CRUSTAL STRUCTURE INFERRED FROM GRAVITY-ANOMALIES IN SOUTH CHINA

The Bouguer gravity anomaly map of the mainland of South China and the South China Sea, compiled from the most recent 1/500,000 and 1/1,000, 000 gravity data is presented. The distribution of faults including th e secondary faults, which can be located by a method based on cross-co rrelation between the theoretical anomaly due to a vertical step and t he second vertical derivative of the Bouguer anomalies, large strike-s lip faults without vertical displacement or with a small throw, which are difficult to identify on the Bouguer gravity map and can be inferr ed from the distribution of the secondary faults, and major faults inf erred from the zones of gravity gradients on the Bouguer gravity map, is presented also. Comparison of the regional gravity anomalies with k nown density interfaces deduced from seismic data and geological maps shows that the main geological factors causing gravity anomalies are g ranites, low-velocity layers in the upper crust, the Moho discontinuit y and the upper crustal thickness variations, The broad trend of gravi ty anomalies depends mainly on the Moho and the upper crustal thicknes s variations. We have attempted to isolate approximately the anomalies due to the upper crustal thickness variations by subtracting the anom alies due to the Moho from the regional anomalies, and then the upper crustal thickness in South China can be estimated by inversion of the gravity anomalies.