ISOSTATIC STATUS IN NORTH CHINA (2)--COMPOSITE COMPENSATION AND DEEP STRUCTURE

From the theory of experimental isostasy, an isostatic response function and composite compensation model are derived for that region. There is a quite complicated relationship between topography and gravitational field, in which the topographical relief with a wavelength shorter than 300 km is of relatively high isostatic response. In the isostatic compensation process of North China local compensation is essential, and the ratio of its gravitational effects to that of the total compensation is 90%, in contrast to this, the thickness of equivalent elastic plate of regional compensation is small, being only 18 km. This indicates that in North China there exist serious crustal cracking and prominent rheological property in the lower crust. It is found that the distribution of isostatic anomaly shows blockwise pattern, furthermore the direction of isostatic adjustment is not completely identical to that of Cenozoic tectonic movement. Some isostatic anomalies come from inhomogeneous internal loads distributed in the shallower crust or at the surface, and can not be interpreted simply in terms of under- or over-compensation. The vertical derivative of isostatic anomaly distinctly reveals several important tectonic boundaries within North China. In the aspect of deep structure, the isostatic compensation process takes place in the lower crust and especially in the upper mantle. The seismic risk in North China is also discussed from the view point of isostaty.