocene paleopoles from the Baoshan Terrane (Huang and Opdyke, 1993; Kornfeld et al., 2014). We therefore conclude that the high-temperature ChRM is a primary remanence acquired at the time of eruption of the Late Triassic Niuhetang basaltic lavas. The corresponding paleomagnetic pole of the high-temperature component has a VGP dispersion of about 12.8°, which is comparable with distributions predicted from global paleosecular variation (PSV) models during the last 5 Ma at latitudes of 10–20° (Johnson et al., 2008). Using the analytical method from Deenen et al. (2011), the 119 available VGPs yield an A95 = 2.2°, which falls into the range of A95max = 4.04, A95min = 1.77 for N = 119 and suggests that PSV has been approximately averaged by this collection. Meanwhile, the ChRM was isolated from all the lower (11 sites), middle (1 site), and upper (3 sites) members of the Niuhetang basaltic lavas and records at least one polarity reversal event (Table 1). In addition, as shown in Fig. 2c, all the sampling sites in our study were drilled from individual lava flow and in particular the three members of the Niuhetang Formation are distinguished by either different lithological associations or terrestrial sediment layers with typical plant and gastropod fossils (YBGMR,

357

J. Zhao et al. / Journal of Asian Earth Sciences 111 (2015) 350–364

(a)

tilt-corrected D =122.8 I =26.9 k =34.5 α95 =6.6 N=15

tilt-corrected D =117.0 I =26.6 k =53.0 α9 5 =5.3 N=15

in-situ D =119.8 I =58.6 k =15.9 α9 5 =9.9 N=15

k m ax

84.8%[81.3%,88.2%]

k max

(d)

92.5%[88.4%,96.6%]

(e)

k

k

0 -50

0

100

150

%

Unfolding percent(%)

Unfolding percent(%)

Fig. 6. Equal-area projections of site-mean observations from the Niuhetang Formation (a) before and (b) after tilt correction; (c) equal-area projection of the site-mean observations after a two-step unfolding correction; (d, e) incremental unfolding analysis using Watson and Enkin (1993) for general tilt-corrected (d) and two-step unfoldingcorrected (e) site-mean observations, respectively.

1984). We therefore believe that our sampling of the Niuhetang basaltic lavas has embraced a time interval larger than at least a million years so that the PSV should have been adequately averaged by this collection. This newly-obtained Late Triassic paleomagnetic pole suggests a paleolatitude of 15°N for the southern part of the Baoshan Terrane during eruption of the Niuhetang basaltic lavas. 5.2. Orientation of the East Paleotethys Suture Zone As shown in Fig. 1b, the Changning-Menglian-Inthanon (CMI) ophiolite belt separating the Sibumasu and Indochina blocks (e.g. Sone and Metcalfe, 2008) extends nearly north-to-south in present geographic coordinates. Noting that substantial paleomagnetic data have shown that the terranes in west Yunnan have experienced a clockwise (CW) rotation relative to the stable Eurasian Plate ranging from 20° to 135° due to the India-Asia collision (Li et al., 2012 and references therein; Tong et al., 2013; Kornfeld et al., 2014). This post-India-Asia collisional large-scale rotational motion is compatible with the observed southeastward escape of upper-crustal blocks (e.g. Chen et al., 2000; Shen et al., 2005; Gan et al., 2007). Therefore, the question of whether the CMI ophiolite belt suffered significant India-Asia collision-induced vertical

axis rotation is a key issue for late Paleozoic to early Mesozoic paleogeographic reconstruction of the Sibumasu and Indochina blocks. Since no available paleomagnetic data have been obtained directly from the CMI ophiolite belt where structural complexities are in any case likely to render such an approach difficult, we consider the available Devonian to Jurassic paleomagnetic results from the Sibumasu and Indochina blocks (Table 2). Firstly, Fang et al. (1989) reported a Devonian paleomagnetic pole from limestones in the northern part of the Baoshan Terrane. This pole is defined by positive fold and reversal tests and indicates a high paleolatitude of 42°S for the Devonian Baoshan Terrane, which seems to be compatible with Early Permian paleolatitudes reported by Huang and Opdyke (1991) and Ali et al. (2013). However, a comparison between this preliminary paleomagnetic pole and the Early Permian pole (Huang and Opdyke, 1991) suggests a vast CW rotation of 169° for northern Baoshan during Devonian to Early Permian times. This vast rotation is more than twice the amount of that observed from west Australia of East Gondwana during the period between 390–380 Ma and 290–280 Ma (e.g. Torsvik et al., 2012) and considered to be of local rather then regional significance. Secondly, Huang and Opdyke (1991) reported a paleomagnetic study for the Early Permian Woniusi Formation from both northern and southern parts of the Baoshan Terrane. It

358

J. Zhao et al. / Journal of Asian Earth Sciences 111 (2015) 350–364

Table 2 Summary of Late Paleozoic to Cenozoic paleomagnetic data from the Baoshan-Sibumasu, Simao-Indochina and South China blocks. ID

Sampling information Locality

Lat. (°N)

Age

N

Test

Lon. (°E)

Observed direction

Paleomagnetic pole

Rotation

Ref.

Reference

PGF

Kornfeld et al. (2014) Huang and Opdyke (1991) Fang et al., (1989)

Dec. (°)

Inc. (°)

a95

Lat. (°N)

Lon. (°E)

A95 (°)

(°)

42.2

47.0

7.8

52.6

175.7

42.2

17.2

257.5

10.1/ 6.5 9.3

13.1

1

Northern part of Baoshan, Sibumasu 1 Baoshan 26.0 98.9

Oli

12

2

Baoshan

25.2

99.3

Pl

13

F

29.2

61.0

6.7

3

Baoshan

25.0

99.0

D

7

R

198.0

62.0

5.8

66.5

313.8

7.0/ 9.0

169.6

2

Southern part of Baoshan, Sibumasu 4 Luxi 24.3 98.4

Jm

6

5 6

35.2

11.3

0.5

166.6

12.2

99.7

PGF

21.7 33.9

160.9 229.6

6.1 11.6

22.2 ± 10.6 7.5 ± 13.1

4 5

Huang and Opdyke (1993) This study Huang and Opdyke (1991)

23.9 23.9

99.1 99.2

Tu Pl

15 6

F, R

122.8 114.5

26.9 63.9

6.6 7.8

Shan State, Sibumasu 7 Kalaw

20.7

96.5

J–K

13

F

44.7

23.4

6.1

47.2

190.6

4.8

44.7

PGF

Richter and Fuller (1996)

