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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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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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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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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Oroclinal origin of the Simao Arc 207
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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Oroclinal origin of the Simao Arc 209
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
Geophysical Journal International C© 2012 RAS
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
Geophysical Journal International C© 2012 RAS
Oroclinal origin of the Simao Arc 211
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
Geophysical Journal International C© 2012 RAS
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
Geophysical Journal International C© 2012 RAS
Oroclinal origin of the Simao Arc 213
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
Geophysical Journal International C© 2012 RAS
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.
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.
Akciz, S., Burchfiel, B.C., Crowley, J.L., Yin, J. & Chen, L., 2008. Ge-ometry, kinematics, and regional significance of the Chong Shan shearzone, Eastern Himalayan Syntaxis, Yunnan, China, Geosphere, 4, 292–314.
Besse, J. & Courtillot, V., 2002. Apparent and true polar wander and thegeometry of the geomagnetic field over the last 200 Ma, J. geophys. Res.,107(B11), 2300, doi:10.1029/2000JB000050.
Bureau of Geology and Mineral Resources of Yunnan province, 1990. Re-gional Geology of Yunnan province (BGMRY) Geol. Mem. Ser. 1, No. 21,Geological Publishing House, Beijing. 728pp.
Chen, H., Dobson, J., Heller, F. & Hao, J., 1995. Paleomagnetic evidencefor clockwise rotation of the Simao region since the Cretaceous: a con-sequence of India-Asia collision, Earth planet. Sci. Lett., 134, 203–217.
Cogne, J.P., 2003. PaleoMac: a MacintoshTM application for treating pa-leomagnetic data and making plate reconstructions, Geochem. Geophys.Geosyst., 4, 1007, doi:10.1029/2001GC000227.
Copley, A., 2008. Kinematics and dynamics of the southeasetern margin ofthe Tibetan Plateau, Geophys. J. Int., 174, 1081–1100.
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.
Fisher, R.A., 1953. Dispersion on a sphere, Proc. R. Soc., 217, 295–305.
Flesch, L.M., Holt, W.E., Silver, P.G., Stephenson, M., Wang, C.-Y. & Chan,W., 2005. Constraining the extent of crust–mantle coupling in central Asiausing GPS, geologic, and shear wave splitting data, Earth planet. Sci. Lett.,238, 248–268.
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 westernYunnan, China, Earth planet. Sci. Lett., 117, 29–42.
C© 2012 The Authors, GJI, 190, 201–216
Geophysical Journal International C© 2012 RAS
Oroclinal origin of the Simao Arc 215
Gan, W., Zhang, P., Shen, Z.-K., Niu, Z., Wang, M., Wan, Y., Zhou,D. & Cheng, J., 2007. Presnt-day crustal motion within the TibetanPlateau inferred from GPS measurements, J. geophys. Res., 122, B08416,doi:10.1029/2005JB004120.
Heuret, A. & Lallemand, S., 2005. Plate motions, slab dynamics and back-arc deformation, Phys. Earth planet. Inter., 149, 31–51.
Houseman, G. & England, P., 1993. Crustal thickening versus lateral ex-pulsion in the Indian-Asian continental collision, J. geophys. Res., 98,12 233–12 249.
Huang, R., Wang, Z., Pei, S. & Wang, Y., 2009. Crsutal ductile flow andits contribution to tectonic stress in Southwest China, Tectonophys, 473,476–489.
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.
International Association of Geomagnetism and Aeronomy, Working GroupV-MOD., 2010. International Geomagnetic Reference Field: the eleventhgeneration, Geophys. J. Int., 183, 1216–1230.
Kirschvink, J.L., 1980. The least-squares line and plane and the anal-ysis of palaeomagnetic data, Geophys. J. R. Astron. Soc., 62, 699–718.
Lacassin, R., Replumaz, A. & Leloup, 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 shear zone (Yunnan,China), Tertiary transform boundary of Indochina, Tectonophysics, 251,3–84.
