oroclinal origin of the simao arc in the shan-thai block inferred from the cretaceous palaeomagnetic...

Geophysical Journal InternationalGeophys. J. Int. (2012) 190, 201–216 doi: 10.1111/j.1365-246X.2012.05467.x

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Oroclinal origin of the Simao Arc in the Shan-Thai Block inferredfrom the Cretaceous palaeomagnetic data

Koichiro Kondo,1 Chuanlong Mu,2 Tatsuhiro Yamamoto,1 Haider Zaman,3

Daisuke Miura,4 Masahiko Yokoyama,1 Hyeon-Seonh Ahn1 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, China3Department of Geology and Geophysics, College of Science, King Saud University, Riyadh, Saudi Arabia4Abiko Research Laboratory, CRIEPI, 1646 Abiko, Chiba, Japan

Accepted 2012 March 18. Received 2012 March 18; in original form 2011 September 30

S U M M A R YAn active oroclinal bending is discovered in the Shan-Thai Block to the south of the easternHimalayan syntaxis. To investigate the evolution of the Simao Arc using palaeomagnetic tech-niques, the Middle Cretaceous red beds of the Nanxin Formation were sampled at the Zhengwan(22.8◦N, 100.9◦E) and Dadugang (22.4◦N, 101.0◦E) localities in the southern Simao Basin.Most of the studied samples revealed the presence of characteristic remnant magnetization withunblocking temperatures around 680 ◦C. A primary nature for this magnetization is interpretedon the basis of a positive fold and reversal test. Tilt-corrected mean directions calculated forZhengwan and Dadugang localities are characterized by large easterly deflected declination;D = 51.8◦, I = 47.9◦, ks = 45.0, α95 = 6.9◦, N = 11 and D = 64.1◦, I = 48.1◦, ks = 36.0,α95 = 7.3◦, N = 12, respectively. Steep inclination values at both these localities with respectto those expected are in the range previously reported from the Shan-Thai Block, confirmingtheir southward displacement by 6.2◦ ± 1.7◦ as part of the Shan-Thai Block. Combinationof the present data (two localities) with those previously reported from Simao Basin (sevenlocalities) reveals a positive palaeomagnetic oroclinal test, indicating that the present-day arc-like geometry of the Simao Basin was formed by oroclinal bending. Comparison with recentGPS and structural data suggest that formation of the Simao curvature started after the earlyPliocene (after 4 Ma) and continuing until the present. Origin of the Simao Arc is ascribedto southwestward movement of the crustal material across the Ailao Shan-Red River Fault(around the eastern Himalaya syntaxis), which was formed by westward movement of thedecollement with progressive eastward deepening of the Lanping-Simao Basin. Decouplingbetween the upper and the middle–lower crusts is a requisite condition for the arc formationon the continent.

Key words: Palaeomagnetism applied to tectonics; Palaeomagnetism applied to geologicprocesses; Tectonics and landscape evolution; Asia.

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

One of the peculiar topographic geometries on the Earth’s surfaceis the large-scale arcs and sinusoidal shapes, ranging in length fromhundreds to thousands of kilometres. One of the categories in thesearc shaped geometry is the island arc, which widely occurs in thesouthwestern Pacific Ocean (including the Aleutian, Kuril, Japan,Ryukyu, New Hebrides, Tonga and Kermadec arcs), the NortheastIndian Ocean (the Sunda, and Banda arcs), the Mediterranean, (theBetic-Rif, Calabrian and Hellenic arcs), the Caribbean Sea (theLesser Antilles arc) and the Scotia Sea (the Scotia arc). Severaltypes of tectonic models have been proposed to describe the arc

shape geometries of the island arcs (Schellart & Lister 2004; Heuret& Lallemand 2005), but all of them have been explained withinthe framework of interactions between the continental and oceanicplates.

Another category is the arcuate and sinusoidal shapes on thecontinents, particularly those appeared in the mountain belts. Al-though, the Alaskan and Bolivian oroclines have been observed inthe North and South American continents, respectively, the Alps,Himalayas, Andes and the Carpathian arcs are distributed over theEurasian continent (e.g. Weil & Sussman 2004). Suture and faultzones in the Tibetan plateau (the Indus-Zangbo, Jinsha and Bangon-Nujian suture zones and the Jiali, Xianshuihe and Red River Fault

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202 K. Kondo et al.

Figure 1. Topographic map of the Tibetan plateau and surrounding areas with simplified tectonic zonations (modified from Tapponnier et al. 2001). The boxedarea refers to Fig. 2. CD is for Chuan Dian Fragment. The topographic map was produced using the GMT software of Wessel & Smith (1998).

zones) also display curved shape geometry (Fig. 1). Tectonic modelsreported from the Asian continent (Tapponnier et al. 1982; House-man & England 1993; Royden et al. 2008) imply that the arc-shapedmountain belts and suture zones were formed as a result of conti-nental deformation. Because, lithospheric plates are too rigid to bedeformed by tectonic regime, the process of arc formation over thecontinent is beyond the concept of plate tectonic theory. The studyof arc formation on the continent, thus, provides information on thedynamics of continental structure and sheds light on new tectonictheorem including the plate tectonics.

Region around the eastern Himalayan syntaxis is a suitable placeto study continental deformation, because indentation of India in toAsia is still progressing, which induces deformation related forcesin to this area. Recently, deformational features of this area havebeen studied by geodetic (GPS) and geophysical techniques (Fleschet al. 2005; Lev et al. 2006; Gan et al. 2007; Rippe & Unsworth2010). Arc formation associated with continental deformationwill be studied using geological, geophysical and palaeomagenticdata.

Here, we focus on N–S directed curvilinear belt (the Chongshan-Lancang-Chiang Mai belt), which is located in the central Shan-Thai Block (Fig. 2). The Lancin River, which flows almost parallelto this belt, clearly delineates its curvilinear shape (Lacassin et al.1998; Akciz et al. 2008). This belt, which has been recognizedas a tectonic boundary between the Lanping-Simao and BaoshanTerranes of the Shan-Thai Block (Ren et al. 1980; Wu et al. 1995;Wang et al. 2006; Akciz et al. 2008), is composed of four arcuatedomains: the southwestward convex Lanping and Simao arcs andthe northeastward convex Wulianshan and Mengla arcs (Wang et al.2006). Study on the kinematic and structural evolution of thesecurvilinear belts will provide clues to our understanding of arc-shaped formation in the Asian continent as a result of Indian Platecollision and subsequent indentation.

