Abstract
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.
Highlights
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
With support from the project ‘‘China Seismic Array,’’ we found the Suhait-Etuokeqi gravity profile of about 250 km, which spans the Alxa block, Jilantai rift zone, Helanshan uplift, Yinchuan basin, and Ordos block
By analyzing the results of the normalized full gradient (NFG) and of the background structure and geology, we found that the transition zones of the NFG are consistent with the distribution of faults, and their dip angles coincide with each other, especially F6, F7, F8, and F9
Summary
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. 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. 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. 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. Earthquake activity is strong in this region; the Pingluo M 8 earthquake occurred in 1739, but its seismogenic structure has been controversial. Mao and Yao (1982), Li and Wan (1984) and Wu et al. Gravity anomaly
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