The Extension Of The Arc Tectonic Belt In The Northeastern Margin Of The Tibet Plateau And The Evolution Of The Yinchuan Basin In The Western Margin Of The North China

In the Cenozoic era, China continent has been affected by both the penetrating of the India plate and the subduction of the Pacific plate. Driven by both geodynamic systems, the tectonic deformation in the eastern and western China is completely different. The Tibet Plateau and the North China block are the typical and representative structures of the east and west China, respectively. The study of the relationship between these two structures is an important subject of continental dynamics.Ningxia is located in the northeast margin of the Tibet Plateau and the western margin of the North China. It is an ideal site to study the above-mentioned subject of continental dynamics. The sinistral strike-slip fault, dextral strike-slip faults, thrust faults and normal faults were all developed in the Ningxia region due to the interaction motion of the Tibet, Alxa, Ordos blocks. Two distinct deformational styles are developed in the southern and northern Ningxia region. In the southern of Ningxia region, the dformation is composed of sinistral strike-slip and thrust faults, which belong to the Tibet Plateau domain. In the northern Ningxia region,the Yinchuan Basin and the Helan Mountain form a typical basin and range structure,which belongs to the North China tectonic domain. The south and north part of Ningxia is divided by a right-lateral shear zone, which is composed of three secondary faults. In this paper, we focus on the deformation of the boundary shear zone. By studing of the evolution of the Yinchuan Basin and the extension of the arc tectonic belt since late Cenozoic, we discuss the relationship between the deformation occurred in the south and the north of Ningxia. Finally, the study provides a reference for tectonic deformation in the mainland of China. In this paper, main findings are shown as following.Field geological survey and preliminary geological mapping indicates that Western Helan Mountain, Sanguankou-Niushoushan, and Eastern Luoshan faults had strong earthquake activity and similar strike-slip rates since late Quaternary. These three faults are connected with each other and form a special right-lateral shear zone. Geometrically the fault zone is uncontinuou, strike and dipping direction of secondary faults vary, when they act as different feological boundaries. The Western Helan Mountain fault is the boundary of Alxa block and the Helan Mountain, which is a thrust fault, dips to the east and shows westward thrusting in the outcrop sections. The Sanguankou fault obliquely cut through the Helan Mountain, dipping to the southwest and thrusting northeastward. The Liumugao fault is the south boundary of Helan Mountain and Yinchuan Basin, dipping to the northeast with vertical normal motion. The Guanmahu fault, which is the boundary between the northeastern margin of the Tibet Plateau and the Yinchuan Basin, dips to the northeast and has the vertical normal motion. The Eastern Luoshan fault is the boundary of the northeastern margin of the Tiebet Plateau and Ordos block, dips to the east in the north part, but dips to different directions in the south part. This complex fault system formed the boundary between the arc-shaped tectonics belts of the northeastern margin of the Tibetan Plateau and the basin riange tectonics of the western margin of the North China Basin, near the Ningxia area.Field geological survey suggests that Sanguankou fault is dextral strike slip fault, instead of previously reported sinistral strike-sliping on fault. Especially for, the geological boundaries formed in the Paleozoic, Mesozoic and Cenozoic were right-laterally displaced by the fault, the ridges and gullies were also systematically displaced. The largest right-lateral strike-slip displacement is approximately 961 m along the Sanguankou fault. The average horizontal and vertical slip rates of the Sanguankou fault are 0.35 mm/a and 0.09 mm/a since the late Quaternary, respectively, estimated by topographic measurements and sediment dating data. The horizontal and vertical slip rates obtained from paleoearthquake studies are 0.34 mm/a and 0.1 mm/a, respectively, which is comparable with the above rates. The average horizontal slip rate and the vertical slip rate of the Liumugao fault since the late Quaternary are 0.59 mm/a and 0.03 mm/a, respectively. Since the late Quaternary, there are three paleoseismic events occurred on the Sanguankou fault occurred, the first event occurred at 25.1-27.63 