ISOSTATIC STATUS IN NORTH CHINA (1)--METHOD AND LOCAL COMPENSATION

Follwing Airy and Pratt principles, five kinds of local compensation models are analysed with emphasis and a rapid 3-D gravity formula is utilized to calculate isostatic anomaly for sixty-six models with different parameters. It is noted that isostatic gravity maps appear nearly identical in their main patterns and features. The optimum compensation model in North China is one of modified Airy models in which the different density distribution in the surface, upper crust and lower crust are taken into account and the standard crustal thickness is 50 km. The position of complete compensation interface is located in the upper mantle. North China platform as a whole is under the sub-isostatic equilibrium status with isostatic anomaly of 1810-5 m/s2 on an average. The distribution of isostatic gravity anomaly shows an obvious blockwise pattern. Most positive anomaly areas occur over the eastern part, the Jiao-Liao block, Mt. Yan block and northern margin of Hebei-Shandong block, whereas negative area occurs in the Shanxi graben. Comparison of models indicates that the Moho discontinuity is not suitable to be taken as a compensation interface, and the compensation effects in Airy model are better than in Pratt model, which is consistent with the feature of dominantly layered structure with less lateral inhomogeneity in crust. Some results about composite compensation, the basic characteristics of isostatic gravity anomaly and deep structure will be published later in the second paper.