Tectonic deformation around the eastern Himalayan syntaxis

Geophys. J. Int. (2008) 175, 713–728 doi: 10.1111/j.1365-246X.2008.03885.x

GJI

Tec

toni

csan

dge

ody

nam

ics

Tectonic deformation around the eastern Himalayan syntaxis:constraints from the Cretaceous palaeomagnetic data of theShan-Thai Block

Kenji Tanaka,1 Chuanlong Mu,2 Ken Sato,1 Kazuhiro Takemoto,1 Daisuke Miura,3

Yuyan Liu,4 Haider Zaman,1 Zhenyu Yang,5 Masahiko Yokoyama,1 Hisanori Iwamoto,1

Koji Uno6 and Yo-ichiro Otofuji11Department of Earth and Planetary Sciences, Faculty of Science, Kobe University, Kobe, Japan. E-mail: [email protected] Institute of Geology and Mineral Resources (CGS), Chengdu 610082, China3Abiko Research Laboratory, CRIEPI, 1646 Abiko, Chiba, Japan4China University of Geosciences, Wuhan 430074, China5Department of Earth Sciences, Nanjing University, Nanjing, China6Faculty of Education, Okayama University, Okayama, Japan

Accepted 2008 June 10. Received 2008 June 10; in original form 2008 February 4

S U M M A R YLower to Middle Cretaceous red sandstones were sampled at four localities in the Lanpin-Simao fold belt of the Shan-Thai Block to describe its regional deformational features. Mostof the samples revealed a characteristic remanent magnetization with unblocking temperaturesaround 680 ◦C. Primary natures of magnetization are ascertained through positive fold test.A tilt-corrected formation-mean direction for the Jingdong (24.5◦N, 100.8◦E) locality, whichis located at a distance of 25 km from the Ailaoshan–Red River Fault, revealed northerlydeclination with steep inclination (Dec./Inc. = 8.3◦/48.8◦, α95 = 7.7◦, N = 13). However,mean directions obtained from the Zhengyuan (24.0◦N, 101.1◦E), West Zhengyuan (24.0◦N,101.1◦E) and South Mengla (21.4◦N, 101.6◦E) localities indicate an easterly deflection indeclination; such as Dec./Inc. = 61.8◦/46.1◦, α95 = 8.1◦ (N = 7), Dec./Inc. = 324.2◦/−49.4◦,α95 = 6.4◦ (N = 4) and Dec./Inc. = 51.2◦/46.4◦, α95 = 5.6◦ (N = 13), respectively. The palaeo-magnetic directions obtained from these four localities are incorporated into a palaeomagneticdatabase for the Shan-Thai Block. When combined with geological, geochronological andGPS data, the processes of deformation in the Shan-Thai Block is described as follows: Sub-sequent to its rigid block clockwise rotation of about 20◦ in the early stage of India–Asiacollision, the Shan-Thai Block experienced a coherent but southward displacement along theRed River Fault prior to 32 Ma. This block was then subjected to a north–south compressivestresses during the 32–27 Ma period, which played a key role in shaping the structure ofChongshan-Lancang-Chiang Mai Belt. Following this some local clockwise rotational motionhas occurred during the Pliocene-Quaternary time in central part of the Shan-Thai Block as aresult of internal block movements along the reactivated network of faults.

Key words: Palaeomagnetism applied to tectonics; Geomorphology; Continental tectonics:compressional; Asia.

1 I N T RO D U C T I O N

As explained in the literature, oceanic part of the lithosphere gen-erally behaves as a single rigid block, whereas behaviour of thecontinents after their collision with each other is rather a com-plex phenomenon. Although, relevance of plate tectonic concept todescribe continental deformation remains under debate, collisionand subsequent indentation of India into Asia during the last 50Myr (Rowley 1996; Aitchison et al. 2007) can be seen as excellent

examples of the continental tectonics. As a result of these pro-cesses the affects of continental deformation, ranging from 100 to1000 km in scale, appeared around the eastern Himalayan syntaxes.Detailed picture of the tectonic deformation from this region willshed further light on establishing a plausible tectonic model.

The present day deformational features of the Asian Continentwith an order of 1000 km can be viewed by a snapshot availablethrough the GPS data, which clearly show clockwise rotationalmovement around the eastern Himalayan syntaxis in East Asia

C© 2008 The Authors 713Journal compilation C© 2008 RAS

714 K. Tanaka et al.

Figure 1. (a) A simplified tectono-geographical map of southeast Asia showing the positions of main sutures and faults CD: Chuan Dian Fragment. (b)Structural sketch map of the Shan-Thai Block and neighbouring areas (modified from Leloup et al. 1995; Laccasin et al. 1996; Shen et al. 2005). Shadedareas indicate the distribution of Jurassic to Cretaceous red beds. Dotted zone is the Chongshan-Lancang-Chiang Mai Belt. Observed mean declination ateach sampling locality in the Shan-Thai and neighbouring blocks is indicated by arrows (Jurassic: open arrow, Cretaceous: solid arrow). Abbreviations are: J,Jingdong, ZY, Zhengyuan, SM, South Mengla, LLF, Longling fault and LRF, Lancang River Fault.

(Wang et al. 2001a; Zhang et al. 2004). Older deformational events(in terms of geology) since the onset of collision can be seen throughpalaeomagnetic declination record. The available palaeomagneticrecord from East Asia also indicates clockwise rotational motionaround the eastern Himalayan syntaxis (Funahara et al. 1992, 1993;Huang & Opdyke 1993; Otofuji et al. 1998; Sato et al. 1999, 2007;Yang et al. 2001; Yoshioka et al. 2003; Aihara et al. 2007). This rota-tional motion has mainly limited in the Shan-Thai Block, IndochinaBlock and Chuan Dian Fragment (Fig. 1).

Further inspection of the declination record (Yoshioka et al. 2003;Aihara et al. 2007) suggests that the Shan-Thai Block and the ChuanDian Fragment were both subjected to intrablock differential motionwith an order of 100 km as well as a coherent block rotation (Fig. 1).This internal tectonic deformation characterizes another componentof tectonic deformation in the East Asia as a result of continuedIndian Plate indentation. Since internal deformation is a higher orderapproximation of tectonic phenomenon in the continent collision,study of this aspect can provide a detailed clue about the mechanicalnature of the upper and lower crusts around eastern Himalayansyntaxis (Royden et al. 1997; Copley and McKenzie 2007; Ganget al. 2007; Vergnolle et al. 2007).

In this paper, attention is focused on the internal deformationalregimes of the Shan-Thai Block. Although the subject of inter-nal deformation in the target block has already been touched inprevious palaeomagnetic studies (Aihara et al. 2007; Sato et al.2007), reliable palaeomagnetic datasets are still not enough to de-lineate deformational features in a systematic way. Keeping in viewa necessity for further data, we have collected palaeomagnetic sam-ples from the Lower to Middle Cretaceous red beds at Jingdong(24.5◦N, 100.8◦E), Zhengyuan-West Zhengyuan (24.1◦N, 101.1◦E),and South Mengla (21.4◦N, 101.9◦E) localities. These localitiesare positioned from north to south along 101◦E longitude in theLanping-Simao fold-thrust belt of the Simao Basin. The purposeof our study is to precisely delineate deformational pattern of theShan-Thai Block using a newly edited palaeomagnetic database.

2 G E O L O G I C A L S E T T I N G A N DS A M P L I N G

The Shan-Thai Block, which received strong tectonic deformationin the Cenozoic era, is situated to the southeast of eastern Himalayansyntaxis (Wang & Burchfiel 2000; Wang et al. 2001a; Zhang

C© 2008 The Authors, GJI, 175, 713–728

Journal compilation C© 2008 RAS

Tectonic deformation of SE Asia 715

et al. 2004). As shown in the Fig. 1, this block is separated fromthe Yangtze Block by a NW–SE trending Red River Fault to thenortheast, while N–S trending Sagaing Fault and the Gaoligongshear zone are separating it from the West Burma Block in thewest and northwest, respectively. Towards southeast, the Shan-ThaiBlock is separated from the Indochina Block by Dien Bien Phu Fault(Fig. 1b).

As reported by Ren et al. (1980), the Shan-Thai Block is madeup from tectonic collages of two continental fragments, such asthe Baoshan Terrane and the Simao Terrane. These two terranesare separated from each other by the Chagning-Memglian suturezone (BGMRY 1990; Wu et al. 1995). Clear Gondwanic affinityhas been determined for the Baoshan Terrane, while a Cathaysianorigin is assigned to Simao Terrane (Wu et al. 1995). The Simao Ter-rane is composed of Proterozoic basement and Palaeozoic marinestrata. Mesozoic to Cretaceous continental sediments rest uncon-formably over Pre-Mesozoic strata in the Lanpin-Simao fold beltarea. The Cretaceous sequence of this fold belt is of terrestrialorigin and is subdivided in to four different formations, they are:the Lower Cretaceous Jinxing Formation, the Middle CretaceousNanxin Formation, the Upper Cretaceous Hutousi Formation, andthe Upper Cretaceous Mankuanhe Formation in ascending order(BGMRY 1990; Leloup et al. 1995).