ka BP, the second event occurred at 17.34-18.09 ka BP, the third event occurred at 8.18-8.32 ka BP. The intervals of the three events were 8.65 ka, 9.47 ka, 8.25 ka, with an average recurrence period of 8.8 ka. The horizontal coseismic displacement was 3m, and the vertical coseismic displacement was 0.9 m. The paleoseismic recurrence model of the Sanguankou fault belongs to the characteristic earthquake model, the elapsed time is close to the average recurrence intervals, hence, we should pay more attention to the seismic potential of the Sanguankou fault in near future.Western Helan Mountain fault is a Holocene Active fault of dextral strike-slip with thrust component.The largest strike slip displacement of the fault is not less than 800 m, and the average slip rate and a vertical slip rate of the fault respectively is 0.28 mm/a and 0.11 mm/a since late Quaternary. The combined trench located at the middle section of the fault revealed three paleoearthquakes. The first event occurred at 30.54-30.6 ka BP, the second event occurred at 10.15-11.24 ka BP, the third event occurred at 6.16-4.83 ka BP. The recurrence interval is close to 5000 a. Between the first event and the second event, it seems some events are missing. The elapsed time of the latest event is close to the recurrence period, hence we should also pay attention to the seismic potential of the Western Helan Mountain fault in near future.The southern segment of the Eastern Luoshan fault, from Miaoshan to Xiquan, was not connected to Eastern Niushoushan fault, but extending northward, arrived in the Shuangjigou near the south Yinchuan Basin. The northern segment of the Eastern Luoshan fault, from Sunjiatan to Shuangjigou, also has a significant dextral strike-slip characteristics, a series of gully were simultaneously displaced by the fault, the horizontal slip rate of the fault since Late Quaternary is from 1.31 mm/a to 1.54 mm/a, i.e., 1.43 ± 0.11 mm/a. The re-evaluation horizontal slip rate of the southern segment of the Eastern Luoshan fault is greater than and close to 1.4 mm/a since late Quaternary, which is consistent with theslip rates of the northern segment.The sedimentation and evolution of the Yinchuan basin are mainly controlled by 4 major normal faults, from east to west are the Eastern Helan Mountain fault, Luhuatai buried fault, Yinchuan buried fault, and Yellow River fault. The vertical slip rate in the middle and northern segment of the Eastern Helan Mountain fault since the late Quaternary was 1.3 mm/a, and the southern segment was 0.1 mm/a. OSL samples test results shows that the last rupture in the Eastern Helan Mountain fault occurred in about post 600-700 a BP. By restoring the displacement of the Great Wall and analyzing of the piles layer show that the Great Wall was really displaced, which was built before the 500 a. Luhuatai buried fault intersects with Eastern Helan Mountain fault at 5 km underground. The southern segment of Luhuatai buried fault is the late Pleistocene active fault and its north is Holocene active fault with a vertical slip rate of 0.18 mm/a. Yinchuan buried fault terminated northward at Yaofu Town and Holocene active segment show length less than 36 km, the last surface rupture occurred shortly prior to 3400 ± 78 a BP, the vertical slip rate of the fault was 0.22 mm/a since late Quaternary. Trenching, drilling and shallow seismic exploration results show that the fault had the fracture feature recurrence in situ and the significant growth characteristics, and the rupture did not rupture to the ground is not due to the absorption of the loose. Although the Yellow River fault cut the lower crust in the deep, but the activity is not strong in the shallow. Our studies showed that the Hongyazi scarps in the northern segment of the fault are not a fault scarp. Drilling results show that the fault has not been active since 3625 years ago. The cumulative displacement is only 0.96 m during the past 30 ka. The vertical slip rate of the fault in north and south were 0.04 mm/a and 0.24 mm/a, respectively, the southern section of activity is relatively strong. Therefore, the Eastern Helan Mountain fault is major and most active normal fault in Yinchuan Basin, which is also the seismogenic structure of the 1739 Pingluo 8 earthquake.The boundary fault zone in the north and south of Ningxia has experienced two stages of tectonic deformation in the late Cenozoic.First stage tectonic deformation that occurred in the late Pliocene, about 2.7 Ma, crustal shortening and regional uplift caused by the folding. This was due to the northward penetrating of the