Large southward motion and clockwise rotation of Indochina throughout the Mesozoic: Paleomagnetic and detrital zircon U–Pb geochronological constraints

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Large southward motion and clockwise rotation of Indochina throughout the Mesozoic: Paleomagnetic and detrital zircon U–Pb geochronological constraints Yonggang Yan a,b , Baochun Huang c,a,∗ , Jie Zhao f , Donghai Zhang c , Xiaohui Liu d,b , Punya Charusiri e , Apivut Veeravinantanakul e a

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 c Key Laboratory of Orogenic Belt and Crust Evolution, Ministry of Education, School of Earth and Space Sciences, Peking University, Beijing 100871, China d Key Laboratory of Continental Collision and Plateau Uplift, Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Beijing 100101, China e Earthquake and Tectonic Geology Research Unit (EATGRU), c/o Department of Geology, Faculty of Science, Chulalongkorn University, Bangkok 10330, Thailand f State Key Laboratory of Continental Dynamics, Department of Geology, Northwest University, Northern Taibai Str. 229, Xi’an 710069, China b

a r t i c l e

i n f o

Article history: Received 21 June 2016 Received in revised form 15 November 2016 Accepted 20 November 2016 Available online xxxx Editor: A. Yin Keywords: paleomagnetism U–Pb geochronology Indochina Mesozoic Tectonics

a b s t r a c t We report a combined paleomagnetic and U–Pb geochronologic study of sedimentary rocks from the Huai Hin Lat and Nam Phong formations of Mesozoic age in NE Thailand in order to provide independent constraints on the tectonic movement of the Indochina Block during convergence of the major blocks now comprising East Asia. The maximum allowable depositional age of the two formations is estimated to be 227 Ma and 215 Ma, respectively, from detrital zircon U–Pb geochronologic analysis which also indicates a sediment source transition in the Khorat Plateau Basin during the Middle–Late Jurassic. A formation mean paleomagnetic direction of D g / I g = 21.4◦ /38.1◦ (k g = 19.5, α95 = 9.6◦ ) before and D s / I s = 43.0◦ /48.0◦ (k s = 47.4, α95 = 6.1◦ , N = 13) after tilt correction is derived from samples with different lithologies, bedding attitudes, magnetic carriers and polarities and yields a positive fold test. Hence, the magnetization is likely primary. The revised Mesozoic APWP of the Indochina Block yields paleolatitudes (for a reference site of 22◦ N, 102◦ E) of 33.4 ± 7.2◦ N during the Norian Late Triassic, 25.9 ± 5.9◦ N during the Late Triassic to Early Jurassic, 23.9 ± 8◦ N during the Late Jurassic to Early Cretaceous, 27.5 ± 3.2◦ N during the Early Cretaceous and 24.5 ± 4.9◦ N by the Late Cretaceous; corresponding declinations are 45.2 ± 8.6◦ , 38.0 ± 6.6◦ , 36.3 ± 8.8◦ , 29.6 ± 3.6◦ and 24.9 ± 5.4◦ respectively. These data indicate a significantly southward displacement accompanied by clockwise rotation during the Mesozoic. A reconstruction of the Indochina Block within the now well-studied merging process of South China and North China indicates that the Indochina Block was located at a higher latitude than the South China Block during the Norian stage of Late Triassic times whilst no significant relative poleward displacement apparently occurred during the Early Jurassic to Early Cretaceous interval. Our study supports a post-Cretaceous tectonic extrusion model with a southeastward displacement of Indochina with respect to the South China Block estimated to be 1000 ± 850 km since the Late Cretaceous. © 2016 Elsevier B.V. All rights reserved.

1. Introduction Paleomagnetic studies of the Indochina Block to constrain its Mesozoic geography have been undertaken for several decades and are an essential complement to tectonic reconstructions (Barr and Macdonald, 1979; Bunopas, 1981; Achache and Courtillot, 1985;

*

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.epsl.2016.11.035 0012-821X/© 2016 Elsevier B.V. All rights reserved.

Maranate and Vella, 1986; Yang and Besse, 1993; Richter and Fuller, 1996; Charusiri et al., 2006; Hall et al., 2008; Hall, 2012; Singsoupho et al., 2014). The tectonic consequences of movements of the Indochina Block during the Mesozoic have long been debated because paleo-positions relative to the other Eastern Asia blocks have been drastically transformed by the Cenozoic India– Asia collision. In most reported Pangea reconstruction models, the Indochina Block has been considered to be attached to the South China Block traveling with it during the convergence of Eastern Asia blocks (Collins, 2003; Scotese, 2004; Golonka, 2007; Metcalfe, 2013); the validity of this proposition has not yet been

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properly tested due to a lack of reliable paleomagnetic data from the Indochina Block. Current studies on Pre-Triassic rocks indicate the widespread impact of remagnetization in the Indochina Block (Chen and Courtillot, 1989; Yang and Besse, 1993; Richter and Fuller, 1996), which could be attributed to effects of the Indosinian Orogeny (Ridd et al., 2011). Investigations covering Late Triassic to Early Jurassic rocks show a general agreement between different authors (Achache and Courtillot, 1985; Maranate and Vella, 1986; Yang and Besse, 1993; Bhongsuwan and Elming, 2000; Singsoupho et al., 2014) and a significant southward movement since the Early Jurassic has been suggested. Although studies of Early Jurassic to Late Jurassic units are lacking, studies of Late Jurassic to Early Cretaceous rocks have provided plentiful and apparently reliable paleomagnetic data (Maranate and Vella, 1986; Yang and Besse, 1993; Bhongsuwan and Elming, 2000; Charusiri et al., 2006; Takemoto et al., 2009; Singsoupho et al., 2014). Studies of Middle to Late Cretaceous rocks however, show a discrepancy between different studies which are likely due to internal deformation of the Indochina Block (Takemoto et al., 2005; Charusiri et al., 2006; Otofuji et al., 2012; Singsoupho et al., 2014). Most paleomagnetic studies of the Indochina Block have focused on Jurassic to Cretaceous age rocks, but because Pangea was already fragmenting in the Early Jurassic (ca. 200–180 Ma), reliable paleomagnetic results derived from units prior to the continental breakup are needed to quantify the Mesozoic paleogeography. In this paper, we present new paleomagnetic and U–Pb geochronologic data from Late Triassic sedimentary rocks from the Indochina Block in Thailand and synthesize the data with previous published results with the aim of interpreting the tectonic movement of the Indochina Block relative to the other Eastern Asia blocks. 2. Geologic settings and sampling The Indochina Block is located within the Sundaland domain of Southeast Asia (Fig. 1) and is bounded by a suite of complex suture zones including the Changning–Menglian, and Inthanon (with the latter possibly extending to the Bentong–Raub suture in the eastern Malay Peninsula) to the west, the Song Ma suture zones to the northeast and an indistinct suture to the east which locates offshore in the South China Sea and extends to SW Borneo. This sector formerly comprised the eastern part of Yunnan south of the Red River fault, central Sundaland, eastern Malay Peninsular and SW Borneo (Sone and Metcalfe, 2008; Metcalfe, 2013). The Mesozoic rocks of the Indochina Block in NE Thailand are dominated by continental sediments divided into seven formations based on lithostratigraphy, including (in ascending stratigraphic order) the Huai Hin Lat, Nam Phong, Phu Kradung, Phra Wihan, Sao Khua, Phu Phan and Khok Kruat formations. The Huai Hin Lat Formation overlies deformed Permian or older strata unconformably and is overlain by the Nam Phong Formation which is dominated by sandstone, shale and limestone and is considered to be Early– Middle Norian in age on the basis of a diverse and abundant fauna and flora content (Ridd et al., 2011). The Nam Phong Formation overlies the Huai Hin Lat Formation unconformably and is generally considered to be of Rhaetian Age (Chonglakmani and Sattayarak, 1978; Bunopas, 1992; Racey et al., 1996; Ridd et al., 2011). Oil geologists have suggested that the Nam Phong Formation can be subdivided into the Upper Nam Phong and Lower Nam Phong formations from recognition of an unconformity within the Nam Phong Formation recorded both in subsurface well and seismic sections (Racey et al., 1996, 1999; Ridd et al., 2011). The low-angle unconformity in the Nam Phong Formation is difficult to identify at the redbed outcrops due to similarities in bedding structures and

