祁连山北阿拉善地体特征及其大地构造演化

祁连山北阿拉善地块在大地构造位置上处于中亚造山带、塔里木板块、祁连山-秦岭造山带和华北板块之间.阿拉善地体在太古代-元古代作为一个相对独立的小地体发育, 其结晶基底岩性与塔里木板块、华北板块存在一定程度差异.受加里东构造运动影响, 板块与地体表现为南聚北散特点; 在中奥陶世早期阿拉善地体与华北板块拼为一体, 进入板块整体演化阶段.中海西构造运动期, 北部早二叠世古亚洲洋关闭, 发育新增地壳, 形成北缘沟弧盆体系; 晚海西构造运动期, 阿拉善地体向南俯冲, 祁连山洋关闭, 地壳增生, 阿拉善地体与塔里木板块、华北板块焊接在一起.新特提斯构造运动对其影响巨大, 南缘表现为"左旋"走滑, 北缘表现为"右旋"走滑, 古生代所形成构造带均受到新生代北东向构造的强烈改造.阿拉善地体频繁的构造运动, 致使其南北部相对稳定的燕山期所形成的煤系中小盆地群(阿拉善-银额盆地群、北山盆地群、河西走廊盆地群、祁连山东部盆地群、祁连山西部盆地群、阿尔金盆地群、柴达木周缘盆地群), 也相应发生快速沉降与快速抬升过程, 后期均遭受强烈改造与破坏, 不仅造就烃源岩欠发育, 就是造成储集层致密, 总体不利于油气的生成、运移、聚集成藏, 油气勘探前景欠佳.

Geological characteristics and tectonic evolution of Alxa terranein the northern Qilian Mountain

Alxa terrane is located among Tarim block, North China block and Qilian-Qinling orogen. Alxa terrane was independently developed during the Archean-Proterozoic. Its crystalline basement displays certain differences with those of the Tarim plate and the North China plate in the lithology. Influenced by the Caledonian event, the Alxa terrane and its adjacent plates are characterized by convergent in the south and divergent in the north. In the Early Middle Ordovician, the Alxa terrane collided with the North China plate, and they share a common evolutionary stage as a unified plate. During the Middle Hercynian period, the Early Permian paleo-Asian ocean to its north was closed and juvenile crust was developed, forming a trench-arc-basin system. In the Late Hercynian period, the Alxa terrane subdued toward the south, the Qilian ocean was closed, generating abundant juvenile crust, and Alxa terrane was amalgamated with the Tarim and North China plates. Neo-Tethys tectonics played a significant influence on Alxa terrane, as suggested by the left-lateral strike-slip at its southern margin and the right-lateral strike-slip at its northern margin. In addition, the Paleozoic structures were modified by the Cenozoic NE-trending structural deformations. The frequent tectonic activities in Alxa terrane led the relatively stable Yanshanian coal-bearing medium-small basins(including Alxa-Yin'e Basin group, Beishan Basin group, Hexi Corridor Basin group, East Qilian Basin group, West Qilian Basin group, Altun Basin group, Chaidamu Basin group)to begin to subside or uplift rapidly. Afterward, these basin groups experienced strong modifications, which resulted in either lack of source rock or tight reservoir. As a whole, these factors are not to the advantage of the generation, migration and accumulation of hydrocarbons, showing poor prospects for petroleum exploration.