The Gravity and Magnetic Field Characteristics of North China Area and Its Tectonic Significance.

English Abstract:

The Bouguer gravity and aeromagnetic anomaly data sets of North China Craton are finely processed and the method such as anomaly separation, reduction to equation pole by varying declination angles, tectonic feature extraction are used to obtain enhancing and comparative results. With comprehensive understanding of MT and other geological results, the identification of tectonic units and faults by gravity and magnetic field are preformed and the 3D distribution of density contrast and equivalent susceptibility in the scale of lithosphere are obtained.

The framework of regional gravity field in North China can be summarized as “East high and west low for regional gravity field, three vertical and two horizontal for gradient gravity zone”. The magnetic field has high amplitude, complex for negative and positive characteristics and also has different kinds of trends.

From the northeast to southwest four orogneic belts and eight tectonic units can be identified according to the corresponding gravity and magnetic field. Four orogneic belts include central Asian orogneic belt, Yinshan orogenic belt, central orognic belt and Qinling orogenic belt. Eight tectonic units consist of Alashan block, Qinlian block, East Kulun block, Songpan block, Eastern block，Western block, Yangtze Block and Sulu block. In addition, eleven main faults can be recognized by gravity and magnetic lineament.

The 0-40km and 0-15km 3D subsurface distributions of density contrast equivalent susceptibility in North China Craton are obtained by 3D gravity inversion and 3D correlation for gravity and magnetic data respectively. The analysis shows that the central orogenci belts region has an overall low density contrast. With increasing depth, density contrast changes by a larger range. The south, north, middle abnormal variations are not identical. This discrepancy may be illustrated by the south, north and central orogneic belt experienced different geological process, and also implies that the lithosphere destruction in North China may have extended from the eastern block to the area. Similarly, the magnetic distribution of North China Craton is not uniform for shallow to deep region in each block units. The boundaries of most tectonic belts are approximately located at the transitional belts of high and low magnetic distribution.