Ocean Plate Stratigraphy in East and Southeast Asia
Ancient accretionary wedges have been recognised by the presence of glaucophane schist, radiolarian chert and mélange. Recent techniques for the reconstruction of disrupted fragments of such wedges by means of radiolarian biostratigraphy, provide a more comprehensive history of ocean plate subduction and successive accretion of ocean floor materials from the oceanic plate through offscraping and underplating. Reconstructed ocean floor sequences found in ancient accretionary complexes in Japan comprise, from oldest to youngest, pillow basalt, limestone, radiolarian chert, siliceous shale, and shale and sandstone. Similar lithologies also occur in the mélange complexes of the Philippines, Indonesia, Thailand and other regions. This succession is called ‘Ocean Plate Stratigraphy’ (OPS), and it represents the following sequence of processes: birth of the oceanic plate at the oceanic ridge; formation of volcanic islands near the ridge, covered by calcareous reefs; sedimentation of calcilutite on the flanks of the volcanic islands where radiolarian chert is also deposited; deposition of radiolarian skeletons on the oceanic plate in a pelagic setting, and sedimentary mixing of radiolarian remains and detrital grains to form siliceous shale in a hemipelagic setting; and sedimentation of coarse-grained sandstone and shale at or near the trench of the convergent margin. Radiolarian biostratigraphy of detrital sedimentary rocks provides information on the time and duration of ocean plate subduction. The ages of detrital sediments becomes younger oceanward as younger packages of OPS are scraped off the downgoing plate. OPS reconstructed from ancient accretionary complexes give us the age of subduction and accretion, direction of subduction, and ancient tectonic environments and is an important key to understanding the paleoenvironment and history of the paleo-oceans now represented only in suture zones and orogenic belts.
Lower Permian conodonts from Palaeo-Tethys Ocean Plate Stratigraphy in the Chiang Mai-Chiang Rai Suture Zone, northern Thailand
Lower Permian (lower Sakmarian) conodonts are reported from a coherent section of Ocean Plate Stratigraphy, and from a limestone block in the Palaeo-Tethys suture zone between Lamphun and Lampang, south of Chiang Mai, northern Thailand. Conodont species from both the pelagic limestones of the OPS section and the limestone block are deep-water forms with distinctive biogeographic affinities comparable to faunas of the Urals and North American Mid-Continent. A new name, Chiang Mai-Chiang Rai Suture Zone, is here proposed for the Palaeo-Tethys suture in northern Thailand that forms the boundary between the Sibumasu Terrane and the Sukhothai Arc terrane. The Inthanon Zone of northern Thailand is interpreted as representing a fold and thrust belt west of the Chiang Mai-Chiang Rai Suture Zone, which comprises Sibumasu Terrane continental margin rocks and remnant klippen of Chiang Mai-Chiang Rai Suture Zone rocks thrust as a nappe westward during the Triassic collision between Sibumasu and the Sukhothai Arc/Indochina Terrane.