Gravity anomaly and crustal density structure in Jilantai rift zone and its adjacent region

Earthquake Science, Aug 2016

This paper deals with the interpretation of Bouguer gravity anomalies measured along a 250 km long Suhaitu-Etuokeqi gravity profile located at the transitional zone of the Alxa and Ordos blocks where geophysical characteristics are very complex. The analysis is carried out in terms of the ratio of elevation and Bouguer gravity anomaly, the normalized full gradient of a section of the Bouguer gravity anomaly (G h ) and the crustal density structure reveal that (1) the ratio of highs and lows of elevation and Bouguer gravity anomaly is large between Zhengyiguan fault (F4) and Helandonglu fault (F6), which can be explained due to crustal inhomogeneities related to the uplift of the Qinghai-Tibet block in the northeast; (2) the main active faults correspond to the G h contour strip or cut the local region, and generally show strong deformation characteristics, for example the Bayanwulashan mountain front fault (F1) or the southeast boundary of Alxa block is in accord with the western change belt of G h , a belt about 10 km wide that extends to about 30 km; (3) Yinchuan-Pingluo fault (F8) is the seismogenic structure of the Pingluo M earthquake, and its focal depth is about 15 km; (4) the Moho depth trend and Bouguer gravity anomaly variation indicates that the regional gravity field is strongly correlated with the Moho discontinuity.

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Gravity anomaly and crustal density structure in Jilantai rift zone and its adjacent region

Earthq Sci ( Gravity anomaly and crustal density structure in Jilantai rift zone and its adjacent region 0 G. Wu (&) C. Shen H. Tan G. Yang The Key Laboratory of Earthquake Geodesy, Institute of Seismology , China Earthquake Administration, Wuhan 430071 , China This paper deals with the interpretation of Bouguer gravity anomalies measured along a 250 km long Suhaitu-Etuokeqi gravity profile located at the transitional zone of the Alxa and Ordos blocks where geophysical characteristics are very complex. The analysis is carried out in terms of the ratio of elevation and Bouguer gravity anomaly, the normalized full gradient of a section of the Bouguer gravity anomaly (Gh) and the crustal density structure reveal that (1) the ratio of highs and lows of elevation and Bouguer gravity anomaly is large between Zhengyiguan fault (F4) and Helandonglu fault (F6), which can be explained due to crustal inhomogeneities related to the uplift of the Qinghai-Tibet block in the northeast; (2) the main active faults correspond to the Gh contour strip or cut the local region, and generally show strong deformation characteristics, for example the Bayanwulashan mountain front fault (F1) or the southeast boundary of Alxa block is in accord with the western change belt of Gh, a belt about 10 km wide that extends to about 30 km; (3) YinchuanPingluo fault (F8) is the seismogenic structure of the Pingluo M earthquake, and its focal depth is about 15 km; (4) the Moho depth trend and Bouguer gravity anomaly variation indicates that the regional gravity field is strongly correlated with the Moho discontinuity. Yinchuan-Helanshan uplift; Jilantai rift zone; Gravity profile; Bouguer gravity anomaly; Crustal density structure; Normalized full gravity gradient; Seismogenesis 1 Introduction The Jilantai rift zone and its adjacent region are located at the border of the Ordos and Alxa blocks, with the stable Alxa block and Ordos block to the west and east, respectively, and in the middle of it is a tectonic deformation transitional belt. The previous research results in the area focus on the Helanshan-Yinchuan basin. Tongen and Jialiu (1982) analyzed the deep structure surrounding the North-South seismic belt and discussed the crust and upper mantle structure feature of the Helanshan-Liupanshan seismic zone. Chai et al. (2006) gave the blind fault activity characteristics in Yinchuan and the surrounding area comprehensively. Fang et al. (2009) studied the relationship between the Huanghe, Yinchuan, and Helanshandonglu faults and their shallow and deep structure using a deep seismic reflection method. Other researchers (Zheng 2011; Yin et al. 2013) then obtained the electrical structure of the Ordos block and JiangyuanNingxia region. All of this research has mainly focused on the deep seismic reflection method and magnetotelluric sounding method in the area, but there are few research findings based on gravity. There is also a lack of density structure research in the region. Gravity profile detection has been widely used; we can obtain the surface gravity anomaly with high precision, small spacing, and crossing fault observation, and then invert the density distribution of the ground material and estimate the geological structure distribution in the crust and media deformation. For example, Halls and Mound (1998) , and Nitescu and Halls (2002) established the faultdeveloping situation in the Kapuskasing structural zone with gravity profile and GPS data. Meng et al. (1990) , Wang et al. (1997) , and Yang et al. (2015) all found crustal block structures using gravity profile data and so on. With support from the project ‘‘China Seismic Array,’’ we found the Suhait-Etuokeqi gravity profile of about 250 km (the blue star line is shown in Fig. 1), which spans the Alxa block, Jilantai rift zone, Helanshan uplift, Yinchuan basin, and Ordos block. In this article, we analyze the distribution features of the gravity anomaly along the profile, which reflects the characteristics of the density of the hierarchical structure of the crust. 2 Geological setting The research area is located in the northwest of Ordos, as shown in Fig. 1. The Jilantai rift zone is mainly in a NorthNorth-West direction and is inside the north China Craton Plate tectonic deformation belt, where its tectonic movement is associated with the mutual movement of the Ordos and Alxa blocks. Since the Cenozoic era, the change of stress in the north China has made the tensions of the Alxa and Ordos blocks affect each other, which means that the region’s tectonic movement has priority over the stretching deformation within the mainland (Hong et al. 1987; Yin and Harrison 2000; Xu et al. 2000; Mao and Yao 1982) . Most of the research area is covered by Quaternary sediments. The covering of quaternary sediments is very serious in the Alxa block part and the outcrop conditions are poor, where it is mainly Variscan. The faults are very developed, and they distribute (...truncated)


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Guiju Wu, Chongyang Shen, Hongbo Tan, Guangliang Yang. Gravity anomaly and crustal density structure in Jilantai rift zone and its adjacent region, Earthquake Science, 2016, pp. 235-242, Volume 29, Issue 4, DOI: 10.1007/s11589-016-0158-z