Simao, Indochina 8 Yunlong 9 Yunlong 10 Xiaguan

25.8 25.8 25.6

99.4 99.4 100.2

Km Km Km

20 29 9

F F, R F, R

40.2 38.3 6.9

49.9 50.7 47.7

3.9 3.4 8.6

54.6 56.7 83.6

171.3 170.1 152.7

4.4 4.0 10.0

40.2 38.3 6.9

PGF PGF PGF

11

Weishan

25.4

100.2

Ju

5

7.3

25.3

10.4

76.3

250.0

10.4

7.3

PGF

12

Yongping

25.5

99.5

Kl

12

F

42.0

51.1

15.7

50.9

167.3

20.6

42.0

PGF

13 14 15 16 17

Jingdong Jinggu Jinggu Jinggu Jinggu

24.5 23.6 23.4 23.4 23.4

100.8 100.5 100.4 100.5 100.9

Kl–m Jm Kl Km Km

13 10 3 7 8

F

8.3 83.3 84.4 115.8 79.4

48.8 36.8 39.6 36.0 43.3

7.7 5.4 17.8 6.3 9.1

81.2 14.0 13.6 13.9 18.9

145.8 173.6 171.5 161.3 170.0

8.9 4.2 8.9

8.3 83.3 84.4 115.8 79.4

PGF PGF PGF PGF PGF

18 19 20 21

Zhenyuan West Zhenyuan Pu’er Mengla

24.1 24.1 23.0 21.6

101.1 101.1 101.0 101.4

Kl–m Kl–m Kl–m Km

7 4 25 10

F F F

61.8 144.2 59.9 60.8

46.1 49.4 45.2 37.8

8.1 6.4 5.1 7.6

34.7 25.7 35.8 33.7

172.7 135.2 173.1 179.3

8.1 7.7 5.6 8.2

61.8 144.2 59.9 60.8

PGF PGF PGF PGF

22 23 24

South Mengla Nan Phong Saly

21.4 19.2 21.6

101.6 101.0 101.9

Kl–m Jl–u Ju–Kl

14 11 19

F F F

51.2 32.2 28.8

46.4 33.3 32.1

5.6 12.2 8.8

43.6 60.1 63.4

172.1 186.5 193.9

6.1 11.7 7.4

51.2 32.2 28.8

PGF PGF PGF

Sato et al. (1999) Yang et al. (2001) Huang and Opdyke (1993) Huang and Opdyke, 1993 Funahara et al., (1993) Tanaka et al. (2008) Chen et al. (1995) Chen et al. (1995) Chen et al. (1995) Huang and Opdyke (1993) Tanaka et al. (2008) Tanaka et al. (2008) Sato et al. (2007) Huang and Opdyke (1993) Tanaka et al. (2008) Aihara et al. (2007) Takemoto et al. (2009)

Central and southern part of Indochina 25 Lai Chau 22.3 103.4

Ku

5

F

12.2

40.1

4.7

78.7

188.0

5.1

12.2

PGF

26

Yen Chau

21.0

104.4

Ku

8

F

3.2

26.7

12.9

83.2

255.6

10.8

3.2

PGF

27

Borikhanxay

18.5

103.8

Ju–Kl

18

F

42.1

46.9

7.9

50.7

169.7

8.7

42.1

PGF

28

18.2

103.9

Km

14

31.8

28.7

3.5

59.4

190.8

3.5

31.8

PGF

29

Amphoe Bung Kuan Nam Nao

16.5

103.0

Kl

10

28.1

40.5

2.4

62.7

173.3

2.4

28.1

PGF

30

Nam Nao

16.5

103.3

Ju

10

31.8

28.7

3.5

64.8

178.1

2.3

31.8

PGF

31

16.5– 17.2 16.5– 17.2 16.5

102.5– 104.1 102.5– 104.1 106.0

Km

8

31.4

27.1

9.4

59.7

192.7

9.4

31.4

PGF

Kl

4

31.8

38.3

5.7

59.7

178.2

5.7

31.8

PGF

Charusiri et al. (2006)

33

Muan Sakon Nakhon Muan Sakon Nakon Muang Phin

Yang and Besse (1993) Yang and Besse (1993) Charusiri et al. (2006)

J (Km)

23

30.8

39.9

3.0

60.5

178.6

3.0

30.8

PGF

34

Da Lat

105.0– 109.4 101.8

J–K

21

14.5

33.3

6.3

74.2

171.1

5.9

14.5

PGF

35

10.4– 12.5 16.7

Jl

8

F

39.5

46.3

7.1

54.4

175.5

7.3

39.5

PGF

36

16.7

101.8

Tu/Jl

13

F

40.4

47.8

4.7

53.6

173.3

4.9

40.4

PGF

37

16.7

101.8

Tu

5

F

42.2

50.2

6.7

52.1

169.8

7.3

42.2

PGF

Takemoto et al. (2009) Chi and Dorobek (2004) Yang and Besse (1993) Yang and Besse (1993) Yang and Besse (1993)

32

Yongde Yongde

99.7

F

F

Takemoto et al. (2005) Takemoto et al. (2005) Takemoto et al. (2009) Charusiri et al. (2006)

359

J. Zhao et al. / Journal of Asian Earth Sciences 111 (2015) 350–364 Table 2 (continued) ID

Sampling information Locality

Lat. (°N)

Age Lon. (°E)

N

Test

Observed direction Dec. (°)

Inc. (°)

a95

Paleomagnetic pole

Rotation

Lat. (°N)

Lon. (°E)

A95 (°)

(°)

South China 38

Jl–m

79.9

221.8

6.3

39 40 41 42

Tu Tm Pm–Tl P/T

52.0 50.8 42.7 48.8

187.3 227.7 215.9 227.7

– 5.2 3.3 3.1

43

Pm

52.7

246.4

9.1

44

Pl

65.3

265.2

6.5

Ref.