The Simao Arc, which is about 250 km long, displays the mostsignificant arcuate feature between the Wulianshan and Menglaarcs. A salient convex southwestward structure, similar to the cur-vature of the Simao Arc, has been observed in the Lamping-SimaoTerrane. Folds and faults of the Mesozoic continental beds in the

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Oroclinal origin of the Simao Arc 203

Figure 2. Structural sketch map for the Shan-Thai Block (consist of the Baoshan and Lanping-Simao Terranes) and the Chuan Dian Fragment (after Leloupet al. 1995). Black arrows indicate the Jurassic to Cretaceous palaeomagnetic declinations from the Shan Thai Block and surrounding area (Otofuji et al. 2010).Red arrows indicate declinations from the present study. Thin lines adjacent to arrows indicate the trend of declination with respect to north. The present andprevious studies with their respective localities (1–9) are listed in Table 3.

Simao Basin make a sinusoidal shape, which is roughly parallel tothe curvature of the Chongshan-Lancang-Chiang Mai belt (Leloupet al. 1995). Palaeomagnetism is a primary technique for decipher-ing the formation of curved structural trends, because it providesa quantitative estimate of vertical-axis rotation subsequent to mag-netic remanence acquisition. The Mesozoic red beds of the NanxinFormation were collected for palaeomagnetic analysis. This forma-tion has been found suitable, because the structural/bedding controlin the area is well established, and the rocks are not affected bymetamorphism (Leloup et al. 1995; Weil & Sussman 2004; Tanakaet al. 2008).

In this paper, we focus our attention on the evolution of the ar-cuate Simao Belt. Although, some results from the northern part ofthis belt with NE–SW trending axes have been reported by Tanakaet al. (2008), reliable palaeomagnetic data from the southern part of

the belt with NW–SE trending axes are still not enough to delineatedeformational features in a systematic manner. Additional palaeo-magnetic samples have been collected from the Early Cretaceousred beds at the Zhengwan and Dadugang localities, the southernSimao Basin (Fig. 2).

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

The Shan-Thai Block is located in the northwestern IndochinaPeninsula. As shown in Fig. 1, this block is separated from theYangtze Block by the NW–SE trending Ailao Shan-Red River Fault,from the West Burma Block by the N–S trending Sagaing Fault and

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204 K. Kondo et al.

Figure 3. Simplified geological sketch map of the sampled localities (i.e., the Zhengwan at 22.8◦N, 100.8◦E and the Dadugang at 22.4◦N, 101.0◦E) in theSimao Basin. Fold axes of the anticlines are indicated by diverging arrows. Bedding attitudes at each sampling site from both localities are shown in the insets.

the Gaoligong shear zone and from the Indochina Block by the DienBien Phu Fault.

The Lanping-Simao Terrane in eastern Shan-Thai contains theProterozoic basement and the Palaeozoic marine strata (BGMRY1990). The Lanping-Simao Basin in this terrane consists of theMesozoic (Upper Triassic to Eocene) continental red beds, whichunconformably overly the Pre-Mesozoic strata. These Mesozoicred beds are generally affected by the NNW–SSE trending foldsand thrusts, but an arcuate trend is maintained in the Simao Basin(Leloup et al. 1995). Timing of fold and thrust formation remainscontroversial. Although, fairly recent fold and trust growth has beenclaimed by Leloup et al. (1995) on the basis of their study on rampanticlines, other researchers (Wang & Burchfiel 2000; Socquet &Pubellier 2005; Akciz et al. 2008) have suggested an older period(between the Paleocene and Oligocene) for these activities.

The Cretaceous sequence in the Simao Basin is subdivided intofour different formations: the Lower Cretaceous Jinxing Forma-tion, the Middle Cretaceous Nanxin Formation, the Upper Creta-ceous Hutousi Formation and the Upper Cretaceous MankuanheFormation (BGMRY 1990; Leloup et al. 1995). The Jinxing For-mation is mainly composed of greyish sandstone intercalated withpurplish-red, greyish and green mudstone. The presence of richLamellibranchiate (such as Estheria, Darwinula, Gasterpods andSporopollen) indicates the Early Cretaceous age for this forma-tion. The Middle Cretaceous Nanxin Formation mainly consists

of purplish-red sandstone, which conformably overlies the EarlyCretaceous Jinxing Formation. This formation is overlain by theUpper Cretaceous Hutousi Formation. The occurrences of Estheria,Cypridea(c), Gasterpods, and Sporopollen indicate an age of MiddleCretaceous for the Nanxin Formation. The Upper CretaceousMankuanhe Formation is distributed only in the Pu’er-Mengla area.

As shown in Fig. 3, the Middle Cretaceous red sandstones andsiltstones of the Nanxin Formation have been sampled for palaeo-magnetic study at the Zhengwan (22.8◦N, 100.8◦E) and Dadugang(22.4◦N, 101.0◦E) localities, located to the south and west of Simao,respectively. These two localities are 40 km apart and the NanxinFormation in these localities reveals monoclinal structure withNNW–SSE trending axes (∼N24◦W at the Zhengwan and ∼N45◦Wat the Dadugang). Palaeomagnetic samples were collected from 11sites at the Zhengwan locality and from 12 sites at the Daduganglocality. Dipping attitudes in the range of 41◦–107◦ and 4◦–46◦ havebeen observed at the Zhengwan and Dadugang localities, respec-tively, which are sufficient enough to be used for palaeomagneticfold test.

About 10 block samples, oriented with a magnetic compass, werecollected at each site from a single outcrop over a horizontal dis-tance of up to 18 m. The present declination at each samplingsite has been evaluated using International Geomagnetic ReferenceField (International Association of Geomagnetism and Aeronomy,Working Group V-MOD 2010).

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3 L A B O R AT O RY P RO C E D U R E

One or more specimens, 25 mm in diameter and 22 mm in height,were prepared from each sample. Natural remnant magnetization(NRM) of each specimen was measured using a non-shielded 2GEnterprises cryogenic magnetometer at the palaeomagnetic labo-ratory of the Kobe University. Stepwise thermal demagnetizationwas carried out up to 690◦C using Natsuhara TDS-1 thermal de-magnetizer, where the residual field in the furnace was less than5 nT. Magnetic behaviour of each specimen after complete demag-netization procedure was plotted on Zijderveld diagrams (Zijderveld1967). Principal component analysis (Kirschvink 1980) was usedto determine directional behaviour of different magnetization com-ponents, whereas Fisherian statistics (Fisher 1953) have been usedto calculate site-mean directions.

