East flank of the Sibumasu block in NW Thailand and Myanmar and its possible northward continuation into Yunnan: a review and suggested tectono-stratigraphic interpretation

• An accretionary-complex tectono-stratigraphic model is invoked.
• Inthanon Zone comprises overthrust oceanic rocks over in situ Sibumasu-flank rocks.
• Sibumasu east flank is speculatively projected north into Yunnan.

The east flank of the Sibumasu block was a passive continental margin, and in NW Thailand is marked by the absence of the autochthonous Middle Permian–Triassic platform carbonates which are widespread across the rest of Sibumasu further west. Instead, the carbonates are represented by hemipelagic cherts, mudstones and sandstones including turbidites. During the northward drift of Sibumasu, following its Early Permian rifting from Gondwana, an accretionary complex was present where Palaeotethyan pelagic rocks as old as Devonian were subducted beneath the Sukhothai volcanic arc. At the time of Sibumasu’s collision with the Sukhothai arc, beginning in the Middle Triassic, the accretionary complex was thrust westwards across the east flank of Sibumasu. It is suggested that in the Late Triassic the thrust pile which had been the accretionary complex underwent erosion and was the source of terrigenous clastic rocks deposited further west in a foredeep basin.The boundary of Sibumasu’s east flank with the Permo-Triassic carbonate platform further west is the arcuate Mae Ping-Nam Teng Fault system. Notwithstanding later Cenozoic strike-slip displacement, those faults (as well as the Mae Yuam Fault) are interpreted to have had an earlier history of westward-directed Indosinian thrusting.Northwards in Myanmar and Yunnan the Sibumasu Permo-Triassic carbonate shelf continues as the Shan Plateau and Baoshan Block. The east flank is represented by the Changning–Menglian Belt, and the Palaeotethys ‘cryptic suture’ in Thailand possibly joins with the Lancangjiang Suture.