Lev, E., Long, M.D. & van der Hilst, R.D., 2006. Seismic anisotropy in East-ern Tibet shear wave splitting reveals changes in lithospheric deformation,Earth planet. Sci. Lett., 251, 293–304.
Lowrie, W., 1990. Identification of ferromagnetic minerals in a rock bycoercivity and unblocking temperature properties, Geophys. Res. Lett.,17, 159–162.
McElhinny, M.W., 1964. Statistical significance of the fold test in palaeo-magnetism, Geophys. J. R. Astron. Soc., 8, 338–340.
McFadden, P.L., 1990. A new fold test for palaeomagnetic studies, Geophys.J. Int., 103, 163–169.
McFadden, P.L. & McElhinny, M.W., 1990. Classification of the reversaltest in palaeomagnetism, Geophys. J. Int., 103, 725–729.
Otofuji, Y. et al., 2007. Spatial gap between Lhasa and Qiangtang blocksinferred from Middle Jurassic to Cretaceous paleomagnetic data, Earthplanet. Sci. Lett., 262, 581–593.
Otofuji, Y., Yokoyama, M., Kitada, K. & Zaman, H., 2010. Paleomagneticversus GPS determined tectonic rotation around eastern Himalayan syn-taxis in East Asia, J. Asian Earth Sci., 37, 438–451.
Pullaiah, G., Irving, E., Buchan, K.L. & Dunlop, D.J., 1975. Magnetizationchanges caused by burial and uplift, Earth planet. Sci. Lett., 28, 133–143.
Ren, J.S., Jiang, C.F., Zhang, Z.K. & Qin, D.Y., 1980. The Tectonic Evolutionof China, Science Press, Beijing, 124pp.
Richter, B. & Fuller, M., 1996. Palaeomagnetism of the Shibumasu andIndochina blocks: implications for the extrusion tectonic model, inTectonic Evolution of Southeast Asia, Geol. Soc. Lond. Spec. Publ.106, pp. 203–224, eds Hall, R. & Blundell, D., The Geological Society,London.
Rippe, D. & Unsworth, M., 2010. Quantifying crustal flow in Tibet withmagnetotelluric data, Phys. Earth planet. Inter., 179, 107–121.
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.
Royden, L.H., Burchfiel, B.C. & van der Hilst, R.D., 2008. The geologicalevolution of the Tibetan Plateau, Science, 321, 1054–1058.
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, western
Yunnan, China: evidence of internal deformation of the Indochina block,Earth planet. Sci. Lett., 258, 1–15.
Sato, S., Yang, Z., Tong, Y., Fujihara, M., Zaman, H., Yokoyama, M.,Kitada, K. & Otofuji, Y., 2011. Inclination variation in the Late Jurassicto Eocene red beds formations of Southeast Asia: lithological to localityscale approach, Geophys. J. Int., 186, 471–491.
Schellart, W.P. & Lister, G., 2004. Tectonic models for the forma-tion of arc-shaped convergent zones and backarc basins, in OrogenicCurvature: Integrating Paleomagnetic and Structural Analyses, Geol.Soc. Am. Spec. Paper 383, pp. 237–258, eds Sussman, J.A. & Weil,A.B., Geological Society of America, Boulder, CO.
Schwartz, S.Y. & Van der Voo, R., 1983. Paleomagnetic evaluation of theorocline hypothesis in the central and southern Appalachians, Geophys.Res. Lett., 10, 505–508.
Shen, Z.K., Lu, J., Wang, M. & Burgmann, R., 2005. Contemporary crustaldeformation around the southeast borderland of the Tibetan Plateau, J.geophys. Res., 110, B11409, doi:10.1029/2004JB003421.
Socquet, A. & Pubellier, M., 2005. Cenozoic deformation in western Yunnan(China-Myanmar border), J Asian Earth Sci., 24, 495–515.
