Variations in Late Cenozoic Recent strike-slip and oblique-extensional geometries, within Indochina: The influence of pre-existing fabrics
From Yunnan to Northern Thailand, Late Cenozoic-Recent faults strike predominantly NNE-SSW, N-S to NNE-SSW and NE-SW to ENE-WSW. Associated sedimentary basins are aligned NE-SW to N-S. The regional fault patterns are commonly interpreted as strike-slip dominated deformation throughout the area. Releasing bend and en echelon stepping patterns on faults bounding sedimentary basins indicate sinistral displacement on NE-SW to ENE-WSW trending faults. Yet, in the escape tectonics model left lateral displacement on the NE-SW to ENE-WSW faults is thought to have occurred late in the Miocene, whilst earlier motion was dextral. However, in Yunnan the NE-SW Shmax direction required for dextral motion on the N-S Sagaing, Nanting and Gaoligong fault zones is consistent with sinistral motion on ENE-WSW striking faults, which is still their sense of motion today. In Northern Thailand the dextral-sinistral switch model during the Miocene is not tenable because the Fang basin is of Late Oligocene-Pliocene age, and requires similar age sinistral motion on the ENE-WSW Mae Chan fault in order to have opened. In an alternative model, Northern Thailand is interpreted to have evolved predominantly by oblique extension. The Golden Triangle area marks a transition from transtensional deformation in the north to oblique extension in the south. The activation of pre-existing fabrics strongly affects both strike-slip and extensional faults and has given rise to the similar extensional and strike-slip fault patterns. Multiple episodes of basin inversion in Northern Thailand during the Miocene require short-term changes in stress pattern. To produce the inferred changes in stress pattern it is suggested that stresses radiating out from the Himalayan syntaxis exert a strong influence, but were not the only important forces acting on the region.