南海中部和北部海域重力异常特征与地壳构造关系

1976年,中国科学院南海海洋研究所与国家海洋局南海分局共同协作使用“向阳红”五号海洋调查船,利用西德GSS-2型海洋重力仪和我国的CHHK-1型核子旋进式磁力仪,在南海珠江口外海域(北纬22°—17°、东经113°50′—115°10′),进行约3000公里的海洋重力、磁力和测深。设计的测线方向为南北向,测线距为10海里。

THE RELATION BETWEEN THE CHARACTERISTICS OF THE GRAVITY ANOMALIES AND THE CRUST STRUCTURE IN THE CENTRAL AND NORTHERN REGIONS OF THE SOUTH CHINA SEA

The data collected in the gravitational and geomagnetic measurements conducted in the South China Sea (A22°-12°N,φ109°-117°30’E) during recent years show that the different types of crustal structures in the regions studied exhibit different geophysical characteristics.The crust structure of the continental shelf and the islands is similar to that of the continental margin.The Bouguer’s gravity anomalies are of low values.The free-air gravity anomalies are mostly positive.The isostatic gravity anomalies range from +13 to +20 mgal.and the depth of Mohorovicic discontinuity is generally about 30 km.The magnetic anomalies are of low values,which change slowly,and have the superpositions of the sub-anomalies.All these obviously reflect the characteristics of the continental crust.The amplitudes of the Bouguer’s gravity anomalies of the continental slope and the island slope show a larger fluctuation,about 110 mgal.The free-air gravity anomalies are comparatively weak positive or negative anomalies.The depth of the Mohorovicic discontinuity is of about 20 km.The signs of magnetic anomalies also change alternately between positive and negative.All these reflect that the earth crust structure is transitional type from continental to oceanic.The values of the Bouguer’s gravity anomalies of the deep-sea basin are larger than "middle value" shown in the Fig.1,among which those of "higher values" account for 2/5 of the deep-sea basin area of the survey region.The depth of the Mohorovicic discontinuity,except that of the several isolated sea-mountains,is of about 10 km.According to the assumption based on the characteristics of the magnetic field,the basement is composed of the oceanic type of basalt reflecting that the crustal structure hero is similar to oceanic.