Geomorphologic Indices Of Drainage Basins On The Eastern Piedmont Of Helanshan Mountain And Its Tectonic Implication

The study area is located in the Helan Mountain and Yinchuan Basin west of Ordos and is characterized by late Cenozoic rapid uplift of the mountain and over 1 km thick sediments of the basin.The East Helan Shan fault as the boundary of the Helanshan and the Yinchuan Basin is very active in late Qunaternary.In 1793,a large earthquake hit the Yingchuan Basin and ruptured the East Helan Shan fault.However,the difference of the vertical activity on the north and south segments of this fault remains debate due to the lack of tectonic geomorphologic markers and well-dated sediments.The studies on alluvial landforms and active tictonics in the bedrock mountain streams show that the alluvial geomorphology can well record the tectonic activity information.As the boundary of the basin and mountain,the eastern piedmont of the Helan Mountain provides a good site to study the link between tectonic activity and the evolution of geomorphology.Based on the data of active tectonics,geology and the Cenozoic tectonic evolution in the Helan Mountain and Yinchuan Basin,we took a supplementary field investigation in southernmost part of this fault and summarize the activity differences between the north and south segments of the East Helan Shan fault since the Late Cenozoic.Multiple sets of digital elevation model(DEM)data were used to extract multiple tectonic geomorphic indicators from the Helan Shan drainage Basin,and the average erosion degree of drainage basins was assessed by a proxy index of atmospheric genesis 10Be.Combined with the characteristics of regional lithology and precipitation,the controlling factors of the geomorphic indices are analyzed.Finally,the Tectonic significance of various geomorphological indicators is discussed.The results show that:(1)TanDEM-X 12m DEM and SRTM 30m DEM data are more suitable for assessing the geomorphological feature in this region.The results from two sets of DEMs show that basin slope,topographic relief,hypsometric integral,normalized steepness index,fault facet and bedrock channel knickpoint in the eastern peidmont of th Helan Mountain all exhibit a similar trend,suggesting a stronger tectonic uplift in the southeastern segment.The spatial variation characteristics of geomorphological indices have a poor correlation with lithology and precipitation,showing that these variaty of geomorphological indices are driven by tectonic uplift on the East Helan Shan fault.(2)The factors such as slope,relief,normalized steepness index,and the average slope of the fault triangle facet in the southern section are larger than the northern section,which is consistent with the vertical activity of the East Helan Shan fault since the mid-Pleistocene.The river undercutting depth estimated by the bedrock channel knickpoints may relate to tectonic uplift of the East Helan Shan since the late Cenozoic.The overall trend of the average erosion degree of the basin derived from the proxy index is slightly worse,but if we consider the Suyukou and Hongguo parts only,this result can reflect differential vertical activity on the south and north segment of the East Helan Shan fault.(3)Different geomorphic units have different time sacle for the steady state after tectonic perturbations.The normalized steepness index responds quickly to the perturbation signal,and the result is consistent with the vertical activity of the fault since the middle of the late Pleistocene.However,the hypsometric integral takes a longer time to reach the steady state,which probably fails to reflect the active fault activity information.That suggests that the stronger vertical activity on the East Helan Shan fault has been transferred from north segment in late Cenozoic into south segment in the middle Quaternary.