CRUSTAL THICKNESS AND DENSITIES IN THE UPPER MANTLE BENEATH CHINATHE RESULTS OF THREE DIMENSIONAL GRAVITY INVERSION

Based on 11 average Bouguer gravity anomalies of China three-dimensional gravity inversion for the whole country is carried out by Parker-Oldenburgs algorithm. The potential field formula of Parker (1973) is an exact mathematical expression and requires less computing time. In the present study, a great deal of current seismic data are utilized at control points and some improvements in the inversion are made to give reasonable results. The distribution of crustal thickness in China is obtained and the densities in the upper mantle down to a depth of 120 km are first presented as well.The results indicate that crustal thickness in China increases from 30-40 km for the eastern coast up to 68 km for most of the Tibetan Plateau, and changes smoothly in eastern China but appears more complicated in the West. There exist some correlations between the crustal thicknesses and geological structures. The upper mantle densities beneath the Tibetan Plateau and the western region of China are 3.40-3.65 g/cc in general, higher than 3.23-3.30 g/cc for the extensive east part of China. The eastern gravity gradient zone reflects the existence of a deep tectonic zone in the lower crust and the upper mantle beneath China, which may mean not only a crustal thickness gradient but also a higher density zone in the upper mantle. Isostatic adjustment is one of the reasons responsible for the uplift of the Tibetan Plateau.The author infers primarily that the upper mantle beneath China could be divided into three regions: the Tibetan plateau, the Middle-transition region and the Normal-east regions according to the lateral inhomogeneities, from which some geophysical phenomena are discussed.