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

Lower to Middle Cretaceous red sandstones were sampled at four localities in the Lanpin-Simao fold belt of the Shan-Thai Block to describe its regional deformational features. Most of the samples revealed a characteristic remanent magnetization with unblocking temperatures around 680 degrees C. Primary natures of magnetization are ascertained through positive fold test. A tilt-corrected formation-mean direction for the Jingdong (24.5 degrees N, 100.8 degrees 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 degrees/48.8 degrees, alpha(95) = 7.7 degrees, N = 13). However, mean directions obtained from the Zhengyuan (24.0 degrees N, 101.1 degrees E), West Zhengyuan (24.0 degrees N, 101.1 degrees E) and South Mengla (21.4 degrees N, 101.6 degrees E) localities indicate an easterly deflection in declination; such as Dec./Inc. = 61.8 degrees/46.1 degrees, alpha(95) = 8.1 degrees (N = 7), Dec./Inc. = 324.2 degrees/-49.4 degrees, alpha(95) = 6.4 degrees (N = 4) and Dec./Inc. = 51.2 degrees/46.4 degrees, alpha(95) = 5.6 degrees (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 degrees 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.