藏南聂拉木—门布重力剖面的初步地质解释

THE PRELIMINARY INTERPRETATIONOF THE GRAVITY PROFILE OF NIELAMU-MENBU, SOUTHERN TIBET

一、前言 目前世界上地壳构造上升运动比较强烈的地区,主要集中在:(1)海岭地区;(2)岛弧—地槽地区;(3)年轻的山脉地带。近几年来,由于海底的广泛地球物理测量和海底扩张学说的出现以及板块理论的进一步研究,使得与前面二种区域上升有关的许多重要问题变得清楚一些。而对后一种区域即山脉地带形成的上升力量,则了解的较少。山脉地带地壳上升的力源问题早在一个世纪之前就开始研究了。

Based on the gravity measurements carried out in 1981, the crustal thickness and the geological structure of a portion of southern Tibet were interpreted qualitatively and quantitatively. The Bouguer anomalies and the free-air anomalies were analysed by using the isostatic thorem, and the crustal thickness was estimated by means of the gravity sounding formula,It is concluded that in the Himalayas the Moho-surface is deeper in the north than in the south. The inclination of the Moho-surface is about 10? The Himalayas, at present, seem to be not in gravity equilibrium and the isostatic compen-sation is still the principal form of the crustal activities in the area. The crust underneath the Himalayas is thinner than that expected from isostatic compensation. The lack of isostatic compensation and the inclination and smooth of the Moho-surface suggest that the Himalayas are currently under the influence of large-scale upward tectonic forces. The collision of the Indian plate against the Asian plate is probably the main factor. In addition, the inclination of the Moho-surface of the Indian plate near the suture line of the two plates suggests that the Indian plate plunges into the Asian plate in about 20°. The analysis of the gravity anomalies in the Himalayas indicates that the isostatic level of the lower Himalayas is better than that of the higher so that the crust of the former is relatively stable.The faults in the Himalayas are mainly crustal faults. Thus, there is no significant influence on the Moho-surface.