Characteristics Of Tectonic Activity Along The Southern Boder Faults Of The Weihe Graben,Central China

The Cenozoic graben systems around the tectonically stable Ordos Block,have been considered as ideal places for investigating active deformation and structural evolution of rift basins due to high seismicity.The Weihe Graben is loactaed at its southern margin,adjacent to the Tibetan Plateau,which has received much attention from geologists.The magnitude 8.5 Huaxian earthquake of 1556 occurred in its southeastern corner,resulting in serious damage and more than 830,000 deaths.Previous studies have mostly focused on the active structures in the eastern and northern parts of this graben.By contrast,in the southwest,tectonic activity along the northern margin of the Qinling Mountains has not been systematically investigated yet due to the lack of prominent tectonic landforms and fault outcrops.In this study,based on digital elevation models(DEMs),we carried out geomorphological analysis to evaluate the relative tectonic activity along the whole South Border Fault(SBF).On the basis of field observations,high resolution DEMs acquired by small unmanned aerial vehicles(sUVA)using structure-for-motion techniques,radiocarbon(14C)age dating as well as cosmogenic nuclide(10Be)dating of river sands collected near catchment outlests,we demonstrate that:1)Tectonic activity along the SBF changes along strike,being higher in the eastern sector;2)Seven major segment boundaries have been assigned,where the fault changes its strike and has lower tectonic activity;3)The Huaxian-Huayin segment of the Huashan Piedmont Fault(HPF)characterized by almost pure normal slip has been active during the Holocene,which together with the Northern Margin Fault of the Weinan Loess Tableland(NMF-WLT)have been inferred as the main seismogenic structures of the 1556 Huaxian earthquake;4)Tectonic activity in SE Weihe Graben has a tendency to migrate from the basin boundary fault(Lantian-Huaxian segment of the HPF)towards the rift center(Lishan Piedmont Fault and NMF-WLT);5)The catchment-averaged erosion rates are 0.03-0.09 mm/yr,which might be affected by many factors including tectonics and lithology,which still deserve more detailed investigations.The results would further our understanding of the seismic risk in densely-populated Weihe Graben and the structure features of active normal faults.