The Lower Cretaceous Jinxing Formation is mainly composedof grey to green feldspathic sandstone in the lower part and lay-ers of sandstones, siltstones, and mudstones in the upper part. Thepresence of rich Lamellibranchiate, such as the Estheria, Darwin-ula, Gasterpods and Sporopollen, assign an age of Early Cretaceousto the Jinxing Formation (BGMRY 1990). The Middle CretaceousNanxin Formation mainly consists of purplish red sandstone, whichconformably overlies the Lower Jinxing Formation. The NanxinFormation is then overlain by the Upper Cretaceous Hutousi For-mation. The occurrence of Estheria, Cypridea(c), Gasterpods andSporopollen indicate an age of Middle Cretaceous for the NanxinFormation (BGMRY 1990). A disconformity between the Middleand Upper Eocene continental molasses (Leloup et al. 1995; Wang& Burchfiel 1997) and unconformity between the Upper EoceneDenghei Formation and Mengla Formation has been observed inthe Simao Basin (BGMRY 1990), indicating the Late Eocene as aprobable time for folding activities in this area.

As shown in Figs 2(a) and (b), Cretaceous red sandstones andsiltstones were sampled at three different areas along 101◦E lon-gitude; Jingdong area (24.5◦N, 100.8◦E), Zhengyuan area (24.0◦N,101.1◦E) and South Mengla area (21.4◦N, 101.6◦E). The Jingdongarea is located in the central part of the Wulianshan belt, which is atabout 25 km from the Red River Fault. Since the Nanxin Formationaround Jingdong area forms an E–W trending anticlinal structure,palaeomagnetic samples were collected from both limbs of the foldat 15 different sites. The Zhengyuan area forms part of the north-ern Simao belt, which is located at a distance of 50 km from theRed River Fault. Here the Nanxin Formation was sampled at twolocalities (Zhengyuan and West Zhengyuan) separated by severalN–S trending strike-slip faults. At Zhengyuan locality, 14 sites weresampled from both limbs of the N–S trending anticlinal structure,while at West Zhengyuan locality five sites were collected from thewest dipping monoclinal structure. The Mengla area is located insouthern part of the Simao Belt and is positioned at about 300 km tosouth of the Zhengyuan area. In Mengla locality samples were col-lected at nine sites from the Nanxin Formation and at 11 sites fromthe Jinxing Formation, where an attitude of monoclinal structurewith N–S trending axis and a dip of 2◦–63◦ (from northeastward toeastward) has been observed.

About eight block samples, oriented with magnetic compass,were collected at each site. The present declination value at eachsampling site was evaluated from an International GeomagneticReference Field (Mandea & Macmillan 2000).

3 PA L A E O M A N G E T I S M

3.1 Laboratory procedure

One or more specimens, 25 mm in diameter and about 22 mm inheight, were prepared from each sample in the palaeomagnetic lab-oratory of the Kobe University. Remanent magnetization of eachspecimen was measured using a 2G Enterprises cryogenic magne-tometer. Stepwise thermal demagnetization was carried out up to690 ◦C using the Natsuhara TDS-1 thermal demagnetizer; the resid-ual field in the furnace was less than 5 nT. The magnetic behaviour ofeach specimen after complete demagnetization procedure was plot-ted on a Zijderveld diagram (Zijderveld 1967). Principal compo-nent analysis (Kirschvink 1980) was used to determine directionalbehaviour of different magnetization components, and site-meandirections were calculated using Fisherian statistics (Fisher 1953).

Progressive acquisition of isothermal remanent magnetization(IRM) up to 2.7 T and thermal demagnetization of composite IRMs(Lowrie 1990) were performed on a 2G pulse magnetizer in or-der to identify ferromagnetic minerals. For composite IRMs, hard,medium and soft components were treated in DC fields of 2.7T,0.4T and 0.12T, respectively.

3.2 Demagnetization results with their mean directions

3.2.1 Jingdong locality

Specimens of the Middle Cretaceous Nanxin Formation from thislocality revealed initial natural remanent magnetization (NRM) in-tensities between 0.6 and 6.8 mA m−1. Most of the specimensgenerally exhibited two components of magnetization. After theremoval of low temperature component by 350 ◦C, the high tem-perature component appeared and then linearly decayed towards theorigin by 690 ◦C (Fig. 3).

The low temperature component is observed in the samples ofall 13 sites. The formation mean direction of this component isDec./Inc. = 0.4◦/42.3◦, k = 95.9, α95 = 4.3◦ (N = 13) in geographiccoordinate, which is almost identical to the geocentric axial dipolefield (D = 0, I = 42.3◦). This type of behaviour strongly indicates aviscous remanent magnetization (VRM) origin for this component.

The high temperature component is also identified in 13 sites,where the α95 value for each site mean direction is less than 17◦

[Table 1(a)]. As evident from the stereographic projection, site meandirections of these 13 sites have clearly developed two visible group-ings in geographic coordinates; showing northerly declinations inboth cases but with steep downward (D ∼ 20◦, I ∼ 75◦) and shal-low (D ∼ 20◦, I ∼ 30◦) inclinations (Fig. 4). However, after tiltcorrection these two groups merged in to a single one.

Fold tests were applied to 9 out of 13 sites (from SK50 to SK60)because they were collected from both limbs of the synform. Forthe McFadden’s fold test (McFadden 1990), a calculated value (ξ 2)is 5.1488 in geographic coordinates and 1.341 after tilt correc-tion, while critical value (ξ c) is 3.4970 at 95 per cent confidencelevel. We recognize that the formation mean direction at 100 percent un-tilting represents the characteristic mean remanent direction(ChRM). Combining with the remaining four sites, a tilt-correctedformation mean direction of Dec./Inc. = 8.3◦/48.8◦, α95 = 7.7◦

(N = 13) is calculated for Jingdong area in the Middle Cretaceous.

C© 2008 The Authors, GJI, 175, 713–728



Figure 2. Geological map of the study area (after BGMRY 1990). (a) Jingdong and Zhengyuan areas and (b) South Mengla locality. Sampling sites are shownby closed circles (Nanxin Formation) and open circles (Jingxing Formaiton). Strike and dip of strata for each sampling site are shown in the insets. Ailao-Shan,Metamorphic rocks; P, Paleozoic; T, Triassic; J, Jurassic; K, Cretaceous; E, Eocene and N, Neogene.

3.2.2 Zhengyuan area (Zhengyuan and West Zhengyuan localities)

Specimens of the Middle Cretaceous Nanxin Formation fromZhengyuan and west Zhengyuan localities have initial NRM in-

tensities between 1.4 and 15.2 mA m−1 (Fig. 3). Fifteen percent ofthe measured samples shows uni-vectorial behaviour during pro-gressive thermal demagnetization procedure, where the remanentmagnetization was unblocked in a temperature range between 590

C© 2008 The Authors, GJI, 175, 713–728



Figure 3. Vector end-point diagrams for the representative red bed specimens from Jingdong, Zhenyuen, West Zhengyuan and South Mengla lcalities duringthermal demagnetization experiments (in geographic coordinates). Solid (open) symbols are projections on horizontal (vertical) plane.

and 690 ◦C. The remaining samples exhibited two component be-haviours in their Zijderveld plots. After the unblocking of low tem-perature component by 350 ◦C, the high temperature componentwas broken down in a temperature range between 350 and 690 ◦C.

Although the low temperature component is identified in 62 spec-imens of 19 sites (both localities), no significant site mean directionsare obtained because of highly scattering behaviour.Zhengyuan locality. The high temperature component fromZhengyuan area, which we consider as ChRM, is identified onlyin seven sites [Table 1(b)]. When plotted on stereographic projec-tion, the ChRM directions from the seven sites display two group-ings before tilt correction; a westerly declination with moderate

inclination (D ∼ 260◦, I ∼ 50◦) and an easterly declination withshallow inclination (D ∼ 70◦, I ∼ 20◦) (Fig. 4). During the pro-gressive untilting procedure, these two directions get closer to eachother and eventually emerged as a single population at 100 per centuntilting with Dec./Inc. = 61.8◦/46.1◦, α95 = 8.1◦, (N = 7). Theoptimal concentration of the DC tilt test (Enkin 2003) is achievedat 92.0 ± 10.1 per cent untilting, which indicates a positive result at95 per cent confidence level. According to the McFadden method(McFadden 1990), the fold test is also positive at 95 per cent confi-dence level.West Zhengyuan locality. The high temperature component isidentified in four sites of the West Zhengyuan locality, where the

C© 2008 The Authors, GJI, 175, 713–728



Table 1. Palaeomagnetic results of red bed samples collected from Cretaceous formations of the (a) Jindong, (b) Zhengyuan, (c) West Zhengyuan and (d)South Mengla localities.