Reference

Yang and Besse (2001) Huang et al. (2008a) Su et al. (2005) Su et al. (2005) Yang and Besse (2001) Yang and Besse (2001) Lin and Fuller (1990)

# Abbreviations are: Lat./Long., latitude and longitude of sampling area; N, number of sites used in paleomagnetic statistics; Dec. and Inc., declination and inclination; a95 and A95, radius of circle of 95 percent confidence of observed direction and paleomagnetic pole; F and R, reversal and fold test; PGF, Present Geomagnetic Field; Rotational difference is evaluated by comparing observed paleomagnetic declination with that expected from reference pole (Ref.), positive and negative values represent CW and CCW rotation, respectively; D, C, P, T, J and Oli represent Devonian, Carboniferous, Permian, Triassic and Oligocene, respectively with l = Early, m = Middle, and u = Late.

is noteworthy that this study found a large declinational difference of 85° between the northern and southern parts of the Baoshan Terrane, suggesting a CW rotation of the southern Baoshan of 85° since the Early Permian. For the northern Baoshan Terrane, comparison between Early Permian (Huang and Opdyke, 1991) and Oligocene poles (Kornfeld et al., 2014) indicates little or no significant counterclockwise (CCW) rotation (13.1°) between Early Permian and Oligocene times followed by a CW rotation of 42.2° relative to present geomagnetic field (PGF) after Oligocene times. The later significant CW rotation is likely to have resulted from the India-Asia collision. For the southern Baoshan Terrane however, a paleomagnetic comparison between Early Permian (Huang and Opdyke, 1991), Late Triassic (this study), and Jurassic (Huang and Opdyke, 1993) poles indicates a CCW rotation of 7.5° ± 13.1° between Early Permian and Late Triassic times, a CW rotation of 22.2° ± 10.6° from Late Triassic to Jurassic times, and a CW rotation of 99.7° (relative to PGF) after Jurassic times. Although some remarkably large rotations are identified here, neither northern nor southern part of the Baoshan Terrane appear to have been subjected to large-scale vertical-axis rotational motion between Permian and Jurassic times. In other words, the large-scale post-Permian CW rotation of the southern Baoshan Terrane relative to the northern Baoshan Terrane (Huang and Opdyke, 1991) should have occurred later than the Jurassic and is most probably related to crustal rotational deformation around the East Himalaya Syntaxis (Huang and Opdyke, 1993; Yang and Besse, 1993; Otofuji et al., 2010; Tong et al., 2013; Kornfeld et al., 2014). More broadly, differential India-Asia collision-induced CW rotations have also been observed between the northern and southern parts of the Simao Terrane of the Indochina Block. As shown in Fig. 8, available Jurassic and Cretaceous paleomagnetic data from the Jinggu (Huang and Opdyke, 1993; Chen et al., 1995), Zhenyuan (Tanaka et al., 2008), Mengla (Huang and Opdyke, 1993; Tanaka et al., 2008), and Pu’er (Sato et al., 2007) areas of southern Simao indicate large-scale Cenozoic CW rotation ranging from 50° to 144° (Table 2). Meanwhile, Tanaka et al. (2008) further argue for a Cenozoic CW rotation of 90° relative to the PGF resulting in an approximately easterly-deflected Jurassic– Cretaceous declination for southern Simao. However, Cretaceous paleomagnetic data from the Yunlong (Sato et al., 1999; Yang et al., 2001) and Yongping (Funahara et al., 1993) areas of northern Simao suggest relatively smaller Cenozoic CW rotation of 38° to 42°, and even little or no significant rotation (7–8°) in the Xiaguan, Weishan (Huang and Opdyke, 1993), and Jingdong areas, any small magnitude rotations here could have a local tectonic explanation (Tanaka et al., 2008).

For the other parts of the Sibumasu and Indochina blocks, the Cenozoic rotation pattern is a little more complicated. As summarized by Otofuji et al. (2012), preliminary Jurassic–Cretaceous paleomagnetic results from the Kalaw area of East Myanmar (Richter and Fuller, 1996) yielded a Cenozoic CW rotation of 45° (relative to PGF) for the Shan State Terrane; while the Cretaceous paleomagnetic results from the Khorat Basin (Yang and Besse, 1993; Charusiri et al., 2006; Takemoto et al., 2009) implied a Cenozoic CW rotation (relative to PGF) of 30° for the central part of the Indochina Block. However, only little or marginal significant Cenozoic CW rotation of 11° (relative to PGF) was observed from the Da Lat area of southeastern tip of the Indochina Peninsula (Otofuji et al., 2012). For this noticeable divergence in rotational motion, the original authors interpreted it in terms of a second phase CCW rotation of 27° ± 10° of the Kontum Terrane relative to the Khorat Basin and it most probably resulted from the 32–17 Ma sinistral movement of the Kontum Terrane along the East Vietnam Boundary Fault. In summary, with the exception of the Kontum Terrane in southeastern tip of the Indochina Block, large-scale Cenozoic vertical-axis CW rotations have been observed from different terranes located on both sides of the CMI belt. In particular, relatively abundant Jurassic to Cretaceous paleomagnetic data from the Baoshan and Simao terranes suggest that the northern part of these two terranes experienced a Cenozoic CW rotation of 40° (relative to PGF); whereas the southern part may have been subjected to a much larger Cenozoic CW rotation of 90° relative to the PGF (Fig. 8). Therefore, when we make a rotational motion correction to the northern and southern segments of the Changning-Menglian suture according to the above differential Cenozoic CW rotation observed in the northern and southern parts of the Baoshan and Simao terranes respectively, the currently NW–SE to NE–SW directed Changning-Menglian suture will have an orientation of approximately east-to-west extended (Fig. 8). This implies that the eastern branch of the Paleotethys Ocean is likely to have subducted northward and closed along a nearly east-to-west directed suture zone. As a result, direct comparison of paleolatitudes of the terranes/blocks located on both sides of the East Paleotethys Ocean should provide robust constraints on the timing and location of the closure of the East Paleotethys Ocean. 5.3. The closure of the East Paleotethys Ocean Based on available paleopoles from the Sibumasu, Indochina and South China blocks (Table 2), the South China and Indochina blocks (reference site: Lincang, 100.1°E, 23.9°N) is constrained to have moved from a paleolatitude near the paleo-equator to a