4 PA L A E O M A G N E T I S M :D E M A G N E T I Z AT I O N R E S U LT SW I T H M E A N D I R E C T I O N S

4.1 The Zhengwan locality

Specimens from the Zhengwan locality reveal initial NRM intensi-ties between 0.67 and 13.8 mA/m. Most of the studied specimensgenerally exhibit a two components behaviour. After the removal ofa low-temperature component (LTC) between 250 ◦C and 350 ◦C,a high-temperature component (HTC) is isolated with linear decaytowards the origin up to 690 ◦C (Fig. 4).

Magnetization related to the LTC is identified in 55 samples. Amean direction of D = 358.3◦, I = 40.0◦, α95 = 11.3◦ (N = 55) isestimated for this component in the geographic coordinates, which

Figure 4. Orthogonal vector plots (Zijderveld diagrams) of demagnetized samples from the Zhengwan (a) and Dadugang (b) localities of the Nanxin Formation.Numbers adjacent to demagnetization path are heating steps in degree Celsius. Open (solid) symbols show projection on to vertical (horizontal) plane. Alldirections are plotted in geographic coordinates.

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206 K. Kondo et al.

is almost parallel to the axial geocentric dipole direction (D = 0◦,I = 40.0◦). This type of behaviour indicates that the LTC was ac-quired as a secondary magnetization during the Bruhnes Chron.In the blocking diagrams of Pullaiah et al. (1975), an unblockingtemperature of 350 ◦C has been linked to a relaxation time of about1 Myr for hematite, during which an axial geocentric dipole direc-tion is recorded as a viscous remnant magnetization (VRM) by thehematite grains.

The HTC from this locality, which is considered as a character-istic remnant magnetization (ChRM), is identified in samples from11 sites (Table 1). When plotted in stereographic projection, theirdirections indicate two groupings before tilt-correction (Fig. 5); awesterly declination with positive inclination (D ∼ 270◦, I ∼ 30◦)

and an easterly declination with negative inclination (D ∼ 70◦, I ∼–30◦). As a result of the progressive untilting procedure, these twogroups of directions behaved in antipodal manner. The tilt-correctednormal (D = 50.2◦, I = 45.9◦, Kg = 53.0, α95 = 7.7◦, N = 8) andreversed (D = 236.1◦, I = –53.2◦, Ks = 29.8, α95 = 23.0◦, N = 3)polarity mean directions from these two groups reveal a positivereversal test (McFadden & McElhinny 1990) with classification C’.An angular distance (γ ) obtained by this procedure is 8.2◦, whichis less than the critical angle γ c = 15.6◦. After flipping the reversedpolarity directions into normal, an optimal concentration of the DCtilt test (Enkin 2003) is achieved at 90.3 ± 17.6 per cent unfolding,indicating a positive result at the 95 per cent confidence level. Theformation mean direction for the 11 sites (D = 51.8◦, I = 47.9◦,

Table 1. Palaeomagnetic results of the high-temperature component identified in Middle Cretaceous rocks of the Nanxin Formation. Bedding at-titude is given as strike and dip angles. N is the number of samples measured/used for the calculation of site or formation mean directions.D and I are declination and inclination, k is the Fisherian precision parameter (Fisher 1953); α95 is the radius of the cone at 95 per cent confidencelevel about the mean direction. VGP is virtual geomagnetic pole and A95 is the radius of the cone at 95 per cent confidence level about the VGP.

Site LocalityBeddingattitude Polarity N In situ Tilt corrected k α95

(◦N) (◦E) (◦) (◦) D (◦) I (◦) D (◦) I (◦) (◦)

[Zhengwan locality]SK13 22.76 100.82 319.2 96 N 10 244.5 39.9 35 42.1 151.2 3.9SK14 22.76 100.82 317.2 98 N 9 229.6 29.5 45.8 52.4 16.6 13SK15 22.76 100.83 332.2 98 N 10 255 28 45.6 52.5 33 8.5SK16 22.75 100.83 352.2 99 N 10 269.9 30.7 73.9 49.8 40.3 7.7SK20 22.76 100.84 321.2 107 R 10 61.2 –29.2 221 –42.9 69.4 5.8SK21 22.76 100.84 336.2 97 R 10 73.4 –27.9 237.2 –54.6 10.1 16.0SK22 22.76 100.84 336.2 64 R 9 53.2 –55.8 261.9 –58.6 227.8 3.4SK23 22.77 100.94 352.2 54 N 10 1.4 66.7 56.2 29.5 123.6 4.4SK24 22.77 100.94 351.2 41 N 10 356.9 66.4 49.5 42 89.7 5.1SK26 22.77 100.94 343.2 50 N 10 309.2 77.3 58.8 46.3 326.6 2.7SK27 22.77 100.95 337.2 49 N 10 307.3 71 43.2 48.2 31.3 8.8

[Mean] 22.76 100.9Normal polarity N 8 269.2 59.1 6.0 24.6

50.2 45.9 53.0 7.7Reversed polarity R 3 63.7 –37.8 20.6 27.9

236.1 –53.2 29.8 23.0All sites N + R 11 267.5 57.4 5.2 22.1

51.8 47.9 45.0 6.9

(◦N) (◦E) Lat. (◦) Long. (◦) A95

VGP 22.76 100.9 11 42.4 170.1 7.8

[Dadugang locality]SK01 22.45 101.03 302.2 41 N 10 64.5 74.7 45.8 33.0 26.4 9.6SK02 22.45 101.03 300.2 15 N 9 123.3 64.7 91.8 61.7 33.5 9.0SK03 22.42 101.00 4.2 13 N 10 79.8 56.0 83.9 43.4 84.7 5.3SK04 22.42 101.00 346.2 13 N 7 70.3 61.8 72.5 49.0 195.4 4.3SK05 22.42 101.00 294.2 15 N 7 73.3 47.9 63.9 37.7 71.3 7.2SK06 22.45 101.03 320.2 42 N 9 208.3 73.0 63.6 62.4 24.5 10.6SK07 22.43 101.02 329.2 46 N 10 162.5 76.6 77.0 45.2 80.9 5.4SK08 22.40 101.00 313.2 4 N 9 54.5 51.3 53.6 47.4 87.4 6.0SK09 22.42 101.00 219.2 28 N 10 64.6 44.0 36.8 48.7 120.0 4.4SK10 22.45 101.03 334.2 38 N 9 113.2 84.4 69.8 48.0 54.8 7.0SK11 22.42 101.00 303.2 16 N 8 57.9 58.6 50.2 43.6 201.9 4.3SK12 22.45 101.03 345.2 37 N 9 55.3 82.3 71.2 45.4 30.9 9.4

[Mean] 22.42 101.0All sites N 12 78.1 69.0 64.1 48.1 16.1 11.2

36.0 7.3

(◦N) (◦E) Lat. (◦) Long. (◦) A95

VGP 22.42 101 12 32.3 169 8.6

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Figure 5. Equal-area projections of the site mean directions for the Zhengwan (a) and Dadugang (b) localities. Solid symbols are projections on to lowerhemisphere. Mean direction is shown by red star with 95 per cent confidence circle.