(a) Site and formation mean directions from the Jingdong palaeomagnetic localitySite Locality Polarity n/N In situ Tilt-corrected k α95 (◦)

(◦N) (◦E) (◦) Dec. (◦) Inc. (◦) Dec. (◦) Inc. (◦)

[High-temperature component]Nanxin Fm.SK47 24.54 100.83 149.2 48 Normal 8/8 23.5 28.7 342.3 58.4 82.1 6.1SK48 24.54 100.83 158.2 46 Normal 8/8 26.5 25.7 352.8 51.8 76 6.4SK49 24.54 100.83 139.2 46 Nomal 8/8 21.3 30.1 342.1 64 20.7 12.5SK50 24.57 100.83 297.2 43 Normal 8/8 225.6 85.4 24.8 51.3 18.7 13.1SK51 24.54 100.83 294.2 21 Normal 8/8 12.3 56.7 16.1 36 97 5.7SK52 24.57 100.83 111.2 29 Normal 8/8 10.5 29.6 3.7 57.8 141.6 4.7SK53∗ 24.57 100.82 290.2 30 0/8 – – – – – –SK54 24.57 100.82 281.2 36 Normal 7/8 21.1 71.9 15 36.1 26 12.1SK55 24.57 100.82 277.2 33 Normal 7/8 44.5 64.8 25.5 35.1 14.6 16.4SK56 24.57 100.82 289.2 39 Normal 8/8 50.3 74.7 29 37.4 32.8 9.8SK57 24.57 100.82 264.2 28 Nomal 8/8 45.1 68.9 17.8 45.8 47.3 8.1SK58 24.57 100.82 269.2 33 Normal 7/8 303.9 72.9 339 45.2 15 16.1SK59 24.57 100.82 290.2 20 Normal 9/9 3.4 68.2 10.7 48.7 16.7 13SK60∗ 24.57 100.82 271.2 35 Normal 1/2 229.5 60 302.2 64.1 – –SK61 24.54 100.83 150.2 43 Normal 8/8 25.2 20.5 1.2 51.2 89.9 5.9Formation mean direction

24.5 100.8 13/15 20.7 55.9 9.6 14.113/15 8.3 48.8 30.1 7.7

(b) Site and formation mean directions from the Zhengyuan palaeomagnetic locality

[High-temperature component]Nanxin Fm.SK28∗ 24.15 101.02 324.2 89 Normal 2/7 222.9 28.2 74.5 60.8 – –SK29∗ 24.15 101.02 324.2 82 Normal 4/6 244.0 35.1 37.1 61.5 4.5 48.9SK30∗ 24.15 101.02 333.2 87 Nomal 2/6 247.3 52.3 59.7 40.5 – –SK31 24.15 101.02 339.2 93 Normal 6/6 256.8 55.2 64.0 31.5 30.2 12.4SK32∗ 24.15 101.02 334.2 99 Normal 3/7 268.3 52.4 48.1 25.2 9.6 42.1SK33∗ 24.15 101.02 343.2 91 Normal 4/7 249.3 20.7 82.5 68.0 4.2 50.9SK34∗ 24.15 101.02 314.2 86 Normal 2/7 236.1 50.1 33.6 42.8 – –SK35 24.15 101.02 336.2 92 Normal 4/6 264.2 45.9 49.8 39.5 120.6 8.4SK36 24.15 101.01 345.2 91 Normal 6/6 267.2 44.7 63.3 43.0 37.7 11.1SK37 24.15 101.01 341.2 91 Normal 5/6 267.2 39.8 53.0 46.6 113.6 7.2SK38∗ 24.15 101.02 337.2 92 Reverse 1/6 180.4 −9.8 166.9 23.3 – –SK44 24.02 100.97 203.2 44 Normal 5/7 83.6 19.3 62.5 56.1 21.4 16.9SK45 24.02 100.97 191.2 38 Normal 5/6 87.5 20.6 77.3 56.8 19.2 17.9SK46 24.02 100.97 204.2 43 Normal 6/6 83.1 12.5 66.8 46.8 564.4 2.8Formation mean direction

24.0 101.0 7/14 94.7 88.2 – – 1.9 63.37/14 – – 61.8 46.1 56.3 8.1

(c) Site and formation mean directions from the West Zhengyuan palaeomagnetic locality

[High-temperature component]Nanxin Fm.SK39 24.03 100.96 200.2 38 Reverse 6/6 307.9 −12.0 316.4 −47.4 78.4 7.6SK40 24.03 100.96 238.2 33 Reverse 6/6 328.9 −20.0 329.4 −53.0 70.9 8.0SK41 24.02 100.96 196.2 43 Reverse 6/6 316.1 −11.2 331.1 −46.2 53.2 9.3SK42∗ 24.02 100.96 228.2 21 0/6 – – – – – –SK43 24.02 100.97 203.2 65 Reverse 6/6 310.1 11.0 319.9 −50.3 230.0 4.4Formation mean direction

4/5 315.6 −8.2 – – 25.3 ###24.0 101.0 4/5 – – 324.2 −49.4 208.6 6.4

direction cluster (α95) in each site is less than 10◦ [Table 1(c)]. Ingeographic coordinates, these four sites gave a northwesterly decli-nation and upward shallow inclination (Dec./Inc. = 315.6◦/−8.2◦,α95 = 18.6◦) (Fig. 4). However, after tilt correction the precision

parameter (k) increases by 8.2 (kg = 25.3, ks = 208.6) and a forma-tion mean direction of Dec./Inc. = 324.2◦/−49.4◦, α95 = 6.4◦ (N =4) is obtained. Using the DC tilt test of Enkin (2003), the optimalconcentration is achieved at 105.4 ± 85.2 per cent untilting. The

C© 2008 The Authors, GJI, 175, 713–728



Table 1. (Continued.)

(d) Site and formation mean directions from the South Mengla palaeomagnetic localitySite Locality Polarity n/N In situ Tilt-corrected k α95 (◦)

(◦N) (◦E) (◦) Dec. (◦) Inc. (◦) Dec. (◦) Inc. (◦)

[High-temperature component]Nanxin Fm.XM01 21.01 101.46 6.6 63 Normal 6/6 311.3 54.3 63.3 52.8 33.1 11.8XM02 21.01 101.46 6.6 63 Normal 4/6 294.9 36.8 43.5 71.7 136.9 7.9∗XM03 21.01 101.46 6.6 63 0/6 – – – – – –XM04 21.40 101.52 16.6 26 Normal 4/6 20.0 49.3 46.0 41.6 11.5 28.4XM05 21.40 101.63 3.6 57 Normal 6/6 333.4 50.1 43.7 43.5 110.5 6.4XM06 21.40 101.63 3.6 57 Normal 5/6 339.2 55.3 50.9 40.2 92.3 7.0∗XM19 21.43 101.57 338.6 17 0/6 – – – – – –∗XM20 21.43 101.57 36.6 38 Normal 3/6 46.3 39.5 69.6 24.9 15.7 32.2Formation mean direction

5/8 329.6 53.1 13.5 21.65/8 49.7 50.1 33.5 13.4

Jingxing Fm.XM08 21.40 101.63 29.6 49 Normal 3/4 18.7 46.5 64.0 35.0 73.5 9.0

Reverse 2/2 208.3 −47.3 248.4 −29.6 – –N + R 5/6 21.0 50.2 68.8 35.2 63.6 15.6

XM09 21.40 101.63 26.6 41 Normal 6/6 354.6 49.2 49.6 53.0 174.9 5.1XM10 21.40 101.63 16.6 39 Normal 6/6 357.4 45.0 38.2 44.1 76.2 7.7XM11 21.40 101.63 16.6 39 Normal 6/6 359.5 52.5 48.4 46.8 56.3 9.0XM12 21.40 101.63 37.6 42 Normal 6/6 9.7 42.3 54.1 47.0 32.3 12.0XM13 21.40 101.63 18.6 43 Normal 6/6 6.7 45.7 47.9 38.4 61.3 8.6XM14 21.40 101.62 34.6 38 Normal 6/6 13.3 46.1 55.4 46.3 145.4 5.6XM15 21.40 101.62 28.6 52 Normal 6/6 4.0 39.1 51.5 40.0 104.6 6.6∗XM16 21.40 101.61 357.6 56 0/6 – – – – – –∗XM17 21.40 101.61 8.6 54 0/6 – – – – – –∗XM18 21.40 101.59 6.6 2 Normal 2/6 39.0 35.6 40.2 34.5 – –Formation mean direction

8/11 5.8 46.6 121.8 5.08/11 52.0 44.2 44.1 5.9

Overall mean direction (Nanxin + Jingxing Formations)13/19 353.6 50.3 19.7 9.613/19 51.2 46.4 55.7 5.6

Notes: The magnetization directions of high temperature components are listed in this Table. N and n are number of samples collected and used forpalaeomagnetic calculation, respectively. Dec. and Inc. are declination and inclination, respectively; k is the Fisherian precision parameter (Fisher 1953); α95

is the radius of cone at 95 per cent confidence level about the mean direction. Sites with asterisk (∗) are not used in the calculation of mean direction for hightemperature component.

fold test of McFadden (McFadden 1990) is positive at 95 per centconfidence level.

A mean direction with reversed polarity is obtained be-cause of negative inclination values. However, its antipodal di-rection with large clockwise deflected declination from north(Dec./Inc. = 144.2◦/49.4◦, α95 = 6.5◦, N = 4) is recognized asthe ChRM direction for West Zhengyuan locality in the MiddleCretaceous.

3.2.3 South Mengla locality

Initial NRM intensities in the specimens of this locality are in arange between 0.5 and 7.5 mA m−1. In the specimens of LowerCretaceous Jinxing Formation and the Middle Cretaceous NanxinFormation, where an initial NRM intensity of >1.5 mA m−1 is ob-served, single to two-components behaviour appeared during pro-gressive thermal demagnetization (Fig. 3). After the removal oflow-temperature component by 500 ◦C, the NRM of most spec-imens decayed linearly to the origin (as shown in the Zijderveldplot). Unblocking temperature of the high-temperature component

varies between 670 and 690 ◦C. Results of the remaining specimensshow an unstable demagnetization behaviour in the initial stage ofthermal treatment as well as after the removal of low-temperaturecomponent.