it is unclear whether the samples collected from the Nam Phong Formation (this study and Yang and Besse, 1993) belong to the Upper or Lower Nam Phong Formation, of Rhaetian and Early to Late Jurassic ages respectively. The overlying member is the Phu Kradung Formation of uncertain age. Carter and Bristow (2003) suggested a Late Jurassic to Early Cretaceous depositional age from Fission Track (FT) and U–Pb geochronology investigations. A recent synthesis (Cuny et al., 2013) suggested a Late Jurassic age for most of the formation based on fossil vertebrate evidence and Early Cretaceous age for the uppermost part. In this paper, the Phu Kradung Formation is considered to be Late Jurassic to Early Cretaceous in age. The overlying successions are the Phra Wihan, Sao Khua, Phu Phan and Khok Kruat formations, generally interpreted to be the Early Cretaceous (Berriasian to Aptian) (Racey et al., 1996; Racey and Goodall, 2009; Ridd et al., 2011). Late Triassic samples were collected from the Huai Hin Lat and Nam Phong formations (very likely formed prior to the breakup of the Pangea) at 13 sites along the main Road 2016–2216 southwest of Wang Saphung (Fig. 2). Samples were drilled using a portable gasoline-powered hand-drill and oriented by magnetic compass or both magnetic and sun compasses when sunshine was available. 45% of the orientation data were oriented by both methods and identified a local magnetization anomaly of −1◦ in agreement with the World Magnetic Model 2015 (Chulliat et al., 2014; see www.magnetic-declination.com). 3. Detrital zircon U–Pb geochronology 3.1. Analytical techniques In the laboratory the field core samples, generally 2.5 cm in diameter and 5–10 cm in length, were cut into standard cylindrical specimens, ∼2.0–2.2 cm in length. Fresh end materials from each of the two formations were used to separate zircons for resolving the formation mean ages. Zircons were separated by conventional heavy liquid and magnetic separation techniques. Transmitted and reflected light images were used to characterize the zircon grains. Detrital zircon U–Pb geochronology was analyzed at the Institute of Tibetan Plateau Research, Chinese Academy of Sciences (Beijing), using a New Wave Research UP193FX Excimer laser coupled with an Agilent 7500a Inductively-Coupled-Plasma Mass Spectrometer (LA-ICP-MS). A laser of 35 μm spot size and ∼8–10 J/cm2 energy was used in this analysis. More detailed analytical procedures and the configuration of the LA-ICP-MS is provided in Cai et al. (2012). In this study, mass fractionation and instrumental bias were monitored using the Plesovice zircon (Slama et al., 2008) and NIST SRM 610 silicate glass standards (Pearce et al., 1997). The Harvard zircon 91500 with the recommended age 1064 ± 0.3 Ma (Wiedenbeck et al., 1995) was used as an external standard. Ages were calculated using GLITTER 4.0 (Griffin et al., 2008). Common Pb corrections were made following the method described by Andersen (2002). The relative age-probability diagrams were generated using the Isoplot Program 4 (Ludwig, 2008) and showed each age and distribution at the 2-σ level, after the exclusion of analyses with discordance >10%. The U–Pb geochronologic results in this study are summarized in Supplementary data. 3.2. Results 3.2.1. Huai Hin Lat Formation (sample HHL24X) More than 1000 detrital zircon grains were separated from end materials of the Huai Hin Lat Formation, of which 100 grains were used to conduct U–Pb geochronology analysis. The sizes of the zircons are generally 80–120 μm and yield good quality results: 99 analyses with discordance <10% yield an age population ranging from Neoarchean to Late Triassic with a major peak at ca.

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Fig. 1. A digital elevation model with schematic tectonic units of Southeast Asia and surrounding regions (modified from Takemoto et al., 2009; Metcalfe, 2013; Burrett et al., 2014). Abbreviations: Fm.: Formation, F.: Fault, E. Malaysia: Eastern Malaysia.

240 Ma and two minor peaks at 330 and 435 Ma, respectively. Ninety-two percent of the analyses are younger than 500 Ma, followed by a great gap (500–1000 Ma), and only 8% of grains are older than 1100 Ma (Fig. 3a, 9a; Supplementary data). This indicates that magmatic rocks from the Indosinian Period are the main sources of zircons in the Huai Hin Lat Formation. Although the youngest zircon grain (HHL24X-024) is 213 ± 2 Ma, the youngest peak (defined as three or more grains contributing to a single age-probability peak at 2-σ level) with a mean age of 227 Ma is interpreted to be the maximum allowable depositional age of the

Huai Hin Lat Formation, consistent with the estimate from paleontologic studies (Fig. 3a_1, 3a_2). 3.2.2. Nam Phong Formation (sample NP23X) The zircon grains separated from the Nam Phong Formation are generally small (ca. 70–100 μm) and a few core-rim structures. One hundred analyses of 96 grains are generally concordant (Fig. 3b) and only 4 analyses which are >10% discordant are excluded from our interpretation. Ninety-six analyses yield an age distribution ranging from Early Paleoproterozoic to Late Triassic with a major

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Fig. 2. Geological map of the sampling area in Northeast Thailand (revised from 1:5,000,000 geological map of Thailand, DMR, 2011; Ridd et al., 2011). Locations-sites: 1 – Th 232, Th233; 2 – Th240, Th241, Th242, Th243, Th244, Th245; 3 – Th237a. Th238, Th239; 4 – Th42; 5 – Th44; 6 – Th45; 7 – Th46; 8 – Th13; 9 – Th12; 10 – Th16; 11 – Th15; 12 – site 4; 13 – site 5; 14 – site 14; 15 – site 3; 16 – site 19, see Table 1 and 2. Abbreviations: post-K: post Cretaceous, pre-P: Pre Permian, Ku: Upper Cretaceous, Kl: Lower Cretaceous, Ju: Upper Jurassic, Jl: Lower Jurassic, Tu: Upper Triassic.

Table 1 Summary of sampling information and characteristic remanence from the Huai Hin Lat Formation rocks in the Indochina Block. Reference

N

Is (◦ )

ϕp

Strike/dip

Ds (◦ )

λp

ϕs

Ig (◦ )

α95

λs

Dg (◦ )

κ

Site ID Th232 Th233 Th240 Th241 Th242 Th243 Th244 Th245 Th12 3 4 5 19

16.9564 16.9619 17.0491 17.0489 17.0488 17.0495 17.0492 17.0484 16.67 16.67 16.67 16.67 16.67

101.3407 101.3573 101.4630 101.4634 101.4632 101.4628 101.4630 101.4633 101.83 101.83 101.83 101.83 101.83

36/28 43/30 45/36 40/35 46/36 38/34 42/35 44/38 152/36 103/10 105/12 110/20 20/13

7 7 9 9 9 9 9 9 8 13 12 14 9

26.8 38.1 5.6 11.6 9.6 5.3 353.1 6.9 231.0 36.7 32.8 32.5 29.1

63.9 44.5 29.1 41.3 44.0 41.0 28.0 31.2 −13.9 46.2 31.5 29.7 35.9

75.7 65.7 31.4 47.4 52.1 40.6 16.6 36.7 227.0 41.6 35.6 36.6 37.0

55.7 39.6 46.0 48.2 54.4 50.5 50.0 46.5 −43.2 54.6 41.8 49.1 33.1

130.6 64.0 36.0 131.8 244.2 77.1 82.5 120.9 23.4 69.7 11.1 79.0 60.0

5.3 7.6 8.7 4.5 3.3 5.9 5.7 4.7 11.7 5.0 13.6 4.5 6.7

21.3 28.4 59.3 45.1 40.5 50.6 69.6 54.6 45.4 48.7 55.9 54.0 54.7

158.3 174.5 166.3 166.9 159.7 162.7 146.2 167.3 172.2 157.1 173.3 163.4 184.0

this study this study this study this study this study this study this study this study Yang and Besse (1993) Chen and Courtillot (1989) Chen and Courtillot (1989) Chen and Courtillot (1989) Chen and Courtillot (1989)

