Late Quaternary Tectonic Activity Characteristics Of The Northwestern Margin Of The Ordos Block

In the early Cenozoic(~50Ma),the collision between the Indian plate and the Eurasian plate,and the subsequent wedge of the Indian plate,led to a significant shortening of the Tibet Plateau,forming the largest plateau in the world.In addition,the plateau is still expanding outwards today,but in the foreland position of the plateau,the expansion is blocked by a number of relatively stable rigid blocks,such as the Sichuan Basin in the east,the Ordos block in the northeast,the Alashan block and the Tarim Basin in the north.The effects of the uplift and expansion of the plateau are not limited to the interior and boundary of the plateau.The expansion of the Tibet Plateau can be transformed into a variety of tectonic activities by interacting with these blocks and being transmitted to further areas.We refer those tectonic activities that are not in direct contact with the plateau but are caused by the expansion of the plateau as the remote effect of the expansion of the Tibet Plateau,such as the activation of the Tianshan Mountains in the northern Tarim Basin.The northwestern margin of the Ordos block is located in the area where the three blocks of the Tibet Plateau,Alashan and Ordos interact,and the area of the forefront of the Tibet Plateau extending to the northeast--the Niushou Mountain-Luoshan area,about 50-450 km,is an excellent location to study the remote effect of the expansion of the Tibet Plateau to its northeast.Then,what kinds of remote influences do the expansion of the Tibet Plateau bring to the study area? How these effects are reflected? What's the mechanism of these?The study area mainly developed four active basins,namely Hetao,Jilantai,Wuhai and Yinchuan,as well as the Alashan uplift,the Helanshan uplift and the Ordos uplift.On the basis of summarizing the previous research results of Yinchuan Basin and Hetao Basin,this thesis makes a detailed study on the geometrical distribution,activity mode and sliding rate of the main faults in the Wuhai Basin,and draws the fine structure of the Wuhai Basin.By comparing the geometry structure and activities of the Hetao Basin,Wuhai Basin and Yinchuan Basin,the relationship between them is discussed.Combined with the modern stress field data,GPS observation data and fault activity characteristics of the study area,a relatively complete active image of northwestern Ordos block is obtained.The dynamic mechanism of tectonic deformation and its relationship with the northeastward expansion of the Tibet Plateau are discussed.This thesis mainly achieved the following understandings:Deep and shallow seismic detection revealed that the Yellow River buried Fault(YRF)in the Wuhai Basin consists of two nearly parallel normal faults(the EYF and WYF).The section showed that the WYF is a normal fault,inclines eastward,and terminates on the ZWPF(the westward boundary fault)at a depth of 8–10 km.The EYF is a westward normal fault that terminates on the eastward WYF at a depth of 1.2–1.5 km.The two faults form a Y-type graben in the section.The Wulan Township drings reveal that the up-breakpoint of the EYF is about 26 meters.The Tongda Street drings reveal that the up-breakpoint of the WYF is about 50.5 meters.Both of them have revealed that four earthquakes(E1–E4)occurred on the YRF at 25.6 ± 0.11 ka BP,39.5 ± 0.45–41.7±0.57 ka BP,58.25 ± 7.13 ka BP,and 111 ± 1.21 ka BP,respectively.We calculated the vertical slip rate of the YRF as 0.12–0.17 mm/a.Since a period of time much longer than the recurrence cycle has passed since the last tectonic activity on the YRF,we believe that the YRF might currently pose a risk.As the eastern boundary fault of Wuhai Basin,the ZWPF mainly appears as a west dipping normal fault on the sections.In the northern Wuhai Basin,the boundary faults controlled basin still show as normal faults,and the NW-trending reverse fault and the Huoluotu anticline are developed in the basin.Based on vertical displacement of matched lacustrine layers on both sides of the fault and the age of the layer,vertical slip rates have been calculated for various sections of the ZWPF,giving a vertical slip rate for the ZWYF of between 0.5±0.2 to 1.0±0.2 mm/a.The Fenghuang Ridge section and the northern part of the Gangdeershan section had the highest rates of vertical slip,which is consistent with the settlement center of Wuhai Basin.Based on the scratches developed on the Sandaokan section,we estimate the right-lateral sliding rate of the ZWPF to be 1.1-5.8 mm/a.The NNE-direction movement of the weak strata in the hangingwall of the fault is blocked by the mountain in the north of the basin,forming NW-trending reverse faults and Huoluotu anticline.Given its importance to regional structural models,we recommend a more detailed study of its horizontal slip rates.Through the vertical drop of the same lacustrine layer on both sides of the fault,it is calculated that the average vertical slip rate(since 65 ka)of the NE-trending Langshan piedmont fault(LPF)is 1.8±0.2 to 3.2±0.2mm/a.In Qingshan Town,the middle section of the fault,the vertical slip rate of the fault reaches the maximum and decreases to both sides.In the northwest corner of the Linhe Basin,the NE-trending Langshan Piedmont Fault(LPF)and the EW-trending Seertengshan Piedmont Fault(SPF)are connected by several stepped fault sections.In the northeast corner of the Linhe Basin,the EW-trending western segment of SPF and NW-trending eastern segment of SPF are connected by a large triangular relay ramp.According to the calculation,we obtain the geometric image of the block motion with the Alashan block as the reference in the northwestern margin of Ordos.The movement rate of the Ordos block relative to the northern Alashan block is 1.31±0.15 mm/a,and the direction is SE143.7°.The movement rate of the Helanshan block relative to the northern Alashan block is 0.132±0.012 mm/a,and the direction is SE123°.The Bayanhaote block moves relative to the Alashan block to the NEE76° at a rate of 0.231±0.022 mm/a.In general,under the strong differential extrusion of the Tibet Plateau to the Alashan block and the Ordos block in the direction of northeastern,the deformation of the study area was dominant by the ‘S' type Yinchuan-Wuhai-Jilanthai-Hetao Basin stretch zone.At the same time,the internal secondary blocks of Alashan block produces relative motion,but the magnitude of the movement is small.