The low temperature component is isolated from eight sites.The mean direction of this component is Dec./Inc. = 4.4◦/38.5◦

α95 = 6.1◦, N = 8 in geographic coordinates. This mean directionis almost parallel to the geocentric axial dipole field direction inthe study area (D = 0, I = 38.1◦), and is attributed to a recentlyacquired VRM.

The high temperature component is obtained from eight sitesof the Jingxing Formation and six sites of the Nanxin Formation[Table 1(d)]. A tilt-corrected formation mean directions for Jingxingand Nanxin formations are Dec./Inc. = 52.0◦/44.2◦ α95 = 5.9◦ andDec./Inc. = 49.7◦/50.1◦, α95 = 13.4◦, respectively. Since meandirections of these two formations are almost identical, a combinedmean is estimated to obtain an average ChRM direction for SouthMengla locality. This combined mean direction (including 13 sites)is Dec./Inc. = 353.6◦/50.3◦, α95 = 9.6◦ in geographic coordinatesand Dec./Inc. = 51.2◦/46.4◦ α95 = 5.6◦ in stratigraphic coordinate

C© 2008 The Authors, GJI, 175, 713–728



α α

αα

αα

α α

Figure 4. Equal-area projections of the site mean directions (circles) forhigh-temperature components with 95 per cent confidence limit (shaded cir-cle) before and after tilt correction for Jindong, Zhengyuan, West Zhengyuanand South Mengla localities. Solid triangle represents the present direction ofthe Earth’s magnetic field. Solid and open symbols correspond to projectionon lower and upper hemisphere, respectively.

(Fig. 4). An optimal concentration of the DC tilt test (Enkin 2003)is achieved at 112.3 ± 42.2 per cent untilting, indicating a positivefold test at 95 per cent confidence level. The fold test of McFadden(McFadden 1990) is also positive at 95 per cent confidence level.We recognize that formation mean direction obtained from the com-bined data set (including both the Jingxing and Nanxin formations)at 100 per cent un-tilting is the ChRM direction for South Menglaarea in Early to Middle Cretaceous.

3.3 Rock magnetism

The IRM acquisition curve and backfield demagnetization of eachspecimen ensures a predominance of hematite as the magneticcarrier in the redbeds of the Cretaceous strata in the Jingdong,Zhengyuan, West Zhengyuan and South Mengla localities (Fig. 5);the saturation field is more than 2.7T and the coercivity of the re-manent is 500–1000 mT. Thermal demagnetization of compositeIRMs suggests that unblocking temperature is around 680 ◦C in allthe components (Fig. 5). The high-temperature components in thestudied localities are probably carried by hematite.

4 D I S C U S S I O N

New reliable palaeomagnetic directions are obtained from the Cre-taceous red beds collected at four different localities. Pre-foldingorigin for these palaeomagnetic data sets are ascertained throughpositive fold test. Almost northerly palaeomagnetic direction ofDec./Inc. = 8.3◦/48.8◦, k = 30.1, α95 = 7.7◦ is obtained from thered beds of Jingdong locality, while easterly deflected directionis estimated for Zhengyuan (Dec./Inc. = 61.8◦/46.1◦, k = 56.3,α95 = 8.1◦), West Zhengyuan (Dec./Inc. = 144.2◦/49.4◦, α95 =6.5◦) and South Mengla (Dec./Inc. = 51.2◦/46.4◦, k = 55.7, α95 =5.6◦) localities. The ChRM directions obtained from all four local-ities are incorporated into a single database of the Shan-Thai Block(Table 2).

Palaeomagnetic data are compared with data set of the SouthChina Block (SCB) in order to detect tectonic movement with re-spect to the SCB. We compile a highly reliable palaeomagnetic dataset from Cretaceous igneous rocks and redbeds in coastal areas of theSCB (Morinaga & Liu 2004; Li et al. 2005) and establish a palaeo-magnetic pole for the SCB (Table 2). This Cretaceous palaeopoleposition of the SCB falls close to the Cretaceous APWPs of Eurasiaor Europe (Table 2) (Besse & Croutillot 1991, 2002; Schenttino &Scotese 2005). Because we discuss tectonic deformation around theEastern Himalayan Syntaxis, we choose a plaeomagnetic pole ofthe SCB as the reference one instead of that of remote area such asEurasia and Europe.

4.1 Southward displacement of the Shan-Thai Block

The here presented Cretaceous palaeomagnetic directions of theShan-Thai block show relatively steeper inclination values thanthose expected from the Cretaceous pole positions of the SCB,indicating a southward displacement of the Shan-Thai Block assuggested in the geological literature (Leloup et al. 1995).

We evaluate the inclination flattening for each studied localitywith respect to expected inclination calculated from palaeomagneticdata of the SCB. The flattening ranges within alimited magnitudefrom −7.3 to −11.6 for the available nine reliable Cretaceous pleo-magnetic data sets (those who passed the fold tests) from the Yun-lung (two data sets), Xiaguan, Yongping, Jindong, Zhengyuan, WestZhengyuan, Pu’er and South Mengla areas (Table 2). An arithmetic

C© 2008 The Authors, GJI, 175, 713–728



Figure 5. IRM acquisition curve, back-field demagnetization (left-hand box) and thermal demagnetization of composite IRMs (2.7T, 0.4T and 0.12T inDC field for three perpendicular axes of a sample) (right-hand box) for the representative specimens of Jiangdong (SK49-6), Zhengyuan (SK28-3), WestZhenyuana (SK40-8) and South Mengla (XM01-3, XM15-1) localities. Each specimen slightly increases and does not reach saturation IRM at field 2.7T.Thermal demagnetization curves of all the components show the hematite unblocking temperature.

mean of the flattening is calculated for these nine reliable observedCretaceous palaeomagnetic data sets. The mean flattening and its95 per cent confidence limit is −9.5 ± 1.1◦, respectively. Negativeflattening with the limited magnitude suggests that the Shan-Thaiblock was located at higher latitude in Cretaceous time as a unitblock.

Assuming the Cretaceous palaeolatitude of 21.8◦N for theZhengyuan locality as a representative position of Shan-Thai blockexpected from the Cretaceous paleomagnetic pole of the SCB, theflattening of −9.5 ± 1.1◦ for the Shan-Thai Block corresponds toamount of southward latitudinal displacement of 7.3 ± 1.0◦. Wenotice that this magnitude in displacement may significantly un-derestimated because Cretaceous palaeomagenetic directions of theShan-Thai block are reported from the redbeds which show some-times shallow inclination. Nonetheless, we conclude that the Shan-Thai block experienced southward displacement as a unit block.

4.2 Rotational aspect of the Shan-Thai Block

A wide range of reliable declination data (D = 50.1◦∼144.2◦) ob-tained through present study from Zhengyuan, West Zhengyuan

and South Mengla confirms large easterly deflected declination. Al-though an easterly deflected declination (between 60◦ and 116◦)has already been reported from the red beds of Jinggu and Menglaareas of the Lanping-Simao fold belt, but the corresponding foldtests were inconclusive/or positive but only at sample level (Huang& Opdyke 1993; Chen et al. 1995). However, a recently reporteddata from Pu’er area of the Lanping-Simao belt (Sato et al. 2007)have a positive fold test at data set level.

Contrary to easterly deflected declination obtained from the cen-tral part of the Shan-Thai block, we have also discovered northerlydeclination in the rocks of Jingdong area, which is located in thenorthern part of this block (Fig. 1). Previously, northerly declina-tions, that is, D = 6.7◦ and 7.3◦, have been reported from the Creta-ceous Nanxin Formation of Xiaguan area (25.6◦N and 100.2◦E) andthe Jurassic Bazhulu Formation of Weishan area (25.4◦N, 100.2◦E),respectively (Huang & Opdyke 1993). In the former case both thefold and reversal tests were passed, while in the latter case fold testwas inconclusive due to monoclinal attitude of the sampling strata.The above mentioned three localities (including Jingdong, Xiaguanand Weishan), which gave northerly declination, are distributedwithin a restricted zone between 24.5◦N and 25.6◦N latitude. Since,

C© 2008 The Authors, GJI, 175, 713–728


722 K. Tanaka et al.T

able

2.C

reta

ceou

san

dJu

rass

icpa

laeo

mag

neti

cre

sult

sav

aila

ble

from

Sha

n-T

haiB

lock

and

SC

B.

Loc

alit

yna

me

Age

NFo

ldte

stO

bser

ved

dire

ctio

nV

GP

Rot

atio

nF

latt

enin

gR

efer

ence

Lat

.(◦ N

)L

on.(

◦ E)

Dec

.(◦ )

Inc.

(◦)

α95

(◦)

Lat

.(◦ N

)L

on.(

◦ E)

A95

(◦)

(◦)

(◦)

Rel

ativ

eto

Sha

n-T

haib

lock

Yun

long

25.8

99.4

K2

20Po

sitiv

e40

.249

.93.

954

.617

1.3

4.4

29.5

±6.

6−9

.4±

4.6

AS

ato

etal

.(19

99)

Yun

long

25.8

99.4

K2

29Po

sitiv

e38

.350

.73.

456

.717

0.1

4.0

27.6

±6.

2−1

0.2

±4.

3A

Yan

get

al.(

2001

)X

iagu

an25

.610

0.2

K2

9Po

sitiv

e6.

947

.78.