360

J. Zhao et al. / Journal of Asian Earth Sciences 111 (2015) 350–364

higher paleolatitude around 20°N in the Northern Hemisphere in the Early Permian to Late Triassic with little vertical-axis rotational motion. However, the Baoshan Terrane of the Sibumasu Block formerly occupied a paleolatitude as high as 42°S in the Southern Hemisphere during the Early Permian (Ali et al., 2013; Huang and Opdyke, 1991), and moved rapidly northward to low latitudes around 15°N in the Northern Hemisphere by Late Triassic times. As shown in Fig. 7, the Late Triassic to Jurassic paleomagnetic poles from the Sibumasu, Indochina and South China blocks are distributed along a small circle centered on the reference site of Lincang. This is the paleomagnetic signature of regional block rotation and indicates that these blocks occupied similar paleolatitudes during Late Triassic to Jurassic times; they may have collided with each other by the Late Triassic. The view that the Sibumasu, Indochina, and South China blocks had collided by the Late Triassic is consistent with substantial geological evidence suggesting the main East Paleotethys Ocean had closed by the Late Triassic. Firstly, the 200–230 Ma Lincang granite on the east fringe of the Changning-Menglian suture zone indicates a continent–continent collision between the Baoshan and Simao terranes no later than the Late Triassic (Dong et al., 2013). Secondly, the chronostratigraphic range of pelagic sediments in the CMI suture zone ranges from Middle Devonian to Late

Anisian/Early Ladinian stages of the Middle Triassic in which the youngest horizon is represented by the Triassocampe deweveri radiolarian assemblage (Feng et al., 1999; Feng, 2002). In contrast the Mae Sariang Group in the Inthanon suture zone, consisting of Middle–Late Triassic radiolarian cherts and turbiditic clastics, is non-pelagic and represents deposits more proximate to the Sibumasu margin (Kamata et al., 2002). Geochemical and geochronological study (Wang et al., 2010) of the Xiaodingxi (214 ± 7 Ma) and Manghuihe (210 ± 22 Ma) volcanic sequences, representative of the Lancangjiang igneous zone and dominated by alkaline basalts and basaltic andesites, indicates that the Lancangjiang igneous zone was erupted in a post-collisional extensional setting, confirming again the commencement of the Baoshan-Simao collision should have occurred at least prior to formation of this igneous zone. Thirdly, paleontological studies (Fang, 1994; Hisada et al., 2001; Jin, 2002; Wang and Sugiyama, 2002) show that the Sibumasu Block fauna exhibits a progressive transition to non-marine provinciality from the peri-Gondwanan Indoralian Province (i.e. the Asselian Bandoproductus– Punctocyrtella–Tomiopsis brachiopod assemblage) in the Early Permian to an endemic Sibumasu Province in the Middle Permian and then into an equatorial Cathaysian Province in the Late Permian.

Fig. 7. Equal area projections of the Late Paleozoic to Mesozoic paleomagnetic poles of the Baoshan Terrane and surrounding blocks. The dashed gray line shows the direction of expected declination from reference site.

361

J. Zhao et al. / Journal of Asian Earth Sciences 111 (2015) 350–364

1

8 9

GLGSZ

Z SCS

10 12 11

2

3?

RS

SR

AL 13

Z

4

ture

18

n su

19

glia

14

15 16

17

Cha

ngn

ing-

Men

6

5

20

21

22 Sampling site

Fig. 8. Sketch geological map showing major units and faults in the Baoshan area modified after Wang et al. (2006). Arrows represent declination deviations (rotations about vertical axes) from the present-day meridian (dashed lines). The long blue/orange rectangle shows orientations of the East Paleotethys Suture Zone at present/before the India-Asia collision-induced rotational deformation. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

362

J. Zhao et al. / Journal of Asian Earth Sciences 111 (2015) 350–364

On the other hand, substantial geological evidence suggests that the Jinshajiang-Ailaoshan-Song Ma-Song Chay Ocean (Faure et al., 2014), separating the Indochina Block in the south from the South China Block in the north (c.f. Section 5.2), should have closed at least by the Late Triassic. Recent geochronological and geochemical study of the Xin’anzhai monzogranite (251.6 ± 2.0 Ma) and the Tongtiange leucogranite (247.5 ± 2.2 Ma) in the Ailaoshan suture zone indicates that emplacement of these igneous intrusions marked the termination of accretion of Indochina to the South China Block and commencement of the Indosinian Orogeny (Liu et al., in press). This in turn suggests that the northern branch of the East Paleotethys may have closed by latest Permian to earliest Triassic times. Meanwhile, as summarized by Lai et al. (2014), the Middle Triassic regional unconformity, supported by detrital U–Pb zircon ages from West Ailaoshan sandstones and the Middle to Late Triassic post-collisional granitoids along the Jinshajiang (ca. 235–230 Ma, Zhu et al., 2011) and Truong Son (ca. 230–200 Ma; Liu et al., 2012) regions, implies that the Jinshajiang-Ailaoshan-Song Ma Ocean should have completely closed by the Late Triassic. Furthermore, field structural observations and systematic analysis of published data in North Vietnam indicated a Middle Triassic age for the South China Block-Indochina collision, which is strongly supported by a Late Triassic regional unconformity, postorogenic stitching granitoids, and a Early–Middle syntectonic metamorphism (see Faure et al., 2014 for details). 6. Conclusions A paleomagnetic study of the Late Triassic Niuhetang basaltic lavas from the Baoshan Terrane of the Sibumasu Block yields a paleomagnetic pole at 160.9°E, 21.7°S (A95 = 6.1°) with positive fold and reversal tests. Together with a significant deviation from younger paleopoles from this terrane and a compatible VGP dispersion (12.8°) to the global PSV model during the last 5 Ma, this indicates that the time-averaged geomagnetic field has been adequately sampled. We therefore believe that this pole is a representative record of the paleomagnetic field during the time of eruption of the Niuhetang basaltic lavas. The new Late Triassic paleomagnetic pole suggests that the Baoshan Terrane was situated in the Northern Hemisphere around 15°N during eruption of these lavas. Further comparison with available paleomagnetic results from the Baoshan Terrane suggests that this terrane experienced little or non-significant CCW rotation during the Early Permian/Late Triassic to Jurassic interval; but a more important observation is that the southern part of the terrane experienced a post-Jurassic CW rotation of 90° relative to the northern sector. The significant differential Cenozoic CW rotational motion between northern and southern parts of the Baoshan and Simao terranes indicates that the currently nearly north-to-south directed East Paleotethys suture zone is very likely to have extended approximately west to east during the suturing of the East Paleotethys Ocean. Therefore paleolatitudinal overlap of the Baoshan and Indochina blocks is a key issue for identifying the timing and position of the East Paleotethys Ocean. The Late Triassic paleomagnetic pole suggests that the Baoshan Terrane occupied similar paleolatitudes to the South China and Indochina blocks during the Middle to Late Triassic thus indicating that the East Paleotethys Ocean closed no later than the Late Triassic at low latitudes of 15°N. Acknowledgments This work was financially supported by a National Natural Science Foundation of China (NSFC) project (41190071) of Major

NSFC Program (41190070) ‘‘Reconstruction of East Asian Blocks in Pangaea’’. We are grateful to Jinjiang Zhang and John D.A. Piper for their constructive discussions and suggestions and to Liwei Chen and Jianjun Li for field assistance. We are also greatly indebted to Michel Faure, Yo-ichiro Otofuji and another anonymous reviewer for careful reviews that greatly improved the manuscript.

