Gravity Anomalies in East Nepal and their Implications to the Crustal Structure of the Himalayas
Gravity Anomalies in East Nepal and their Implications to the Crustal Structure of the Himalayas
复制标题
尼泊尔东部重力异常及其对喜马拉雅山地壳结构的影响
DOI：
10.1111/j.1365-246x.1974.tb05455.x
复制
发表时间：
1974
影响因子：
2.8
通讯作者：
M. Kono
中科院分区：
文献类型：
--
作者：
M. Kono
作为1970年日本珠峰峰考察科学方案的一部分，在尼泊尔东部的145个点进行了重力测量。由于重力站的位置和高程是借助无液高度计在地图上确定的，它们的标准误差分别约为400米和33米。相应地，本文给出的重力异常误差为5-15 mgal。从这些数据中获得了自由空气、布格、普拉特-海福德和艾里-海斯卡宁异常。后三个异常的计算中应用了Hayford区F-0的地形校正。地形校正量在许多站超过40毫伽。根据尼泊尔东部的地质地貌带，将重力数据分为三组：（a）高喜马拉雅山，（B）低喜马拉雅山，和（c）山麓。不同带的自由空气异常和布格异常反映了不同的异常-高程关系。特别是个别带的布格异常与高度几乎没有相关性，这表明在波长小于100 km的地区，等厚线不占优势。Pratt-Hayford和Airy-Heiskanen均衡异常与高程呈弱正相关。在这种情况下，细分区域之间的趋势没有显著差异。布格异常在垂直于喜马拉雅构造走向的直线上的投影与沿着该直线测得的距离呈显著的线性关系。虽然由于南北走向的河流的侵蚀，该地区的地形相当崎岖，但基底表面似乎相当光滑，布格异常是由喜马拉雅山几乎二维的结构造成的。喜马拉雅山下的地壳比均衡理论所预期的要薄得多。因此，喜马拉雅山似乎是均衡的，而不是在较长和较短的波长平衡。缺乏均衡平衡和存在非常平滑，倾斜的底层表明，喜马拉雅山目前受到大规模构造力量的影响，可能是由于印度板块对亚洲板块的碰撞。
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.