Tibetan Plateau.The second stage of the tectonic deformation occurred in the early Quaternary, showed the lateral extrusion of the secondary blocks, which was the result of the combined action of the outward growth of the Tibetan plateau and the counterclockwise rotation of Ordos Block. The extension of the arc-shape tectonic belt in the northeastern margin of the Tibet Plateau was gradually growing outward by the sub-blocks that occurred in this stage. The margin reached the Haiyuan fault, Tianjingshan fault, and SGK-NSSF at 10 Ma, 5.4 Ma, and 2.7 Ma, respectively. The activity status of the sub-blocks controlled the motion at the boundaries and faults. Lateral extrusion happened when the sub-blocks were obstructed, which might be the main reason that the boundary fault zones show different character,The Yinchuan basin has a double layer extension mode in the crust, and the deep crustal movement controls the shallow tectonic activity. In the lower crust, the two ductile shear zone converts the horizontal extension of the upper mantle to the downward vertical movement. The vertical movement of the lower crust between the two ductile shear zones is decoupled from the upper and lower crust, and shear slip occurs on the Conrad discontinuity. By sliding adjustment along the Conrad discontinuity and the Eastern Helan Mountain fault, most of the crustal vertical motion in the bottom of the upper crust produced levels of conjugated extension between Yinchuan fault and the Eastern Helan Mountain fault, resulting in a serial of brittle normal faults in the upper crust. Helan Mountain experienced three stages of uplift since the late Cenozoic, the southwestard inclined lifting and uplift at 10-12 Ma, the regional uplift in the southern sectionat the end of Pliocene and the fault block difference uplift during Quaternary. Tilting uplift of the east side of Helan Mountain may be controlled by the uplift of the lower crust ductile fault. The uplift of the west side of Helan Mountain was controlled by the Alxa block oblique wedging. Under the influence of these two tectonic forces, the Helan Mountain unevenly uplifted.The structure of the south and north Ningxia is two relatively independent structural systems and has a independent tectonic evolution.The southern Ningxia is controlled by northward penetrating of the Tibet Plateau, and the northern Ningxia is controlled by the lithosphere stretching of North China. Both of them tend to be synchronized in important tectonic events. In 10-12 Ma, Yinchuan Basin began to shrink, and Helan Mountain began to rapidly tilted uplifted, at the same time, the expansion of the Tibet Plateau arrived to the Haiyuan fault. In 5.4 Ma, Yinchuan Basin is extended and the plateau extended to Tianjingshan fault. In the early Quaternary, about 2.6 Ma, Yinchuan Basin began to shrink again, and the plateau expansion arrived near the boundary fault zone. The formation of the Northern Ningxia should be earlier than the south, and the deformation of the south will affect the north Ningxia. The influence in the early stage may be indirectly realized through the deep tectonic action of the Ordos block. In the later stage, the structural deformation of the shallow layer is extended and arrived at the northern directly. Since Quaternary, the block movement is dominated near Ningxia and adjacent regions. The differential movement of Tibet Plateau, Ordos and Alxa blocks caused the different fault characters at different locations to equilibrium the deformations of the blocks. In the south of Ningxia, sinistral strike-slip faults and thrust faults combined together to absorb the sinistral shearbetween the Tibet Plateau and Alxa block as well as the compression to Ordos block. In the north of Ningxia, dextral strike-slip faults and normal faults combined together to absorb the dextral shear movement between the Ordos and Alxa block.The study of the tectonic deformation in the south and north of Ningxia late Cenozoic shows that the extension of the Tibet Plateau has limited influence on the North China, and the present boundary of the two is still located in the western margin of North China. The formation and evolution of the Yinchuan Basin in the western margin of the North China is mainly controlled by the Pacific plate subduction dynamical system. After the extension of the Tibet Plateau to the western margin of the North China, the influence of tectonic deformation on the local area has been established. This influence may be realized through the deep tectonic action, and can be done directly by the shallow tectonic deformation in the latter.