The Ordos block and its adjacent areas are roughly located at 105°-115°E, 34°-42°N, bordered on the east by Lvliang Mountain, on the west by the Zhuozi Mountain and on the south by Weibei mountainous region, spreading on the north to the Yellow River (Fan Junxi, 2002). Except for a bunch of arc-shaped faults at the southwest corner, the block is surrounded by the Hetao subsidence zone, Shanxi subsidence zone, Weihe subsidence zone and Jartai-Yinchuan subsidence zone. Structural differentiation between the Ordos block and its adjacent areas is obvious, which forms a series of folds and compression faults on its periphery, with complicated structure. At present, with the accumulation of geodetic deformation data, a number of scholars have analyzed activity characteristics in this region. Hu Huimin (1996) analyzed vertical crustal deformation and tectonic stress field of Ordos and its adjacent areas. Zhang Sixin et al.(2000, 2012) analyzed vertical differential movement near the Weihe subsidence zone using cross-fault leveling data, believing that the recently active small earthquakes around Ordos block are related to the transient instability caused by the weakening of extrusion stress of the Qinghai-Tibet subplate and relative enhancement of extrusion stress of the North China subplate after the Wenchuan earthquake. Deng Mingjing (2003), by analysing nearly a decade of leveling data of the southwestern margin of Ordos, indicated that the region showed mainly inherited movement characterized by rise of mountainous areas and subsidence of basins. Zhang Zhanyang et al. (2016) also used precise leveling data and found that the northeastern margin of Ordos block was recently in a period of subsidence, and a Ⅴ-shaped subsidence funnel appeared.
Restricted by the observation environment and economic development, geodetic data on the northwestern margin of Ordos is very lacking, and vertical deformation characteristics of this region are mostly obtained from geological survey data. The piedmont fault of Lang Mountain, piedmont fault of Serteng Mountain, piedmont fault of Ural Mountain, northern margin fault of Ural Mountain and Daqingshan fault are distributed along the northern margin of Ordos, controlling the Hetao subsidence zone (Li Jianhua, 2005). The eastern Helanshan piedmont fault, Lingwu fault along the Yellow River and Dengkou-Benjing fault along the western margin of Ordos control the Jartai-Yinchuan subsidence zone (as shown in Fig. 1). Since the 1970s, the region has witnessed the 1979 Wuyuan MS6.0 earthquake, the 1996 Baotou MS6.4 earthquake and the 2015 Bayinmuren MS5.8 earthquake, among which, the Wuyuan MS6.0 earthquake and the Baotou MS6.4 earthquake took place in the downfaulted basins along the northern margin of Ordos, where seismic activity is controlled by downfaulted basins and active faults, with shallow focal depths, generally located in the upper crust. In this paper, the vertical deformation characteristics of this region were analyzed based on 3 periods of precise leveling observation data.
Three periods of precise leveling data are used to obtain the vertical deformation field, the years of 1976-1980, 1990-1997 and the year of 2, 014, with route lengths of 2, 682km, 2, 709km and 2, 898km respectively and data scope of 105.5°-111°E, 37°-41.5°N. The piecewise dynamic linear rate model is adopted to obtain the vertical deformation field based on the reference Qidong 49 base in Ordos for adjustment, and general information of adjustment is shown in table 1. Unit weight mean square errors in the adjustment results are all less than 1mm, indicating that the accuracy of data in the region meets requirements. After adjustment, mutation points with too large a rate or too great a difference with adjacent points are removed, and the results are gridded by using the multi-surface function method, and finally vector and contour diagrams of vertical deformation in this region during two periods are obtained, as shown in Fig. 2 and Fig. 3.
From vertical deformation diagrams of 2 periods (Fig. 2 and Fig. 3), it can be seen that regional deformation characteristics shown in 2 diagrams are basically consistent, that is, basins subside comparatively and most mountainous areas are uplifted, marked as obvious inherited movements. Among these, the subsidence rate of the Yinchuan basin, located between the eastern Helanshan piedmont fault and the Lingwu fault along Yellow River, has slowed down, and the obtained subsidence rate in the region from 1980 to 1990 is 2-4mm/a, from 1990 to 2014 is 2-3mm/a. Vertical deformation diagrams of 2 periods both show that the Hetao basin, located between the Yinshan fault block uplift and the Ordos fault block uplift, comparatively subsides, of which, the Linhe basin sinks most obviously, with a sink rate of about 2-3mm/a, which is basically consistent with the understanding learned by geological survey that the subsidence of the Linhe basin is the most intense among the three subsidence centers of Hetao. The interior of Ordos is relatively rising, and the calculated uplift rate in 1990-2014 is basically the same as that of 1980-1990, which is 0-1mm/a, indicating that the interior of the block is relatively stable.