13 13

21.4

19.5 47.4

9.6 6.1

K = 33.9

A 95 = 7.2

48.7

165.9

Huai Hin Lat Formation – Norian of Upper Triassic (◦ N)

Mean of sites Mean of VGPs

(◦ E)

13

38.1 43.0

48.0

(◦ )

(◦ N)

(◦ E)

Abbreviations: λs /ϕs : latitude and longitude of sampling site; N: number of samples or sites used in the mean calculation; Strike/dip: strike azimuth and dip of bed; D g , I g (D s , I s ): declination and inclination before (after) tilt correction; κ & α95 : precision parameter and the radius of the cone of 95% confidence for mean-direction; λ p /ϕ p : latitude and longitude of virtual geomagnetic pole (VGP); K & A 95 : precision parameter and the radius of the cone of 95% confidence for formation mean VGP. (1) McElhinny’ fold test (1964): ks/kg = 2.43 > F(24,24) = 1.98; indicate positive fold test at 95% confidence level. (2) McFadden’s fold test (1990): ξ2 = 9.193 before and ξ2 = 0.067 after tilt correction, critical ξ , at 95% = 4.200 and at 99% = 5.860; indicate positive fold test at 99% confidence level. (3) Watson and Enkin’s fold test (1993): optimal concentration at 101.2 ± 11.7% unfolding percentage, indicate positive fold test.

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Table 2 Summary of sampling information and characteristic remanence from the Nam Phong Formation rocks in the Indochina Block. Nam Phong Formation – Rhaetian of Upper Triassic to Lower Jurassic Site ID

λs

ϕs

Strike/dip

N

Dg (◦ )

Ig (◦ )

Ds (◦ )

Is (◦ )

Th237a Th238 Th239 Th13 Th14 Th15 Th16 Th42 Th44 Th45 Th46

17.06869 17.06961 17.06917 16.67 16.67 16.67 16.67 16.67 16.67 16.67 16.67

101.5977 101.5984 101.5985 101.83 101.83 101.83 101.83 101.83 101.83 101.83 101.83

329/15 333/15 333/15 160/40 110/25 110/3 240/8 280/16 81/32 100/21 168/30

8 7 6 9 7 5 8 7 9 6 4

16.6 40.4 28.5 232.7 203.2 197.7 53.6 55.4 32.6 32.7 50.6

43.8 47.6 45.0 −2.3 −9.3 −35.1 39.1 55.4 14.2 20.7 22.9

23.9 44.9 34.8 226.7 203.8 197.6 47.3 43.1 44.0 38.0 38.4

32.0 33.6 32.1 −40.7 −30.8 −38.0 37.7 42.4 36.3 39.7 48.2

11 11

36.5

(◦ N)

Mean of sites Mean of VGPs

(◦ E)

31.3 36.4

11

37.8

κ

α95

λp

ϕp

Reference

31.3 59.3 28.1 11.8 16.4 16.1 42.4 29.0 144.3 69.4 25.1

11.2 7.9 14.3 20.0 14.5 23.5 8.6 11.4 4.3 8.1 18.7

67.2 47.3 56.8 45.8 67.3 72.7 45.2 49.1 48.2 53.8 52.7

187.3 182.8 185.7 175.4 183.6 173.4 178.4 173.4 180.0 176.3 165.2

this study this study this study Yang and Besse Yang and Besse Yang and Besse Yang and Besse Yang and Besse Yang and Besse Yang and Besse Yang and Besse

14.7 68.5

12.3 5.6

K = 61.5

A 95 = 5.9

55.2

178.0

(◦ )

(◦ N)

(◦ E)

(1993) (1993) (1993) (1993) (1993) (1993) (1993) (1993)

Abbreviations are given in Table 1. (1) McElhinny’ fold test (1964): ks/kg = 4.66 > F(20,20) = 2.94; indicate positive fold test at 99% confidence level. (2) McFadden’s fold test (1990): ξ2 = 9.750 before and ξ2 = 1.702 after tilt correction, critical ξ , at 95% = 3.865 and at 99% = 5.378; indicate positive fold test at 99% confidence level. (3) Watson and Enkin’s fold test (1993): optimal concentration at 95.5 ± 11.8% unfolding percentage, indicate positive fold test.

peak at ca. 240 Ma, three minor peaks at ca. 350, 480 and 2400 Ma and a broad age group between ca. 800 and 1200 Ma (Fig. 6b; Supplementary data). The youngest ages peak (215 Ma) is considered to be the maximum allowable depositional age of the Nam Phong Formation (Fig. 3b_1, 3b_2). 4. Paleomagnetic analysis 4.1. Laboratory methods Representative fresh end material was selected for rock magnetic analysis to identify the composition and domain state of the dominant magnetic carriers. Thermomagnetic analyses (susceptibility versus temperature) were conducted on representative samples in argon from room temperature to 700 ◦ C using an AGICO Multi-function Kappabridge (MFK1). Acquisition and back-field demagnetization of Isothermal Remanent Magnetization (IRM) and hysteresis loop measurements were carried out using a Micromag 3900 Vibrating Sample Magnetometer at room temperature with an applied field range of ±1.0 T or ±1.5 T. Following the evaluation of rock magnetic behaviors, thermal demagnetization was applied to the standard cylindrical specimens using an ASC Model TD-48 oven (residual magnetic field <10 nT) and a 2G-755R cryogenic magnetometer in the Paleomagnetism and Geochronology Laboratory (PGL) of the Institute of Geology and Geophysics, Chinese Academy of Sciences. The magnetometer is installed in a magnetically shielded room with the background field weaker that 300 nT. Demagnetization results were processed using principle component analysis (Kirschvink, 1980) or great-circle fitting of magnetizations (McFadden and McElhinny, 1988), and mean directions were calculated using Fisher statistics (Fisher, 1953). 4.2. Results

Fig. 3. U–Pb Concordia diagrams and age histogram for detrital zircons from the Huai Hin Lat and Nam Phong Formations. (a_1) and (a_2), (b_1) and (b_2): Concordia diagrams and age histograms between 200–400 Ma of the Huai Hin Lat, Nam Phong Formations.