683

.615

2.7

10.0

−3.9

±11

.2−7

.3±

7.7

AH

uang

&O

pdyk

e(1

993)

Wei

shan

25.4

100.

2J3

57.

325

.310

.476

.325

0.0

10.4

−11.

4±

11.0

−9.3

±10

.1B

Hua

ng&

Opd

yke

(199

3)Y

ongp

ing

25.5

99.5

K1

12Po

sitiv

e42

.051

.115

.750

.916

7.3

20.6

31.3

±20

.9−1

1.0

±13

.0A

Fun

ahar

aet

al.(

1993

)Ji

ngdo

ng24

.510

0.8

K1-

213

Posi

tive

8.3

48.8

7.7

81.2

145.

88.

9−2

.4±

10.4

−9.8

±7.

1A

Thi

sst

udy

Lux

i24

.398

.4J2

699

.735

.211

.3−0

.511

6.6

12.2

81.6

±10

.8−1

3.0

±10

.7B

Hua

ng&

Opd

yke

(199

3)Z

heng

yuan

24.1

101.

1K

1-2

7Po

sitiv

e61

.846

.18.

134

.717

2.7

8.1

51.1

±10

.4−7

.5±

7.3

AT

his

stud

yW

estZ

heng

yuan

24.1

101.

1K

1-2

4Po

sitiv

e32

4.2

−49.

46.

4−2

5.7

135.

27.

713

3.5

±9.

0−1

0.8

±6.

2A

Thi

sst

udy

Jing

gu23

.610

0.5

J210

83.3

36.8

5.4

14.0

173.

64.

264

.6±

7.9

−23.

9±

7.1

BC

hen

etal

.(19

95)

Jing

gu23

.410

0.4

K1

–84

.439

.617

.813

.617

1.5

–73

.8±

19.2

−2.2

±14

.7A

Che

net

al.(

1995

)Ji

nggu

23.4

100.

5K

2–

295.

8−3

6.0

6.3

−13.

916

1.3

–10

5.2

±7.

71.

4±

6.1

AC

hen

etal

.(19

95)

Jing

gu23

.410

0.9

K2

879

.443

.39.

118

.917

0.0

8.9

68.8

±11

.0−5

.8±

8.1

AH

uang

&O

pdyk

e(1

993)

Pu’

er23

.010

1.0

K1-

225

Posi

tive

59.9

45.2

5.1

35.8

173.

15.

649

.3±

7.3

−8.2

±5.

4A

Sat

oet

al.(

2007

)M

engl

a21

.610

1.4

K2

1060

.837

.87.

633

.717

9.3

8.2

50.3

±8.

9−2

.8±

7.1

AH

uang

&O

pdyk

e(1

993)

Sou

thM

engl

a21

.410

1.6

K1-

214

Posi

tive

51.2

46.4

5.6

43.6

172.

16.

140

.7±

9.9

−11.

6±

5.8

AT

his

stud

yP

hong

Sal

y21

.610

1.9

J3-K

119

Posi

tive

29.8

32.7

9.1

62.4

192.

37.

619

.2±

9.7

2.5

±8.

1A

Take

mot

o(2

006)

Nan

19.2

101.

0J1

-311

Posi

tive

32.2

33.3

12.2

60.1

186.

511

.713

.4±

13.1

−28.

3±

11.4

BA

ihar

aet

al.(

2007

)K

alaw

20.7

96.5

J-K

13Po

sitiv

e44

.723

.46.

147

.219

0.6

4.8

27.3

±7.

9−1

8.2

±7.6

BR

iche

ter

&F

ulle

r(1

996)

Sou

thC

hina

Blo

ck(S

CB

)A

Cre

tace

ous

Nan

jing

32.0

119.

0K

1076

.317

2.6

10.3

Ken

teta

l.(1

986)

Anh

ui30

.811

8.2

K4

83.8

200.

314

.6G

ilde

ret

al.(

1999

)Z

heji

ang

29.0

119.

7K

1083

.224

7.9

10.6

Mor

inag

a&

Liu

(200

4)Z

heji

ang

28.4

119.

9K

977

.119

9.4

7.0

Mor

inag

a&

Liu

(200

4)F

ujia

n26

.411

7.8

K8

76.3

197.

112

.3M

orin

aga

&L

iu(2

004)

Fuj

ian

25.7

116.

8K

681

.420

8.7

10.5

Mor

inag

a&

Liu

(200

4)F

ujia

n25

.111

6.4

K5

85.5

167.

89.

9M

orin

aga

&L

iu(2

004)

Gua

ngdo

ng24

.111

5.8

K7

81.9

215.

66.

1M

orin

aga

&L

iu(2

004)

Hon

gK

ong

22.2

114.

1K

1269

.321

1.2

8.9

Lie

tal.

(200

5)M

ean

80.0

201.

94.

0B

Jura

ssic

Wan

gcha

ng32

.110

6.2

J36

66.6

236.

47.

6B

aiet

al.(

1998

)B

azho

ng31

.810

6.7

J311

67.4

236.

07.

0Y

okoy

ama

etal

.(20

01)

Jian

gyan

g30

.410

4.5

J324

61.3

222.

74.

2Y

okoy

ama

etal

.(20

01)

mea

n64

.323

1.2

6.7

Eur

asia

100

Ma

76.7

197.

15.

0B

esse

and

Cro

util

lot(

1991

)11

0M

a77

.120

1.9

9.4

Sch

entt

ino

&S

cote

se(2

005)

110

Ma

(syn

thes

is)

80.7

180.

4S

chen

ttin

o&

Sco

tese

(200

5)E

urop

e11

0M

a82

.120

3.8

8.7

Bes

se&

Cro

util

lot(

2002

)

Not

es:

Lat

.and

Lon

.are

lati

tude

and

long

itud

e.A

ge:J

,Jur

assi

c,J1

,Low

erJu

rass

ic,J

2,M

iddl

eJu

rass

ic,J

3,U

pper

Jura

ssic

;K,C

reta

ceou

s,K

1,L

ower

Cre

tace

ous;

K2,

Upp

erC

reta

ceou

s.N

isnu

mbe

rsof

site

sus

edfo

rpa

laeo

mag

neti

cst

atis

tics

.Dec

.and

Inc.

are

decl

inat

ion

and

incl

inat

ion,

resp

ectiv

ely.

α95

and

A95

are

radi

iof

cone

of95

per

cent

confi

denc

eab

outt

hem

ean

dire

ctio

nan

dth

eV

GP,

resp

ectiv

ely.

Rot

atio

nan

dfl

atte

ning

are

eval

uate

dby

aco

mpa

riso

nbe

twee

nth

eob

serv

edpa

laeo

mag

neti

cde

clin

atio

nsan

dth

ose

expe

cted

from

Cre

tace

ous

(A)

and

Jura

ssic

(B)

pala

eom

agne

tic

pole

sof

the

SC

B.U

ncer

tain

tyin

rota

tion

and

flat

teni

ngar

eca

lcul

ated

afte

rD

emar

est(

1983

).

C© 2008 The Authors, GJI, 175, 713–728



Figure 6. (a) An amount of palaeomagnetic rotation at each studied locality in the Shan-Thai Block with respect to South China Block is shown by solidarrows together with image of the Chongshan-Lancang-Chiang Mai belt. Deflection of arrows from north indicates an amount of rotation. (Jurassic: open arrow,Cretaceous: solid arrow) (LRF, Lancang River Fault; NFT, Nantinghe Fault). (b) Contours for the amount of rotation in the Shan-Thai Block. These contoursare plotted with 10◦ interval by inputting rotational data from 19 different locations (including Yunlong (two data set), Xiaguan, Weishan, Yongping, Jindong,Luxi, Zhengyuan (two data set), Jinggu (four data sets), Pu’er, Mengla, South Mengla, Phong Saly, Nan and Kalaw localities) using the General Mapping Toolsof Wessel and Smith (1991). We assume null rotation for several areas located on the tectonic boundaries between the Shan-Thai and neighbouring blocks.Although rigid clockwise rotation of about 20◦ is expected for the Shan-Thai Block, large localized clockwise rotation is obtained from the central triangleshaped area.

this zone is located in the northern part of the Shan-Thai Blockat a distance of only 30 km from the Red River–Ailaoshan Fault,northerly declination may be related to some specific localized tec-tonic features.

Using a combination of the here presented and previously re-ported Cretaceous palaeomagnetic data, an amount of rotation forthe studied localities in the Shan-Thai Block is estimated with re-spect to the SCB (Table 2). Tectonic rotation at five other localities isalso estimated using the Jurassic palaeomagnetic data. The rotationaspect for each studied locality is plotted in Fig. 6(a).

Relatively large rotational motion has occurred in the centralpart of the Shan-Thai Block along the Chongshan-Lancang-ChiangMai belt, which is clearly manifested in the rotation versus latitudeplot (Fig. 7a). With the exception of data from Jingdong, Xiaguanand Weishan localities (from the Ailaoshan–Red River fault zone),the magnitude of clockwise rotation is larger than 13◦. Rotationalmotion of more than 40◦ is observed in a limited central part ofthe studied block between 24.1◦N and 21.6◦N latitude. On the basisof third order polynomial fit, the location of maximum rotationalmotion is found to be 23.7◦N latitude.