The diagram of vertical deformation in 1980-1990 (Fig. 2) shows that monitoring points located in the Hetao rift basin are in a state of rapid subsidence and rate at subsidence center reaches 3mm/a, which forms a distinct rate gradient belt, indicating that the region is in a period of rapid strain accumulation. This finding is consistent with the result that in west Baotou, the Yinshan block, Ordos block and Hetao subsidence zone has formed a uniform high-strain body, which is obtained by Guo Liangqian et al. (2002) using cross-fault vertical deformation and GPS observation data in 1992-1995. The northern margin of Ordos is located in the junction area between the Ordos platform and Yinshan-Yanshan fold belt, forming a strong contrasting geomorphologic landscape of rift basins and alpine valleys, with intense tectonic activities. The nearly EW-dipping, S-trending piedmont fault of Serteng Mountain located in this region experienced a normal faulting in 1979, resulting in the Wuyuan MS6.0 earthquake (Wen Xueze, 2014), and in 1996, the Baotou M6.4 earthquake occurred in the section of acute angle structure controlled by the northern margin fault of Ural Mountain and Sertengshan fault. The Jartai-Yinchuan subsidence zone located in the western margin of Ordos is also in a state of rapid subsidence, among which, the Yinchuan basin, located between the eastern Helanshan piedmont fault and the Lingwu fault along Yellow River, is at the center of subsidence, with a rate of -4mm/a. The interior of the Ordos block remains steady, with vertical movement rate hovering at the zero contour line.
Fig. 3 is the diagram of vertical deformation during 1990-2014. Because the monitoring route is different, with few monitoring points of the same name, there is a monitoring blank space near Wuhai. However, in view of the whole survey area, monitoring points in the Yinchuan basin, the Jartai basin, the Linhe basin, the Baiyanhua basin and the Hubao basin are still in a state of rapid subsidence, and monitoring points inside the Ordos block show that the whole block demonstrates steady performance, at a rate of 0-1mm/a, which is submerged in the error.3 ANALYSIS OF CROSS-FAULT LEVELING DATA
Leveling data used in this article is mostly from the first-order leveling network laid out for the national information infrastructure, and most leveling routes avoid active faults with intense tectonic activities, so only 2 level profiles are listed here crossing the northern margin fault of Ural Mountain and Dengkou-Benjing fault (Fig. 4). Leveling routes meet the piedmont fault of Ural Mountain at small angles, orthogonal to the northern margin fault of Ural Mountain, and the profiles extend from west to east about 50km on both sides of the fault. The observation values of the 1980-1990 and 1990-2014 level profiles show that there is no obvious difference in the vertical movement rate of blocks on both sides of the northern margin fault of Ural Mountain and the piedmont fault of Ural Mountain, and rate difference is less than 0.5mm/a, indicating that vertical movement of fault is not evident. The Dengkou-Benjing fault is a buried fault speculated based on aeromagnetic data, and it is exposed on the surface only near Dengkou at the northeastern segment, forming a cliff of 20-50m, and presumably there was activity in the Late Pleistocene. Of all leveling data collected presently, level routes laid out in 1980 and 1990 happen to pass through the Dengkou-Benjing fault, near Dengkou County, while the level route constructed in 2014 is changed, therefore only the curve of observation values of level profile of 1980-1990 is drawn here (Fig. 5). This level profile, from north to south, extends about 60km on both sides of the fault. The profile shows that the rate difference between blocks on both sides of the Dengkou-Benjing fault is not obvious, and the vertical movement rate of the fault is less than 0.5mm/a.
Structural differentiation between the Ordos block and its adjacent areas is obvious, and tectonic movements mostly occur in down-faulted basins and fault belts around the block. In the western margin of Ordos, a NNE-NE compressed structural zone is formed along the Jartai-Yinchuan subsidence zone and its adjacent areas, and its tectonic framework greatly impacts the formation and evolution of Cenozoic structures. The northern margin of Ordos is controlled by Yinshan block uplift, Hetao subsidence zone and Ordos block, with intense tectonic activities. It can be seen from the analysis based on leveling data that the current vertical deformation in down-faulted basins around Ordos is controlled by the regional geological structure, and the differential movement is characterized by inherited motions. Basins are comparatively subsiding, and most mountainous areas are uplifted. Among them, the Ordos block, the Alxa block, the Yinshan, the Helanshan and Zhuozishan mountain ranges are usually rising, and the Linhe, Baiyanhua, Hubao and Jartai-Yinchuan basins show relative subsidence, with an evident deformation gradient.
This paper has been published in Chinese in the Journal of Technology for Earthquake Disaster Prevention, Volume 12, Number 3, 2017.
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