The results can be divided into two categories according to compositions of the magnetic minerals. First, in the sandstones from both the Huai Hin Lat and Nam Phong formations, thermomagnetic analyses show a significant drop of susceptibility at 550–600 ◦ C and ∼675 ◦ C (Fig. S1a, S1b); the wasp-waist hysteresis curves (Fig. S2a, S2e) suggest the presence of components with contrasting coercivities (Roberts et al., 1995; Tauxe et al., 1996). Coercivity spectra analyses (Heslop et al., 2002) were conducted on

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the IRM curves which increase gradually and do not saturate completely even when the applied field reached 1.5 T (Fig. S2b, S2f). These curves reveal two distinct components, of which the mean coercivities were located in the ranges 20–30 mT and 400–500 mT (Fig. S2d, S2h), respectively. These observations indicate that magnetic remanence is dominated by both magnetite and hematite in the first group. The limestone and siltstone samples from the Huai Hin Lat Formation constitute the second group. Those samples saturate at approximately 300 mT in the IRM acquisition curves (Fig. S2j) and the hysteresis curves close at very low applied fields (less than 200 mT) (Fig. S2i). Those samples have Bcr/Bc values of 2.02–2.84 and Mr/Ms values of 0.07–0.13, and coercivity spectra analysis indicates mainly one coercivity component at ∼30 mT (Fig. S2l and see Dunlop, 2002). Thermomagnetic analyses show a substantial drop of susceptibility at ∼580 ◦ C (Fig. S1c, S1d). Therefore, the rock magnetic analyses indicate that the magnetic carriers of the second group are dominated by low-coercivity PSD magnetite. 4.2.1. Huai Hin Lat Formation (Norian stage of Upper Triassic) One hundred and five pieces of samples were collected from 10 sites of the Huai Hin Lat Formation, and stable characteristic remanences (ChRM) were isolated from 8 sites (Table 1). Magnetization carriers in site Th232 and Th233 are magnetite and hematite according to the rock magnetization analyses. After the elimination of a low temperature component, high temperature components isolated between 500 ◦ C and 690 ◦ C that decay toward the origin of orthogonal vector plots are used to calculate the site mean direction (Fig. S3a–c). The samples of sites Th240, Th241, Th242, Th243, Th244 and Th245 are mainly siltstones and limestones. Following a very low temperature component removed below 65 ◦ C, a relatively low unblocking temperature is observed. Commonly, 80% of the NRM is erased by 450 ◦ C and completely at approximately 500 ◦ C for most samples or 540–590 ◦ C for a few. Medium to high temperature components isolated between 200 ◦ C and 500 ◦ C (or higher) are most stable and are used to calculate the ChRM directions (Fig. S3d–g). In addition, five other high quality sites (Th12, 3, 4, 5, 19, see Table 1; Fig. 4a, 4b) reported from this area (Achache and Courtillot, 1985; Chen and Courtillot, 1989; Yang and Besse, 1993) share similar demagnetization characteristics with our sites Th240, Th241, Th242, Th243, Th244 and Th245 of this study. All the thirteen sites are located within a limited area between 16.67–17.05◦ N, 101.34–101.83◦ E in the Loei–Phetchabun fold belt. Underlying them is the strongly deformed Permian strata. Our eight new sites have much steeper bedding dip angles, as well as different bedding dip azimuths, enhancing their value for application of the paleomagnetic fold test (Table 1; Fig. 4c). We include all thirteen sites in the calculation of the formation mean direction yielding a direction of D g / I g = 21.4◦ /38.1◦ (k g = 19.5, α95 = 9.6◦ ) before and D s / I s = 43.0◦ /48.0◦ (k s = 47.4, α95 = 6.1◦ , N = 13) after tilt correction (Fig. 4a, 4b). The data cluster is significantly improved after tilt correction with ks/kg = 2.43 > F (24, 24) = 1.98 (McElhinny, 1964). The fold test identifies a positive result at the 95% confidence level (in-situ ξ2 = 9.193, tilt-corrected ξ2 = 0.067, ξ2 critical value = 4.200 at 95% confidence level for N = 13) (McFadden, 1990); further analysis by stepwise tectonic correction (Watson and Enkin, 1993) yields an optimal clustering at unfolding of 101.2% with 95% uncertainty ranging from 89.2% to 112.7% (Fig. 4c) and indicates that the ChRM is a pre-folding magnetization. 4.2.2. Nam Phong Formation (Rhaetian stage of Upper Triassic to Jurassic) Yang and Besse (1993) studied the Nam Phong Formation and suggested a primary magnetization measured in 8 sites, includ-

ing five with normal polarity and three with reversal polarity. Aiming to confirm the consistency of paleomagnetic results reported by different workers, we have collected four additional sites and isolated ChRMs from three of them (Th237a, Th238, Th239; Table 2). Following removal of low temperature components by 300 ◦ C, medium to high temperature components isolated between 300 ◦ C to 670 ◦ C or 690 ◦ C are interpreted to comprise the ChRMs. Site mean directions were calculated by combining principle component analyses (Fig. S3i; Kirschvink, 1980) and remagnetization great-circles (Fig. S3i; McFadden and McElhinny, 1988). The magnetic directions of the three sites (Th237a, Th238, Th239) are not significantly different from the eight sites reported by Yang and Besse (1993). All the eleven sites are located within a limited area between 16.67–17.07◦ N, 101.60–101.83◦ E in the Loei–Phetchabun fold belt which is formed during the Indosinian Orogeny. By adding our three new sites, the new formation mean direction of the eleven sites is D g / I g = 36.5◦ /31.3◦ (k g = 14.7, α95 = 12.3◦ ) before and D s / I s = 36.4◦ /37.8◦ (k s = 68.5, α95 = 5.6◦ , N = 11) after tilt correction (Fig. 4d, 4e). The data clustering improves after tilt correction with ks/kg = 4.66 > F(20,20) = 2.94 (McElhinny, 1964) and the data pass a fold test (McFadden, 1990) at 99% confidence level with ξ2 statistic equal to 9.750 before and 1.702 after tilt correction; the critical value is 5.378 at the 99% confidence level. Further analysis yields an optimal clustering with 95.5% unfolding and 95% uncertainty ranging from 83.6% to 107.2% (Fig. 4f) (Watson and Enkin, 1993). Aiming to better constrain the age of the ChRMs, sites distribution analysis of the Huai Hin Lat and Nam Phong formations is carried out which is shown in Fig. 2. The bedding dip azimuth of the fourteen sites in location 1–5 is southeast or northeast, and the nine sites in location 6–15 dip towards southwest or northwest. The analyzing area located in the Loei–Phetchabun fold belt is interpreted to be a syncline and the folding axis maybe stretch northeast through the Nam Non area. This interpretation is supported by the seismic sections reported in Ridd et al. (2011), in which a syncline is shown for the Mesozoic strata (from Huai Hin Lat to Khok Kruat Formation) in the western Khorat Basin which is located south to the analyzing area. On the basis of the seismic section analysis, the syncline is interpreted to be formed in the Mid-Cretaceous Event, above which the rocks of the Maha SaraKham and Phu Tok formations are not deformed. 4.2.3. Sao Khua Formation (Berriasian to Barremian stage of Lower Cretaceous) Bhongsuwan and Elming (2000) recalculated a formation mean magnetic direction from five new sampling sites and ten sites reported by Yang and Besse (1993), which provided a positive fold test at the 99% confidence level. This mean direction is not significantly different from that reported by Charusiri et al. (2006) calculated from four sampling sites and interpreted to be pre-folding from a positive fold test at the 95% confidence level (McElhinny, 1964). Among the nineteen sites reported, seven are located in the Phu Phan Uplift at the northeast Khorat Basin and twelve at the northwest Khorat Basin. Most of these sites show relatively low dip angles, which indicate a weak deformation history in the Mid-Cretaceous and Himalayan Orogeny events. To get a more convincing result, we provide a new recalculated formation mean direction of D g / I g = 26.4◦ /41.7◦ , (k g = 28.1, α95 = 6.4◦ ) before and D s / I s = 28.6◦ /39.6◦ , (k s = 116.2, α95 = 3.1◦ , N = 19) after tilt correction (Fig. 5a, 5b) by combining all the reported 19 sampling sites (Yang and Besse, 1993; Bhongsuwan and Elming, 2000; Charusiri et al., 2006; see Table S1). The data clustering is improved significantly after tilt correction with ks/kg = 4.14, indicating a positive fold test (McFadden, 1990) at the 99% confidence level with ξ2 statistic of 10.042 before and 5.835 after tilt correction; the critical value is 7.112 at the 99% confidence level.

