Angular Unconformity of the Late Quaternary Strata in the Hetao Basin, North of the Ordos Block (West China): Timing and Its Tectonic Implications

Following the uplift of the Tibet Plateau and the continuous subduction of the Pacific Plate, graben faulting began to appear around the Ordos Block in the Cenozoic. The Hetao Basin is a Cenozoic rift basin between the Ordos Block and the Yinshan Mountains, and Late Quaternary sedimentary strata, which have lacustrine facies, are widely distributed inside this basin. However, the evolution of the Hetao Basin and its related fault systems has been debated for a long time due to the lack of tectonic evidence. In this study, four sections named Haolaigou, Bianqianghao, Huhesala, and Hazigai are selected along the north margin of the Hetao Basin. With the lithology and structural analysis of the Upper Pleistocene series in these sections, two new angular unconformities are found within the 10 m thick sedimentary sequence of the lacustrine sediments. Based on the dating results, we speculate that these two upper and lower angular unconformities are formed between 33 ka BP and 40 ka BP, and 60 ka BP and 80 ka BP, respectively. The angular unconformities also provide tectonic constraints for the latest and ongoing tectonic activity in the Quaternary. This tectonic movement begins at around 80 ka BP and causes two different unconformities of the lower strata with varying degrees of deformation (tilt) but also leads to the final death of the ancient lake.

Using Google Earth Images to Extract Dense Landslides Induced by Historical Earthquakes at the Southwest of Ordos, China

As historical earthquake records are simple, determining the source parameters of historical strong earthquakes over an extended period is difficult. There are numerous uncertainties in the study of historical earthquakes based on limited literature records. Co-seismic landslide interpretation combined with historical documents can yield the possibility of reducing these uncertainties. The dense co-seismic landslides can be preserved for hundreds to thousands of years in Loess Plateau, North China; furthermore, there are notable attribute differences between earthquake landslides and rainfall-triggered landslides. Along the southwestern margin of the Ordos Block, only one severe earthquake has been recorded in the past 3,000 years. The records of “Sanchuan exhaustion and Qishan collapse” provide clues for an investigation of the 780 BC Qishan earthquake. In this study, combined with historical documents, current high-resolution Google Earth images were used to extract historical landslides along the southwestern of the Ordos Block. There were 6,876 landslides with a total area of 643 km2. The landslide-intensive areas were mainly distributed along the Longxian–Qishan–Mazhao Fault in the loess valley area on the northeastern side of the fault. Loess tableland and river terraces occur on the southwest side of the fault; dense landslides have not been examined due to the topographical conditions in this area. By analyzing the spatial distribution of historical earthquake damage in this region, comparing the characteristics of rainfall-triggered landslides, and combining existing dating results for bedrock collapse and loess landslides, the interpretation of dense historical landslides can be linked to the Qishan Earthquake. The interpretation results are associated with historical records. Analyses of current earthquake cases show that the distribution of dense landslides triggered by strong earthquakes can indicate the episeismic area of an earthquake. In addition, the non-integrated landslide catalog without small- and medium-scale coseismic landslides can be used to effectively determine the source parameters of historical strong earthquakes and perform quantitative evaluations. This study evaluates the focal parameters of the 780 BC Qishan earthquake based on interpretations of the spatial distribution range of historical landslides as representations of the range of the extreme earthquake zone.