References Acharyya, S.K., 1998. Break-up of the greater Indo-Australian continent and accretion of blocks framing South and East Asia. J. Geodyn. 26, 149–170. Aihara, K., Takemoto, K., Zaman, H., Inokuchi, H., Miura, D., Surinkum, A., Paiyarom, A., Phajuy, B., Chantraprasert, S., Panjasawatwong, Y., Wongpornchai, P., Otofuji, Y., 2007. Internal deformation of the Shan-Thai block inferred from paleomagnetism of Jurassic sedimentary rocks in Northern Thailand. J. Asian Earth Sci. 30, 530–541. Ali, J.R., Cheung, H.M.C., Aitchison, J.C., Sun, Y.D., 2013. Palaeomagnetic reinvestigation of Early Permian rift basalts from the Baoshan Block, SW China: constraints on the site-of-origin of the Gondwana-derived eastern Cimmerian terranes. Geophys. J. Int. 193 (2), 650–663. http://dx.doi.org/10.1093/gji/ggt012. Brookfield, M.E., 1996. Reconstruction of western Sibumasu. J. Geol., 65–80 Cao, S.Y., Liu, J.L., Leiss, B., Neubauer, F., Genser, J., Zhao, C.Q., 2011a. Oligo-Miocene shearing along the Ailao Shan-Red River shear zone: constraints from structural analysis and zircon U/Pb geochronology of magmatic rocks in the Diancang Shan massif, SE Tibet, China. Gondwana Res. 19, 975–993. Cao, S.Y., Neubauer, F., Liu, J.L., Genser, J., Leiss, B., 2011b. Exhumation of the Diancang Shan metamorphic complex along the Ailao Shan-Red River belt, southwestern Yunnan, China: evidence from 40Ar/39Ar thermochronology. J. Asian Earth Sci. 42, 525–550. Carter, A., Roques, D., Bristow, C., Kinny, P., 2001. Understanding Mesozoic accretion in Southeast Asia: significance of Triassic thermotectonism (Indosinian orogeny) in Vietnam. Geology 29, 211–214. Chan, L.S., Wang, C.Y., Wu, X.Y., 1984. Paleomagnetic results from some PermianTriassic rocks from southwestern China. Geophys. Res. Lett. 11, 1157–1160. Charusiri, P., Imsamut, S., Zhuang, Z., Ampaiwan, T., Xu, X., 2006. Paleomagnetism of the earliest Cretaceous to early Late Cretaceous sandstones, Khorat Group, Northeast Thailand: implications for tectonic plate movement of the Indochina block. Gondwana Res. 9, 310–325. Chen, H., Dobson, J., Heller, F., Hao, J., 1995. Paleomagnetic evidence for clockwise rotation of the Simao region since the Cretaceous: a consequence of India-Asia collision. Earth Planet. Sci. Lett. 134, 203–217. Chen, Z., Burchfiel, B.C., Liu, Y., King, R.W., Royden, L.H., Tang, W., Wang, E., Zhao, J., Zhang, X., 2000. Global positioning system measurements from eastern Tibet and their implications for India/Eurasia intercontinental deformation. J. Geophys. Res.: Sol. Earth 105, 16215–16227. Chi, C.T., Dorobek, S.L., 2004. Cretaceous palaeomagnetism of Indochina and surrounding regions: Cenozoic tectonic implications, In: Malpas, J., Fletcher, C.J.N., Ali, J.R. (Eds.), Aspects of the Tectonic Evolution of China, Geological Society Special Publication No. 226, pp. 273–287. Collins, W.J., 2003. Slab pull, mantle convection, and Pangaean assembly and dispersal. Earth Planet. Sci. Lett. 205, 225–237. Day, R., Fuller, M., Schmidt, V.A., 1977. Hysteresis properties of titanomagnetites: grain-size and compositional dependence. Phys. Earth Planet. In. 13, 260–267. Deenen, M.H.L., Langereis, C.G., van Hinsbergen, D.J.J., Biggin, A.J., 2011. Geomagnetic secular variation and the statistics of palaeomagnetic directions. Geophys. J. Int. 186, 509–520. Dong, G.C., Mo, X.X., Zhao, Z.D., Zhu, D.C., Goodman, R.C., Kong, H.L., Wang, S., 2013. Zircon U–Pb dating and the petrological and geochemical constraints on Lincang granite in Western Yunnan, China: implications for the closure of the Paleo-Tethys Ocean. J. Asian Earth Sci. 62, 282–294. Dunlop, D.J., 2002. Theory and application of the Day plot (Mrs/Ms versus Hcr/Hc) 1. Theoretical curves and tests using titanomagnetite data. J. Geophys. Res.: Sol. Earth 107, EPM 4-1–EPM 4-22. Fang, Z.J., 1994. Biogeographic constraints on the rift-drift-accretion history of the Sibumasu block. J. SE Asian Earth Sci. 9, 375–385. Fang, W., Van der Voo, R., Liang, Q.Z., 1989. Devonian paleomagnetism of Yunnan Province across the Shan Thai-South China Suture. Tectonics 8, 939–952. Faure, M., Lepvrier, C., Nguyen, V.V., Vu, T.V., Lin, W., Chen, Z., 2014. The South China block-Indochina collision: where, when, and how? J. Asian Earth Sci. 79 (Part A), 260–274. Feng, Q.-L., 2002. Stratigraphy of volcanic rocks in the Changning-Menglian Belt in southwestern Yunnan, China. J. Asian Earth Sci. 20, 657–664. Feng, Q.L., Ge, M.C., Xie, D.F., Ma, Z.D., Jiang, Y.S., 1999. Stratigraphic sequence and tectonic evolution in passive continental margin, Jinshajiang belt, northwestern Yunnan Province, China. Earth Sci.-J. China Univ. Geosci. 24, 553–557 (in Chinese). Fisher, R., 1953. Dispersion on a sphere. Proc. Roy. Soc. Lond. A217, 295–305. Funahara, S., Nishiwaki, N., Murata, F., Otofuji, Y., Wang, Y.Z., 1993. Clockwise rotation of the Red River fault inferred from paleomagnetic study of Cretaceous rocks in the Shan-Thai-Malay block of western Yunnan, China. Earth Planet. Sci. Lett. 117, 29–42.

