Gravity Anomalies in East Nepal and their Implications to the Crustal Structure of the Himalayas

Abstract

Summary Gravity measurements were carried out at 145 points in eastern Nepal as a part of the scientific programme of the Japanese Mount Everest Expedition 1970. As the position and elevation of the gravity stations were determined by maps with the aid of an aneroid altimeter, they have standard errors of about 400 m and 33 my respectively. Correspondingly, the gravity anomalies presented in this paper have errors of 5-15 mgal. Free-air, Bouguer, Pratt-Hayford and Airy-Heiskanen anomalies were obtained from these data. A terrain correction of Hayford zones F-0 was applied in the calculation of the latter three anomalies. The terrain correction amounts to more than 40 mgal at many stations. The gravity data were divided into three groups according to geological-morphological zones of east Nepal; (a) Higher Himalayas, (b) Lower Himalayas, and (c) Foothills. Different anomaly-elevation relations were indicated by free-air and Bouguer anomalies of different zones. Especially, Bouguer anomalies in individual zones show almost no correlation with the heights, suggesting that isostasy does not prevail in the wavelength of less than 100 km. Pratt-Hayford and Airy-Heiskanen isostatic anomalies show a weak positive correlation with the elevation. In this case the trend is not significantly different among the subdivided zones. It was observed that Bouguer anomalies projected on a line perpendicular to the structural trend of the Himalayas in this region show a remarkable linear relation with the distance measured along this line. Although the topography in this area is quite rugged because of the erosion due to the north-south running rivers, the surface of the substratum seems to be quite smooth and Bouguer anomalies are caused by almost two-dimensional structure of the Himalayas. The crust under the Himalayas is much thinner than that expected from isostatic equilibrium. Thus the Himalayas seem to be isostatically not in equilibrium in longer as well as shorter wave lengths. The lack of isostatic equilibrium and the existence of very smooth, inclined substratum suggest that the Himalayas are currently under the influence of large scale tectonic forces, probably due to collision of the Indian plate against the Asian plate.