Using the General Mapping Tools of Wessel & Smith (1991),contours of rotational angle is drawn by in-putting 19 data setsfrom 13 localities (listed in Table 2). As shown by closed circlesin Fig. 6(b), these are Yunlong (two data set), Xiaguan, Weishan,

Yongping, Jindong, Luxi, Zhengyuan (two data set), Jinggu (fourdata sets), Pu’er, Mengla, South Mengla, Phong Saly, Nan andKalaw localities. After assigning null to several localities on theboundary of Shan-Thai Block, we applied a solution using a tensionparameter T I = 0.4 (Smith & Wessel 1990; Wessel & Smith 1991) todepict first order approximation of rotation. The following rotationalfeatures can be explained by Fig. 8(b): (1) Contours of clockwiserotation of 20◦ covers almost every area of the Shan-Thai Block,(2) large rotation of more than 40◦ is only observed within triangleshaped area of 400 km × 300 km × 300 km, covering a zonebetween 22◦N–25◦N latitude and 98◦E–101◦E longitude.

The above-mentioned rotational motion not only includes a com-ponent of rigid block rotation but also some factors related to in-ternal deformation. We conclude that the Shan-Thai Block initiallyexperienced a rigid block clockwise rotation of about 20◦, and fol-lowed by an internal intrablock deformation with additional local-ized rotation of more than 30◦ within the limited triangle shapedcentral part.

4.3 Correlation between the tectonic rotation andcurvature of the Chongshan-Lancang-Chiang Mai Belt

Most of the studied palaeomagnetic localities in the Shan-ThaiBlock are distributed in the Lanping-Simao fold/thrust belt along

C© 2008 The Authors, GJI, 175, 713–728



Figure 7. Deformational features of the Shan-Thai Block through; (a) the amount of rotation as a function of latitude (b) strike of folds and faults fromLanping-Simao fold/thrust belt (observed at each sampling locality) as a function of latitude and (c) a correlation between an amount of rotation and tangentialstrike attitudes.

the Chongshan-Lancang-Chiang Mai belt (Fig. 6a). Trends of foldsand faults in the Lanping-Simao fold/thrust belt make a sinusoidalshape which is roughly parallel to the curvature of this Chongshan-Lancang-Chiang Mai belt. The most significant of these shapes isrepresented by four arcuate tectonic belts; the Lanping and Simaobelts which are convex to the southwest, and the Wulianshan andMengla belts which are convex to the northeast. The curvilinearshape of the shear zone-belt seems to have played a key role inmaking a sense of tectonic rotation. The logic behind this phe-nomenon is that localities with large clockwise tectonic rotationare positioned in a westward convex belt, whereas those with smallrotation are located in eastward convex belt.

We examined a correlation between tectonic rotation and cur-vature of this sinusoidal shape. At first, strike of the tangentialdirection along this shape is measured as a function of latitude(Fig. 7b). Deflection of strike with respect to north reaches its max-imum value at about 24◦N latitude. Fitting a third order polynomial,the maximum value is observed at 23.7◦N latitude. Almost compa-

rable variation between the magnitude of tectonic rotation and theattitude of strike (as a function of latitude) suggest a good corre-lation between the curvature of the belt and the tectonic rotation.This correlation is clearly demonstrated in a rotation versus strikeplot (Fig. 7c), where a positive correlation with a correspondingcoefficient of 0.84 is obtained.

This type of behaviour strongly suggests that the sinusoidal shapeof the Lanping-Simao fold/thrust belt and the Chongshan-Lancang-Chiang Mai belt has been developed as a result of tectonic ro-tation in the Lanping-Simao fold/thrust belt. In the light of thisinterpretation, if we restore pre-rotation geological features of theChongshan-Lancang-Chiang Mai belt, the current curvilinear shapewill change in to a straight geological picture. Left-lateral motionalong the Mae Chan Fault and right-lateral motion along the Lan-cang River Fault have probably played an important role in giving asinusoidal shape to this belt (Fig. 6a). We therefore, conclude thatthe effect of tectonic deformation in the Shan-Thai Block is reflectedin the curvilinear shape of the Chongshan shear system (CSSS in

C© 2008 The Authors, GJI, 175, 713–728



Figure 8. Model explaining different stages of deformation in the Shan-Thai block, (a) In the early stage of India–Asia collision, the Shan-Thai Blockexperienced a clockwise rotation of about 20◦, (b) Prior to 32 Ma, this block has begun to undergo a coherent southward displacement along the Red RiverFault, but a peak time for this motion was between 28 and 17 Ma, (c) During a period between 32 and 27 Ma, a sinusoidal shape of the Chongshan-Lancang-Chiang Mai belt formed under the influence of north–south compressive regimes (white arrows) (LRF, Lancang River Fault; MCF, Mae Chan Fault; NFT) and(d) During the Pliocene-Quaternary, localized microblocks separated by a network of faults within the Simao fold belt (drawn by circle) in the Lanping-Simaofold/thrust belt experienced a clockwise rotation as a result of re-activation along these faults.

Fig. 6a)-Changning-Menglian suture as well as orogenic bending inthe Lanping-Simao fold/thrust belt, which in turn is responsible fornorth–south shortening and east–west extension in the block.

4.4 Timing of tectonic deformation in the Shan-ThaiBlock

We summarize tectonic evolution of the Shan-Thai Block in the fol-lowing four important steps: (1) A rigid body clockwise rotation ofabout 20◦ in the initial stage, (2) followed by Rigid body southwarddisplacement of 7.3 ± 1.0◦, (3) formation of curvilinear shape in thesinuous Chongshan-Lancang-Chianmai belt as a result of localizedrotation and (4) localized clockwise rotation of more than 30◦ inthe central part.

Using a combination of available data (including geological,geochronological, GPS and palaeomagnetic), the timings for eachindividual deformational event is updated.

Some of the initial tectonic models about the study area predictnorthward migration of the deformational activities around easternHimalayan syntaxis as a result of continuous indentation of India into Asia (Tapponnier et al. 1982; Houseman & England 1986). Thisdiachronism of northward progression in deformation is observedthrough fault plane solution and analysis of deformational struc-tures (Lacassin et al. 1997; Wang & Burchfiel 2000; Bertrand et al.2001). While clockwise rotation is clearly observed in the ChuanDian Fragment by GPS data, relatively small amount of clockwiserotation has been recorded in the Shan-Thai Block (Wang et al.2001a: Shen et al. 2005; Gang et al. 2007). Therefore, the present

position of the Chuan Dian Fragment, which is located to the north-east of the syntaxis, can be considered as a setting analogous to thelocation of Shan-Thai Block in the Middle Cenozoic. We concludethat a rigid block rotation of about 20◦ for the Shan-Thai Block isattributed to a tectonic phenomenon happening in the early stageof India–Asia collision. Since that time, slow and steady clockwiserotational motion has occurred continuously and is still going on inthe area. Because the Khorat Plateau in the Indochina block was alsosubjected to clockwise rotation by 16◦–18◦ since Cretaceous (Yang& Besse 1993; Charusiri et al. 2006), wider region over the realmof the Shan-Thai block experienced similar clockwise rotation byabout 20◦.

The above mentioned diachronical view thus suggests that largesouthward displacement of the Shan-Thai Block with respect toSCB is ascribable to Eocene-Miocene left-lateral movement alongthe Ailaoshan–Red River shear zone, because the present day south-ward displacement of the Chuan Dian fragment is controlled by aleft-lateral movement along the Xianshuihe-Xiaojiang fault sys-tem. Recent geochronological studies assigned an ages of 34–30Ma (Th-Pb), >32 Ma (U-Pb) and 28–17 Ma (40Ar/39Ar) to left-lateral shearing activities in the Ailaoshan–Red River shear system(Leloup et al. 1995, 2001; Wang et al. 1998, 2000, 2001b; Gilleyet al. 2003). This variation in ages is justifiable in a sense that move-ment of the Shan-Thai Block was initiated at or before the 32 Ma,while major part of southward displacement has occurred between28 and 17 Ma.

Recently two phases of tectonic events has been postulated re-garding deformational activities in the Shan-Thai Block (Morley2002; Bertrand & Rangin 2003; Socquet & Pubellier 2005). An

C© 2008 The Authors, GJI, 175, 713–728



earlier phase in this context is related to Eocene–Oligocene ductiledeformation along the Chongshan shear system accompanied byextensional processes in the South China Sea. The later phase isrelated to a slowing down of sinistral and reversal motions alongthe Ailaoshan–Red River shear system. Recently reported 40Ar/39Ardata suggest that the Chongshan shear system has been subjected tosinistral strike-slip movement between 32 and 27 Ma (Leloup et al.1995; Wang et al. 2006). Since strike slip movement can producesignificant bendings and stepovers in the fault zone area (Twiss &Moores 2007), shearing activities may be taken as answer to theformation of curvature along the Chongshan-Lancang-Chiang Maibelt. This stage of shearing related deformation has probably oc-curred during left-lateral motion along the Ailaoshan–Red Rivershear zone.

