Manifestation Of New Tectonic Activity On The Proluvial Landform In The Northern Yumu Mountain Marginal Fault

Located in the middle part of the northern part of the Tibetan Plateau,the active fault system of the Qilian Mountains ——Hexi Corridor is a strongly active tectonic area since the Quaternary,which is composed of three fault zones and a basin zone.Among them,the northern Qilian Mountain marginal fault zone is a NWW trend and still in active tectonic fault zone,which is composed of 9 secondary faults.The northern Yumu mountain marginal fault is one of them.The northern Yumu mountain marginal fault lies on the proluvial fan of the north in the Yumu mountain.At the same time,the tectonic activity of Yumu mountain area is very strong.Therefore,study on the northern Yumu mountain marginal fault,not only lay the foundation for seismic risk assessment in the region,and also have important guiding significance to ensure the safety of national major projects,including west-east gas pipelines,Lanzhou Xinjiang Railway and Lianhuo highway.Through field study,interpretation of remote sensing images of the northern Yumu mountain marginal fault and micro topography measurements to the fault scarp on the proluvial fan by differencial GPS and laser range finder,we obtain the data of the proluvial fan stages,fault scarp identification and the vertical offset of the faults in different periods.Through the analysis,it is concluded that the following points:(1)the northern Yumu mountain marginal fault distributes in the north of Yumu mountain uplift.From east to west,the fault successively passes by Liyuan river,Liyuanxiaokouzi and other places,and checks in Yuanshanzi,which is about 58.3km.According to the tend of fault scarp,the northern Yumu mountain marginal fault can be divided into three sections.The Yumu mountain marginal fault on the whole is low angle thrust fault,whose trend is from NXX to NX,and which dips SW,also its eastern local is dextral strike slip since Late Quaternary.(2)The proluvial fan in the study area can be divided into six periods.Among them,the sixth period proluvial fan limits distribution and storage is limited.Meanwhile,the first phase to the fifth phase of proluvial fan develop widely and are well preserved.In the respect of times,the sixth period proluvial fan may be the Middle Pleistocene and the fifth to third may be the late Pleistocene.Moreover,the second to the first period of proluvial fan may be Holocene.(3)The vertical offset of fault scarp in the different proluvial fan at the same area is different.The height of fault scarp in an older proluvial fan is higher than that in the new proluvial fan,which illustrates that the fault exists multiple activity.Yumu Mountain had uplifted intermittently at least four times since the late Quaternary.The fault scarp of the same period of proluvial fan have different height,which reflects different fault activity rate in the different segments.And Mutougou in the middle section and Liyuanxiaokouzi in the east are biggest about activity rate.The Yumu mountain has the characteristics of migration changes in different periods of intense tectonic activity area,lateral migration is from east to West.