Response Of Stream Geomorphic Indices And Lacustrine Sediments To The Activity Of Daqingshan Piedmont Fault

The Hetao Basin in the northern margin of Ordos Block has developed a lot of active fault structures.Since the last century,earthquakes above M6 have occurred for many times,and the fault system in the northern margin of Hetao Basin has attracted much attention.However,it is difficult to study the difference of fault activities because of the complex and changeable topography and sedimentary strata of the piedmont fault,as well as the difficulty of accurate dating.It is found that the evolution of stream landform is usually influenced by regional tectonic activities and the evolution of stream landform also records the tectonic history of the region.The stream geomorphic indices can well record the information of fault activity.As a traditional method for palaeoseismic studies,trenching can be combined with dating techniques to identify palaeoseismic events and the earthquake recurrence interval.However,when using trenches to study palaeoearthquakes,factors such as the active tectonic background of the earthquake-caused structure,the lithology on both sides of the fault,the geomorphology location and type and the samples and methods for dating will affect the location of the trench.Thus,trenches should be carefully selected and used to identify the impact of ancient earthquakes.In recent years,scholars have made considerable progress in using other methods to reveal the palaeoseismic information of faults.Moreover,the history of fault activity may have been recorded in the lacustrine sediment adjacent to the fault.The lake sediments are easily affected by seismic waves and the disturbance of sedimentary characteristics can record the palaeoearthquake events.In recent years,stream geomorphic indexes and lacustrine sediments have gradually become common methods to study fault activity.The Daqingshan piedmont fault is located in the northern margin of Hetao Basin and has been active strongly since Pleistocene.The region is rich in tectonic landforms and less affected by climate factors,and the Hasuhai Lake developed in the middle of the fault in front of the Daqingshan Mountain.It is an ideal place to carry out research on the response of stream landforms and lake paleoearthquakes to active faults.In the present study,following the segmentation regime of previous studies,we divide the fault zone into five segments,namely,the Baotou,Tuyouqi West,Tuzuoqi West,Bikeqi,and Hohhot segments,and we discuss the relationship between the drainage basin geomorphology and the piedmont fault activity in the Daqingshan area using 30 m spatial resolution Shuttle Radar Topography Mission(SRTM)digital elevation model(DEM)data.We use a range of geomorphic indices to examine the drainage basins in the Daqingshan area,including the channel steepness index(ksn),slope,hypsometric integral(HI),relief degree of land surface(RDLS),and stream lengthgradient index(SL),extracted with ArcGIS and MATLAB,and we also consider local lithologic and climate aspects.Hasuhai Lake is adjacent to the middle segment of the Daqingshan piedmont fault in Inner Mongolia.The region belong to a temperate continental semi-arid climate with little interference.Sediment samples and samples for dating were collected from three trenches excavated on the periphery of the Hasuhai Lake.Based on the variation characteristics of grain size and magnetic susceptibility of sediments,the environmental change characteristics and paleoseismic events of Hasuai lake are analyzed.The 14C dating results and variations in the grain size distribution,grain size components and magnetic susceptibility of sediments caused by seismic events obtained in this study were compared with those caused by a series of palaeoseismic events at the middle segment of the Daqingshan piedmont fault reported by previous studies using trenches,knickpoints and alluvial soil profile records.The main conclusions of this study include the following aspects:(1)the difference of tectonic activities in different fault segments is the main factor affecting the fluvial geomorphic changes;(2)The fault activity is strong in the middle section,but decreases in the east and west sections;(3)The sediments in the Hasuhai have been subjected to weak hydrodynamics since the Holocene;(4)Seven palaeo-seismic events have been identified since 12,000 years by using the variation characteristics of lacustrine deposition samples collected from the lake.