Kg = 45.0, α95 = 6.9◦, N = 11) is considered as a primary MiddleCretaceous ChRM direction for the Zhengwan locality.

4.2 The Dadugang locality

Initial NRM intensities observed in the specimens of the Daduganglocality range between 1.4 and 33.8 mA/m, which is almost doubleof that obtained from the Zhengwan locality. Most of the specimensgenerally exhibit two magnetization components, where the LTC isunblocked by 400 ◦C, and the HTC is observed through linear decayto the origin by 690 ◦C (Fig. 4).

Except the results from sites SK07 and SK12, an in situ mean di-rection of D = 11.0◦, I = 46.6◦, α95 = 15.7◦ (N = 23) is calculatedfor the LTC of this locality, which is subparallel to the axial geocen-tric dipole direction (D = 0◦, I = 39.5◦). Similar to the Zhengwanlocality, this VRM related LTC is probably carried by hematite. Anenigmatic LTC direction (D = 140◦ ∼ 180◦, I = –40 ∼ 60◦) isobserved in the remaining two sites (SK07 and SK12) from thislocality.

The HTC is identified in 12 sites from the Dadugang locality,where α95 for each site mean direction is found to be less than 10.6◦

(Table 1). As evident from equal area projection (Fig. 5), the insitu site mean directions from these 12 sites indicate an elongateddistribution between D = 64◦, I = 44◦ and D = 208◦, I = 73◦.However, with the application of tilt-correction these site meandirections clustered around D = 64.1◦, I = 48.1◦, α95 = 7.3◦, with

increase in precision parameter (k) by a factor of 2.2 (kg = 16.1versus ks = 36.0).

The DC tilt test (Enkin 2003) applied to all 12 sites from theDadugang locality gives an optimal concentration at 79.5 ± 32.1per cent unfolding, which indicates a positive fold test at the 95per cent confidence level. Following the fold tests of McFadden’s(1990) and Cogne (2003), a value of 7.3355 is obtained for ξ 2 in thegeographic coordinates and 2.0876 in the stratigraphic coordinates,whereas critical values (ξ c) of 3.865 and 5.378 have been obtainedat 95 per cent and 99 per cent confidence levels, respectively. We,therefore, interpret the formation mean direction at 100 per centunfolding as a ChRM for the Dadugang locality.

5 RO C K M A G N E T I S M

Rock magnetic properties have been investigated using 10 speci-mens from five sites, covering both the Zhengwan and Daduganglocalities (Fig. 6). Progressive acquisition of isothermal remnantmagnetization (IRM) was performed up to a maximum field of 2.7T using 2G–pulse magnetizer. Thermal demagnetization of three-component IRMs (2.7 T, 0.4 T and 0.12 T along z, y and x axes ofeach specimen, respectively) was conducted to isolate unblockingtemperature spectra according to the method of Lowrie (1990).

The IRM acquisition curves for most of the studied specimensreveal hematite as a dominant magnetic carrier in the Cretaceousred beds from the Zhengwan (Fig. 6a) and Dadugang (Fig. 6b)localities; where the saturation field is more than 2.7 T. Thermaldemagnetization of three-component IRMs suggests an unblocking

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Figure 6. Rock magnetic experiments on selected specimens from the Nanxin Formation. IRM acquisition and thermal demagnetization of three-axis IRMs(imparted by DC fields of 2.7 T, 0.4 T and 0.12 T along three perpendicular axes) curves for the Zhengwan (a) and Dadugang (b) localities.

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temperature around 680 ◦C for all three components (Fig. 6). Theserock magnetic parameters indicate hematite as a carrier of HTC inthe studied red bed samples.

6 D I S C U S S I O N

6.1 Reliability of the present palaeomagnetic results

In addition to the standard demagnetization procedures performedfor this study, reliability of the palaeomagnetic data has been testedby palaeomagnetic stability tests. The positive fold and reversal testsreveal a primary origin for the HTC in the studied samples. Reliabil-ity and primary nature of the present results is further supported bythe observed inclinations (Io), i.e. 47.9◦ and 48.1◦ for the Zhengwanand Dadugang localities, respectively (Table 2), which are 8.1◦ and8.8◦ (Io–Iexp) steeper than the inclinations expected (Iexp) from theEurasian palaeomagnetic pole (Besse & Courtillot 2002) for theselocalities. These are compatible to a difference between the ob-served and expected Cretaceous inclinations (7.4◦ ± 2.6◦) reportedfrom the Shan-Thai Block (K1 and K2 in Table 2), which has beendeclared primary as a result of the positive fold test. This compat-ibility between the present and previously reported data sets (Satoet al. 2011) confirm a widespread presence of steep inclinations inthe region, which tectonically suggest a southward displacement ofthe Shan-Thai Block (Leloup et al. 1995).

Combining the present data with previous palaeomagnetic re-sults provides a more accurate estimate of the magnitude of south-ward displacement for the Shan-Thai Block. An arithmetic meanof flattening is calculated for 12 reliable Cretaceous (K1 and K2)palaeomagnetic data sets (including 10 from previous studies andtwo from this study). An average flattening for these 12 sites andits 95 per cent confidence limit is –7.9◦ ± 2.1◦. Negative value offlattening suggests that the Shan-Thai Block was located at higherlatitude (by 6.2◦ ± 1.7◦) as a composite unit during the Cretaceous.

6.2 Oroclinal bending of the Simao Arc

Formation of arcuate belts has been categorized into two end-member-groups, that is, primary arcs and secondary oroclines (Weil& Sussman 2004). Primary arcs are formed without any vertical-axis rotations, whereas in the case of secondary orocline an orig-inally linear belt adopts a curved shape as a result of secondarydeformation accompanied by vertical-axis rotations. Because,palaeomagnetism has been satisfactorily used to study oroclinalstructure in the past (Schwartz & Van der Voo 1983; Weil & Sussman2004), the same is applied to the arc like fold–thrust belt in the SimaoBasin through this study.