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Fig. 4. Equal-area projection of the site mean directions of the ChRMs before (a, d) and after (b, e) tilt corrections for the Huai Hin Lat and Nam Phong formations. Mean directions of normal and reversal polarities are marked by blue and green confidence ellipsoids, respectively. Small circles filled with green and red colors are new data in this study and that reported by Achache and Courtillot (1985) and Yang and Besse (1993); (c, f) increment unfolding analysis using Watson and Enkin (1993)’s method, (g) Distribution of Watson (1983)’s V w for the Huai Hin Lat and Nam Phong Formations simulated using the Pmagpy Python software package (version Pmagpy-2.220, see https://earthref.org/PmagPy/). The two data sets are significantly different because Watson’s V w (the dashed blue line) is larger than the V crit value. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Further analysis indicates an optimal clustering at 98.4% unfolding with 95% uncertainty ranging from 90.9% to 105.9% (Fig. 5c) (Watson and Enkin, 1993). Since no detailed demagnetization and field test information were given, the results reported by Maranate and Vella (1986) are not included here in the calculation of the mean direction of the Sao Khua Formation, although their result is not significantly different from the new one. Paleomagnetic studies have been conducted on the Sao Khua Formation by Maranate and Vella (1986), Yang and Besse (1993), Bhongsuwan and Elming (2000) and Charusiri et al. (2006). Yang and Besse (1993) and Bhongsuwan and Elming (2000) considered it to be part of the Upper Jurassic, whereas Maranate and Vella (1986) and Charusiri et al. (2006) regarded it as part of the Lower Cretaceous. According to Hahn (1982), Racey et al. (1996) and Racey and Goodall (2009), a Berriasian–Barremian (Early Cre-

taceous) age suggested by Ridd et al. (2011) is accepted in this study. 4.2.4. Khok Kruat Formation (Aptian stage of Lower Cretaceous) Similar to the Sao Khua Formation, the Khok Kruat Formation in northwest and northeast Khorat Basin has been studied by paleomagnetists for several decades (Maranate and Vella, 1986; Yang and Besse, 1993; Bhongsuwan and Elming, 2000; Charusiri et al., 2006), but no positive fold test has yet been obtained due mainly to prevailing low bedding dip angle (generally less than 10◦ ). We suggest a new recalculated formation mean magnetic direction of D g / I g = 27.3◦ /37.8◦ , (k g = 219.9, α95 = 2.7◦ ) before and D s / I s = 28.9◦ /39.5◦ , (k s = 204.3, α95 = 2.8◦ , N = 14) after tilt correction (see Table S2, Fig. 5d, 5e) by combining ten sites reported by Yang and Besse (1993), three sites by

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Fig. 5. Equal-area projection of the site mean directions of the ChRMs before (a, c) and after (b, d) tilt corrections for the Sao Khua and Khok Kruat Formations. Symbols are same as in Fig. 4. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Charusiri et al. (2006) and one site by Bhongsuwan and Elming (2000). The change of the precision parameter before and after tilt correction is not significant (ks/kg = 0.93) which is due to minimal variations in dip angle. The primary origin of this magnetization is supported by the primary depositional fabric (Fig. S4) discussed by Bhongsuwan and Elming (2000), and indirectly by the positive fold tests from the underlying formations as analyzed in Yang and Besse (1993). 5. Discussion 5.1. Late Triassic U–Pb zircon ages of the Indochina Block The pattern of the zircon age distribution in this study can be compared with the other investigations on the Late Triassic rocks from different areas of the Indochina Block (Fig. 6). Burrett et al. (2014) reported Late Triassic U–Pb zircon data sets from Central Vietnam (Fig. 6c), Truong Son Terrane (Fig. 6d), and Eastern Malaysia (Fig. 6e) which are generally considered to be part of the Indochina Block. In these data sets (Fig. 6a–e), apart from the dominant Late Triassic peaks, the Early Paleozoic minor peak is the most noticeable age distribution area. The other two minor peaks, ca. 900 and 1100 Ma, discovered from the Truong Son and Central Vietnam respectively, are shared by the Nam Phong Formation (Fig. 6b) in this study. The U–Pb geochronology analysis on the rocks from the Saraburi Group (Fig. 6f) shows a major peak at 423.5 Ma (Arboit et al., 2016), which is consistent with the Paleozoic peaks in the Late Triassic data sets (Fig. 6a–e). These characteristics suggest that the dominant Indosinian zircons are very likely derived from the widely exposed magmatic rocks in the Indosinian Orogenic Belt between the Indochina and Sibumasu blocks. Precambrian zircon populations are similar between different areas. We synthesize the Late Triassic U–Pb zircon ages from the Indochina Block (Fig. 6a–e) in Fig. 7f and compare these data with

results from rock units with different temporal or spatial affiliation. The Late Triassic (Fig. 7f) and Middle Jurassic (Fig. 7e) age spectra of Indochina share similar distribution characteristics with the Late Triassic from the Lampang Group (Fig. 7h) in northwestern Thailand and Early Cretaceous from the Loei Belt (Fig. 7g). The age populations are dominated by Indosinian grains and include only very few of Precambrian age. The absence of peaks of 700–800, 1800–1900 and 2300–2500 Ma suggests isolation of Indochina from North China–South China detrital influence, thus the occurrence of these peaks in the Indochina Block after the Late Jurassic implies a transition in the sedimentary source. Noticeably, the Late Jurassic age spectra from the Khorat Plateau (Indochina) (Fig. 7d) shows five peaks (ca. 230–250, 430, 780, 1800, 2400–2500 Ma) and matches well with the age spectra from the Songpan–Ganzi terrane (Fig. 7c) and the South China Block (Fig. 7a, 7b). Stratigraphic studies indicate both eastward and westward-directed paleocurrents in the Nam Phong Formation, whereas southeastward and westward directions in the Phu Kradung to Phu Phan Formation, and westward in the Khok Kruat Formation (Heggemann et al., 1990; Racey et al., 1996; Racey and Goodall, 2009) are consistent with a sedimentary source transition throughout the stratigraphic section. In this study, the source transition is confirmed for the Khorat Plateau (Indochina), which shared common sources with the Loei Belt and Lampang Group to the west before the Middle Jurassic, and with the Songpan–Ganzi Terrane and South China Block after the Late Jurassic. The former source is interpreted to be the widely exposed magmatic rocks in the Indosinian Orogenic belt between the Indochina and Sibumasu blocks, and the later source is presumed to be the Qinling–Dabie Orogen between the South China and North China blocks on the basis of comparative geological studies (Yin and Nie, 1993; Weislogel et al., 2006, 2010; Luo et al., 2014; Liu et al., 2015; Zhang et al., 2015a, 2015b) and the reconstruction of their relative positions in Fig. 10.

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Fig. 6. U–Pb age probability plots for samples from (a) Nam Phong Formation, this study, (b)Huai Hin Lat Formation, this study, (c) Central Vietnam (Burrett et al., 2014), (d) Truong Son terrane (Burrett et al., 2014), (e) Eastern Malaysia (Burrett et al., 2014) and (f) Saraburi Group (Arboit et al., 2016).

