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)