西藏日喀则拉堆—乃东与毕定—甲舍拉断裂的活动性分析研究

Activity Analysis of the Ladui—Naidong and Biding—Jiashela Faults in Xigaze,Tibet

综合采用地质地貌调查、探槽开挖及OSL测年等,对离日喀则市最近的拉堆—乃东和毕定—甲舍拉断裂的活动性进行研究,结果表明:拉堆—乃东断裂在距今约3万年前停止了活动,毕定—甲舍拉断裂则至少在5万年前停止了活动,即二者自晚更新世晚期已不再活动——不属于全新世活动断裂,这与部分学者认为拉堆—乃东断裂为全新世活动断裂的结论不同。研究获得的最新断层活动资料可为该地区的地震预测提供科学依据。

About 50~60 million years ago,the Indian subcontinent plate collided with Eurasia plate,which resulted in the Tibetan Plateau,the highest and most active in the world.The Xigaze region of southern Tibet,which crosses the Yarlung Zangbo River suture zone,is the most significant tectonic boundary in southern Tibet.Geoscientists have conducted several studies on the evolution of geological structure,rocks and sedimentary stratigraphy,and paleoclimate and paleoenvironment in southern Tibet,and large amounts of geophysical data have been collected from this region.However,the studies on active structure and the active faults data collected from the Xigaze area are rather insufficient.In this study,based on detailed field geomorphology surveys,trench studies,and optically stimulated luminescence(OSL)data,we investigated the activities of Ladui-Naidong and Biding-Jiashela faults,which lie in the suburban area of the Xigaze city.The trench profile near Dari village and the OSL ages obtained from this trench indicate that the Ladui-Naidong fault possibly ceased to be active approximately 30 ka ago,indicating that the fault is not a Holocene active fault.Our result on Ladui-Naidong fault activity directly contradicts those of previous studies,which claim the fault is a Holocene active fault,based on scanning electron microscopy(SEM)analysis on the quartz surface of fault gouge.Likely,the trench excavated near Nadang village suggests that the fault was deeply buried and did not reach the surface area.From the uninfluenced strata OSL ages,it can be deduced that the Biding-Jiashela fault ceased to be active at least 50 ka ago.In addition,the earthquake distribution and shallow artificial seismic survey results suggest that the two target faults are less active or inactive,which further confirms our geological observations.In the regional scale,the other E-W-striking faults also seem to be inactive,while the N-S-striking fault systems,such as Shenzha-Dingjie and Yadong-Gulu rifts,are very active.Such structural framework is probably because of the long,complicated tectonic evolutional history and the particular geological stress background.