S U M M A R YLower to Middle Cretaceous red sandstones were sampled at four localities in the LanpinSimao fold belt of the Shan-Thai Block to describe its regional deformational features. Most of the samples revealed a characteristic remanent magnetization with unblocking temperatures around 680 • C. Primary natures of magnetization are ascertained through positive fold test. A tilt-corrected formation-mean direction for the Jingdong (24.5 • N, 100.8 • E) locality, which is located at a distance of 25 km from the Ailaoshan-Red River Fault, revealed northerly declination with steep inclination (Dec./Inc. = 8.3 • /48.8 • , α 95 = 7.7 • , N = 13). However, mean directions obtained from the Zhengyuan (24.0 • N, 101.1 • E), West Zhengyuan (24.0 • N, 101.1 • E) and South Mengla (21.4 • N, 101.6 • E) localities indicate an easterly deflection in declination; such as Dec./Inc. = 61.8 • /46.1 • , α 95 = 8.1 • (N = 7), Dec./Inc. = 324.2 • /−49.4 • , α 95 = 6.4 • (N = 4) and Dec./Inc. = 51.2 • /46.4 • , α 95 = 5.6 • (N = 13), respectively. The palaeomagnetic directions obtained from these four localities are incorporated into a palaeomagnetic database for the Shan-Thai Block. When combined with geological, geochronological and GPS data, the processes of deformation in the Shan-Thai Block is described as follows: Subsequent to its rigid block clockwise rotation of about 20 • in the early stage of India-Asia collision, the Shan-Thai Block experienced a coherent but southward displacement along the Red River Fault prior to 32 Ma. This block was then subjected to a north-south compressive stresses during the 32-27 Ma period, which played a key role in shaping the structure of Chongshan-Lancang-Chiang Mai Belt. Following this some local clockwise rotational motion has occurred during the Pliocene-Quaternary time in central part of the Shan-Thai Block as a result of internal block movements along the reactivated network of faults.

SUMMARY An active oroclinal bending is discovered in the Shan‐Thai Block to the south of the eastern Himalayan syntaxis. To investigate the evolution of the Simao Arc using palaeomagnetic techniques, 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 with unblocking temperatures around 680 °C. A primary nature for this magnetization is interpreted on the basis of a positive fold and reversal test. Tilt‐corrected mean directions calculated for Zhengwan 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 respect to those expected are in the range previously reported from the Shan‐Thai Block, confirming their southward displacement by 6.2°± 1.7° as part of the Shan‐Thai Block. Combination of the present data (two localities) with those previously reported from Simao Basin (seven localities) 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 recent GPS and structural data suggest that formation of the Simao curvature started after the early Pliocene (after 4 Ma) and continuing until the present. Origin of the Simao Arc is ascribed to 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 the decollement with progressive eastward deepening of the Lanping‐Simao Basin. Decoupling between the upper and the middle–lower crusts is a requisite condition for the arc formation on the continent.

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