To judge kinematic models of oroclinal formations, an oroclinaltest (Schwartz & Van der Voo 1983; Yonkee & Weil 2010) and afold test (McFadden’s 1990) are applied (Figs 7 and 8). At first, anamount of rotation in the study area is estimated and then recon-structed for bringing the arcuate shape in to straight bedding attitude.For this purpose, two methods have been adopted; (1) geometricalapproximation and (2) estimation from geological structure. In ge-ometrical approximation (Fig. 8), the Simao Arc is assumed to bean arc of a reference circle, which has a radius of 166 km and itscentre is at 23.33◦N latitude 101.20◦E longitude. This arc of a circleis then cut by a straight line of NW20◦ strike (So), which passes viatwo points (24.10◦N, 100.50◦E and 21.9◦N, 101.35◦E) in the SimaoArc. For a secondary orocline, this line would represent an originalstraight belt before the formation of curvature. Regional strike (S) is

defined as tangential direction to a concentric circle of the referencepoint 1, which passes through the study area (Table 3). Because, thenorthern Zhengyuan locality (marked as Zhengyuan N in Table 3) issituated in the area close to the inflection point (between the Simaoand Wulianshan Arcs); regional strike is estimated from a line tan-gential to major structural trends (fold axes) of the Mesozoic cover(Leloup et al. 1995). Finally, deviation of regional strike (S) fromthat of a reference straight line (strike = So) is calculated (Table 3),which corresponds to an amount of clockwise rotation around thevertical axis for each locality. This rotation is then used to take backthe present arcuate structure in to its original straightened form.

As shown in Fig. 7, an oroclinal or strike test of Schwartz &Van der Voo (1983) has been applied. Amounts of tectonic rotationare plotted against deviations (S – So) of the regional strike (S)from reference strike (So). The amount of rotation for each localityis defined as deviation (D – Do) of the observed declination (D)from reference declination (Do). The reference declination is thatexpected from the European APWP (Besse & Crourtillot 2002).Because, the declination data are required to properly apply thisapproach; nine data sets (eight from the Cretaceous and one fromthe Jurassic) have been used for this purpose (Table 2).

A linear regression line is drawn for the data in the plot of (D – Do)versus (S – So) (Fig. 7). As shown in figure, the line is expressedin terms of rotation, that is, (D – Do) = 53.8 + 1.12 × (S – So).A value of 53.8 is total amount of tectonic rotation for the SimaoBasin. A slope of 1.12 together with a correlation coefficient (R)larger than 0.85 (R2 = 0.72) points to a significantly reliable levelof palaeomagnetic oroclinal test.

As an alternative approach, a folding test is applied to the Cre-taceous palaeomagnetic directions (tilt-corrected) available fromeight localities of the Simao Basin, because the arc formation in thearea was instigated by vertical-axis folding. As shown in Fig. 8(a),these directions indicate an arc-type distribution with respect to thepresent arcuate geometry. To estimate palaeomagnetic directions forthe original straightened attitude, a CCW rotation around a verticalaxis has been applied using the strike deviation (S – So) parame-ters. The newly corrected palaeomagnetic directions (Fig. 8b) showtight clustering with high precision parameter (kstraight = 43.9). Anincrease in precision parameter (kstraight/karc = 3.18) strongly ad-vocates a positive fold test at the 95 per cent confidence level(McElhinny 1964). As mentioned above, the fold tests (McFad-den 1990; Cogne 2003) gave estimates of 6.21151 and 1.66638 forξ 2 parameter in the geographic coordinates and stratigraphic coor-dinates, respectively. In addition, the critical values of 3.298 and4.562 are obtained for ξ c at 95 per cent and 99 per cent confidencelevels, respectively. We, therefore, claim that the formation meandirection from the original straightened beds represents a ChRM forthe Simao Basin. This concludes that the straight line with NW20◦

strike (So) is statistically significant as an original geometry beforethe arc formation at 95 per cent confidence.

These two tests, thus, attribute the formation of the Simao Arc tooroclinal bending associated with appreciable vertical-axis rotationin the study area.

6.3 Timing and mechanism of oroclinal bendingin the Simao Arc

Tectonic activities related to fold and thrust formation in the SimaoBasin gives hints on the timing of oroclinal bending. The axes ofmany folds in the Simao Basin show arcuate trends that are convex

C© 2012 The Authors, GJI, 190, 201–216


210 K. Kondo et al.

Tab

le2.

Jura

ssic

toC

reta

ceou

spa

laeo

mag

neti

cre

sult

scu

rren

tlyav

aila

ble

from

the

Sha

n-T

haiB

lock

.Lat

.and

Lon

g.ar

ela

titu

dean

dlo

ngit

ude,

resp

ectiv

ely.

Age

s:J

,Jur

assi

c,J1

,Ear

lyJu

rass

ic,J

3,L

ate

Jura

ssic

,K,C

reta

ceou

s,K

1,E

arly

Cre

tace

ous;

K2,

Lat

eC

reta

ceou

s.N

,num

ber

ofsi

tes

used

for

pala

eom

agne

tic

stat

isti

cs.D

and

Iar

ede

clin

atio

nan

din

clin

atio

n,re

spec

tivel

y.α

95is

ara

dius

ofth

eco

neof

95pe

rce

ntco

nfide

nce

abou

tthe

mea

ndi

rect

ion,

resp

ectiv

ely.

Fiel

dte

sts;

reli

abil

ity

ofda

tais

asce

rtai

ned

thro

ugh

posi

tive

fold

test

.An

amou

ntof

rota

tion

(D–

Do)

isev

alua

ted

byco

mpa

ring

the

obse

rved

pala

eom

agne

tic

decl

inat

ions

wit

hth

ose

(Do)

expe

cted

from

the

Eur

asia

nA

PW

Ps

(Bes

se&

Cou

rtil

lot

2002

).In

clin

atio

nsh

allo

win

g(fl

atte

ning

)an

dno

rthw

ard

lati

tudi

nal

disp

lace

men

tar

eev

alua

ted

byco

mpa

ring

the

obse

rved

pala

eom

agne

tic

incl

inat

ion

(Iob

s)an

dpa

laeo

lati

tude

(λob

s)w

ith

thos

eex

pect

edfr

omth

eA

PW

P;

shal

low

ing

=I e

xp–

I obs

and

nort

hwar

dla

titu

dina

ldi

spla

cem

ent=

λex

p–

λob

s.U

ncer

tain

ty(D

elta

)in

rota

tion

and

lati

tudi

nald

ispl

acem

enti

sca

lcul

ated

usin

gth

em

etho

dof

Dem

ares

t(19

83).

Loc

alit

yA

geN

Roc

kty

peFo

ldte

stO

bser

ved

dire

ctio

nL

at.d

ispl

ace

Ref

eren

ceN

ame

Lat

.(◦ N

)L

ong.

(◦E

)D

(◦)

I(◦

)α

95(◦

)R

otat

ion

(D–

Do)

Del

taS

hall

owin

gD

elta

(Nor

thw

ard)

Del

ta

Sha

n-T

haiB

lock

Yun

long

25.8

99.4

K2

20R

edbe

dsPo

sitiv

e40

.249

.93.

