蒙古国塔木察格盆地南贝尔凹陷早白垩世断裂发育和盆地演化

Early Cretaceous Fracturing and Basin Evolution of the South Buir Sag of the Tamtsag Basin in Mongolia

塔木察格盆地南贝尔凹陷早白垩世的断裂发育可分为早、晚两期。早期(铜钵庙组-南屯组沉积期)为北东向同生断层的伸展(或张扭)活动,可细分为4幕,分别发生在铜钵庙组、南一段下部、南一段上部和南二段沉积期。因北东东-近东西向断裂的左行剪切变换,其南、北两侧发育的北东向盆地有不同的特征,北为2个相背而倾的箕状断陷,南为2个相向而倾的箕状断陷。晚期为近南北向断裂的右行剪切,分2幕:第I幕发生在大磨拐组沉积的早-中期,除新生断裂外还有南二段沉积期形成的张扭性断裂的继承性活动;第II幕发生在大磨拐河组沉积晚期-伊敏组沉积早期。早期的北东向断裂晚期再度活动并遭受了不同程度的改造。基于上述认识分两阶段重塑了南贝尔凹陷早白垩世的演化,并与塔南凹陷进行了对比,发现东部次凹的构造活动性大于西部次凹,即:后者以北东向断裂为主而前者以近南北向断裂为主,北东东-近东西向断裂在东部次凹发育更好,南屯组沉积期末的隆起造成东部剥蚀的地层厚度更大。

The fractural development of the South Buir Sag of the Tamtsag Basin during the Early Cretaceous could be divided into two phases. The fractural activities in the early phase, namely, the depositional period of the Tongbomiao-Nantun Formations, are characterized by the NE-orientated synsedimentary extensional （or transtensile） faulting. It could be further divided into four episodes, occurring in the depositional periods of the Tongbomiao Formation, the lower part of No.1 Member of the Nantun Formation, the upper part of No.1 Member of the Nantun Formation, and the No.2 Member of the Nantun Formation, respectively. Owing to the sinistral shearing transit of the NEE-/near WE-striking fractures, the NE-orientated basins at either side of the transit belt have varied features. To the north of the transit belt, there developed two normal faults dipped back-to-back. Moreover, each fault controlled a half-graben developing on its upper plate, and the two half-grabens are separated by the Sudert Fault-Rise. To the south of it there occurred two normal faults dipped face-to-face, and two half-grabens on the hanging walls were joined each other on their southern terminals. The fractural mobilization in the late phase displayed a dextral shearing of the near NS-striking fractures, and two shear episodes could be distinguished. The first episode took place in the early-middle stage of the depositional period of the Damoguaihe Formation, during which some neogenic shear faults created, and in addition, the transtensile faults formed in the No.2 Member of the Nantun Formation rejuvenated. The second episode occurred in the latest depositional period of the Damoguaihe Formation and the earliest sedimentary stage of the Yimin Formation. The early phase NE-orientated faults remobilized in the late phase and reformed in varying degrees. Corresponding to the two-phase fractural activities, the two-phase basin evolution is reconstructed, and compared with that of the Tanan Sag to the south. A peculiarity for the South Buir Sag is that the East Sub-sag has stronger tectonic mobility than the West Sub-sag. The NE-orientated faults are relatively well preserved in the West Sub-sag, while in the East Sub-sag the structural framework is composed of the nearly NS-striking fractures as well as the NEE-/near WE-striking fractures. Furthermore, subsequent to the deposition of the Nantun Formation, the basin closed and the regional uplift, moreover, the eroded stratigraphic thickness in the East Sub-sag is much larger than that of the West Sub-sag.