5.2. Mesozoic Apparent Polar Wander Path (APWP) of the Indochina Block Paleomagnetists have worked for several decades to build an Apparent Polar Wander Path (APWP) for the Indochina Block. The key Mesozoic paleomagnetic poles were summarized by Maranate and Vella (1986) and revised successively by Chen and Courtillot (1989), Yang and Besse (1993), Bhongsuwan and Elming (2000), Charusiri et al. (2006) and Singsoupho et al. (2014). In this study, we add new data to the Huai Hin Lat and Nam Phong formations and synthesize the key Mesozoic paleomagnetic data of the Indochina Block. 5.2.1. Late Triassic to Early Jurassic Chen and Courtillot (1989) argued that the paleomagnetic results of the Huai Hin Lat Formation reported initially by Achache and Courtillot (1985) might be remagnetized. Their argument was due mainly to the badly defined ChRM of site 8 which was used to conclude the widespread remagnetization of the Khorat basin. To solve this problem, from the top to the bottom of the same sampled section, a sequence of normal, reversed (five samples) and normal polarity is found at site TH12 by Yang and Besse (1993). In this paper, a data set which can pass a positive fold test at 95%

Fig. 7. U–Pb age probability plots for samples of (a) the Late Jurassic (Luo et al., 2014) and (b) Late Triassic (Luo et al., 2014; Zhang et al., 2015a) from the South China Block, (c) Late Jurassic from the Songpan–Ganzi Belt (Weislogel et al., 2006; Zhang et al., 2015b), (d) Late Jurassic from the Khorat area (Burrett et al., 2014), (e) Middle Jurassic from the Cambodian and Khorat area (Burrett et al., 2014), (f) synthesis of the Late Triassic data from the Indochina Block, (g) Early Cretaceous from the Loei Belt (Burrett et al., 2014) and (h) Late Jurassic from the Lampang Group NW Thailand (Gao et al., 2014).

confidence level is obtained from 13 sites, which includes samples of different lithologies, bedding attitudes, magnetic carriers and polarities. It indicates that the ChRM is very likely to be prefolding in origin. A reliable paleomagnetic result from the Nam Phong Formation (Yang and Besse, 1993) has been referred to as the oldest quantitative estimate for interpreting the tectonic movement of the Indochina Block. In this paper, we obtain a new recalculated formation mean direction by adding three new sites to the data set. This new recalculated paleomagnetic pole (55.2◦ N, 178.0◦ E with K = 61.5 and A 95 = 5.9◦ ) is not significantly different from the

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Fig. 8. Equal area projections of the paleomagnetic poles from the (a) Huai Hin Lat and Nam Phong Formations, (b) Phu Kradung and Phra Wihan Formations, (c) Sao Khua Formation and (d) Phu Phan and Khok Khuat Formations. Recalculated formation mean poles are marked with purple color. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

initial result reported by Maranate and Vella (1986) and is similar to the coeval pole from Laos reported by Singsoupho et al. (2014). It becomes a next essential step to test whether the mean directions of the Huai Hin Lat and Nam Phong formations are significantly different from each other, because the distributions of the two data sets, only judging by appearance, are not distinct from each other (Fig. 4b, 4e). When applying a F -test proposed by Watson (1956), the observed Watson’s F statistic (F w = 6.23) exceeds the critical value (F crit = 3.2). Further applying of a superior statistic (V w ) proposed by Watson (1983) suggests that the V w for the two data sets is 12.9 and clearly larger than critical value V crit = 6.4 (Fig. 4g). Both of the two tests suggest that the paleomagnetic directions derived from the two formations do not share a common mean direction at the 95% confidence level. The paleomagnetic pole derived from the Huai Hin Lat Formation (48.7◦ N, 165.9◦ E with K = 33.9 and A 95 = 7.2◦ ) is clearly different from that of the Nam Phong Formation (55.2◦ N, 178.0◦ E with K = 61.5 and A 95 = 5.9◦ ), which is key to understand the poleward movements in the Late Triassic (Fig. 9). The Huai Hin Lat Formation is separated by the Indosinian II Unconformity with the Nam Phong Formation which is subdivided into the Lower and Upper Nam Phong formation by the Indosinian III Unconformity. The pre-folding origin nature of the ChRMs of the two formations strongly suggests they are acquired prior to the re-

gional folding events including the Mid-Cretaceous and Himalayan Orogeny (Ridd et al., 2011). Because of discovery the Indosinian III Unconformity which increased the age uncertainty of the Nam Phong Formation, it is actually not sure whether the ChRM of the Nam Phong Formation is acquired prior the Indosinian III Unconformity. However, the extremely gentle attitudes of the Late Jurassic to Early Cretaceous formations indicates the rocks in this region is weakly deformed by the Mid-Cretaceous and Himalayan Orogeny events. Therefore, the ChRM of the Nam Phong Formation is very likely to be acquired during its depositional process. Furthermore, the different distributions (both before and after tilt correction) of the magnetic directions from the two formations indicate that the ChRM of the Huai Hin Lat Formation is very likely to be acquired prior to the Indosinian II Unconformity, whose age is approximately constrained to be at ca. 215 Ma by the detrital zircon U–Pb geochronological analysis in this study. 5.2.2. Late Jurassic to Early Cretaceous Bhongsuwan and Elming (2000) reported a data set derived from four sites which indicated a positive fold test (McElhinny, 1964) from the Phu Kradung Formation and suggested an early Middle Jurassic paleomagnetic pole (56.6◦ N, 181.5◦ E with A 95 = 8.0◦ ) whose age was revised to be Late Jurassic to Early Cretaceous (Fig. 8b; Fig. 9). Within the four sites, the direction of site 5 is

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Fig. 9. Equal area projections of the Mesozoic paleomagnetic poles and 95% confidence circles of the Indochina, South China and North China Blocks. Abbreviations: TuINC: Upper Triassic Indochina; Ju–KlINC: Upper Jurassic to Lower Cretaceous Indochina; KlINC: Lower Cretaceous Indochina; KuINC: Upper Cretaceous Indochina; TmSCB: Middle Triassic South China; Jl–mSCB: Lower to Middle Jurassic South China; JuSCB: Upper Jurassic South China; KlSCB: Lower Cretaceous South China; KuNCB-SCB: Lower Cretaceous North-South China; TmNCB: Middle Triassic North China; TuNCB: Upper Triassic North China; JmNCB: Middle Jurassic North China; JuNCB: Upper Jurassic North China; KlNCB: Lower Cretaceous North China.

quite separated from the other three sites before tilt correction and tightly cluster after tilt correction. Since only four sites were used in the fold test which critically depends on the quality of site 5, the authors emphasized the predominantly oblate magnetic susceptibility fabric as additional evidence for a primary origin because this form of magnetic fabric is often interpreted to indicate foliation coincident with the bedding planes characteristic of a primary compaction or deposition fabric (Fig. S4). Actually, the interpretation is far from robust and needs to be tested. 5.2.3. Early Cretaceous Paleomagnetic studies of the Phra Wihan Formation have been carried out by Maranate and Vella (1986) and Bhongsuwan and Elming (2000) (Fig. 8b). Since there is little difference in the bedding attitudes, the fold test is insignificant for the Phra Wihan Formation (Bhongsuwan and Elming, 2000). The data set is similar to that reported by Maranate and Vella (1986). Data sets derived from the Sao Khua Formation that pass fold tests were independently reported by Bhongsuwan and Elming (2000) and Charusiri et al. (2006). Their mean magnetic directions were similar to the results of Yang and Besse (1993) and Maranate and Vella (1986). To get a more convincing result, a new recalculated mean direction is provided in this paper by synthesizing all the data with demagnetization details from Yang and Besse (1993), Bhongsuwan and Elming (2000) and Charusiri et al. (2006) (Fig. 8c). No data set has yet been reported with positive field test from the Phu Phan and Khok Khuat formations (Fig. 8d) since at outcrops the bedding orientations have very low dip angles (commonly less than 10◦ ). Because the mean magnetic directions from the four Lower Cretaceous formations are similar to each other (Table 3), we conclude that the new paleomagnetic pole (62.6◦ N, 174.4◦ E with K = 109.5 and A 95 = 3.2◦ ) of the Sao Khua Formation is the most reliable Early Cretaceous pole in the studying area (Fig. 9).