931

.26.

2–9

.55.

7–7

.65.

9S

ato

etal

.(19

99)

Yun

long

25.8

99.4

K2

29R

edbe

dsPo

sitiv

e38

.350

.73.

429

.35.

8–1

0.3

5.4

–8.4

5.7

Yan

get

al.(

2001

)X

iagu

an25

.610

0.2

K2

9R

edbe

dsPo

sitiv

e6.

947

.78.

6–2

.111

.0–7

.48.

3–5

.89.

3H

uang

&O

pdyk

e(1

993)

Wei

shan

25.4

100.

2J3

5R

edbe

dsIn

conc

lusi

ve7.

325

.310

.4–8

.010

.026

.29.

918

.99.

9H

uang

&O

pdyk

e(1

993)

Yon

gpin

g25

.599

.5K

112

Red

beds

Posi

tive

42.0

51.1

15.7

28.9

20.8

–8.2

12.7

–6.8

16.6

Fun

ahar

aet

al.(

1993

)Ji

ngdo

ng24

.510

0.8

K1–

213

Red

beds

Posi

tive

8.3

48.8

7.7

–0.7

10.2

–9.9

7.8

-7.8

8.5

Tana

kaet

al.(

2008

)L

uxi

24.3

98.4

J26

Red

beds

Inco

nclu

sive

99.7

35.2

11.3

84.3

11.8

14.7

10.5

11.3

11.1

Hua

ng&

Opd

yke

(199

3)Z

heng

yuan

N24

.210

1.0

K1–

27

Red

beds

Posi

tive

61.8

46.1

8.1

52.8

10.2

–7.7

8.0

–5.8

8.0

Tana

kaet

al.(

2008

)Z

heng

yuan

S24

.010

1.0

K1–

24

Red

beds

Posi

tive

144.

249

.46.

413

5.2

8.8

–11.

07.

0–8

.67.

8Ta

naka

etal

.(20

08)

Jing

gu23

.610

0.5

J210

Red

beds

Inco

nclu

sive

83.3

36.8

5.4

68.3

6.7

12.9

6.8

10.0

6.3

Hua

ng&

Opd

yke

(199

3)Ji

nggu

23.4

100.

4K

1–

Red

beds

Inco

nclu

sive

84.4

39.6

17.8

71.5

19.1

0.9

0.6

Che

net

al.(

1995

)Ji

nggu

23.4

100.

5K

27

Red

beds

Posi

tive

115.

836

.06.

310

6.9

7.4

1.3

0.9

Che

net

al.(

1995

)Ji

nggu

23.4

100.

9K

28

Red

beds

Inco

nclu

sive

79.4

43.3

9.1

70.5

10.8

–5.9

8.7

–4.3

8.5

Hua

ng&

Opd

yke

(199

3)P

u’er

23.0

101.

0K

1–2

25R

edbe

dsPo

sitiv

e59

.945

.25.

151

.07.

0–8

.46.

2–6

.26.

5S

ato

etal

.(20

07)

Zhe

ngw

an22

.810

0.9

K1

11R

edbe

dsPo

sitiv

e51

.847

.96.

939

.08.

7–8

.15.

9–6

.56.

5T

his

stud

yD

adug

ang

22.4

101.

0K

112

Red

beds

Posi

tive

64.1

48.1

7.3

51.3

9.2

–8.8

6.2

–6.9

7.1

Thi

sst

udy

Men

gla

21.6

101.

4K

210

Red

beds

Inco

nclu

sive

60.8

37.8

7.6

51.9

8.6

–3.0

7.7

-2.0

8.1

Hua

ng&

Opd

yke

(199

3)S

outh

Men

gla

21.4

101.

6K

1–2

14R

edbe

dsPo

sitiv

e51

.246

.45.

642

.37.

6–1

1.8

6.5

–8.7

6.8

Tana

kaet

al.(

2008

)P

hong

Sal

y21

.610

1.9

J3–K

119

Red

beds

Posi

tive

29.8

32.7

9.1

17.1

9.5

5.7

7.5

3.8

6.4

Take

mot

oet

al.(

2009

)K

alaw

20.7

96.5

J–K

13R

edbe

dsPo

sitiv

e44

.723

.46.

136

.26.

69.

06.

85.

46.

1R

icht

er&

Ful

ler

(199

6)N

an19

.210

1.0

J1–3

11R

edbe

dsPo

sitiv

e32

.233

.312

.217

.812

.311

.511

.18.

210

.7A

ihar

aet

al.(

2007

)

Eur

ope

(Bes

se&

Cou

rtil

lot2

002)

80M

a(K

2)12

0M

a(K

1)15

0M

a(J

,J1–

J3)

C© 2012 The Authors, GJI, 190, 201–216



Figure 7. A palaeomagnetic oroclinal test, where tectonic rotation (D – Do) with respect to Eurasia is plotted against deviations (S – So) between the regionalstrike (S) and a reference strike (So). Error bar is uncertainty in rotation with 95 per cent confidence level. Numbers adjacent to black bars are locality numbersas listed in Table 3. Also shown is a linear regression line through data sets; (D – Do) = 53.8 + 1.12 × (S – So). A correlation coefficient (R) is 0.85(R2 = 0.72).

westward and parallel to the Simao Arc. Observations of thesefeatures suggest that the arc-shaped folds in the study area formedduring the oroclinal bending of the Simao Arc.

As shown in Figs 2 and 9, the Simao Arc and folds belt is posi-tioned ahead of the curved part of the Ailao Shan-Red River Fault.The curved geometry of the Ailao –Shan-Red River Fault is inferredto be deformational in origin, which has been attributed to inden-tation inflicted by the southward moving Chuan Dian Fragment(Socquet & Pubellier 2005). This crustal fragment experienced adisplacement along the Xianshuihe-Xiaojiang Fault from at least∼4 Ma to the present (Wang & Burchfiel 2000; Shen et al. 2005).According to the GPS data, this crustal block is currently mov-ing southwestward (in the Yunnan province) by pushing the AilaoShan-Red River Fault into the Simao Basin (Shen et al. 2005; Ganet al. 2007). Because, the arcuate shaped fold–thrust belt in theSimao Basin was probably formed simultaneous to curvature build-ing along the Ailao Shan-Red River Fault, a period from about 4 Mato the present is considered as the most likely age for the formationof Simao Arc.

