Rift Volcanism and Rift Basin in Central Myanmar Basin
Keywords:
volcanic, fault, extension, basalt, sedimentary, differentiation, emplacement, assimilationAbstract
The Central Myanmar Basin is made up of Tertiary sedimentary rocks between the Rakhine Western Ranges in the west and Shan Plateau to the east. The basin consists of six sub-basins: Putao, Hukawng, Chindwin, Salin, Pyay and Ayeyawady Delta basin. This 1000 km× 200km tectonic strip in NNW-direction in Central Myanmar Basin is characterized by the presence of numerous ENE to northeast and east-west trending normal faults and north-south to NW-SE and NNW-SSE trending folds and thrusts. This arrangement of structures is indicative of an extensional stress in NNW-SSE direction and a compressional stress in ENE-WSW direction. Tensor solution of earthquakes which occurred in Central Myanmar Basin also indicates a dominant extensional stress direction oriented NNW-SSE. This NNW-SSE extensional direction appears to have favored the formation of rifts in ENE-WSW, NE-SW and E-W direction which were later emplaced with volcanic rocks. These volcanic rocks were derived from mantle material which risen into the several segments of rifts from underneath of the crust during rifting. Majority of these volcanic rocks are different types of basalt, andesite, dolerite, volcanic tuff, volcanic ash, lava flow and rhyolites. The studies of associated sedimentary rocks with the volcanic rocks indicate that extension and volcanic activity began in the north in Early Tertiary and migrated to the south from the Miocene to Quaternary. The Central Myanmar Basin and Central Andaman Basin record an active extensional process that varies laterally from continental rifting in the north in Central Myanmar Basin and sea-floor spreading in the south in Central Andaman Basin. The objective of this paper is to define regional volcanic differentiation pattern and their linear emplacement which are in accordance with regional tectonic stress field and to interpret them in term of tectonic deformation. Bimodal basalt-rhyolite suite with the presence of intermediate composition and compositional variations of volcanic rocks indicate that the volcanic rocks are formed by assimilation, fractional crystallization and the crustal components and magma mixing. Geochemical analyses of these volcanic rocks reveal that they are also enriched in the alkalis, high level of barium and strontium and light rare earth elements.
Associated host rocks of the volcanic material are sedimentary rocks of the Peguan Group (Oligocene-Miocene) and Irrawaddian Group (Pliocene-Pleistocene).
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