J. Zhao et al. / Journal of Asian Earth Sciences 111 (2015) 350–364 Gan, W.J., Zhang, P.Z., Shen, Z.K., Niu, Z.J., Wang, M., Wan, Y.G., Zhou, D.M., Cheng, J., 2007. Present-day crustal motion within the Tibetan Plateau inferred from GPS measurements. J. Geophys. Res.: Sol. Earth 112, B08416. Golonka, J., 2007. Late Triassic and Early Jurassic palaeogeography of the world. Palaeogeogr. Palaeoclimatol. Palaeoecol. 244, 297–307. Hisada, K.-I., Ueno, K., Sugimaya, T., Nagai, K., Wang, X.D., 2001. Confirmation of Dropstones in the Dingjiazhai Formation of the Gondwana-Derived Baoshan Block, West Yunnan. Gondwana Res. 4, 630–631. Huang, K.N., Opdyke, N.D., 1991. Paleomagnetic results from the Upper Carboniferous of the Shan-Thai-Malay Block of Western Yunnan, China. Tectonophysics 192, 333–344. Huang, K.N., Opdyke, N.D., 1993. Paleomagnetic results from Cretaceous and Jurassic rocks of South and Southwest Yunnan: evidence for large clockwise rotations in the Indochina and Shan-Thai-Malay terranes. Earth Planet. Sci. Lett. 117, 507–524. Huang, B.C., Zhou, Y.X., Zhu, R.X., 2008a. Discussions on Phanerozoic evolution and formation of continental China, based on paleomagnetic studies. Earth Sci. Front. 15, 348–359 (in Chinese). Huang, H., Jin, X.C., Shi, Y.K., Yang, X.N., 2008b. Middle Permian Western Tethyan Fusulinids from Southern Baoshan Block, Western Yunnan, China. J. Paleontol. 83, 880–896. Jian, P., Liu, D.Y., Kröner, A., Zhang, Q., Wang, Y.Z., Sun, X.M., Zhang, W., 2009. Devonian to Permian plate tectonic cycle of the Paleo-Tethys Orogen in southwest China (I): geochemistry of ophiolites, arc/back-arc assemblages and within-plate igneous rocks. Lithos 113, 748–766. Jin, X.C., 2002. Permo-Carboniferous sequences of Gondwana affinity in southwest China and their paleogeographic implications. J. Asian Earth Sci. 20, 633–646. Jin, X.C., Huang, H., Shi, Y.K., Zhan, L.P., 2011. Lithologic boundaries in Permian postglacial sediments of the Gondwana-affinity Regions of China: typical sections, age range and correlation. Acta Geol. Sin. – Engl. Ed. 85, 373–386. Johnson, C.L., Constable, C.G., Tauxe, L., Barendregt, R., Brown, L.L., Coe, R.S., Layer, P., Mejia, V., Opdyke, N.D., Singer, B.S., Staudigel, H., Stone, D.B., 2008. Recent investigations of the 0–5 Ma geomagnetic field recorded by lava flows. G-Cubed 9. http://dx.doi.org/10.1029/2007GC001696. Kamata, Y., Hisada, K., Nakornsri, N., Charusiri, P., Sashida, K., Ueno, K., 2002. Triassic radiolarian faunas from the Mae Sariang area, northern Thailand and their paleogeographic significance. J. Asian Earth Sci. 20, 491–506. Kornfeld, D., Eckert, S., Appel, E., Ratschbacher, L., Pfänder, J., Liu, D.L., Ding, L., 2014. Clockwise rotation of the Baoshan Block due to southeastward tectonic escape of Tibetan crust since the Oligocene. Geophys. J. Int. 197 (1), 149–163. http:// dx.doi.org/10.1093/gji/ggu009. Krischvink, J.L., 1980. The least-squares line and plane and the analysis of palaeomagnetic data. Geophys. J. Roy. Astron. Soc. 62, 699–718. Kruiver, P.P., Dekkers, M.J., Heslop, D., 2001. Quantification of magnetic coercivity components by the analysis of acquisition curves of isothermal remanent magnetisation. Earth Planet. Sci. Lett. 189, 269–276. Lai, C.-K., Meffre, S., Crawford, A.J., Zaw, K., Xue, C.-D., Halpin, J.A., 2014. The Western Ailaoshan Volcanic Belts and their SE Asia connection: a new tectonic model for the Eastern Indochina Block. Gondwana Res. 26, 52–74. Leloup, P.H., Lacassin, R., Tapponnier, P., Schärer, U., Zhong, D.L., Liu, X.H., Zhang, L.S., Ji, S.C., Trinh, P.T., 1995. The Ailao Shan-Red River shear zone (Yunnan, China), Tertiary transform boundary of Indochina. Tectonophysics 251, 3–84. Lepvier, C., Maluski, H., Vu, V.T., Leyreloup, A., Phan, T.T., Nguyen, V.V., 2004. The Early Triassic Indosinian orogeny in Vietnam (Truong Sone Belt and Kontum Massif); implications on the geodynamic evolution of Indochina. Tectonophysics 393, 87–118. Li, P.W., Gao, R., Cui, J.W., Ye, G., 2004. Paleomagnetic analysis of eastern Tibet: implications for the collisional and amalgamation history of the Three Rivers Region, SW China. J. Asian Earth Sci. 24, 291–310. Li, S.H., Huang, B.C., Zhu, R.X., 2012. Paleomagnetic constraints on the tectonic rotation of the southeastern margin of the Tibetan Plateau. Chin. J. Geophys. 55 (1), 76–94. http://dx.doi.org/10.6038/j.isn.0001-5733.2012.01.008 (in Chinese). Liao, Z.T., Chen, Y.K., Wei, Z.H., Li, M.H., 2003. Tectonic evolution sine Late Palaeozoic Era in West Yunnan. J. Tongji Univ. 31, 1029–1033 (in Chinese). Lin, J.L., Fuller, M., 1990. Palaeomagnetism, North China and South China Collision, and the Tan-Lu Fault. Philos. Trans. Roy. Soc. A331, 589–598. http://dx.doi.org/ 10.1098/rsta.1990.0091. Liu, B.P., Feng, Q.L., Fang, N.Q., Jia, J.H., He, F.X., 1993. Tectonic evolution of PaleoTethys Poly-Island-Ocean in Changning-Menglian and Lancangjiang Belts, Southwestern Yunnan, China. Earth Sci.-J. China Univ. Geosci. 18, 529–539 (in Chinese). Liu, J., Tran, M.-D., Tang, Y., Nguyen, Q.-L., Tran, T.-H., Wu, W., Chen, J., Zhang, Z., Zhao, Z., 2012. Permo-Triassic granitoids in the northern part of the Truong Son belt, NW Vietnam: geochronology, geochemistry and tectonic implications. Gondwana Res. 22, 628–644. Liu, H.C., Wang, Y.J., Cawood, P.A., Fan, W.M., Cai, Y.F., Xing, X.W., 2015. Record of Tethyan ocean closure and Indosinian collision along the Ailaoshan suture zone (SW China). Gondwana Res. (in press). http://dx.doi.org/10.1016/j.gr.2013.12.013. Maluski, H., Lepvrier, C., Leyreloup, A., Vu, V.T., Phan, T.T., 2005. 40Ar-39Ar geochronology of the charnockites and granulites of the Kan Nack complex, Kon Tum Massif, Vietnam. J. Asian Earth Sci. 25, 653–677. McElhinny, M.W., 1964. Statistical significance of the fold test in palaeomagnetism. Geophys. J. Roy. Astron. Soc. 8, 338–340. McFadden, P.L., McElhinny, M.W., 1988. The combined analysis of remagnetization circles and direct observations in palaeomagnetism. Earth Planet. Sci. Lett. 87, 161–172.

