Formation And Evolution Mechanism Of Langshan Fault And Its Engineering Geological Significance

The Langshan fault sandwiched between the Alxa block to the west and the Erdos block to the southeast,is the major constituent of the peri-Erdos seismic zone.It is also considered as the northern segment of the North-South seismic belt of the mainland China.However,previous studies on the Langshan fault focus on its northeastern segment,and several critical issues remain poorly discussed.E.g.,What are the geometrical features of the Langshan fault? How the Langshan fault evolved through time? Could we correlate the evolution of the Langshan fault with the rising of the Tibetan plateau to the southwest or the subduction of the western Pacific plate to the east? Is there a serious seismic risk in the Langshan fault zone? What is the effect of the Langshan fault on the regional stability for geological engineering? These aforementioned questions remain unanswered and urge further investigation.With the support from the National Science Foundation programme “41172198,Provenance,Deformation and Structural Attribution of Lower Paleozoic on the Eastern and Southern Margins of Alxa” and China Geological Survey programme “12120115069601,1:50000 geological mapping of Langshan Map and Ertuomiao Map”,we have conducted field geological survey and engineering exploration in the Langshan region.On the basis of that,we sampled along the Langshan fault for necessary analytical studies.By summarizing the newly obtained and previous data,we have discussed the regional geology,structural features,basin structure,paleo-stress and thermochronology of the Langshan fault.In addition,we have studied the historical earthquakes in the Langshan region,and made prediction on the seismic hazard of the Langshan fault.Focusing on the possible impact of seismic hazard,we made a comprehensive assessment on the regional stability of the Langshan region.The conclusive remarks are listed as follows:We have furthered the study on the Langshan fault,and proposed an innovative multi-stage and segmental differential movement model regarding the structural evolution of the fault zone.We have systematically investigated the southwestern segment of the Langshan fault,and revealed that the Langshan fault is an SE dipping Cenozoic listric normal fault.The fault propagated along the dip towards the hinterland of the Ordos basin,and is characterized by multi-stage,segmental differential movement.In this study,we proposed an evolving model regarding the formation of the Langshan fault.In this model,(1)the development of the Langshan fault initiated after the Eocene tectono-thermo event(50-45 Ma).The intensity of the fault activity reached its peak during Miocene(ca.10 Ma),which was likely triggered by the far-field effect of the Tibetan plateau and the westerly subduction of the western Pacific oceanic plate.This have shaped the early form of the modern landform;(2)The interaction between the multi-stage Tibet plateau rising and subduction of the Pacific plate formed the modern structural landform of the Langshan region.On the basis of structural analyses on the Langshan fault zone,we have predicted the seismic hazard in the Langshan fault zone.Two possible seismogenic mechanisms have been put forward:(1)the earthquakes generated by the movement of the whole Langshan fault could trigger earthquakes with magnitude of 7.5?8.0,(2)while the faulting of the Ulan Baixing-Dabatu Temple section of the Langshan fault could generate earthquakes of M 7.5.By large,we have adopted the maximum scope of the seismic hazard,and made comprehensive assessment on the regional stability of geological engineering in the Langshan region during a predicted earthquake.The predicted seismic hazard in the Langshan fault could affect a scope of 1.314×105 km2,including the stable and unstable regions,that could be subdivided into 6 types,8 stages,and 11 regions.Baotou,Wuhai and Bayan Nur cities,Wulate Zhongqi and Wulata Houqi counties,the Lin-Ce railway,the Gan-Lin(G242),Jing-Xin(G7),Jing-Zang(G6),Bai-Gu(G211)national highways,and the Hai-Wu(S212),Qing-Wu(S213),Wu-Heng(S311)regional highways are located in the predicted region that could be affected by the seismic hazards occurred in the Langshan fault.We have evaluated the site stabilities of these subjects,and provided suggestion for prevention and reduction of the seismic hazards.In this study,we have established a systematic theory and corresponding methodology for structural analysis,earthquake prediction and regional stability assessment in active fault zones.The assessment should be based on comprehensive study on the structural evolution and earthquake forecasting of the active fault zone.The study should focus on the effect of the seismic hazard on the regional stability and co-genetic geological hazards.By spatial analysis of the principal and secondary factors,we could evaluate the stability of the regions affected by the geological hazards.The study and assessment should focus on the major settlements and major liner constructions.