鄂尔多斯盆地西南缘中生代中晚期构造体制转化过程及其动力学背景

Spatial-temporal evolution and the dynamic background of the translation of Mid-Late Mesozoic tectonic regimes of the southwest Ordos basin margin

逆冲推覆构造对含油气盆地有效圈闭形成、源储体系配置影响明显,且其形成时限及演化过程对油气勘探意义重要。鄂尔多斯盆地西南缘中生代中晚期曾发生过强烈的逆冲推覆作用,但其构造体制转化过程一直缺乏年代学的约束。热年代学是揭示构造带隆升剥露和相邻盆地演化过程的有效手段。本文通过对祁连山东缘钻井基岩的磷灰石裂变径迹分析,结合鄂尔多斯盆地西南缘热史模拟综合分析,探讨了该期构造体制转化的时空过程。结果表明祁连山东缘在中晚侏罗世—早白垩世(174~130 Ma)发生了快速冷却事件,该期抬升冷却事件广泛存在于鄂尔多斯盆地西南缘,且盆地西南缘同期物源体系变化、构造变形、隆升剥蚀和砾岩堆积等响应明显,共同表明在中侏罗世延安期之后(170 Ma)该区可能就已转变为挤压环境,并于晚侏罗世挤压变形和隆升剥蚀达到高峰,一直持续至早白垩早期,具西强东弱、边缘强内部弱特点的挤压变形特点,推测与特提斯构造域北东向汇聚联系密切;然于早白垩世三桥期开始,鄂尔多斯西南缘转化为伸展背景,并沉积了巨厚的下白垩统六盘山群,认为是东亚多板块联合作用的结果。鄂尔多斯盆地西南缘中生代中晚期沉积-构造体制转化时限、过程及其动力学背景的厘定,对探讨鄂尔多斯盆地西南缘构造面貌的形成演化及油气勘探具有重要意义。

The thrust-nappe structure has remarkable influence on the formation of effective traps and the allocation of source and reservoir systems, and its evolution is of great significance for oil and gas exploration. The thrust-nappe occurred strongly in the southwest margin of the Ordos basin since the Mid-Late Mesozoic, but transformation of its tectonic regime lacks chronological constraint. Thermochronology is a useful method to reveal uplift, exhumation and evolution of mountains and basins. Based on the analysis of bedrock apatite fission tracks drilled in the eastern margin of Qilian Shan and a comprehensive study of thermal history modeling in the southwestern Ordos basin, the spatial-temporal evolution of tectonic regime transformation in the southwest margin of Ordos basin is discussed. The thermal history modeling results indicate that the eastern Qilian Shan rapidly cooled during the Middle Jurassic to Early Cretaceous(ca.174~130 Ma). This uplift event was widespread in the southwest margin of Ordos basin. Combined with the simultaneously strong thrust fold deformation, uplift, exhumation and conglomerate deposition of southwest Ordos basin margin, it indicates that that the southwest Ordos basin margin and the eastern Qilian Shan entered the compressional environment after the Yan’an period in the Middle Jurassic(ca.170 Ma). The deformation and rock uplift and exhumation were enhanced during the Late Jurassic. This compression deformation, characterized by weakening from the west to the interior of the basin, lasted until the early Early Cretaceous, and should be closely related to the north-easterly convergence of the Tethys tectonic domain. At the beginning of the Sanqiao period of the Early Cretaceous, the southwest Ordos margin was in an extensional environment and deposited thick conglomerate of the Liupanshan Group due to the convergence of multiple plates in East Asia. The determination of the spatial-temporal evolution and dynamic background of the Mid-late Mesozoic tectonic regime transformation in the southwest margin of the Ordos basin is of great significance to discuss the formation and evolution of the region and petroleum exploration.