The boundary between the Inthanon Zone (Palaeotropics) and the Gondwana-derived Sibumasu Terrane, northwest Thailand—evidence from Permo-Triassic limestones and cherts

The boundary between tropical Permian faunas of the Inthanon Zone and Gondwana faunas in far NW Thailand has been long debated. Both the Late Devonian and the Permian Gondwanan platform margins lie a few kilometres west of the Mae Yuam/Mae Sariang Fault (MYMS FZ). In the Permian, the margin grades eastwards into hemipelagic radiolarites along the MYMS FZ and westwards into the Thitsiphin carbonate platform of Myanmar. The area west of the MYMS FZ is the Northern part of the West Thailand Region (NWTR). Quartz-rich limestones of Roadian age in the NWTR are succeeded by deep-water platform limestones in the NWTR and shallow-water carbonates in Myanmar and contain a distinctive fusulinid fauna including Monodiexodina which does not occur in palaeotropical terranes. A 300-m section of limestone 10 km west of the MYMS FZ contains Wordian microfauna and is placed in a deep shelf to slope environment. Carboniferous to Triassic continental margin, hemipelagic, non-hydrothermal, radiolarian cherts outcrop on either side of the Mae Yuam valley and were deposited on the upwelling margins of an ocean separating the Inthanon Zone and Sibumasu Terrane. The widely accepted allochthon model proposes that the Inthanon Zone Devonian-Triassic radiolarites were pelagic and deposited on a subducting ocean that supported seamounts with Visean to Permian tropical shallow-water carbonates lasting at least 90 my. We suggest an alternative hypothesis where the radiolarites of the Inthanon Zone were continental margin as shown by their geochemistry and deposited in deeper parts of small extensional basins with limited volcanism between long-lived, isolated carbonate platforms.