Takemoto, K. et al., 2009. Tectonic deformation of the Indochina Peninsularecorded in the Mesozoic palaeomagnetic results, Geophys. J. Int., 179,97–111.
Tanaka, K. et al., 2008. Tectonic deformation around the eastern Himalayansyntaxis: constraints from the Cretaceous palaeomagnetic data of theShan-Thai Block, Geophys. J. Int., 175, 713–728.
Tapponnier, P., Xu, Z., Roger, F., Meyer, B., Arnaud, N., Wittlinger, G. &Yang, J., 2001. Oblique stepwise rise and growth of the Tibet plateau,Science, 294, 1671–1677.
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.
Wang, E., Burchfiel, B.C., Royden, L.H., Chen, L., Chen, J., Li, W. &Chen, Z., 1998. Late Cenozoic Xianshuihe-Xiaojiang, Red River and Dalifault systems of southwestern Sichuan and central Yunnan, China,, Geol.Soc. Am. Spec. Paper 327, Geological Society of America, Boulder, CO,108pp.
Wang, E. & Burchfiel, B.C., 2000. Late Cenozoic to Holocene deformationin southwestern Sichuan and adjacent Yunnan, China, and its role information of the southern part of the Tibetan Plateau, Bull. geol. Soc.Am., 112, 413–423.
Wang, Q. et al., 2001. Present-day crustal deformation in China constrainedby global positioning system measurements, Science, 294, 574–577.
Wang, W., Fan, W., Zhang, Y., Peng, T., Chen, W. & 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.
Wang, G., Wan, J., Wan, E., Zheng, D. & Li, F., 2008. Late Cenozoic to recenttranstensional deformation across the Southern part of the Gapligongshear zone between the Indian plate and SE margin of the Tibetan plateauand its tectonic origin, Tectonophysics, 460, 1–20.
Weil, A.B. & Sussman, A.J., 2004. Classifying curved orogens based ontiming relationships between structural development and vertical-axis ro-tations, in Orogenic curvature: Integrating Paleomagnetic and StructuralAnalyses, Geol. Soc. Am. Spec. Paper 383, pp. 1–17, eds Sussman, J.A.& Weil, A.B., Geological Society of America, Boulder, CO.
Wessel, P. & Smith, W.H.F., 1998. New, improved version of the GenericMapping Tools released, EOS, Trans. Am. geophys. Un., 79, 579.
Wu, H.R., Boulter, C.A., Ke, B., Stow, D.A.V. & Wang, Z.C., 1995. TheChangning-Menglian suture zone; a segment of the major Cathaysian-Gondwana divide in South Asia, Tectonophysics, 242, 267–280.
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.
Yonkee, A. & Weil, A.B., 2010. Quantifying vertical axis rotationin curved orogens: correlating multiple data sets with a refinedweighted least squares strike test, Tectonics, 29, TC3012, doi:1029/2008TC002312.
C© 2012 The Authors, GJI, 190, 201–216
Geophysical Journal International C© 2012 RAS
216 K. Kondo et al.
Zhang, B., Zhang, J. & Zhong, D., 2010. Structure, kinematics and agesof transpression during strain-patitioning in the Chongshan sear zone,western Yunnan, China, J. Struct Geol., 32, 445–463.
Zhang, P.-Z. et al., 2004. Continuous deformation of the Tibetan Plateaufrom global positioning system data, Geology, 32, 809–812.
Zhang, X. & Wang, Y., 2009. Crustal and upper mantle velocity structure inYunnan, Southwest China, Tectonophysics, 471, 171–185.
Zijderveld, J.D.A., 1967. A. C. demagnetization of rocks: analysis of results,in Methods in Paleomagnetism, pp. 254–286, eds Collison, D.W., Creer,K.M. & Runcorn, S.K., Elsevier, Amsterdam.
C© 2012 The Authors, GJI, 190, 201–216
Geophysical Journal International C© 2012 RAS