363

McFadden, P.L., McElhinny, M.W., 1990. Classification of the reversal test in palaeomagnetism. Geophys. J. Int. 103, 725–729. Metcalfe, I., 1996. Gondwanaland dispersion, Asian accretion and evolution of eastern Tethys. Aust. J. Earth Sci. 43, 605–623. Metcalfe, I., 2002. Permian tectonic framework and palaeogeography of SE Asia. J. Asian Earth Sci. 20, 551–566. Metcalfe, I., 2006. Paleozoic and Mesozoic tectonic evolution and palaeogeography of East Asian crustal fragments: the Korean Peninsula in context. Gondwana Res. 9, 24–46. Metcalfe, I., 2013. Tectonic evolution of the Malay Peninsula. J. Asian Earth Sci. 76, 195–213. Morley, C.K., 2002. A tectonic model for the Tertiary evolution of strike–slip faults and rift basins in SE Asia. Tectonophysics 347, 189–215. Nam, T.N., Sano, Y., Terada, K., Toriumi, M., Van Quynh, P., Dung, L.T., 2001. First SHRIMP U–Pb zircon dating of granulites from the Kontum massif (Vietnam) and tectonothermal implications. J. Asian Earth Sci. 19, 77–84. Otofuji, Y., Yokoyama, M., Kitada, K., Zaman, H., 2010. Paleomagnetic versus GPS determined tectonic rotation around eastern Himalayan syntaxis in East Asia. J. Asian Earth Sci. 37, 438–451. Otofuji, Y., Tung, V.D., Fujihara, M., Tanaka, M., Yokoyama, M., Kitada, K., Zaman, H., 2012. Tectonic deformation of the southeastern tip of the Indochina Peninsula during its southward displacement in the Cenozoic time. Gondwana Res. 22, 615–627. Richter, B., Fuller, M. 1996. Paleomagnetism of the Sibumasu and Indochina blocks: implications for the extrusion tectonic model. In: Hall, R., Blundell, D. (Eds.), Tectonic Evolution of Southeast Asia, Geological Society Special Publication No. 106, pp. 203–224. Sato, K., Liu, Y.Y., Zhu, Z.C., Yang, Z.Y., Otofuji, Y., 1999. Paleomagnetic study of middle Cretaceous rocks from Yunlong, western Yunnan, China: evidence of southward displacement of Indochina. Earth Planet. Sci. Lett. 165, 1–15. Sato, K., Liu, Y., Wang, Y., Yokoyama, M., Yoshioka, S., Yang, Z., Otofuji, Y., 2007. Paleomagnetic study of Cretaceous rocks from Pu’er, western Yunnan, China: evidence of internal deformation of the Indochina block. Earth Planet. Sci. Lett. 257, 1–15. Scotese, C.R., 2004. A continental drift flipbook. J. Geol. 112, 729–741. Sengör, A.M.C., 1987. Tectonics of the Tethysides: orogenic collage development in a collisional setting. Annu. Rev. Earth Planet. Sci. 15, 213–244. Shen, Z.K., Lü, J.N., Wang, M., Bürgmann, R., 2005. Contemporary crustal deformation around the southeast borderland of the Tibetan Plateau. J. Geophys. Res.: Sol. Earth 110, B11409. Shi, G.R., Archbold, N.W., 1998. Permian marine biogeography of SE Asia. In: Robert, H., Hollyway, J.D. (Eds.), Biogeography and Geological Evolution of SE Asia. Backhys Publishers, Leiden, The Netherlands, pp. 57–72. Sone, M., Metcalfe, I., 2008. Parallel Tethyan sutures in mainland Southeast Asia: new insights for Palaeo-Tethys closure and implications for the Indosinian orogeny. C.R. Geosci. 340, 166–179. Stampfli, G.M., 2013. Response to the comments on ‘‘The formation of Pangea’’ by D.A. Ruban. Tectonophysics 608, 1445–1447. Stewart, S.A., Jackson, K.C., 1995. Palaeomagnetic analysis of fold closure growth and volumetrics. Geol. Soc. Lond. Spec. Publ. 98, 283–295. Su, L., Yang, Z.Y., Sun, Z.M., Yang, T.S., Zaman, H., Takemoto, K., Otofuji, Y., 2005. Regional deformational features of the South China Block inferred from Middle Triassic palaeomagnetic data. Geophys. J. Int. 162, 339–356. Takemoto, K., Halim, N., Otofuji, Y., Tri, T.V., De, L.V., Hada, S., 2005. New paleomagnetic constraints on the extrusion of Indochina: late Cretaceous results from the Song Da Terrane, northern Vietnam. Earth Planet. Sci. Lett. 229, 273–285. Takemoto, K., Sato, S., Chanthavichith, K., Inthavong, T., Inokuchi, H., Fujihara, M., Zaman, H., Yang, Z.Y., Yokoyama, M., Iwamoto, H., Otofuji, Y., 2009. Tectonic deformation of the Indochina Peninsula recorded in the Mesozoic palaeomagnetic results. Geophys. J. Int. 179, 97–111. Tanaka, K., Mu, C.L., Sato, K., Takemoto, K., Miura, D., Liu, Y.Y., Haider, Z., Yang, Z.Y., Yokoyama, M., Hisanori, I., Uno, K., Otofuji, Y.-I., 2008. Tectonic deformation around the eastern Himalayan syntaxis: constraints from the Cretaceous palaeomagnetic data of the Shan-Thai Block. Geophys. J. Int. 175, 713–728. Tapponnier, P., Lacassin, R., Leloup, P.H., Scharer, U., Zhong, D.L., Wu, H.W., Liu, X.H., Ji, S.C., Zhang, L.S., Zhong, J.Y., 1990. The Ailao Shan/Red River metamorphic belt: Tertiary left-lateral shear between Indochina and South China. Nature 343, 431–437. Tong, Y.B., Yang, Z.Y., Zheng, L.D., Xu, Y.L., Wang, H., Gao, L., Hu, X.-Z., 2013. Internal crustal deformation in the northern part of Shan-Thai Block: new evidence from paleomagnetic results of Cretaceous and Paleogene redbeds. Tectonophysics 608, 1138–1158. Torsvik, T.H., Van der Voo, R., Preeden, U., Niocaill, C.M., Steinberger, B., Doubrovine, B.V., van Hinsbergen, D.J.J., Domeier, M., Gaina, C., Tohver, E., Meert, J.G., McCausland, P.J.A., Cocks, R.M., 2012. Phanerozoic polar wander, palaeogeography and dynamics. Earth Sci. Rev. 114, 325–368. Ueno, K., 2003. The Permian fusulinoidean faunas of the Sibumasu and Baoshan blocks: their implications for the paleogeographic and paleoclimatologic reconstruction of the Cimmerian Continent. Palaeogeogr. Palaeoclimatol. Palaeoecol. 193, 1–24. Van der Voo, R., 1990. The reliability of paleomagnetic data. Tectonophysics 184, 1–9. Wang, Y.J., Fan, W.M., Zhang, Y.H., Peng, T.P., Chen, X.Y., Xu, Y.G., 2006. Kinematics and 40Ar/39Ar geochronology of the Gaoligong and Chongshan shear systems, western Yunnan, China: implication for early Oligocene tectonic extrusion of SE Asia. Tectonophysics 418, 235–254.