5.2.4. Late Cretaceous Research on Late Cretaceous rocks from areas far from the Khorat Plateau Basin (Takemoto et al., 2005; Otofuji et al., 2012) have indicated a significant discrepancy to that near the Khorat Plateau Basin (Charusiri et al., 2006; Singsoupho et al., 2014), which is considered to be the central part of the Indochina Block (Table 3). The discrepancies including different interpretations of the rotation and motion, are appear to have been caused by internal deformation within the Indochina Block (Singsoupho et al., 2014). Hence, the data set derived from the Bolikhamxay and Savannakhet areas (Singsoupho et al., 2014) is presented here as the most reliable paleomagnetic result for the Late Cretaceous in this paper (Table 4; Fig. 9). 5.3. Mesozoic paleo-positions and tectonic movements of the Indochina Block A tectonic extrusion model is widely proposed to explain the Mesozoic–Cenozoic tectonic evolution of Indochina Block (Tapponnier et al., 1982, 1986; Yang and Besse, 1993; Replumaz et al., 2004; Charusiri et al., 2006; Aitchison et al., 2007; Ali and Aitchison, 2008; Royden et al., 2008; Otofuji et al., 2012), although it is challenged by geological evidence (Searle, 2006; Hall et al., 2008; Fyhn et al., 2009; Hinsbergen et al., 2011; Hall, 2012). Yang and Besse (1993) suggested a 11.5 ± 6.7◦ paleolatitudinal difference, implying a 1500 ± 800 km motion of the Indochina Block relative to the South China Block along the Red river (or Song Ma) and Xian Shui He fault zones since the Middle Cretaceous. Charusiri et al. (2006) estimated the southeastward translation to be approximately 950 ± 150 km after the Cretaceous. Takemoto et al. (2009) suggested a ca. 7.1 ± 2.2◦ latitudinal motion of central Laos during the Jurassic to Cretaceous, while as calculated by Singsoupho et al. (2014), the latitudinal differences between the Indochina and South China blocks were 10.8 ± 8.8◦ in the Early Jurassic and 11.1

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Table 3 Summarized key paleomagnetic data of the Indochina Block. Formation

Age

N /n

Ds

Is

κ

α95

λp

ϕp

(◦ E)

A95 (dp/dm)

Fold test

Reference

Bolikhamxay+Savannakhet Da Lat Phu Thok Khok Khuat Khok Khuat Khok Khuat Phu Phan–Khok Khuat Phu Phan–Khok Khuat Sao Khua Sao Khua Sao Khua Sao Khua Sao Khua Phra Wihan Phra Wihan Phu Kradung Phra Wihan–Phu Kradung Phu Kradung Nam Phong Nam Phong Nam Phong Huai Hin Lat Huai Hin Lat Huai Hin Lat

Ku Ku Ku Km Kl Kl Kl–m Kl Kl Kl Kl Ju Ju Jm Jm Jm Jm Jl Jl Tu–Jl Tu–Jl Tu Tu Tu

28/318 21/121 18/189 7/21 14/111 10/88 8/42 3/27 19/124 16/48 4/20 10/73 5/31 14/42 7/36 4/29 11/65 15/45 8/54 9/27 11/75 5/56 13/124 5/26

27.0 11.4 31.8 47 28.9 28.1 31.4 30.2 28.6 33 31.8 26.6 30.4 31 27.7 35.0 30.3 33 37.2 34 36.4 39.5 43.0 43.2

27.3 35.4 28.7 29 39.5 40.5 27.1 36.1 39.6 35 38.3 37.3 45.1 41 25 32.1 27.6 29 40.1 41 37.8 44.4 48.0 42.9

36.2 331.7 111.4 19 204.3 398.5 27.6 108.0 116.2 23 149.6 339.2 59.1 10 120 173.1 97.5 31 71.6 20 68.5 85.0 47.4 106

4.6 1.7 3.5 14 2.8 2.4 9.4 11.9 3.1 8 5.7 2.6 10 13 5.5 7.0 4.6 7 6.6 12 5.6 8.3 6.1 7.5

67.0 76.5 59.4 45 62.1 62.7 59.7 61.0 62.6 59 59.7 64.8 59.9 60 62.9 56.6 60.7 58 54.4 58 55.2 52.1 48.7 49.2

180.8 161.2 190.8 189 175.6 173.3 190.8 182.1 174.4 184 178.2 178.1 173.7 174 202.3 181.5 187.4 188 175.6 174 178.0 169.8 165.9 172.3

4.9 1.7 3.5 9/16 2.6 2.4 9.4 8.1 3.2 5/9 5.7 2.3 10.5 10/16 4 8.0 4 4/8 7.3 9/14 5.9 7.3 7.2 7.0

positive positive insignificant – insignificant insignificant insignificant insignificant positive – positive insignificant positive – insignificant positive positive – positive – positive insignificant positive positive

Singsoupho et al. (2014) Otofuji et al. (2012) Charusiri et al. (2006) Maranate and Vella (1986) This study Yang and Besse (1993) Charusiri et al. (2006) Bhongsuwan and Elming (2000) This study Maranate and Vella (1986) Charusiri et al. (2006) Yang and Besse (1993) Bhongsuwan and Elming (2000) Maranate and Vella (1986) Bhongsuwan and Elming (2000) Bhongsuwan and Elming (2000) Bhongsuwan and Elming (2000) Maranate and Vella (1986) Yang and Besse (1993) Maranate and Vella (1986) This study Yang and Besse (1993) This study Achache and Courtillot (1985)

(◦ )

(◦ N)

Abbreviations: N /n: number of sites/samples; D s , I s : declination and inclination after tilt correction; κ & α95 : precision parameter and the radius of the cone of 95% confidence for formation mean direction; λ p /ϕ p : latitude and longitude of virtual geomagnetic pole (VGP); A 95 : the radius of the cone of 95% confidence for formation-mean VGP; dp/dm: axes of ellipse of uncertainty of the pole; Ku: Upper Cretaceous; Km: Middle Cretaceous; Kl: Lower Cretaceous; Kl–m: Lower to Middle Cretaceous; Ju: Upper Jurassic; Jm: Middle Jurassic; Jl: Lower Jurassic; Tu–Jl: Upper Triassic to Lower Jurassic; Tu: Upper Triassic. Table 4 Summarized key paleomagnetic poles of the Indochina, South China and North China Block. Formation

λp

ϕp

(◦ N)

(◦ E)

Ku Kl Ju–Kl Tu–Jl Tu

67.0 62.6 56.6 55.2 48.7

180.8 174.4 181.5 178 165.9

Ku Kl Ju Jl–m Tm

74.3 79.0 71.0 79.9 52.8

Ku Kl Ju Jm Tu Tm

74.3 76.4 74.4 76.4 62.3 61.0

Age (Ma)

A 95

Expected (ref.: 22◦ N, 102◦ E)

Reference

Dec (◦ )

Inc (◦ )

α95 (◦ )

Paleolatitude (◦ )

λ

4.9 3.2 8 5.9 7.2

24.9 29.6 36.3 38.0 45.2

42.4 46.1 41.6 44.1 52.8

5.1 3.1 8.4 6.0 6.3

24.5 27.5 23.9 25.9 33.4

4.9 3.2 8 5.9 7.2

Singsoupho et al. (2014) This study Bhongsuwan and Elming (2000) This study This study

205.1 208.3 215.1 221.8 224.7

5.5 5.9 6.7 5.3 5.1

16.1 11.1 17.9 9.1 30.6

32.5 33.9 25.9 31.0 −0.5

6.5 6.8 8.4 6.3 7.2

17.7 18.5 13.6 16.7 −0.3

5.5 5.9 6.7 5.3 5.1

Yang and Besse (2001) Huang et al. (2008) Huang et al. (2008) Yang and Besse (2001) Su et al. (2005)

205.1 208.8 222.8 234.8 7.7 3.1

5.5 5.1 5.9 5.1 3.8 3.2

16.1 13.7 13.7 10.2 330.9 330.3

32.5 32.3 25.6 23.9 32.1 28.1

6.5 6.0 7.5 6.5 4.5 3.9

17.7 17.5 13.5 12.5 17.4 15.0

5.5 5.1 5.9 5.1 3.8 3.2

Yang and Besse (2001) Yang and Besse (2001) Gilder and Courtillot (1997) Yang and Besse (2001) Huang et al. (2008) Huang et al. (2008)

Indochina Bolikhamxay+Savannakhet Sao Khua Phu Kradung Nam Phong Huai Hin Lat South China

North China

Abbreviations: λ p /ϕ p : latitude and longitude of pole; A 95 : radius of the cone of 95% confidence of pole; Dec: declination expected at the reference site; Inc: inclination expected at the reference site; ref.: reference site; α95 : radius of the cone of 95% confidence of the expected direction; λ: error of paleolatitude; Ku: Upper Cretaceous; Kl: Lower Cretaceous; Ju–Kl: Upper Jurassic to Lower Cretaceous; Ju: Upper Jurassic; Jm: Middle Jurassic; Jl–m: Late–Middle Jurassic; Jl: Lower Jurassic; Tu–Jl: Upper Triassic to Lower Jurassic; Tu: Upper Triassic; Tm: Middle Triassic.