The Simao belt (Fig. 6a), which experienced significantly largeclockwise rotation (60◦<), is attributed to this later phase of de-formation in the Shan-Thai Block. As shown in Fig. 1, the tri-angle shaped area is occupied by a network of left lateral faults,which played an important role in the tectonic adjustment of thisarea (Bertrand & Rangin 2003; Socquet & Pubellier 2005). Fromnorthwest to southeast this network includes the Wanding Fault,the Longling Fault, the Nantinghe Fault, the Menglian Fault, theMenxing Fault, the Nam Ma Fault, the SW–NE trending Mae ChanFault and the NW–SE trending Lancang River Fault (Shen et al.2005). The SW–NE trending Mae Chan Fault exhibits left-lateralmotion, whereas the NW-SE trending Lancang River Fault showsright-lateral motion. The Pliocene-Quaternary slip rate on thesefaults is estimated to be ∼1 mm yr−1 (Lacassin et al. 1998). Sig-nificantly large tectonic rotation of more than 60◦ in the Simaofold/thrust belt is partly accommodated by the localized rotation ofmicroblocks (thrust sheets) through the network of faults during thePliocene-Quaternary (Wang & Burchfiel 2000; Socquet & Pubellier2005; Schoenbohm et al. 2006).

5 C O N C LU S I O N

(1) Lower to Middle Cretaceous red sandstones sampled at Jing-dong, Zhengyuan, West Zhengyuan and South Mengla localities(central part of the Shan-Thai Block) provide a new reliable palaeo-magnetic results. Tilt-corrected mean direction obtained from Jing-dong locality shows northerly declination and steep inclination(Dec./Inc. = 8.3◦/48.8◦, α95 = 7.7◦, N = 13), whereas those fromZhengyuan (24.0◦N, 101.1◦E), West Zhengyuan (24.0◦N, 101.1◦E)and South Mengla (21.4◦N, 101.6◦E) localities indicate an easterlydeflected declination, such as Dec./Inc. = 61.8◦/46.1◦, α95 = 8.1◦ (N= 7), Dec./Inc. = 324.2◦/−49.4◦, α95 = 6.4◦ (N = 4) and Dec./Inc.= 51.2◦/46.4◦ α95 = 5.6◦ (N = 13), respectively. Primary nature ofthe remanent magnetization is ascertained by a positive fold test.

(2) Using an integrated palaeomagnetic database, deformationalactivities of the Shan-Thai Block is characterized by four indepen-dent deformational events; they are, (a) a rigid body southwarddisplacement of about 7.3 ± 1.0◦, (b) a rigid body clockwise rota-tion of about 20◦, (c) An extremely large localized rotation of morethan 40◦ in the triangle shaped central area, covering a zone of 300km × 300 km between 22◦N–25◦N latitude and 98◦E–101◦E lon-gitude and (d) Formation of the curvilinear (sinuous) shape withinthe Chongshan-Lancang-Chiang Mai belt as a result of local scalerotational motion in the Simao Basin.

(3) Combining the available palaeomagnetic results with Geo-logical, geochronological and GPS data, the following deforma-tional processes are proposed for the tectonic evolution of the Shan-

Thai Block (Fig. 8). Soon after the collision of India with Asia, theShan-Thai Block was subjected to rigid block clockwise rotationof about 20◦. Prior to 32 Ma, this block has begun to undergo acoherent southward displacement along the Red River Fault, but apeak time for this motion was between 28 and 17 Ma. Between 32and 27 Ma, the Chongshan-Lancang-Chiang Mai belt obtained asinusoidal shape. In the final stage of these events, localized mi-croblocks within the central part experienced an extremely largeclockwise rotation as a result of Pliocene-Quaternary reactivationalong the network of faults.

A C K N OW L E D G M E N T S

We thank J. R. Ali, D. van Hinsbergen, E. Appel and C. Langereisfor their constructive reviews of the manuscript. This work hasbeen supported by ‘The 21st Century COE Program of Origin andEvolution of Planetary Systems’ through Ministry of Education,Culture, Sports, Science and Technology (MEXT). This researchwas partly supported by the Toyota Foundation and Grant-in aid(Nos. 09041109, 11691129, 14403010 and 18403012) from theMEXT.

R E F E R E N C E S

Aihara, K. et al., 2007. Internal deformation of the Shan-Thai block in-ferred from Paleomagnetism of Jurassic sedimentary rocks in NorthernThailand, J. Asian Earth Sci., 30, 530–541.

Aitchison, J.C., Ali, J.R. & Davis, A.M., 2007. When and where did India andAsia collide? J. geophys. Res., 112, B05423, doi: 10.1029/2006JB004706.

Bai, L., Zhu, R., Wu, H. & Guo, B., 1998. Paleomagnetism of the LateJurassic northern Sichuan basin and preliminary study on the true polarwander, Acta Geophys. Sin., 41, 324–331.

Besse, J. & Courtillot, V., 1991. Revised and synthetic apparent polar wanderpaths of the African, Eurasian, North American and Indian plates, andtrue polar wander since 200 Myr. J. geophys. Res. 96, 4029–4050.

Besse, J. & Courtillot, V., 2002. Apparent and true polar wander and thegeometry of the geomagnetic field over the last 200 Myr. J. geophys. Res.107, B11, 2300, doi: 10.1029/2000JB000050.

Bertrand, G. & Rangin, C., 2003. Tectonic of the western margin of theShan plateau (central Myanmar): implication for the India-Indochinaoblique convergence since the Oligocene, J. Asian Earth Sci., 21, 1139–1157.

Bertrand, G., Rangin, C., Maluski, H., Bellon, H. & CIAC Scientific Party,2001. Diachronous cooling along the Mogok Metamorphic Belt (Shanscarp, Myanmar): the trace of the northward migration of the Indiansyntaxis, J. Asian Earth Sci., 19, 649–659.

Bureau of Geology and Mineral Resources of Yunnan province, 1990. Re-gional Geology of Yunnan province (BGMRY) (Geological Memories Se-ries 1, No.21), 728 pp., Geol. Publ. House, Beijing.

Charusiri, P., Imsamut, S., Zhuang, Z., Ampaiwan, T. & Xu, X., 2006.Paleomagnetism of the earliest Cretaceous to early late Cretaceous sand-stones, Khorat Group, Northeast Thailand: implications for tectonic platemovement of the Indochina block, Gondwana Res., 9, 310–325.

Chen, H., Dobson, J., Heller, F. & Hao, J., 1995. Paleomagnetic evidencefor clockwise rotation of the Simao region since the Cretaceous: a conse-quence of India-Asia collision, Earth planet. Sci. Lett., 134, 203–217.

Copley, A. & McKenzie, D., 2007. Models of crustal flow in the India-Asiacollision, Geophys. J. Int., 169, 683–698.

Demarest, H.H., 1983. Error analysis of the determination of tectonic rota-tion from paleomagnetic data, J. geophys. Res., 88, 4321–4328.

Enkin, R.J., 2003. The direction-correction tilt test: an all-purpose tilt/foldtest for paleomagnetic studies, Earth planet. Sci. Lett., 212, 151–166.

C© 2008 The Authors, GJI, 175, 713–728



Fisher, R.A., 1953. Dispersion on a sphere, Proc. R. Soc. Lond., Ser. A, 217,295–305.

Funahara, S., Nishiwaki, N., Miki, M., Murata, F., Otofuji, Y. & Wang, Y.Z.,1992. Paleomagnetic study of Cretaceous rocks from the Yangtze block,central Yunnan, China: implications for the India-Asia collision, Earthplanet. Sci. Lett., 113, 77–91.

Funahara, S., Nishiwaki, N., Murata, F., Otofuji, Y. & Wang, Y.Z., 1993.Clockwise rotation of the Red River fault inferred from paleomagneticstudy of Cretaceous rocks in the Shan-Thai-Malay block of western Yun-nan, China, Earth planet. Sci. Lett., 117, 29–42.

Gan, W., Zhang, P., Shen, Z.-K., Niu, Z., Wang, M., Wan, Y., Zhou, D. &Cheng, J., 2007. Presnt-day crustal motion within the Tibetan Plateauinferred from GPS measurements, J. geophys. Res., 122, B08416, doi:10.1029/2005JB004120.

Gilder, S.A. et al., 1999. Tectonic evolution of the Tancheng-Lujiang (Tan-Lu) fault via Middle Triassic to Early Cenozoic paleomagnetic data. J.geophys. Res., 104, 15 365–15 390.

Gilley, L.D., Harrison, T.M., Leloup, P.H., Ryerson, F.J., Lovera, O.M. &Wang, J.-H., 2003. Direct dating of left-lateral deformation along theRed River shear zone, China and Vietnam, J. geophys. Res., 108, B2,doi: 10.1029/2001JB001726.

Houseman, G. & England, P., 1986. Finite strain calculations of continen-tal deformation 1. Method and general results for convergent zones, J.geophys. Res., 91, 3651–3663.

Huang, K. & Opdyke, N.D., 1993. Paleomagnetic results from Cretaceousand Jurassic rocks of South and Southwest Yunnan: evidence for largeclockwise rotations in the Indochina and Shan-Thai-Malay terranes, Earthplanet. Sci. Lett., 117, 507–524.

Kent, D.V., Xu, G., Huang, K., Zhang, W.Y. & Opdyke, N.D., 1986. Pale-omagnetism of upper Cretaceous rocks from South China, Earth planet.Sci. Lett., 79, 179–184.

Kirschvink, J.L., 1980. The least-squares line and plane and the analysis ofpalaeomagnetic data, Geophys. J. R. Astron. Soc., 62, 699–718.

Lacassin, R., Scharer, U., Leloup, P.H., Arnaud, N., Tapponnier, P., Liu, X.& Zhang, L., 1996. Tertiary deformation and metamorphism SE of Tibet:The folded Tiger-leap decollement of NW Yunnan, China, Tectonics, 15,605–622.

