六盘山东麓断裂南段断裂沟槽韵律沉积特征对最新活动时代的限定

六盘山位于华北断块西部鄂尔多斯块体与青藏块体祁连断褶带之间的构造转换部位。前人对六盘山东麓断裂南段进行过一定程度的研究,根据地貌特征推断六盘山东麓断裂南段晚更新世以来不活动。根据断裂特征,大致可将六盘山东麓断裂划分为北段、中段和南段。活动断裂带上的断裂沟槽沉积物表现为由多个粗粒层与细粒层交替的韵律层序结构,韵律沉积特征与断裂活动有直接关系。在六盘山东麓断裂南段冶家村西发现一个断裂沟槽韵律沉积剖面,共鉴别出三期韵律沉积组合。通过分析认为本剖面所反映的韵律沉积特征应为构造成因。对断裂沟槽沉积物采样测年的结果表明,该段断层分别在约500 a BP后、6 ka BP后和15 ka BP前有过明显的地质活动。确认六盘山东麓断裂南段晚更新世以来有过多次活动,是一条活动断裂带。

The Recent Active Time of the South Segment of the Eastern Liupanshan Piedmont Fault: Constraints from the Characteristics of Rhythmic Deposits in the Fault Grooves

The Liupanshan region is located in the transitional tectonic zone between the Ordos block of the western North China block and the Qilianshan orogenic belt of the northeastern Qinghai-Tibetan block. Generally the eastern Liupanshan piedmont fault may be divided into northern, middle and southern segments. The groove sedimentation on the active fault is displayed as the structure of the rhythm sequences that consist of the many alternate coarse beds and fine beds. Each circle of rhythmic deposits was caused by a movement of the fault. A profile of the rhythmic deposits of the fault grooves in the south segment of the eastern Liupanshan piedmont fault was discovered. Three circles of the rhythmic deposits were discerned on the profile. The characteristics of the rhythm sedimentation on the profile are considered to have been caused by a few geological events. The results of sampling and dating from the deposits of the fault grooves suggest that the south segment of the eastern Liupanshan piedmont fault expereicned remarkable movements after about 500 a BP, after about 6 ka BP and before about 15 ka BP respectively. It can be confirmed that the recent activity of the south segment of the eastern Liupanshan piedmont fault is evident and this segment of fault has expereinced several movements since the Late Pleistocene. Hence, the south segment of the eastern Liupanshan piedmont fault is active.