± 6.2◦ in the Early Cretaceous, implying a post Cretaceous motion. The different estimates of the time and scale of the relative motion can potentially be reconciled by synthesizing or recalculating the data sets. Our new mean paleomagnetic pole (48.7◦ N, 165.9◦ E with K = 33.9 and A 95 = 7.2◦ ) is the first robust result for the Norian Stage Upper Triassic Huai Hin Lat Formation. The corresponding paleolatitude is 33.4 ± 7.2◦ N at the reference site (22◦ N, 102◦ E) located at the boundary of the Indochina and South China blocks (Fig. 10). The paleolatitudes are 25.9 ± 5.9◦ N and 23.9 ± 8◦ N derived from

the Upper Triassic to Lower Jurassic Nam Phong Formation and Upper Jurassic to Lower Cretaceous Phu Kradung Formation, respectively. We conclude that the Indochina Block moved southward during Jurassic to Cretaceous time, and we extend the starting time of the southward motion to the Norian Stage of the Late Triassic (Huai Hin Lat Formation) (Fig. 10a–c; Table 4). In order to further calculate the relative displacement of the Indochina Block, the corresponding paleomagnetic poles of the South and North China blocks are used as the reference poles. Noting that the Mesozoic APW paths of the South and North

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Fig. 10. A displacement model of the Indochina relative to the South China and North China Blocks during (a) the Norian Age Late Triassic, (b) Late Triassic to Early Jurassic, (c) Late Jurassic to Early Cretaceous, (d) Early Cretaceous, (e) Late Cretaceous and (f) Present. The positions of SCB in (a) and (b) are interpolated based on the Middle Triassic and Early–Middle Jurassic paleomagnetic data. ALS-RRF: Ailaoshan-Red River Fault; INC: Indochina; SCB: South China Block; NCB: North China Block; The reference point is marked with red star. The direction of relative motions are marked with arrows.

China blocks have repeatedly well-constructed by several papers (e.g. Yang et al., 1998; Yang and Besse, 2001; Wu et al., 1998; Huang et al., 2008) before; and there are only a few new data (Wang et al., 2011; Kawamura et al., 2013; Ren et al., 2015) added to the Mesozoic data sets of the North and South China blocks; However, these new data do not change the main patterns or features of the APW paths constructed by Yang and Besse (2001) and Huang et al. (2008). So we chose their APW paths as the reference frame in this paper. Key poles of these blocks are shown in Table 4 and Fig. 9. The reference site (22◦ N, 102◦ E) is located at the boundary of the South China and Indochina blocks, which is also used for the calculation of the expected paleomagnetic directions and paleolatitudes, which is shown in Table 4. The consecutive southward motion of the Indochina Block during the Late Triassic to Late Jurassic is shared by the North China Block and partially by the South China Block (Middle–Late Jurassic to Late Jurassic) (Fig. 10a–c; Table 4). It should be noted that the position of South China in Fig. 10a–b is interpolated from the Middle Triassic and Early–Middle Jurassic poles due to the lack of reliable Late Triassic paleomagnetic data from the South China Block. The paleolatitudes of the Late Jurassic to Early Cretaceous indicate a slightly northward movement of the Indochina, South China and North China blocks (Fig. 10c–d; Table 4). These data indicate that the latitudinal difference between the Indochina and Eastern China (South China and North China) blocks remained about 9–10◦ implying that no significant relative poleward movement occurred during the Early Jurassic to Early Cretaceous. Then the latitudinal difference decreased to 6.9 ± 5.9◦ in the Late Cretaceous indicating relative displacements of 1000 ± 850 km since the Late Cretaceous along the Ailaoshan-Red River Fault (taking the azimuth N45W). This calculation is basically consistent with the post-Cretaceous motion estimates based on paleomganetic data (Yang and Besse, 1993; Yang et al., 1995; Charusiri et al., 2006; Singsoupho et al., 2014) and challenges the interpretation of ca. 600–700 km displacement along the Ailaoshan-Red River Fault based on lateral offsets (Leloup et al., 1995; Replumaz and Tapponnier, 2003; Replumaz et al., 2004; Faure et al., 2014) and a more comprehensive reconstruction which supposed only 250 km of southward displacement (Hall et al., 2008; Hall, 2012). Therefore, two main stage of southward movements of the Indochina block are identified. The First is during Late Triassic to Early Jurassic, and the second is interpreted to be in the Cenozoic on the basis of the latitudinal difference in the Late Cretaceous. Notably, no relative movement is discovered during the Early Jurassic to Early Cretaceous. The inclination shallowing effect in the red beds is well documented (Dupont-Nivet et al., 2010; Lippert et al., 2011; Huang et al., 2013) and also argued by Cogné et al. (2013) who suggested the inclination shallowing is not significant. It should be noted that although the inclination shallowing

may increase the amount of displacement, but it does not affect the conclusion of their relative displacement since it is common for the Indochina, North China and South China Blocks. As synthesized in this study, the expected declinations derived from the poles of the Huai Hin Lat, Nam Phong, Phu Kradung, Sao Khua Formation and Late Cretaceous (Bolikhamxay & Savannakhet areas) are 45.2 ± 8.6◦ , 38.0 ± 6.6◦ , 36.3 ± 8.8◦ , 29.6 ± 3.6◦ and 24.9 ± 5.4◦ respectively, indicating a continuous clockwise rotation of the Indochina Block (Table 4; Fig. 10). The rotation relative to the South China Block (Table 4) may have contributed to the sedimentary source transition noted during Middle–Late Jurassic.

6. Conclusion

On the basis of our paleomagnetic and geochronologic investigations and a synthesis of earlier studies, we conclude as follow: 1. U–Pb geochronologic analyses demonstrate the maximum allowable depositional age of the Huai Hin Lat Formation to be 227 Ma, consistent with the paleontology estimate (Norian Age). The maximum allowable depositional age of the Nam Phong Formation is estimated to be 215 Ma. 2. The paleomagnetic pole (48.7◦ N, 165.9◦ E with K = 33.9 and A 95 = 7.2◦ ) from the Norian Huai Hin Lat Formation is the first reliable result of this age and indicates the paleolatitude to be 33.4 ± 7.2◦ N at the reference site (22◦ N, 102◦ E). The revised Mesozoic APWP indicates a plausible movement history for the Indochina Block which means a long distance southward motion accompanied by a continuous clockwise rotation. 3. Analyses of detrital zircons confirm that a sediment source motion occurred in the Khorat Plateau during the Middle–Late Jurassic with a source interpreted to be the Indosinian Orogenic belt, and later the Qinling–Dabie Orogen. 4. Mainly two stage of movements are identified, Late Triassic to Early Jurassic, and Cenozoic. As a result, the post-Cretaceous extrusion model of the Indochina Block relative to the South China Block is supported by this study. The Indochina Block was located at a much higher latitude than present during the Norian Stage of Late Triassic time and its latitudinal difference relative to the Eastern China blocks (at the reference site) remained at about 9–10◦ during the Early Jurassic to Early Cretaceous interval implying no significant relative displacement. In this study, the relative displacement is calculated to be about 1000 ± 850 km since the Late Cretaceous, which we assume was accommodated along the Ailaoshan-Red River Fault.

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