古亚洲洋南支为复杂的多岛洋，可分为西、中、东3段，其演化具有共同点，如既向北消减又向南消减和晚古生代发生多期的消减，也有沟弧盆系发育特征和时代上的差异，反映了可能存在近北北东向的剪切转换带。二叠纪北东东—东西向的洋盆、北西向的剪切带和北北东向的额尔德尼达来海槽构成“三叉构造”，后者是西部隆起区与东部残留海盆区之间的分界。中段的中戈壁地区的洋壳向北消减，洋消失后在弧前地区形成温都尔希雷特残留海盆。中段的南戈壁地区的洋壳向南消减，形成洋内弧（佐伦弧）和大陆边缘弧（雅干—索果淖弧），洋消失后在在弧后地区分别出现南戈壁和拐子湖残留海盆；晚二叠世两地持续发育海盆，前者形成具巨大经济价值的海相煤田。南戈壁海盆一度东延连接内蒙东部（东段洋壳消失后残留）的哲斯海盆和吴家屯海盆，现东延被东曼达洛包岩浆弧所截。晚二叠世内蒙东部的哲斯海盆闭合，吴家屯海盆退化为2个陆相盆地。蒙古西南部在昌德曼地区形成2条窄长的槽地接受晚二叠世的海侵，堆积相应的煤层。%The south branch of the Paleo-Asian Ocean,being a complicated archipelago,could be divided into the west,middle and east segments by the NNE-orientated transform faults.The oceanic crust created and subducted earlier in the west segment,and western Mongolia became a landmass in the Permi-an.Simultaneously,there developed the Zuunmod oceanic basin and several residual marine basins in central-eastern Mongolia and neighboring regions.The NEE-to WE-orientated oceanic basin,the NW-striking Adaatsag suture (shear zone) and the NNE-striking Erdenedalai trough made up a Permian three-armed structure.The oceanic realm of the middle segment could be divided into two sub-oceanic basins, distributing respectively in central Gobi and southern Gobi areas.The oceanic crust of the former subducted northwards.After the disappearance of the ocean,a residual marine basin with an age of Middle-Late Per-mian,named the Ondorsht basin,occurred in the former fore-arc region.The oceanic crust in southern Gobi area and western Inner Mongolia subducted southwards,accompanied with occurrence of an intra-o-ceanic arc (the Zoolen Arc) and a continent marginal arc (the Yagan-Suoguozhuo Arc).After the disap-pearance of the ocean,the residual marine basins formed in the former back-arc regions during the Middle Permian,and named the South Gobi and Guaizihu basins respectively.A similar tectono-palaeogeographic evolution during the Late Paleozoic occurred in the east segment (the east Mongolia and east Inner Mongo-lia),and the residual Middle Permian marine basins,named the Zhesi and Wujiatun basins,once linked up with the South Gobi basin.During the Late Permian,the marine South Gobi and Guaizihu basins contin-ued to develop,where the thick coal-bearing strata or coarse clastic and volcanic rocks deposited respective-ly.Nowadays,the eastward stretch of South Gobi basin is intercepted by a magmatic arc.The Zhesi basin closed and the continental Linxi basin occurred during the Late Permian in eastern Inner Mongolia.Whereas in southwestern Mongolia,the marine invaded along two narrow swales near Chandmani during the late Late Permian,and deposited the middle-upper part of coal measures of the Tavan Dolgoi Formation.
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