山西大同与晋中盆地全新世活动断裂定量研究中热释光与14C测年方法应用

文章通过对山西断陷系内4条边界活动断裂带开展全新世活动定量研究中实测的67个热释光测年样品及20个14C测年样品的实例分析,讨论了在断错地貌研究和探槽全新世古地震研究中,地层测年样品帮助确定断层活动事件年代、判断探槽内古地震事件的时间跨度、古地震事件的完整性及垂直断距的不确定性等方面发挥的作用;并讨论了少量热释光样品出现年代倒置现象的原因。

THERMOLUMINESCENCE AND CARBON-14 DATING METHODS APPLIED TO QUANTITATIVE STUDIES OF THE HOLOCENE ACTIVE FAULTS IN DATONG AND JINZHONG BASINS, SHANXI

In order to dating Holocene paleoearthquakes, 67 samples for Thermoluminescence (TL) and 20 samples for 14 C were used. Samples were taken near the active faults including the Kouquan fault and the northern piedmont fault of Mt. Hengshan on the western and southern boundary of Datong basin, and the Taigu fault and the Jiaocheng fault on the eastern and western boundary of Jinzhong basin respectively. The important role of 14 C and TL dating in determining the age of dislocated geomorphology and paleo-earthquake was discusses. In the geomorphological investigation 28 TL samples and 3 14 C samples were dated. Among them, 8 TL-samples were taken from the back-border of alluvial fans; 7 TL samples from the bottom of gullies and the first and second order terraces; 13 TL-samples from the front and back border of loess platform. 3 14 C samples were taken from loam layers in the first order terrace on upthrown wall of the fault. Only three TL ages contradict with others, and therefore not used. The dating results indicate that the upper deposits at the alluvial fans back-borders on the downthrow side of the faults in Datong basin formed during Middle Holocene. While the strata at the foot of the upthrown side along the newest dislocated faces of the Taigu and Jiaocheng fault can be dated to Early Holocene. Therefore, the ages of deposits proved that the multiple activities on the Kouquan fault and on the northern piedmont fault of Mt. Hengshan took place along the main fault planes in between bedrock and back margin of alluvial fans. It also proved that the fault planes of Jiaocheng fault and partial segments of the Taigu fault are distributed as step faults. The newest activities of the faults migrate towards the basin. Ten big trenches have been opened along the four faults in the research. Our study on these trenches, showed that fourteen strong earthquakes took place along the four faults during Holocene, four on the Kouquan and Taigu fault and three on the northern piedmont fault of Mt. Hengshan and the Jiaocheng fault. Along the faults, 39 TL and 17 14 C samples were dated. Among them, 12 14 C and 15 TL datings were used for determining the upper or lower-limit of 13 paleo-earthquake events on the four faults. 5 14 C and 16 TL samples were used for determining the age of strata. 8 TL ages showed inversed chronology, three of them were taken from the open fissure wedge and collapse wedge near the fault planes in trenches. Samples taken from the top layer of the trenches may help to determine the age of morphological planes, and that from the bottom may help to determine the time span of paleo-earthquake events, their integrity, and the uncertainty of vertical offset of the fault. The loose deposits between the two sides of faults in the bottom of trenches along the Kouquan fault, the Taigu fault, and the Jiaocheng fault all exhibit a long time-span, indicating that these faults have had multiple activities during the time interval. The oldest deposit on the downthrow side of the three faults is not older than Middle Holocene. It indicates that the paleo-earthquake events during Early Holocene might be missing. Finally, the reasons for a few TL age reversal were also discussed. Besides the age of deposits in the tensile fissure wedges and in collapse wedges is older than the underneath layers; there are also TL ages older than those of 14 C from the same layers as well as the age of upper deposit in a trench older than the lower one. The latter is probably man-made. Another reason could be that sample site in geomorphologic investigation are not deep enough.