364

J. Zhao et al. / Journal of Asian Earth Sciences 111 (2015) 350–364

Wang, X.D., Sugiyama, T., 2002. Permian coral faunas of the eastern Cimmerian Continent and their biogeographical implications. J. Asian Earth Sci. 20, 589–597. Wang, X.D., Sugimaya, T., Nagai, K., 2001a. Permo-Carboniferous Coral Faunas from Paleo-Tethys in the Changning-Menglian Belt, West Yunnan, Southwest China. Gondwana Res. 4, 818–819. Wang, X.D., Ueno, K., Mizuno, Y., Sugiyama, T., 2001b. Late Paleozoic faunal, climatic, and geographic changes in the Baoshan block as a Gondwana-derived continental fragment in southwest China. Palaeogeogr. Palaeoclimatol. Palaeoecol. 170, 197–218. Wang, X.D., Shi, G.R., Sugiyama, T., 2002. Permian of West Yunnan, Southwest China: a biostratigraphic synthesis. J. Asian Earth Sci. 20, 647–656. Wang, Y.J., Zhang, A.M., Fan, W.M., Peng, T.P., Zhang, F.F., Zhang, Y.H., Bi, X.W., 2010. Petrogenesis of late Triassic post-collisional basaltic rocks of the Lancangjiang tectonic zone, southwest China, and tectonic implications for the evolution of the eastern Paleotethys: geochronological and geochemical constraints. Lithos 120, 529–546. Watson, G.S., Enkin, R.J., 1993. The fold test in paleomagnetism as a parameter estimation problem. Geophys. Res. Lett. 20, 2135–2137. Wopfner, H., 1996. Gondwana origin of the Baoshan and Tengchong terranes of West Yunnan, in Tectonic Evolution of Southeast Asia. In: Hall, R., Blundell, D. (Eds.), Geol. Soc. London, Special Publications, vol. 106, pp. 539–547.

Yang, Z.Y., Besse, J., 1993. Paleomagnetic study of Permian and Mesozoic Sedimentary-Rocks from Northern Thailand Supports the Extrusion Model for Indo-China. Earth Planet. Sci. Lett. 117, 525–552. Yang, Z.Y., Besse, J., 2001. New Mesozoic apparent polar wander path for south China: Tectonic consequences. J. Geophys. Res.: Sol. Earth 106, 8493–8520. Yang, Z.Y., Yin, J.Y., Sun, Z.M., Otofuji, Y., Sato, K., 2001. Discrepant Cretaceous paleomagnetic poles between Eastern China and Indochina: a consequence of the extrusion of Indochina. Tectonophysics 334, 101–113. Yunnan Bureau of Geology and Mineral Resources (YBGMR), 1984. Geological map (1:200000) of the People’s Republic of China, Nansan sheet (F-47-III) and Gengma sheet (F-47-IV). Zhu, J.J., Hu, R.Z., Bi, X.W., Zhong, H., Chen, H., 2011. Zircon U–Pb ages, Hf–O isotopes and whole-rock Sr–Nd–Pb isotopic geochemistry of granitoids in the Jinshajiang suture zone, SW China: constraints on petrogenesis and tectonic evolution of the Paleo-Tethys Ocean. Lithos 126, 248–264. Zijderveld, J.D.A., 1967. A.C. demagnetization of rocks: analysis of results. In: Collinson, D.W., Greer, K.M., Runcorn, S.K. (Eds.), Methods on Paleomagnetism. Elsevier, Amsterdam.