Lacassin, R., Maluski, H., Leloup, H., Tapponnier, P., Hinthong, C.,Sorobhkdi, K., Chuaviroj, S. & Charoenravat, A., 1997. Tertiary di-achronic extrusion and eformation of western Indochina: structural andAr/Ar evidence from NW Thailand, J. geophys. Res., 102, 10 013–10 037.

Lacassin, R., Replumaz, A. & Reloup, P.H., 1998. Hairpin river loops andslip-sense inversion on southeast Asian strike-slip faults, Geology, 26,703–706.

Leloup, P.H. et al., 1995. The Ailao Shan-Red River shere zone (Yunnan,China), Tertiary transform boundary of Indochina, Tectonophysics, 251,3–84.

Leloup, P.H., Arnaud, N., Lacassin, R., Kienast, J.R., Harrison, T.M., Trong,P.T., Replumaz, A. & Tapponnier, P. 2001. New constraints on the struc-ture, thermochronology and timing of the Ailao Shan-Red River shearzone SE Asia, J. geophys. Res., 106, 6683–6732.

Li, Y., Ali, J.R., Chan, L.S. & Lee, C.M., 2005. New and revised setof Cretaceous paleomagnetic poles from Hong Kong: implications forthe development of southeast China, J. Asian Earth Sci., 24, 481–493.

Lowrie, W., 1990. Identification of ferromagnetic minerals in a rock bycoercivity and unblocking temperature properties, Geophys. Res. Lett.,17, 159–162.

Mandea, M. & Macmillan, S., 2000. International geomagnetic referencefield-the eighth generation, Earth Planets Space, 52, 1119–1124.

McFadden, P.L., 1990. A new fold test for palaeomagnetic studies, Geophys.J. Int., 103, 163–169.

Morley, C.K., 2002. A tectonic model for the Tertiary evolution of strike-slipfaults and rift basins in SE Asia, Tectonophysics, 347, 189–215.

Morinaga, H. & Liu, Y., 2004. Cretaceous paleomagnetism of the easternSouth China Block: establishment of the stable body of SCB, Earth planet.Sci. Lett., 222, 971–988.

Otofuji, Y., Liu, Y., Yokoyama, M., Tamai, M. & Yin, J., 1998. Tectonicdeformation of the southwestern part of the Yangtze craton inferredfrompaleomagnetism, Earth planet. Sci. Lett., 156, 47–60.

Ren, J.S., Jiang, C.F., Zhang, Z.K. & Qin, D.Y., 1980. The Geotetonic Evo-lution of China, 124 pp., Science Press, Beijing.

Richeter, B. & Fuller, M., 1996. Palaeomagnetism of the Shibumasu andIndochina blocks: implications for the extrusion tectonic model, in Tec-tonic Evolution of Southeast Asia, pp. 203–224, eds Hall, R. & Blundell,D., Geological Society Special Publication No. 106.

Rowley, D.B., 1996. Age of initiation of collision between India and Asia:a review of stratigraphic data, Earth planet. Sci. Lett., 145, 1–13.

Royden, L.H., Burchfiel, B.C., King, R.W., Wang, E., Chen, Z., Shen, F.& Liu, Y., 1997. Surface deformation and lower crustal flow in easternTibet, Science, 276, 788–790.

Sato, K., Liu, Y.Y., Zhu, Z.C., Yang, Z.Y. & Otofuji, Y., 1999. Paleomagneticstudy 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, westernYunnan, China: evidence of internal deformation of the Indochina block,Earth planet. Sci. Lett. 258, 1–15.

Schettino, A. & Scotese, C.R., 2005. Apparent polar wander paths for themajor continents (200 Ma to the present day): a palaeomagnetic referenceframe for global plate tectonic reconstructions, Geophys. J. Int., 163,727–759.

Schoenbohm, L.M., Burchfiel, B.C., Chen, L. & Yin, J., 2006. Mioceneto present activity along the Red River fault, China, in the context ofcontinental extrusion, upper-crust crustal rotation, and lower crustal flow,Geol. Soc. Am. Bull., 118, 672–688.

Shen, Z.K., Lu, J., Wang, M. & Burgmann, R., 2005. Contemporary crustaldeformation around the southeast boarderland of the Tibetan Plateau, J.geophys. Res., 110, B11409, doi: 10.1029/2004JB003421.

Smith, W.H.F. & Wessel, P., 1990. Gridding with continuous curvaturesplines in tension, Geophysics, 55, 293–305.

Socquet, A. & Pubellier, M., 2005. Cenozoic deformation in western Yunnan(China-Myanmar border), J. Asian Earth Sci., 24, 495–515.

Takemoto, K., 2006. Regional extent of the Cenozoic deformation of theIndochina Peninsula: paleomagnetic constraints from the Late Mesozoicred beds, Doctor’s thesis. Kobe University.

Tapponnier, P., Peltzer, G., Le Dain, A.Y., Armijo, R. & Cobbold, P., 1982.Propagating extrusion tectonics in Asia: new insights from simple exper-iments with plasticine, Geology, 10, 611–616.

Twiss, R.J. & Moores, E.M., 2007. Structural Geology, 736 pp., W.H. Free-man and Company, New York.

Vergnolle, M., Calais, E & Dong, L., 2007. Dynamics of conti-nental deformation in Asia, J. geophys. Res., 112, B11403, doi:10.1029/2006JB004807.

Wang, E. & Burchfiel, B.C., 1997. Interpretation of Cenozoic tectonics inthe right-lateral accomodation zone between the Ailao Shan shear zoneand eastern Himalayan syntaxis, Int. Geol. Rev., 39, 191–219.

Wang, E. & Burchfiel, B.C., 2000. Late Cenozoic to Holocene deformationin southwestern Sichuan and adjacent Yunnan, China, and its role information of the southeastern part of the Tibetan Plateau, Geol. Soc. Am.Bull., 112, 413–423.

Wang, E., Burchfiel, B.C., Royden, L.H., Chen, L., Chen, J., Li, W. & Chen,Z. 1998. Late Cenozoic Xianshuihe-Xiaojiang, Red River,and Dali faultsytems of southwestern Sichuan and central Yunnan, China, Spec. Pa.Geol. Soc. Am., 327, 108.

Wang, P.L., Lo, C.H., Chung, S.L., Lee, T.-Y., Lan, C.Y. & Thang, T.V.,2000. Onset timing of left-lateral movement along the Ailao Shan-RedRiver shear zone: 40Ar/39Ar dating constraint from the Nam Dinh area,northern Vietnam, J. Asian Earth Sci., 18, 281–292.

Wang, Q. et al., 2001a. Present-day crustal deformation in China constrainedby global positioning system measurements, Science, 294, 574–577.

Wang, J.-H., Yin, A., T. Harrison, T.M., Grove, M., Zhang, Y.-Q. & Xie,G.-H., 2001b. A tectonic model for Cenozoic igneous activities in theeastern Indo–Asian collision zone, Earth planet. Sci. Lett., 188, 123–133.

C© 2008 The Authors, GJI, 175, 713–728



Wang, Y., Fan, W., Zhang, Y., Peng, T., Chen, X. & Xu, Y., 2006. Kine-matics and 40Ar/39Ar geochronology of the Gaoligong and Chongshanshear systems, western Yunnan, China: implications for early Oligocenetectonic extrusion of SE Asia, Tectonophysics, 418, 235–254.

Wessel, P. & Smith, W.H.F., 1991. Free software helps map and display data,EOS., Trans. AGU 72, 441, 445–446.

Wu, H.R., Boulter, C.A., Baojia, K., Stow, D.A.V. & Wang, Z.C., 1995. TheChangning-Menglian suture zone: a segment of the major Cathaysian-Gondwana divide in Southeast Asia, Tectonophysics, 242, 267–280.

Yang, Z.Y. & Besse, J., 1993. Paleomagnetic study of Permian and Mesozoicsedimentary rocks from Northern Thailand supports the extrusion modelfor Indochina, Earth planet. Sci. Lett., 117, 525–552.

Yang, Z.Y., Yin, J.Y., Sun, Z.M., Otofuji, Y. & Sato, K., 2001. DiscrepantCretaceous paleomagnetic poles between Eastern China and Indochina:

a consequence of the extrusion of Indochina, Tectonophysics, 334, 101–113.

Yokoyama, M., Liu, Y., Halim, N. & Otofuji, Y., 2001. Paleomagnetic studyof Upper Jurassic rocks from the Sichuan basin: tectonic aspects for thecollision between the Yangtze Block and the North China Block. Earthplanet. Sci. Lett., 193, 273–285.

Yoshioka, S., Liu, Y.Y., Sato, K., Inokuchi, H., Su, L., Zaman, H. &Otofuji, Y., 2003. Paleomagnetic evidence for post-Cretaceous internaldeformation of the Chuan Dian Fragment in the Yangtze block: a conse-quence of indentation of India into Asia, Tectonophysics, 376, 61–74.

Zhang, P-Z. et al., 2004. Continuous deformation of the Tibetan Plateaufrom global positioning system data, Geology, 32, 809–812.

Zijderveld, J.D.A., 1967. A. C. demagnetization of rocks: analysis of re-sults, in Methods in Paleomagnetism, eds Collison, D.W., Creer, K.M. &Runcorn, S.K., pp. 254–286, Elsevier, Amsterdam.

C© 2008 The Authors, GJI, 175, 713–728


Documents

Tectonic deformation around the eastern Himalayan syntaxis