New paleomagnetic results from the Permian and Mesozoic rocks in central and northeast Thailand: their implications for the construction of the Indochina block in Pangea

Abstract:

Paleomagnetic studies of the Indochina block, aiming to reconstruct the paleogeography, have been undertaken for several decades. Since the Indochina block is lack of reliable paleomagnetic data to constraint its paleo-positions during the Middle Permian to Upper Triassic, the paleogeography reconstruction is still in debate between different models reported. Here we present new paleomagnetic data of Middle Permian to Upper Triassic sediment rocks from the Indochina block in Thailand, and recalculate paleomagnetic data reported by different authors. We collected the Permian samples in 20 sites distributed in the central Thailand, and Triassic samples from the Huai Hin Lat and Nam Phong formations in 13 sites in the northern Thailand.The magnetic directions of the 11 sites of Permian limestones are not significantly clustered after tilt correction which implying a remagnetized result. Remarkably, in geographic coordinate, the 11 sites were distributed along a circle showing a similar inclination，which is 22.9° implying the paleolatitude to be about 12°. Totally, 13 sites from the Huai Hin Lat formation are included in the calculation of the formation mean direction D_g/I_g = 21.4°/38.1°, k_g = 19.5, α₉₅ = 9.6° before and D_s/I_s = 43.0°/48.0°, k_s = 47.4, α₉₅ = 6.1°, N = 13 after bedding correction. A pre-folding characteristic magnetization is suggested by the positive fold test result derived from the Huai Hin Lat formation, and thus implies a primary remanence of the Norian Stage Upper Triassic rocks. A new Nam Phong formation mean direction derived from 11 sites is D_g/I_g = 36.5°/31.3°, k_g = 14.7, α₉₅ = 12.3°before and D_s/I_s = 36.4°/37.8°, k_s = 68.5, α₉₅ = 5.6°, N = 11 after bedding correction. The two formation mean directions correspond to the magnetic pole positions , Plat./Plon=48.7°N/165.9°E, A₉₅=7.2° and Plat./Plon=55.2°N/178.0°E, A₉₅=5.9°, respectively. A remarkable tectonic movement (~8° southward) of the Indochina block from the age of the Huai Hin Lat formation to the Nam Phong formation is suggested in this study, indicating the Indochina block locates in the west of the South China block at the Norian Age Late Triassic and moved to a new position which is quite near the present at the Rhaetian Age of Late Triassic to Early Jurassic.