Recent origin is inferred for the oroclinal bending through mod-ern GPS/geodetic studies in East Asia (Wang et al. 2001; Zhanget al. 2004; Gan et al. 2007; Copley 2008), which suggests south-ward flow of crustal material into southern Tibet and the Yunnanprovince accompanied by rotational motion around the easternHimalayan syntaxis. Gan et al. (2007) and Copley (2008) haveidentified differential rotation in the southward moving zones of theSimao Basin. Gan et al. (2007) have estimated a CCW rotation of7.5 nanorad/yr in the northern Simao Basin (101.5◦E, 23.5◦N) buta relatively higher rate of CCW rotation (20.7 nanorad/yr) in thesouthern Simao Basin (101.5◦E, 22.5◦N). Comparison of the GPSdata from different parts of the Simao Basin suggests that althoughthe CCW rotation governs the Simao Basin region, local CW rota-

tional motion progresses in the north, which reduces the magnitudeof CCW rotation in this area relative to the south. This mode ofdifferential rotation is consistent with the formation of an arcuatestructure (convex toward west) in the Simao Basin. All these mod-ern geodetic investigations point to ongoing oroclinal bending inthe study area.

An active left lateral Nanting Fault (NTF) and a right lateralLancing Fault (LCF) support the active nature of oroclinal bending,and both of them seem to play an important role in accommodatingthe formation of the Simao Arc (Leloup et al. 1995; Wang et al.1998; Socquet & Pubellier 2005; Wang et al. 2008). Northeasterlymovement of the northern Simao Basin along the NTF developedthe northern Simao Arc, whereas southeasterly moving part of theSimao Basin along the LCF produced its southern counterpart. Allthese features suggest an active oroclinal bending in the Simao Arc.

Formation of the Simao Arc requires westward movement of thecrustal materials in the Simao Basin (Fig. 9). Structural and geolog-ical observation suggests westward displacements on the order of20 ∼ 40 km for the Mesozoic cover in the central part of the SimaoBasin (Leloup et al. 1995). This motion was accommodated byprogressive eastward deepening of the regional decollement alonga series of west-directed thrusting. Based on the information col-lected from the west-directed thrusts together with ramped anticlinefolds and the current topographic relief, recent movements havebeen inferred in the Simao Basin (Leloup et al. 1995). Decouplingbetween the upper and the middle–lower crusts associated with for-mation of the decollement is geophysically inferred. Analysis ofP-wave velocity confirms that surface deformation in the uppercrust does not extend to the lower crust because of mechanical de-coupling (Flesch et al. 2005; Lev et al. 2006; Zhang & Wang 2009;Huang et al. 2009). The presence of a weak middle–lower crust hasalso been inferred from magnetotelluric studies in the Simao Basin

C© 2012 The Authors, GJI, 190, 201–216


212 K. Kondo et al.

Figure 8. The Cretaceous palaeomagnetic directions for the Simao Basin after a simplified tectonic reconstruction of the Simao Arc. Configurations of thestudied area through different ages are given as following. (a) Present: The present configuration of the Lanping, Wulianshan and Simao arcs. An arc of areference circle with a radius of 166 km (red circle) represents the Simao Arc. Strike of a reference line (N20◦W) is shown by blue dashed line. The Cretaceouspalaeomagnetic directions from the Simao Basin are plotted in present configuration. Locality numbers 1–9 are those shown in Fig. 2 and Table 3 NTF; theNanting Fault, LCF; the Lancing Fault. (b) 17 ∼ 4 Ma: The Simao Arc is straightened to the reference line after restoring it to a N20◦W strike. The reconstructedstraight line for Simao is shown by blue line. As shown in the stereographic projection, tight clustering appeared after restoring the palaeomagnetic directionsto the Cretaceous position. (c) 17 Ma<: The corrected Cretaceous palaeomagnetic directions are CCW rotated by 28◦ around a vertical axis to make themparallel to the Cretaceous palaeomagnetic declination of the Lanping Basin. After rotating the Simao Basin by 28◦ in CCW sense, it becomes parallel to theLanping arc.

(Rippe & Unsworth 2010). A possibility of upper crust movementin the Simao Basin toward the west could, therefore, be a driver fororoclinal bending.

6.4 Oroclinal bending before the Simao Arc formation

As described above, an oroclinal bending occurred before the SimaoArc formation in the Shan-Thai Block. A northeastward convexWulio Anshan Arc was formed as a result of tectonic bending alongthe Chongshan-Lancang-Chiang Mai belt before the Simao Arcformation. An easterly deflected declination from the straitenedSimao Arc (Fig. 8b) characterize the Cretaceous palaeomagenticdirection (D = 68.5◦ and I = 45.4◦) in the Simao Basin. A dif-ference of 28.3◦ in declination appeared when compared with themean declination (D = 40.2◦) from the Lanping Basin. It is dif-ficult to explain more than 25◦ of divergence in declination over500 km area by geomagnetic field changes deduced from the presentIGRF (International Association of Geomagnetism and Aeronomy,Working Group V-MOD 2010). Therefore, a CW tectonic rotation

around vertical axis between the Simao and Lanping arcs is consid-ered to be the most possible cause for this significant declinationcontrast.

When the Simao Arc with NW20◦ trend get restorationwith re-spect to the Lanping Arc by rotating the Wulianshan Arc 28◦ aroundthe vertical axis, two arcs form a straight line (Fig. 8c). This showsthat the northern Chongshan-Lancang-Chiang Mai belt, includingthe Lanping, Wulianshan and Simao arcs, has experienced oroclinalbending. A sinistral motion along the Chongshan-Lancang-ChiangMai belt occurred between 32 and 22 Ma (Wang et al. 2006; Akcizet al. 2008; Zhang et al. 2010). However, complicated antitheti-cal strike-slip motions with dextral or sinistral shearing started atca. 17 Ma (Akciz et al. 2008; Zhang et al. 2010). This implies thatbending of the Chongshan-Lancang-Chiang Mai belt took placesometime between 22 and 17 Ma as a result of changes in the strike-slip motion. Timing of this oroclinal bending suggests that decou-pling between the upper and middle–lower crusts already occurredbefore the Simao Arc formation by ongoing indentation of India into Asia.

C© 2012 The Authors, GJI, 190, 201–216



Tab

le3.

The

avai

labl

eJu

rass

icto

Cre

tace

ous

Pala

eom

agne

tic

resu

lts

from

the

Lan

ping

and

Sim

aoB

asin

s.D

and

Iar

ede

clin

atio

nan

din

clin

atio

n,N

isnu

mbe

rof

loca

liti

es,k

isth

eFi

sher

ian

prec

isio

npa

ram

eter

(Fis

her

1953

);α

95is

the

radi

usof

the

cone

at95

per

cent

confi

denc

ele

vel

abou

tth

em

ean

dire

ctio

n.S

–S o

:de

viat

ion

inre

gion

alst

rike

(S)

from

the

refe

renc

est

rike

(So

=34

0◦=

N20

◦ W),

see

text

for

furt

her

expl

anat

ion.

