鄂尔多斯地块中元古代长城纪盆地属性研究

Researches on basin property of Ordos block during Mesoproterozoic Changcheng Period

中元古代长城纪华北克拉通南缘的大地构造背景一直备受关注。学者们主要依据该时期火成岩地球化学性质推断其构造背景,提出了B型俯冲、安第斯型俯冲、非造山环境以及与Columbia超大陆裂解有关的伸展环境。本次研究应用地球物理方法来剖析鄂尔多斯盆地中南部长城系构造格架和地层分布,以探索研究区大地构造背景及盆地属性。二维地震资料显示长城纪构造旋回包含裂陷期和裂后期。裂陷期半地堑主控边界断层活动强度大,下降盘发生旋转掀斜,发育次级地堑、半地堑,同时发育扇三角洲、水下扇沉积,楔状体沉积中心地层厚度大。裂后期断层活动减弱,水体范围扩大,地层超覆在早期楔状地层之上,地层厚度变化小。重力异常反演表明,研究区存在同期的凹陷和凸起,它们可能受控于边界断裂。结合地块周缘长城系沉积特征,我们认为鄂尔多斯地块中元古代长城纪盆地属性为陆内裂陷。

The tectonic setting of southern margin of North China Craton has drawn much attention during Mesoproterozoic Changcheng Period. Researchers have proposed several plausible models of geological settings mostly based on geochemical properties of volcanic rocks. The main views about the geological background are B-type subduction,Andean type subduction,non-orogenic extension and stretching which is related to the breakup of Supercontinent Columbia. This paper focus on tectonic features and stratum distribution of Changcheng Group in the south central Ordos block with geophysical methods meanwhile makes some study on its tectonic setting and basin property. The 2D seismic data indicates tectonic cycle in Changcheng Period with syn-rift and post-rift phase included. In syn-rift phase,the boundary faults act intensely,and the hanging wall rotated,leading to the development of sub-grabens and half-grabens. In the meantime fan delta and subaqueous fans filled in. The formations thicknesses are huge in depocenter of wedges. In post rift phase,the activity of faults was weak,resulting in the formation with consistent thickness,which overlapped on the wedges. Besides,the consequence of gravity anomaly inversion suggests several depressions and salients developed during the same period which were probably controlled by the boundary faults. Combined with the sedimentary characteristics of block margin,we deem the study area to be a rift basin during the Changcheng Period.