Cor

rect

ed-p

alae

omag

enti

cdi

rect

ion

isth

ere

clai

med

dire

ctio

nex

iste

dbe

fore

the

form

atio

nof

the

Sim

aocu

rvat

ure.

∗an

dM

ean∗

∗ :Ju

rass

icda

ta(3

)an

nota

ted

as∗

isno

tus

edfo

rm

ean

dire

ctio

nca

lcul

atio

n(M

ean∗

∗ ).

Loc

alit

yA

geO

bser

ved

dire

ctio

nS

–S o

Cor

rect

eddi

rect

ion

Ref

eren

ces

(Arc

topo

grap

hy)

(Str

aigh

ttop

ogra

phy)

Nam

eL

at.(

◦ N)

Lon

g.(◦

E)

D(◦

)I

(◦)

α95

(◦)

(◦)

D(◦

)I

(◦)

α95

(◦)

Sha

n-T

haiB

lock

Yun

long

Bas

inY

unlo

ng25

.899

.4K

240

.249

.93.

9S

ato

etal

.(19

99)

Yun

long

25.8

99.4

K2

38.3

50.7

3.4

Yan

get

al.(

2001

)Y

ongp

ing

25.5

99.5

K1

42.0

51.1

15.7

Fun

ahar

aet

al.(

1993

)

Mea

nK

1+

K2

40.2

50.6

2.0

N=

3k

=37

63.8

Sim

aoB

asin

(1)

Zhe

ngyu

anN

24.2

101.

0K

1–2

61.8

46.1

8.1

754

.846

.1Ta

naka

etal

.(20

08)

(2)

Zhe

ngyu

anS

24.0

101.

0K

1–2

144.

249

.46.

458

86.2

49.4

Tana

kaet

al.(

2008

)(3

)∗Ji

nggu

23.6

100.

5J2

83.3

36.8

5.4

3053

.336

.8H

uang

&O

pdyk

e(1

993)

(4)

Jing

gu23

.410

0.4

K1

84.4

39.6

17.8

2757

.439

.6C

hen

etal

.(19

95)

(5)

Jing

gu23

.410

0.5

K2

115.

836

.06.

324

91.8

36.0

Che

net

al.(

1995

)(6

)Ji

nggu

23.4

100.

9K

279

.443

.39.

123

56.4

43.3

Hua

ng&

Opd

yke

(199

3)(7

)P

urer

23.0

101.

0K

1–2

59.9

45.2

5.1

158

.945

.2S

ato

etal

.(20

07)

(8)

Zhe

ngw

an22

.810

0.9

K1

51.8

47.9

6.9

–657

.847

.9T

his

stud

y(9

)D

adug

ang

22.4

101.

0K

164

.148

.17.

3–2

084

.148

.1T

his

stud

y

Mea

n∗∗

K1

+K

281

.748

.215

.568

.445

.48.

5N

=8

k=

13.8

k=

43.9

C© 2012 The Authors, GJI, 190, 201–216


214 K. Kondo et al.

Figure 9. A simple model showing formation of the Simao Arc. An arcu-ate shape of this arc was formed as a result of westward displacement (redarrows) of the crust in the Simao Basin associated with indentation by south-ward moving Chuan Dian Fragment along the Xiaojiang Fault since 4 Ma.Mechanical decoupling took place between the upper and middle–lowercrusts (cross-section A–B of the Mesozoic Simao Basin), which moved theupper crust westwards associated with decollement formation in the SimaoBasin. A set of active faults in the area, including a left lateral NantingFault (NTF) and a right lateral Lancing Fault (LCF) consumed the westwardmovement of the upper crust.

Nevertheless, an easterly deflected declination of 40◦ remainedin the palaeomagnetic directions after straitening the northernChongshan-Lancang-Chiang Mai belt (Fig. 8c). Comparison of thisdeclination with the one expected from the Eurasian APWP indi-cates a CW rotation of about 30◦ for the Shan-Thai Block. This isattributed to tectonic deformation in the early stage of India–Asiacollision, as suggested by different authors (Tapponnier et al. 1982;Otofuji et al. 2007; Royden et al. 1997; 2008) in their models.

7 C O N C LU S I O N

New palaeomagnetic investigations have been conducted on theMiddle Cretaceous red beds from the Zhengwan (22.8◦N, 100.9◦E)and Dadugang (22.4◦N, 101.0◦E) localities, which form part of theSimao Basin in the Shan Thai Block. Large easterly deflected de-clinations with steep inclinations, such as D = 51.8◦, I = 47.9◦,ks = 45.0, α95 = 6.9◦, N = 11 and D = 64.1◦, I = 48.1◦, ks = 38.0,α95 = 7.3◦, N = 12, have been calculated from tilt-corrected meandirections for the Zhengwan and Dadugang localities, respectively.Combined with previously reported palaeomagnetic data from theShan Thai Block, the following deformational history is hypothe-sized for the Simao Arc.

(1) The Simao Arc with a radius of 166 km was formed as aresult of oroclinal bending in the Simao Basin. Tectonic deforma-

tion responsible for this bending started about 4 Ma and is stillcontinuing.

(2) An arcuate geometry of the Simao Arc is ascribed to south-westward displacement of the Chuan Dian crustal fragment acrossthe Ailao Shan-Red River Fault, which brought about movement ofthe decollement toward the Simao Arc accompanied by decouplingbetween the upper and middle–lower crusts.

(3) Before the formation of the Simao Arc, the northernChongshan-Lancang-Chiang Mai belt experienced an oroclinalbending between 22 and 17 Ma. Decoupling between the upperand middle–lower crusts of the Shan-Thai Block has occurred as aresult of earlier stage deformation inflicted by Indian Plate indenta-tion in to Asia.

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

We thank A. Ali and A. Weil for their constructive reviews of themanuscript. T. Miyata and K. Yamazaki are warmly thanked fortheir valuable comments. This work has been supported by ‘The21st Century COE Program of Origin and Evolution of PlanetarySystems’ through the Ministry of Eduation, Culture, Sports, Scienceand Technology (MEXT). In addition, this research was partly sup-ported by the Toyota Foundation and Grant-in aid (Nos. 14403010and 18403012, 22403012) from the MEXT.

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oroclinal origin of the simao arc in the shan-thai block inferred from the cretaceous palaeomagnetic...

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